Phosphoric acid
Encyclopedia
Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a mineral (inorganic) acid
having the chemical formula
H
3P
O
4. Orthophosphoric acid
molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way. The term phosphoric acid can also refer to a chemical or reagent
consisting of phosphoric acids, usually orthophosphoric acid.
phosphoric acid is a white solid that melts at 42.35 °C to form a colorless, viscous liquid.
Most people and even chemists refer to orthophosphoric acid as phosphoric acid, which is the IUPAC name for this compound. The prefix ortho
is used to distinguish the acid from other phosphoric acids, called polyphosphoric acids two(ii). Orthophosphoric acid is a non-toxic, inorganic, rather weak triprotic acid
, which, when pure, is a solid
at room temperature
and pressure
. The chemical structure
of orthophosphoric acid is shown above in the data table. Orthophosphoric acid is a very polar
molecule
; therefore it is highly soluble in water. The oxidation state
of phosphorus
(P) in ortho- and other phosphoric acids is +5; the oxidation state of all the oxygen
atoms (O) is −2 and all the hydrogen
atoms (H) is +1. Triprotic means that an orthophosphoric acid molecule can dissociate up to three times, giving up an H+ each time, which typically combines with a water molecule, H2O, as shown in these reactions
:
The anion after the first dissociation, H2PO4−, is the dihydrogen phosphate anion. The anion after the second dissociation, HPO42−, is the hydrogen phosphate anion. The anion after the third dissociation, PO43−, is the phosphate
or orthophosphate anion. For each of the dissociation reactions shown above, there is a separate acid dissociation constant
, called Ka1, Ka2, and Ka3 given at 25 °C. Associated with these three dissociation constants are corresponding pKa1=2.12 , pKa2=7.21 , and pKa3=12.67 values at 25 °C. Even though all three hydrogen
(H) atoms are equivalent on an orthophosphoric acid molecule, the successive Ka values differ since it is energetically less favorable to lose another H+ if one (or more) has already been lost and the molecule/ion is more negatively-charged.
Because the triprotic dissociation of orthophosphoric acid, the fact that its conjugate bases (the phosphates mentioned above) cover a wide pH
range, and, because phosphoric acid/phosphate solution
s are, in general, non-toxic, mixtures of these types of phosphates are often used as buffering agents or to make buffer solution
s, where the desired pH depends on the proportions of the phosphates in the mixtures. Similarly, the non-toxic, anion salt
s of triprotic organic
citric acid
are also often used to make buffers. Phosphates are found pervasively in biology, especially in the compounds derived from phosphorylated sugar
s, such as DNA
, RNA
, and adenosine triphosphate
(ATP). There is a separate article on phosphate
as an anion or its salts.
Upon heating orthophosphoric acid, condensation of the phosphoric units can be induced by driving off the water formed from condensation. When one molecule of water has been removed for each two molecules of phosphoric acid, the result is pyrophosphoric acid
(H4P2O7). When an average of one molecule of water per phosphoric unit has been driven off, the resulting substance is a glassy solid having an empirical formula of HPO3 and is called metaphosphoric acid. Metaphosphoric acid is a singly anhydrous version of orthophosphoic acid and is sometimes used as a water- or moisture-absorbing reagent. Further dehydrating
is very difficult, and can be accomplished only by means of an extremely strong desiccant
(and not by heating alone). It produces phosphoric anhydride, which has an empirical formula P2O5, although an actual molecule has a chemical formula of P4O10. Phosphoric anhydride is a solid, which is very strongly moisture-absorbing and is used as a desiccant
.
For large acid concentrations, the solution is mainly composed of H3PO4. For [A] = 10−2, the pH is close to pKa1, giving an equimolar mixture of H3PO4 and H2PO4−. For [A] below 10−3, the solution is mainly composed of H2PO4− with [HPO42−] becoming non negligible for very dilute solutions. [PO43−] is always negligible. Note that the above analysis does not take into account ion activity coefficients; as such, the pH and molarity of a real phosphoric acid solution may deviate substantially from the above values.
s (the most common) are clear, colourless, odourless, non-volatile
, rather viscous, syrupy liquid
s, but still pourable. Phosphoric acid is very commonly used as an aqueous solution of 85% phosphoric acid or H3PO4. Because it is a concentrated acid, an 85% solution can be corrosive
, although nontoxic when diluted. Because of the high percentage of phosphoric acid in this reagent, at least some of the orthophosphoric acid is condensed into polyphosphoric acids in a temperature-dependent equilibrium
, but, for the sake of labeling and simplicity, the 85% represents H3PO4 as if it were all orthophosphoric acid. Other percentages are possible too, even above 100%, where the phosphoric acids and water would be in an unspecified equilibrium, but the overall elemental mole
content would be considered specified. When aqueous solutions of phosphoric acid and/or phosphate are dilute, they are in or will reach an equilibrium after a while where practically all the phosphoric/phosphate units are in the ortho- form.
, Fe2O3 (rust
) to black ferric phosphate
, FePO4.
"Rust converter" is sometimes a greenish liquid suitable for dipping (in the same sort of acid bath as is used for pickling metal
), but it is more often formulated as a gel
, commonly called naval jelly. It is sometimes sold under other names, such as "rust remover" or "rust killer". As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces.
After treatment, the black ferric-phosphate coating can be scrubbed off, leaving a fresh metal surface. Multiple applications of phosphoric acid may be required to remove all rust. The black phosphate coating can also be left in place, where it will provide moderate further corrosion resistance (such protection is also provided by the superficially similar Parkerizing and blued
electrochemical conversion coating
processes).
) is used to acidify foods and beverages such as various cola
s, but not without controversy regarding its health effects. It provides a tangy or sour taste and, being a mass-produced chemical, is available cheaply and in large quantities. The low cost and bulk availability is unlike more expensive seasonings that give comparable flavors, such as citric acid
which is obtainable from lemon
s and limes
. However, most citric acid in the food industry is not extracted from citrus fruit, but fermented by Aspergillus niger
mold from scrap molasses
, waste starch
hydrolysates
and phosphoric acid.
), has been linked to lower bone density in epidemiological studies. For example, a study using dual-energy X-ray absorptiometry rather than a questionnaire about breakage, provides reasonable evidence to support the theory that drinking cola results in lower bone density. This study was published in the American Journal of Clinical Nutrition
. A total of 1672 women and 1148 men were studied between 1996 and 2001. Dietary information was collected using a food frequency questionnaire that had specific questions about the number of servings of cola and other carbonated beverages and that also made a differentiation between regular, caffeine-free, and diet drinks. The paper cites significant statistical evidence to show that women who consume cola daily have lower bone density. Total phosphorus intake was not significantly higher in daily cola consumers than in nonconsumers; however, the calcium-to-phosphorus ratios were lower.
On the other hand, another study suggests that insufficient intake of phosphorus
leads to lower bone density
. The study does not examine the effect of phosphoric acid, which binds with magnesium
and calcium
in the digestive tract to form salts that are not absorbed, but rather studies general phosphorus intake.
A clinical study by Heaney and Rafferty using calcium-balance methods found no impact of carbonated soft drinks containing phosphoric acid on calcium excretion. The study compared the impact of water, milk, and various soft drinks (two with caffeine and two without; two with phosphoric acid and two with citric acid) on the calcium balance of 20- to 40-year-old women who customarily consumed ~3 or more cups (680 mL) of a carbonated soft drink per day. They found that, relative to water, only milk and the two caffeine-containing soft drinks increased urinary calcium, and that the calcium loss associated with the caffeinated soft drink consumption was about equal to that previously found for caffeine alone. Phosphoric acid without caffeine had no impact on urine calcium, nor did it augment the urinary calcium loss related to caffeine. Because studies have shown that the effect of caffeine is compensated for by reduced calcium losses later in the day, Heaney and Rafferty concluded that the net effect of carbonated beverages—including those with caffeine and phosphoric acid—is negligible, and that the skeletal effects of carbonated soft drink consumption are likely due primarily to milk displacement.
Other chemicals such as caffeine
(also a significant component of popular common cola drinks) were also suspected as possible contributors to low bone density, due to the known effect of caffeine on calciuria. One other study, involving 30 women over the course of a week, suggests that phosphoric acid in colas has no such effect, and postulates that caffeine has only a temporary effect, which is later reversed. The authors of this study conclude that the skeletal effects of carbonated beverage consumption are likely due primarily to milk displacement (another possible confounding factor may be an association between high soft drink consumption and sedentary lifestyle).
Cola consumption has also been associated with chronic kidney disease and kidney stones through medical research. The preliminary results suggest that cola consumption may increase the risk of chronic kidney disease.
and orthodontics
as an etch
ing solution, to clean and roughen the surfaces of teeth where dental appliances or fillings will be placed.
Phosphoric acid is also an ingredient in over-the-counter anti-nausea medications that also contain high levels of sugar
(glucose
and fructose
). This acid is also used in many teeth whiteners to eliminate plaque that may be on the teeth before application.
to produce phosphorus pentoxide
and dissolving the product in dilute phosphoric acid. This produces a very pure phosphoric acid, since most impurities present in the rock have been removed when extracting phosphorus from the rock in a furnace. The end result is food-grade, thermal phosphoric acid; however, for critical applications, additional processing to remove arsenic compounds may be needed.
Elemental phosphorus
is produced by an electric furnace. At a high temperature, a mixture of phosphate ore, silica and carbonaceous material (coke, coal etc...) produces calcium silicate, phosphorus gas and carbon monoxide. The P and CO off-gases from this reaction are cooled under water to isolate solid phosphorus. Alternatively, the P and CO off-gases can be burned with air to produce phosphorus pentoxide
and carbon dioxide.
to tricalcium phosphate
rock, typically found in nature as apatite
.
The reaction is:
The initial phosphoric acid solution may contain 23–33% P2O5, but can be concentrated by the evaporation of water to produce commercial- or merchant-grade phosphoric acid, which contains about 54% P2O5. Further evaporation of water yields superphosphoric acid with a P2O5 concentration above 70%.
Digestion of the phosphate ore using sulfuric acid yields the insoluble calcium sulfate
(gypsum), which is filtered and removed as phosphogypsum
. Wet-process acid can be further purified by removing fluorine to produce animal-grade phosphoric acid, or by solvent extraction and arsenic removal to produce food-grade phosphoric acid.
Mineral acid
A mineral acid is an acid derived from one or more inorganic compounds. A mineral acid is not organic and all mineral acids release hydrogen ions when dissolved in water.-Characteristics:...
having the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
3P
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
4. Orthophosphoric acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way. The term phosphoric acid can also refer to a chemical or reagent
Reagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...
consisting of phosphoric acids, usually orthophosphoric acid.
Orthophosphoric acid chemistry
Pure anhydrousAnhydrous
As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
phosphoric acid is a white solid that melts at 42.35 °C to form a colorless, viscous liquid.
Most people and even chemists refer to orthophosphoric acid as phosphoric acid, which is the IUPAC name for this compound. The prefix ortho
Ortho
Ortho may refer to:In science* An arene substitution pattern in which two substituents occupy adjacent positions on an aromatic ring...
is used to distinguish the acid from other phosphoric acids, called polyphosphoric acids two(ii). Orthophosphoric acid is a non-toxic, inorganic, rather weak triprotic acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
, which, when pure, is a solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
at room temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
. The chemical structure
Chemical structure
A chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as...
of orthophosphoric acid is shown above in the data table. Orthophosphoric acid is a very polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
; therefore it is highly soluble in water. The oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
of phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
(P) in ortho- and other phosphoric acids is +5; the oxidation state of all the oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atoms (O) is −2 and all the hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atoms (H) is +1. Triprotic means that an orthophosphoric acid molecule can dissociate up to three times, giving up an H+ each time, which typically combines with a water molecule, H2O, as shown in these reactions
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
:
- H3PO4(s) + H2O(l) H3O+(aq) + H2PO4−(aq) Ka1= 7.25×10−3
- H2PO4−(aq)+ H2O(l) H3O+(aq) + HPO42−(aq) Ka2= 6.31×10−8
- HPO42−(aq)+ H2O(l) H3O+(aq) + PO43−(aq) Ka3= 3.98×10−13
The anion after the first dissociation, H2PO4−, is the dihydrogen phosphate anion. The anion after the second dissociation, HPO42−, is the hydrogen phosphate anion. The anion after the third dissociation, PO43−, is the phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
or orthophosphate anion. For each of the dissociation reactions shown above, there is a separate acid dissociation constant
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
, called Ka1, Ka2, and Ka3 given at 25 °C. Associated with these three dissociation constants are corresponding pKa1=2.12 , pKa2=7.21 , and pKa3=12.67 values at 25 °C. Even though all three hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(H) atoms are equivalent on an orthophosphoric acid molecule, the successive Ka values differ since it is energetically less favorable to lose another H+ if one (or more) has already been lost and the molecule/ion is more negatively-charged.
Because the triprotic dissociation of orthophosphoric acid, the fact that its conjugate bases (the phosphates mentioned above) cover a wide pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
range, and, because phosphoric acid/phosphate solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
s are, in general, non-toxic, mixtures of these types of phosphates are often used as buffering agents or to make buffer solution
Buffer solution
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...
s, where the desired pH depends on the proportions of the phosphates in the mixtures. Similarly, the non-toxic, anion salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s of triprotic organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
citric acid
Citric acid
Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...
are also often used to make buffers. Phosphates are found pervasively in biology, especially in the compounds derived from phosphorylated sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
s, such as DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
, RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, and adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
(ATP). There is a separate article on phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
as an anion or its salts.
Upon heating orthophosphoric acid, condensation of the phosphoric units can be induced by driving off the water formed from condensation. When one molecule of water has been removed for each two molecules of phosphoric acid, the result is pyrophosphoric acid
Pyrophosphoric acid
Pyrophosphoric acid, also known under the name diphosphoric acid, is colorless, odorless, hygroscopic and is soluble in water, diethyl ether, and ethyl alcohol. It is produced from phosphoric acid by dehydration. Pyrophosphoric acid slowly hydrolyzes in the presence of water into phosphoric...
(H4P2O7). When an average of one molecule of water per phosphoric unit has been driven off, the resulting substance is a glassy solid having an empirical formula of HPO3 and is called metaphosphoric acid. Metaphosphoric acid is a singly anhydrous version of orthophosphoic acid and is sometimes used as a water- or moisture-absorbing reagent. Further dehydrating
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...
is very difficult, and can be accomplished only by means of an extremely strong desiccant
Desiccant
A desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container....
(and not by heating alone). It produces phosphoric anhydride, which has an empirical formula P2O5, although an actual molecule has a chemical formula of P4O10. Phosphoric anhydride is a solid, which is very strongly moisture-absorbing and is used as a desiccant
Desiccant
A desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container....
.
Composition and pH of a phosphoric acid aqueous solution
For a given total acid concentration [A] = [H3PO4] + [H2PO4−] + [HPO42−] + [PO43−] ([A] is the total number of moles of pure H3PO4 which have been used to prepare 1 liter of solution), the composition of an aqueous solution of phosphoric acid can be calculated using the equilibrium equations associated with the three reactions described above together with the [H+][OH−] = 10−14 relation and the electrical neutrality equation. Possible concentrations of polyphosphoric molecules and ions is neglected. The system may be reduced to a fifth degree equation for [H+] which can be solved numerically, yielding:| [A] (mol/L) | pH | [H3PO4]/[A] (%) | [H2PO4−]/[A] (%) | [HPO42−]/[A] (%) | [PO43−]/[A] (%) |
| 1 | 1.08 | 91.7 | 8.29 | 6.20×10−6 | 1.60×10−17 |
| 10−1 | 1.62 | 76.1 | 23.9 | 6.20×10−5 | 5.55×10−16 |
| 10−2 | 2.25 | 43.1 | 56.9 | 6.20×10−4 | 2.33×10−14 |
| 10−3 | 3.05 | 10.6 | 89.3 | 6.20×10−3 | 1.48×10−12 |
| 10−4 | 4.01 | 1.30 | 98.6 | 6.19×10−2 | 1.34×10−10 |
| 10−5 | 5.00 | 0.133 | 99.3 | 0.612 | 1.30×10−8 |
| 10−6 | 5.97 | 1.34×10−2 | 94.5 | 5.50 | 1.11×10−6 |
| 10−7 | 6.74 | 1.80×10−3 | 74.5 | 25.5 | 3.02×10−5 |
| 10−10 | 7.00 | 8.24×10−4 | 61.7 | 38.3 | 8.18×10−5 |
For large acid concentrations, the solution is mainly composed of H3PO4. For [A] = 10−2, the pH is close to pKa1, giving an equimolar mixture of H3PO4 and H2PO4−. For [A] below 10−3, the solution is mainly composed of H2PO4− with [HPO42−] becoming non negligible for very dilute solutions. [PO43−] is always negligible. Note that the above analysis does not take into account ion activity coefficients; as such, the pH and molarity of a real phosphoric acid solution may deviate substantially from the above values.
Chemical reagent
Pure 75–85% aqueous solutionSolution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
s (the most common) are clear, colourless, odourless, non-volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
, rather viscous, syrupy liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s, but still pourable. Phosphoric acid is very commonly used as an aqueous solution of 85% phosphoric acid or H3PO4. Because it is a concentrated acid, an 85% solution can be corrosive
Corrosive
A corrosive substance is one that will destroy or irreversibly damage another surface or substance with which it comes into contact. The main hazards to people include damage to the eyes, the skin, and the tissue under the skin; inhalation or ingestion of a corrosive substance can damage the...
, although nontoxic when diluted. Because of the high percentage of phosphoric acid in this reagent, at least some of the orthophosphoric acid is condensed into polyphosphoric acids in a temperature-dependent equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
, but, for the sake of labeling and simplicity, the 85% represents H3PO4 as if it were all orthophosphoric acid. Other percentages are possible too, even above 100%, where the phosphoric acids and water would be in an unspecified equilibrium, but the overall elemental mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
content would be considered specified. When aqueous solutions of phosphoric acid and/or phosphate are dilute, they are in or will reach an equilibrium after a while where practically all the phosphoric/phosphate units are in the ortho- form.
Preparation of hydrogen halides
Phosphoric acid reacts with halides to form the corresponding hydrogen halide gas (steamy fumes are observed on warming the reaction mixture). This is a common practice for the laboratory preparation of hydrogen halides.| NaCl(s) | + H3PO4(l) → NaH2PO4(s) | + HCl(g) |
| NaBr(s) | + H3PO4(l) → NaH2PO4(s) | + HBr(g) |
| NaI(s) | + H3PO4(l) → NaH2PO4(s) | + HI(g) |
Rust removal
Phosphoric acid may be used as a "rust converter", by direct application to rusted iron, steel tools, or surfaces. The phosphoric acid converts reddish-brown iron(III) oxideIron(III) oxide
Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...
, Fe2O3 (rust
Rust
Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...
) to black ferric phosphate
Iron(III) phosphate
Iron phosphate, also ferric orthophosphate, or ferric phosphate, FePO4, is a phosphate of iron. It is one of the few molluscicides approved for use in the practice of organic farming....
, FePO4.
"Rust converter" is sometimes a greenish liquid suitable for dipping (in the same sort of acid bath as is used for pickling metal
Pickling (metal)
Tarnision is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, rust or scale from ferrous metals, copper, and aluminum alloys. A solution called Tarnision liquor, which contains strong acids, is used to remove the surface impurities...
), but it is more often formulated as a gel
Gel
A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...
, commonly called naval jelly. It is sometimes sold under other names, such as "rust remover" or "rust killer". As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces.
After treatment, the black ferric-phosphate coating can be scrubbed off, leaving a fresh metal surface. Multiple applications of phosphoric acid may be required to remove all rust. The black phosphate coating can also be left in place, where it will provide moderate further corrosion resistance (such protection is also provided by the superficially similar Parkerizing and blued
Bluing (steel)
Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface...
electrochemical conversion coating
Conversion coating
Conversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for...
processes).
Processed food use
Food-grade phosphoric acid (additive E338E number
E numbers are number codes for food additives that have been assessed for use within the European Union . They are commonly found on food labels throughout the European Union. Safety assessment and approval are the responsibility of the European Food Safety Authority...
) is used to acidify foods and beverages such as various cola
Cola
Cola is a carbonated beverage that was typically flavored by the kola nut as well as vanilla and other flavorings, however, some colas are now flavored artificially. It became popular worldwide after druggist John Pemberton invented Coca-Cola in 1886...
s, but not without controversy regarding its health effects. It provides a tangy or sour taste and, being a mass-produced chemical, is available cheaply and in large quantities. The low cost and bulk availability is unlike more expensive seasonings that give comparable flavors, such as citric acid
Citric acid
Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...
which is obtainable from lemon
Lemon
The lemon is both a small evergreen tree native to Asia, and the tree's ellipsoidal yellow fruit. The fruit is used for culinary and non-culinary purposes throughout the world – primarily for its juice, though the pulp and rind are also used, mainly in cooking and baking...
s and limes
Lime (fruit)
Lime is a term referring to a number of different citrus fruits, both species and hybrids, which are typically round, green to yellow in color, 3–6 cm in diameter, and containing sour and acidic pulp. Limes are a good source of vitamin C. Limes are often used to accent the flavors of foods and...
. However, most citric acid in the food industry is not extracted from citrus fruit, but fermented by Aspergillus niger
Aspergillus niger
Aspergillus niger is a fungus and one of the most common species of the genus Aspergillus. It causes a disease called black mold on certain fruits and vegetables such as grapes, onions, and peanuts, and is a common contaminant of food...
mold from scrap molasses
Molasses
Molasses is a viscous by-product of the processing of sugar cane, grapes or sugar beets into sugar. The word molasses comes from the Portuguese word melaço, which ultimately comes from mel, the Latin word for "honey". The quality of molasses depends on the maturity of the sugar cane or sugar beet,...
, waste starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
hydrolysates
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
and phosphoric acid.
Biological effects on bone calcium and kidney health
Phosphoric acid, used in many soft drinks (primarily colaCola
Cola is a carbonated beverage that was typically flavored by the kola nut as well as vanilla and other flavorings, however, some colas are now flavored artificially. It became popular worldwide after druggist John Pemberton invented Coca-Cola in 1886...
), has been linked to lower bone density in epidemiological studies. For example, a study using dual-energy X-ray absorptiometry rather than a questionnaire about breakage, provides reasonable evidence to support the theory that drinking cola results in lower bone density. This study was published in the American Journal of Clinical Nutrition
American Journal of Clinical Nutrition
The American Journal of Clinical Nutrition is a monthly peer-reviewed medical journal in the field of clinical nutrition.According to the Journal Citation Reports, it has a 2009 impact factor of 6.307, ranking it third among 66 journals in the category "Nutrition & Dietetics".The journal was...
. A total of 1672 women and 1148 men were studied between 1996 and 2001. Dietary information was collected using a food frequency questionnaire that had specific questions about the number of servings of cola and other carbonated beverages and that also made a differentiation between regular, caffeine-free, and diet drinks. The paper cites significant statistical evidence to show that women who consume cola daily have lower bone density. Total phosphorus intake was not significantly higher in daily cola consumers than in nonconsumers; however, the calcium-to-phosphorus ratios were lower.
On the other hand, another study suggests that insufficient intake of phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
leads to lower bone density
Bone density
Bone density is a medical term normally referring to the amount of mineral matter per square centimeter of bones. Bone density is used in clinical medicine as an indirect indicator of osteoporosis and fracture risk.This medical bone density is not the true physical "density" of the bone, which...
. The study does not examine the effect of phosphoric acid, which binds with magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
and calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
in the digestive tract to form salts that are not absorbed, but rather studies general phosphorus intake.
A clinical study by Heaney and Rafferty using calcium-balance methods found no impact of carbonated soft drinks containing phosphoric acid on calcium excretion. The study compared the impact of water, milk, and various soft drinks (two with caffeine and two without; two with phosphoric acid and two with citric acid) on the calcium balance of 20- to 40-year-old women who customarily consumed ~3 or more cups (680 mL) of a carbonated soft drink per day. They found that, relative to water, only milk and the two caffeine-containing soft drinks increased urinary calcium, and that the calcium loss associated with the caffeinated soft drink consumption was about equal to that previously found for caffeine alone. Phosphoric acid without caffeine had no impact on urine calcium, nor did it augment the urinary calcium loss related to caffeine. Because studies have shown that the effect of caffeine is compensated for by reduced calcium losses later in the day, Heaney and Rafferty concluded that the net effect of carbonated beverages—including those with caffeine and phosphoric acid—is negligible, and that the skeletal effects of carbonated soft drink consumption are likely due primarily to milk displacement.
Other chemicals such as caffeine
Caffeine
Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants...
(also a significant component of popular common cola drinks) were also suspected as possible contributors to low bone density, due to the known effect of caffeine on calciuria. One other study, involving 30 women over the course of a week, suggests that phosphoric acid in colas has no such effect, and postulates that caffeine has only a temporary effect, which is later reversed. The authors of this study conclude that the skeletal effects of carbonated beverage consumption are likely due primarily to milk displacement (another possible confounding factor may be an association between high soft drink consumption and sedentary lifestyle).
Cola consumption has also been associated with chronic kidney disease and kidney stones through medical research. The preliminary results suggest that cola consumption may increase the risk of chronic kidney disease.
Medical use
Phosphoric acid is used in dentistryDentistry
Dentistry is the branch of medicine that is involved in the study, diagnosis, prevention, and treatment of diseases, disorders and conditions of the oral cavity, maxillofacial area and the adjacent and associated structures and their impact on the human body. Dentistry is widely considered...
and orthodontics
Orthodontics
Orthodontics, orthodontia, or orthodonture is the first specialty of dentistry that is concerned with the study and treatment of malocclusions , which may be a result of tooth irregularity, disproportionate jaw relationships, or both...
as an etch
Etch
Etch may refer to:*Etch : an open source, cross-platform, multi-language framework for building network services released by Cisco Systems into the Apache Software Foundation*Etch, a character from the film Toy Story, based on the Etch A Sketch...
ing solution, to clean and roughen the surfaces of teeth where dental appliances or fillings will be placed.
Phosphoric acid is also an ingredient in over-the-counter anti-nausea medications that also contain high levels of sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
(glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
). This acid is also used in many teeth whiteners to eliminate plaque that may be on the teeth before application.
Preparation
Phosphoric acid can be prepared by three routes – the thermal process, the wet process and the dry kiln process.Thermal phosphoric acid
This very pure phosphoric acid is obtained by burning elemental phosphorusPhosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
to produce phosphorus pentoxide
Phosphorus pentoxide
Phosphorus pentoxide is a chemical compound with molecular formula P4O10 . This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant.-Structure:...
and dissolving the product in dilute phosphoric acid. This produces a very pure phosphoric acid, since most impurities present in the rock have been removed when extracting phosphorus from the rock in a furnace. The end result is food-grade, thermal phosphoric acid; however, for critical applications, additional processing to remove arsenic compounds may be needed.
Elemental phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
is produced by an electric furnace. At a high temperature, a mixture of phosphate ore, silica and carbonaceous material (coke, coal etc...) produces calcium silicate, phosphorus gas and carbon monoxide. The P and CO off-gases from this reaction are cooled under water to isolate solid phosphorus. Alternatively, the P and CO off-gases can be burned with air to produce phosphorus pentoxide
Phosphorus pentoxide
Phosphorus pentoxide is a chemical compound with molecular formula P4O10 . This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant.-Structure:...
and carbon dioxide.
Wet phosphoric acid
Wet process phosphoric acid is prepared by adding sulfuric acidSulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
to tricalcium phosphate
Tricalcium phosphate
Tricalcium phosphate is a calcium salt of phosphoric acid with the chemical formula Ca32. It is also known as tribasic calcium phosphate or "bone ash" ....
rock, typically found in nature as apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
.
The reaction is:
- Ca5(PO4)3X + 5 H2SO4 + 10 H2O → 3 H3PO4 + 5 CaSO4·2H2O + HX
- where X may include OH, F, Cl, and Br
The initial phosphoric acid solution may contain 23–33% P2O5, but can be concentrated by the evaporation of water to produce commercial- or merchant-grade phosphoric acid, which contains about 54% P2O5. Further evaporation of water yields superphosphoric acid with a P2O5 concentration above 70%.
Digestion of the phosphate ore using sulfuric acid yields the insoluble calcium sulfate
Calcium sulfate
Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...
(gypsum), which is filtered and removed as phosphogypsum
Phosphogypsum
Phosphogypsum refers to the gypsum formed as a by-product of processing phosphate ore into fertilizer with sulfuric acid.Phosphogypsum is produced from the fabrication of phosphoric acid by reacting phosphate ore with sulfuric acid according to the following reaction:Phosphogypsum is radioactive...
. Wet-process acid can be further purified by removing fluorine to produce animal-grade phosphoric acid, or by solvent extraction and arsenic removal to produce food-grade phosphoric acid.
Other applications
- Phosphoric acid is used as an external standard for phosphorusPhosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
-31 Nuclear magnetic resonanceNuclear magnetic resonanceNuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
(NMR). - Phosphoric acid is used for high-performance liquid chromatographyHigh-performance liquid chromatographyHigh-performance liquid chromatography , HPLC, is a chromatographic technique that can separate a mixture of compounds and is used in biochemistry and analytical chemistry to identify, quantify and purify the individual components of the mixture.HPLC typically utilizes different types of stationary...
. - Phosphoric acid is used as a chemical oxidizing agent for activated carbonActivated carbonActivated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...
production, as used in the Wentworth Process.
- Phosphoric acid is used as the electrolyteElectrolyteIn chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
in phosphoric acid fuel cells. - Phosphoric acid is used with distilled water (2–3 drops per gallon) as an electrolyte in oxyhydrogen (HHO) generators.
- Phosphoric acid is used as an electrolyteElectrolyteIn chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
in copperCopperCopper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
electropolishingElectropolishingElectropolishing, also known as electrochemical polishing or electrolytic polishing , is an electrochemical process that removes material from a metallic workpiece. It is used to polish, passivate, and deburr metal parts. It is often described as the reverse of electroplating...
for burr removal and circuit board planarization.
- Phosphoric acid is used as a fluxFlux (metallurgy)In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
by hobbyists (such as model railroaders) as an aid to solderingSolderingSoldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...
. - In compound semiconductorCompound semiconductorA compound semiconductor is a semiconductor compound composed of elements from two or more different groups of the periodic table . These semiconductors typically form in groups 13-16 ,...
processing, phosphoric acid is a common wet etching agent: for example, in combination with hydrogen peroxide and water it is used to etch InGaAsIndium gallium arsenideIndium gallium arsenide is a semiconductor composed of indium, gallium and arsenic. It is used in high-power and high-frequency electronics because of its superior electron velocity with respect to the more common semiconductors silicon and gallium arsenide. InGaAs bandgap also makes it the...
selective to InP. - Hot phosphoric acid is used in microfabricationMicrofabricationMicrofabrication is the term that describes processes of fabrication of miniature structures, of micrometre sizes and smaller. Historically the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" or "semiconductor device...
to etch silicon nitrideSilicon nitrideSilicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...
(Si3N4). It is highly selective in etching Si3N4 instead of SiO2, silicon dioxideSilicon dioxideThe chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
. - Phosphoric acid is used as a cleaner by constructionConstructionIn the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...
trades to remove mineral deposits, cementitious smears, and hard water stains. - Phosphoric acid is also used as a chelantChelationChelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....
in some household cleaners aimed at similar cleaning tasks.
- Phosphoric acid is also used in hydroponicsHydroponicsHydroponics is a method of growing plants using mineral nutrient solutions, in water, without soil. Terrestrial plants may be grown with their roots in the mineral nutrient solution only or in an inert medium, such as perlite, gravel, mineral wool, or coconut husk.Researchers discovered in the 18th...
pH solutions to lower the pH of nutrient solutions. While other types of acids can be used, phosphorus is a nutrient used by plants, especially during flowering, making phosphoric acid particularly desirable. - Phosphoric acid is used as a pH adjuster in cosmetics and skin-care products.
- Phosphoric acid can be used as a dispersing agent in detergents and leather treatment.
- Phosphoric acid can be used as an additive to stabilize acidic aqueous solutions within a wanted and specified pH range
- Phosphoric acid is the ingredient that provides acidity to Coca-ColaCoca-ColaCoca-Cola is a carbonated soft drink sold in stores, restaurants, and vending machines in more than 200 countries. It is produced by The Coca-Cola Company of Atlanta, Georgia, and is often referred to simply as Coke...
and PepsiPepsiPepsi is a carbonated soft drink that is produced and manufactured by PepsiCo...
sodas.