Hard water
Encyclopedia
Hard water is water
that has high mineral
content (in contrast with soft water). Hard water has high concentrations of Ca2+
and Mg2+
ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boiler
s, cooling tower
s, and other equipment that handles water. In domestic settings, the hardness of water is often indicated by the non-formation of suds
when soap
is agitated in the water sample.
. Common calcium-containing minerals are calcite
and gypsum
. A common magnesium mineral is dolomite
(which also contains calcium). Rainwater and distilled water are soft, because they also contain few ions.
The following equilibrium reaction describes the dissolving/formation of calcium carbonate
scales:
Calcium and magnesium ions can sometimes be removed by water softeners.
) instead of producing lather. This effect arises because the 2+ ions destroy the surfactant
properties of the soap by forming a solid precipitate (the soap scum). A major component of such scum is calcium stearate
, which arises from sodium stearate, the main component of soap
:
Hardness can thus be defined as the soap-consuming capacity of a water sample, or the capacity of precipitation of soap as a characteristic property of water that prevents the lathering of soap. Synthetic detergent
s do not form such scums.
Hard water also forms deposits that clog plumbing. These deposits, called "scale", are composed mainly of calcium carbonate
(CaCO3), magnesium hydroxide
(Mg(OH)2), and calcium sulfate
(CaSO4). Calcium and magnesium carbonates tend to be deposited as off-white solids on the surfaces of pipes and the surfaces of heat exchanger
s. This precipitation (formation of an insoluble solid) is principally caused by thermal decomposition of bi-carbonate ions but also happens to some extent even in the absence of such ions. The resulting build-up of scale restricts the flow of water in pipes. In boilers, the deposits impair the flow of heat into water, reducing the heating efficiency and allowing the metal boiler components to overheat. In a pressurized system, this overheating can lead to failure of the boiler. The damage caused by calcium carbonate deposits varies depending on the crystalline form, for example, calcite
or aragonite
.
The presence of ion
s in an electrolyte
, in this case, hard water, can also lead to galvanic corrosion, in which one metal will preferentially corrode when in contact with another type of metal, when both are in contact with an electrolyte. The softening of hard water by ion exchange does not increase its corrosivity per se. Similarly, where lead plumbing is in use, softened water does not substantially increase plumbo-solvency.
s, which replace ions like Ca2+ by twice the number of monocations such as sodium
or potassium
ions.
and magnesium carbonate
). When dissolved, these minerals yield calcium and magnesium cations (Ca2+, Mg2+) and carbonate and bicarbonate anions (CO32-, HCO3-). The presence of the metal cations makes the water hard. However, unlike the permanent hardness caused by sulfate
and chloride
compounds, this "temporary" hardness can be reduced either by boiling the water, or by the addition of lime
(calcium hydroxide
) through the process of lime softening
. Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling.
says that "there does not appear to be any convincing evidence that water hardness causes adverse health effects in humans."
Some studies have shown a weak inverse relationship between water hardness and cardiovascular disease
in men, up to a level of 170 mg calcium carbonate per litre of water. The World Health Organization has reviewed the evidence and concluded the data were inadequate to allow for a recommendation for a level of hardness.
Recommendations have been made for the maximum and minimum levels of calcium (40-80 ppm) and magnesium (20-30 ppm) in drinking water, and a total hardness expressed as the sum of the calcium and magnesium concentrations of 2-4 mmol/L.
Other studies have shown weak correlations between cardiovascular health and water hardness.
Some studies correlate domestic hard water usage with increased eczema in children.
The Softened-Water Eczema Trial (SWET), a multicenter randomized controlled trial of ion-exchange softeners for treating childhood eczema, was undertaken in 2008. However, no meaningful difference in symptom relief was found between children with access to a home water softener and those without.
. The total water hardness, including both Ca2+ and Mg2+ ions, is reported in parts per million (ppm) or mass/volume (mg/L) of calcium carbonate (CaCO3) in the water. Although water hardness usually measures only the total concentrations of calcium and magnesium (the two most prevalent divalent
metal ions), iron
, aluminium
, and manganese
can also be present at elevated levels in some locations. The presence of iron characteristically confers a brownish (rust
-like) colour to the calcification, instead of white (the color of most of the other compounds).
Because it is the precise mixture of minerals dissolved in the water, together with the water's pH
and temperature, that determines the behavior of the hardness, a single-number scale does not adequately describe hardness. Descriptions of hardness correspond roughly with ranges of mineral concentrations:
The level of total hardness in water can be evaluated with commercial testing kits, which measure the concentrations of calcium and magnesium. Several scales are used to describe the hardness of water in different contexts. The hardness is indicated by a calculation where both calcium and magnesium values are reported as mg/L (ppm) (Ca x 2.5) + (Mg x 4.12)= Hardness in mg/L
Although most of the above measures define hardness in terms of concentrations of calcium in water, any combination of calcium and magnesium cations having the same total molarity as a pure calcium solution will yield the same degree of hardness. Consequently, hardness concentrations for naturally occurring waters (which will contain both Ca2+ and Mg2+ ions), are usually expressed as an equivalent concentration of pure calcium in solution. For example, water that contains 1.5 mmol/L of elemental calcium (Ca2+) and 1.0 mmol/L of magnesium (Mg2+) is equivalent in hardness to a 2.5 mmol/L solution of calcium carbonate alone (250.2 ppm as CaCO3).
If the actual pH of the water is below the calculated saturation pH, the LSI is negative and the water has a very limited scaling potential. If the actual pH exceeds pHs, the LSI is positive, and being supersaturated with CaCO3, the water has a tendency to form scale. At increasing positive index values, the scaling potential increases.
In practice, water with an LSI between -0.5 and +0.5 will not display enhanced mineral dissolving or scale forming properties. Water with an LSI below -0.5 tends to exhibit noticeably increased dissolving abilities while water with an LSI above +0.5 tends to exhibit noticeably increased scale forming properties.
It is also worth noting that the LSI is temperature sensitive. The LSI becomes more positive as the water temperature increases. This has particular implications in situations where well water is used. The temperature of the water when it first exits the well is often significantly lower than the temperature inside the building served by the well or at the laboratory where the LSI measurement is made. This increase in temperature can cause scaling, especially in cases such as hot water heaters. Conversely, systems that reduce water temperature will have less scaling.
Ryznar saturation index (RSI) was developed from empirical observations of corrosion rates and film formation in steel mains. It is defined as:
Total Hardness levels of Calcium Carbonate in ppm are:
Canberra
: 40; Melbourne: 10 - 26;
Sydney
: 39.4 – 60.1;
Perth
: 29 – 226;
Brisbane
: 100;
Adelaide
: 134 – 148;
Hobart
: 5.8 – 34.4;
Darwin: 31.
provinces (mainly Saskatchewan
and Manitoba
) contain high quantities of calcium and magnesium, often as dolomite
, which are readily soluble in the groundwater that contains high concentrations of trapped carbon dioxide
from the last glaciation. In these parts of Canada, the total hardness in ppm of calcium carbonate equivalent frequently exceed 200 ppm, if groundwater is the only source of potable water. The west coast, by contrast, has unusually soft water, derived mainly from mountain lakes fed by glaciers and snowmelt.
Some typical values are: Montreal
116 ppm, Calgary
165 ppm, Regina
202 ppm, Saskatoon
< 140 ppm, Winnipeg
77 ppm, Toronto
121 ppm, Vancouver
< 3 ppm, Charlottetown PEI 140 – 150 ppm.
is generally considered to be 'very hard', with most areas of England, particularly east of a line between the Severn and Tees estuaries, exhibiting above 200 ppm for the calcium carbonate equivalent. Wales
, Devon
, Cornwall
and parts of North-West England
are softer water areas, and range from 0 to 200 ppm. In the brewing
industry in England and Wales, water is often deliberately hardened with gypsum
in the process of Burtonisation.
Tap water
in Manchester
in England is soft because it comes from Thirlmere
and Haweswater reservoir
s in the Lake District
, and in their headwaters there is no exposure to limestone
or chalk
. Similarly, tap water in Birmingham
is also soft as it is sourced from the Elan Valley Reservoirs
in Wales.
, South Atlantic-Gulf, Pacific Northwest
, and Hawaii
regions. Moderately hard waters are common in many of the rivers of the Tennessee, Great Lakes
, and Alaska regions. Hard and very hard waters are found in some of the streams in most of the regions throughout the country. The hardest waters (greater than 1,000 ppm) are in streams in Texas, New Mexico, Kansas, Arizona, and southern California.
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
that has high mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
content (in contrast with soft water). Hard water has high concentrations of Ca2+
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...
and Mg2+
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...
ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boiler
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
s, cooling tower
Cooling tower
Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely...
s, and other equipment that handles water. In domestic settings, the hardness of water is often indicated by the non-formation of suds
Foam
-Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...
when soap
Soap
In chemistry, soap is a salt of a fatty acid.IUPAC. "" Compendium of Chemical Terminology, 2nd ed. . Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford . XML on-line corrected version: created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN...
is agitated in the water sample.
Sources of hardness
Hardness in water is defined as concentration of multivalent cations. Multivalent cations are cations (positively charged metal complexes) with a charge greater than 1+. They mainly have the charge of 2+. These cations include Ca2+ and Mg2+. These ions enter a water supply by leaching from minerals within an aquiferAquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
. Common calcium-containing minerals are calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
and gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
. A common magnesium mineral is dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
(which also contains calcium). Rainwater and distilled water are soft, because they also contain few ions.
The following equilibrium reaction describes the dissolving/formation of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
scales:
- CaCO3 + CO2 + H2O Ca2+ + 2HCO3-
Calcium and magnesium ions can sometimes be removed by water softeners.
Effects of hard water
With hard water, soap solutions form a white precipitate (soap scumSoap scum
Soap scum is an informal term for the white solid that results from the addition of soap to hard water. Hard water contains calcium or magnesium ions, which react with the fatty acid component of soap to give what are technically called lime soaps:In this reaction, the sodium ion in soap is...
) instead of producing lather. This effect arises because the 2+ ions destroy the surfactant
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
properties of the soap by forming a solid precipitate (the soap scum). A major component of such scum is calcium stearate
Calcium stearate
Calcium stearate is carboxylate of calcium that is found in some lubricants and surfactants. It is a white waxy powder.-Production and occurrence:Calcium stearate is produced by heating stearic acid, a fatty acid, and calcium oxide:...
, which arises from sodium stearate, the main component of soap
Soap
In chemistry, soap is a salt of a fatty acid.IUPAC. "" Compendium of Chemical Terminology, 2nd ed. . Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford . XML on-line corrected version: created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN...
:
- 2 C17H35COO- + Ca2+ → (C17H35COO)2Ca
Hardness can thus be defined as the soap-consuming capacity of a water sample, or the capacity of precipitation of soap as a characteristic property of water that prevents the lathering of soap. Synthetic detergent
Detergent
A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...
s do not form such scums.
Hard water also forms deposits that clog plumbing. These deposits, called "scale", are composed mainly of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
(CaCO3), magnesium hydroxide
Magnesium hydroxide
Magnesium hydroxide is an inorganic compound with the chemical formula Mg2. As a suspension in water, it is often called milk of magnesia because of its milk-like appearance. The solid mineral form of magnesium hydroxide is known as brucite....
(Mg(OH)2), and 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...
(CaSO4). Calcium and magnesium carbonates tend to be deposited as off-white solids on the surfaces of pipes and the surfaces of heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s. This precipitation (formation of an insoluble solid) is principally caused by thermal decomposition of bi-carbonate ions but also happens to some extent even in the absence of such ions. The resulting build-up of scale restricts the flow of water in pipes. In boilers, the deposits impair the flow of heat into water, reducing the heating efficiency and allowing the metal boiler components to overheat. In a pressurized system, this overheating can lead to failure of the boiler. The damage caused by calcium carbonate deposits varies depending on the crystalline form, for example, calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
or aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
.
The presence of ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s in an electrolyte
Electrolyte
In 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 this case, hard water, can also lead to galvanic corrosion, in which one metal will preferentially corrode when in contact with another type of metal, when both are in contact with an electrolyte. The softening of hard water by ion exchange does not increase its corrosivity per se. Similarly, where lead plumbing is in use, softened water does not substantially increase plumbo-solvency.
Softening
For the reasons discussed above, it is often desirable to soften hard water. Most detergents contain ingredients that counteract the effects of hard water on the surfactants. For this reason, water softening is often unnecessary. Where softening is practiced, it is often recommended to soften only the water sent to domestic hot water systems so as to prevent or delay inefficiencies and damage due to scale formation in water heaters. A common method for water softening involves the use of ion exchange resinIon exchange resin
An ion-exchange resin or ion-exchange polymer is an insoluble matrix normally in the form of small beads, usually white or yellowish, fabricated from an organic polymer substrate. The material has highly developed structure of pores on the surface of which are sites with easily trapped and...
s, which replace ions like Ca2+ by twice the number of monocations such as sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
or potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
ions.
Temporary hardness
Temporary hardness is a type of water hardness caused by the presence of dissolved carbonate minerals (calcium carbonateCalcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
and magnesium carbonate
Magnesium carbonate
Magnesium carbonate, MgCO3, is a white solid that occurs in nature as a mineral. Several hydrated and basic forms of magnesium carbonate also exist as minerals...
). When dissolved, these minerals yield calcium and magnesium cations (Ca2+, Mg2+) and carbonate and bicarbonate anions (CO32-, HCO3-). The presence of the metal cations makes the water hard. However, unlike the permanent hardness caused by sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
and chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
compounds, this "temporary" hardness can be reduced either by boiling the water, or by the addition of lime
Lime (mineral)
Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
(calcium hydroxide
Calcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
) through the process of lime softening
Lime softening
Lime softening, also known as Clark's process, is a type of water treatment used for water softening. In the USA, it is used primarily in the Midwest, Florida and Texas. It utilizes the addition of lime to remove hardness ions by precipitation...
. Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling.
Health considerations
The World Health OrganizationWorld Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...
says that "there does not appear to be any convincing evidence that water hardness causes adverse health effects in humans."
Some studies have shown a weak inverse relationship between water hardness and cardiovascular disease
Cardiovascular disease
Heart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...
in men, up to a level of 170 mg calcium carbonate per litre of water. The World Health Organization has reviewed the evidence and concluded the data were inadequate to allow for a recommendation for a level of hardness.
Recommendations have been made for the maximum and minimum levels of calcium (40-80 ppm) and magnesium (20-30 ppm) in drinking water, and a total hardness expressed as the sum of the calcium and magnesium concentrations of 2-4 mmol/L.
Other studies have shown weak correlations between cardiovascular health and water hardness.
Some studies correlate domestic hard water usage with increased eczema in children.
The Softened-Water Eczema Trial (SWET), a multicenter randomized controlled trial of ion-exchange softeners for treating childhood eczema, was undertaken in 2008. However, no meaningful difference in symptom relief was found between children with access to a home water softener and those without.
Measurement
Hardness can be quantified by instrumental analysisInstrumental chemistry
Instrumental analysis is a field of analytical chemistry that investigates analytes using scientific instruments.-Spectroscopy:Spectroscopy measures the interaction of the molecules with electromagnetic radiation...
. The total water hardness, including both Ca2+ and Mg2+ ions, is reported in parts per million (ppm) or mass/volume (mg/L) of calcium carbonate (CaCO3) in the water. Although water hardness usually measures only the total concentrations of calcium and magnesium (the two most prevalent divalent
Divalent
In chemistry, a divalent ion or molecule has a valence of two and thus can form two bonds with other ions or molecules. An older term for divalent is bivalent....
metal ions), iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, and manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
can also be present at elevated levels in some locations. The presence of iron characteristically confers a brownish (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...
-like) colour to the calcification, instead of white (the color of most of the other compounds).
Because it is the precise mixture of minerals dissolved in the water, together with the water's 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...
and temperature, that determines the behavior of the hardness, a single-number scale does not adequately describe hardness. Descriptions of hardness correspond roughly with ranges of mineral concentrations:
| Soft: | 0–60 mg/L |
| Moderately hard: | 61–120 mg/L |
| Hard: | 121–180 mg/L |
| Very hard: | ≥181 mg/L |
The level of total hardness in water can be evaluated with commercial testing kits, which measure the concentrations of calcium and magnesium. Several scales are used to describe the hardness of water in different contexts. The hardness is indicated by a calculation where both calcium and magnesium values are reported as mg/L (ppm) (Ca x 2.5) + (Mg x 4.12)= Hardness in mg/L
- Parts per million (ppm)
Usually defined as one milligram of calcium carbonateCalcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
(CaCO3) per liter of water (the definition used below). - Grains per GallonGrains per GallonGrain per gallon is a unit of water hardness defined as 1 grain of calcium carbonate dissolved in 1 US gallon of water . It translates into 1 part in about 58,000 parts of water or 17.1 parts per million...
(gpg)
Defined as 1 grainGrain (measure)A grain is a unit of measurement of mass that is nominally based upon the mass of a single seed of a cereal. From the Bronze Age into the Renaissance the average masses of wheat and barley grains were part of the legal definition of units of mass. However, there is no evidence of any country ever...
(64.8 mg) of calcium carbonate per U.S. gallon (3.79 litres), or 17.118 ppm - mmol/L (millimoles per litre)
One millimole of calcium (either Ca2+ or CaCO3) per litre of water corresponds to a hardness of 100.09 ppm or 5.608 dGHDGHDegrees of general hardness is a unit of water hardness, specifically of general hardness.General hardness is a measure of the concentration of metal divalent ions such as calcium and magnesium per volume of water...
, since the molar massMolar massMolar mass, symbol M, is a physical property of a given substance , namely its mass per amount of substance. The base SI unit for mass is the kilogram and that for amount of substance is the mole. Thus, the derived unit for molar mass is kg/mol...
of calcium carbonate is 100.09 g/mol. - Degrees of General Hardness (dGHDGHDegrees of general hardness is a unit of water hardness, specifically of general hardness.General hardness is a measure of the concentration of metal divalent ions such as calcium and magnesium per volume of water...
)
One degree of General Hardness is defined as 10 milligrams of calcium oxide per litre of water, which is the same as one German degree (17.848 ppm). - Various alternative "degrees":
- ClarkThomas Clark (chemist)Thomas Clark was a British chemist. Clark was born in Ayr; he became well known for the discovery of the phosphate of soda, and the process of softening hard water....
degrees (°Clark)/English degrees (°e or e)
One degree Clark is defined as one grainGrain (measure)A grain is a unit of measurement of mass that is nominally based upon the mass of a single seed of a cereal. From the Bronze Age into the Renaissance the average masses of wheat and barley grains were part of the legal definition of units of mass. However, there is no evidence of any country ever...
(64.8 mg) of calcium carbonate per Imperial gallon (4.55 litres) of water, equivalent to 14.254 ppm. - German degrees (Deutsche Härte, °dH or dH)
One degree German is defined as 10 milligrams of calcium oxideCalcium oxideCalcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
per litre of water. This is equivalent to 17.848 milligrams of calcium carbonate per litre of water, or 17.848 ppm. - French degrees (letter written in lower-case to avoid confusion with degree Fahrenheit — not always adhered to)
One degree French is defined as 10 milligrams of calcium carbonate per litre of water, equivalent to 10 ppm. - American degrees
One degree American is defined as one milligram of calcium carbonate per litre of water, equivalent to 1 ppm.
- Clark
Although most of the above measures define hardness in terms of concentrations of calcium in water, any combination of calcium and magnesium cations having the same total molarity as a pure calcium solution will yield the same degree of hardness. Consequently, hardness concentrations for naturally occurring waters (which will contain both Ca2+ and Mg2+ ions), are usually expressed as an equivalent concentration of pure calcium in solution. For example, water that contains 1.5 mmol/L of elemental calcium (Ca2+) and 1.0 mmol/L of magnesium (Mg2+) is equivalent in hardness to a 2.5 mmol/L solution of calcium carbonate alone (250.2 ppm as CaCO3).
Indices
Several indices are used to describe the behaviour of calcium carbonate in water, oil, or gas mixtures.Langelier Saturation Index (LSI)
The Langelier Saturation Index (sometimes Langelier Stability Index) is a calculated number used to predict the calcium carbonate stability of water. It indicates whether the water will precipitate, dissolve, or be in equilibrium with calcium carbonate. In 1936, Wilfred Langelier developed a method for predicting the pH at which water is saturated in calcium carbonate (called pHs). The LSI is expressed as the difference between the actual system pH and the saturation pH:- LSI = pH (measured) - pHs
- For LSI > 0, water is super saturated and tends to precipitate a scale layer of CaCO3.
- For LSI = 0, water is saturated (in equilibrium) with CaCO3. A scale layer of CaCO3 is neither precipitated nor dissolved.
- For LSI < 0, water is under saturated and tends to dissolve solid CaCO3.
If the actual pH of the water is below the calculated saturation pH, the LSI is negative and the water has a very limited scaling potential. If the actual pH exceeds pHs, the LSI is positive, and being supersaturated with CaCO3, the water has a tendency to form scale. At increasing positive index values, the scaling potential increases.
In practice, water with an LSI between -0.5 and +0.5 will not display enhanced mineral dissolving or scale forming properties. Water with an LSI below -0.5 tends to exhibit noticeably increased dissolving abilities while water with an LSI above +0.5 tends to exhibit noticeably increased scale forming properties.
It is also worth noting that the LSI is temperature sensitive. The LSI becomes more positive as the water temperature increases. This has particular implications in situations where well water is used. The temperature of the water when it first exits the well is often significantly lower than the temperature inside the building served by the well or at the laboratory where the LSI measurement is made. This increase in temperature can cause scaling, especially in cases such as hot water heaters. Conversely, systems that reduce water temperature will have less scaling.
Ryznar Stability Index (RSI)
The Ryznar stability index (RSI) uses a database of scale thickness measurements in municipal water systems to predict the effect of water chemistry.Ryznar saturation index (RSI) was developed from empirical observations of corrosion rates and film formation in steel mains. It is defined as:
- RSI = 2 pHs – pH (measured)
- For 6,5 < RSI < 7 water is considered to be approximately at saturation equilibrium with calcium carbonate
- For RSI > 8 water is under saturated and, therefore, would tend to dissolve any existing solid CaCO3
- For RSI < 6,5 water tends to be scale forming
Puckorius Scaling Index (PSI)
The Puckorius Scaling Index (PSI) uses slightly different parameters to quantify the relationship between the saturation state of the water and the amount of limescale deposited.Other indices
Other indices include the Larson-Skold Index, the Stiff-Davis Index, and the Oddo-Tomson Index.Hard water in Australia
Analysis of water hardness in major Australian cities by the Australian Water Association shows a range from very soft (Melbourne) to very hard (Adelaide).Total Hardness levels of Calcium Carbonate in ppm are:
Canberra
Canberra
Canberra is the capital city of Australia. With a population of over 345,000, it is Australia's largest inland city and the eighth-largest city overall. The city is located at the northern end of the Australian Capital Territory , south-west of Sydney, and north-east of Melbourne...
: 40; Melbourne: 10 - 26;
Sydney
Sydney
Sydney is the most populous city in Australia and the state capital of New South Wales. Sydney is located on Australia's south-east coast of the Tasman Sea. As of June 2010, the greater metropolitan area had an approximate population of 4.6 million people...
: 39.4 – 60.1;
Perth
Perth, Western Australia
Perth is the capital and largest city of the Australian state of Western Australia and the fourth most populous city in Australia. The Perth metropolitan area has an estimated population of almost 1,700,000....
: 29 – 226;
Brisbane
Brisbane
Brisbane is the capital and most populous city in the Australian state of Queensland and the third most populous city in Australia. Brisbane's metropolitan area has a population of over 2 million, and the South East Queensland urban conurbation, centred around Brisbane, encompasses a population of...
: 100;
Adelaide
Adelaide
Adelaide is the capital city of South Australia and the fifth-largest city in Australia. Adelaide has an estimated population of more than 1.2 million...
: 134 – 148;
Hobart
Hobart
Hobart is the state capital and most populous city of the Australian island state of Tasmania. Founded in 1804 as a penal colony,Hobart is Australia's second oldest capital city after Sydney. In 2009, the city had a greater area population of approximately 212,019. A resident of Hobart is known as...
: 5.8 – 34.4;
Darwin: 31.
Hard water in Canada
PrairiePrairie
Prairies are considered part of the temperate grasslands, savannas, and shrublands biome by ecologists, based on similar temperate climates, moderate rainfall, and grasses, herbs, and shrubs, rather than trees, as the dominant vegetation type...
provinces (mainly Saskatchewan
Saskatchewan
Saskatchewan is a prairie province in Canada, which has an area of . Saskatchewan is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, and on the south by the U.S. states of Montana and North Dakota....
and Manitoba
Manitoba
Manitoba is a Canadian prairie province with an area of . The province has over 110,000 lakes and has a largely continental climate because of its flat topography. Agriculture, mostly concentrated in the fertile southern and western parts of the province, is vital to the province's economy; other...
) contain high quantities of calcium and magnesium, often as dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
, which are readily soluble in the groundwater that contains high concentrations of trapped carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
from the last glaciation. In these parts of Canada, the total hardness in ppm of calcium carbonate equivalent frequently exceed 200 ppm, if groundwater is the only source of potable water. The west coast, by contrast, has unusually soft water, derived mainly from mountain lakes fed by glaciers and snowmelt.
Some typical values are: Montreal
Montreal
Montreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...
116 ppm, Calgary
Calgary
Calgary is a city in the Province of Alberta, Canada. It is located in the south of the province, in an area of foothills and prairie, approximately east of the front ranges of the Canadian Rockies...
165 ppm, Regina
Regina, Saskatchewan
Regina is the capital city of the Canadian province of Saskatchewan. The city is the second-largest in the province and a cultural and commercial centre for southern Saskatchewan. It is governed by Regina City Council. Regina is the cathedral city of the Roman Catholic and Romanian Orthodox...
202 ppm, Saskatoon
Saskatoon
Saskatoon is a city in central Saskatchewan, Canada, on the South Saskatchewan River. Residents of the city of Saskatoon are called Saskatonians. The city is surrounded by the Rural Municipality of Corman Park No. 344....
< 140 ppm, Winnipeg
Winnipeg
Winnipeg is the capital and largest city of Manitoba, Canada, and is the primary municipality of the Winnipeg Capital Region, with more than half of Manitoba's population. It is located near the longitudinal centre of North America, at the confluence of the Red and Assiniboine Rivers .The name...
77 ppm, Toronto
Toronto
Toronto is the provincial capital of Ontario and the largest city in Canada. It is located in Southern Ontario on the northwestern shore of Lake Ontario. A relatively modern city, Toronto's history dates back to the late-18th century, when its land was first purchased by the British monarchy from...
121 ppm, Vancouver
Vancouver
Vancouver is a coastal seaport city on the mainland of British Columbia, Canada. It is the hub of Greater Vancouver, which, with over 2.3 million residents, is the third most populous metropolitan area in the country,...
< 3 ppm, Charlottetown PEI 140 – 150 ppm.
Hard water in England and Wales
Information from the British Drinking Water Inspectorate shows that drinking water in EnglandEngland
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
is generally considered to be 'very hard', with most areas of England, particularly east of a line between the Severn and Tees estuaries, exhibiting above 200 ppm for the calcium carbonate equivalent. Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
, Devon
Devon
Devon is a large county in southwestern England. The county is sometimes referred to as Devonshire, although the term is rarely used inside the county itself as the county has never been officially "shired", it often indicates a traditional or historical context.The county shares borders with...
, Cornwall
Cornwall
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
and parts of North-West England
North West England
North West England, informally known as The North West, is one of the nine official regions of England.North West England had a 2006 estimated population of 6,853,201 the third most populated region after London and the South East...
are softer water areas, and range from 0 to 200 ppm. In the brewing
Brewing
Brewing is the production of beer through steeping a starch source in water and then fermenting with yeast. Brewing has taken place since around the 6th millennium BCE, and archeological evidence suggests that this technique was used in ancient Egypt...
industry in England and Wales, water is often deliberately hardened with gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
in the process of Burtonisation.
Tap water
Tap water
Tap water is a principal component of "indoor plumbing", which became available in urban areas of the developed world during the last quarter of the 19th century, and common during the mid-20th century...
in Manchester
Manchester
Manchester is a city and metropolitan borough in Greater Manchester, England. According to the Office for National Statistics, the 2010 mid-year population estimate for Manchester was 498,800. Manchester lies within one of the UK's largest metropolitan areas, the metropolitan county of Greater...
in England is soft because it comes from Thirlmere
Thirlmere
Thirlmere is a reservoir in the Borough of Allerdale in Cumbria and the English Lake District. It runs roughly south to north, with a dam at the northern end, and is bordered on the eastern side by the A591 road and on the western side by a minor road....
and Haweswater reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...
s in the Lake District
Lake District
The Lake District, also commonly known as The Lakes or Lakeland, is a mountainous region in North West England. A popular holiday destination, it is famous not only for its lakes and its mountains but also for its associations with the early 19th century poetry and writings of William Wordsworth...
, and in their headwaters there is no exposure to limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
or chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
. Similarly, tap water in Birmingham
Birmingham
Birmingham is a city and metropolitan borough in the West Midlands of England. It is the most populous British city outside the capital London, with a population of 1,036,900 , and lies at the heart of the West Midlands conurbation, the second most populous urban area in the United Kingdom with a...
is also soft as it is sourced from the Elan Valley Reservoirs
Elan Valley Reservoirs
The Elan Valley Reservoirs are a chain of man-made lakes and reservoirs in the Elan Valley in Powys, Mid Wales , using the rivers Elan and Claerwen...
in Wales.
Hard water in the United States
More than 85% of American homes have hard water. The softest waters occur in parts of the New EnglandNew England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...
, South Atlantic-Gulf, Pacific Northwest
Pacific Northwest
The Pacific Northwest is a region in northwestern North America, bounded by the Pacific Ocean to the west and, loosely, by the Rocky Mountains on the east. Definitions of the region vary and there is no commonly agreed upon boundary, even among Pacific Northwesterners. A common concept of the...
, and Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
regions. Moderately hard waters are common in many of the rivers of the Tennessee, Great Lakes
Great Lakes
The Great Lakes are a collection of freshwater lakes located in northeastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth by total surface, coming in second by volume...
, and Alaska regions. Hard and very hard waters are found in some of the streams in most of the regions throughout the country. The hardest waters (greater than 1,000 ppm) are in streams in Texas, New Mexico, Kansas, Arizona, and southern California.
See also
- Carbonate hardnessCarbonate hardnessCarbonate hardness, or carbonate alkalinity is a measure of the alkalinity of water caused by the presence of carbonate and bicarbonate anions. It is usually expressed either as parts per million , or in degrees KH...
- dGHDGHDegrees of general hardness is a unit of water hardness, specifically of general hardness.General hardness is a measure of the concentration of metal divalent ions such as calcium and magnesium per volume of water...
- Water softener
- Water qualityWater qualityWater quality is the physical, chemical and biological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which...
- Water treatmentWater treatmentWater treatment describes those processes used to make water more acceptable for a desired end-use. These can include use as drinking water, industrial processes, medical and many other uses. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the...
- Water purificationWater purificationWater purification is the process of removing undesirable chemicals, materials, and biological contaminants from contaminated water. The goal is to produce water fit for a specific purpose...
External links
- Akzo Nobel, Langelier Saturation Index (LSI) Calculator
- Water hardness unit converter, Converter for hardness of water