Hydroxyl value
Encyclopedia
In chemistry
, the hydroxyl value is a measure of the content of free hydroxyl
groups in a compound, typically a fat, oil, or natural or synthetic ester
. This value is a useful measure of the degree of esterification in ester synthesis. The method involves acetylation
of the free hydroxyl groups of the compound with acetic anhydride
in pyridine
solvent http://www.astm.org/Standards/D1957.htm. After completion of the reaction, water is added, the remaining acetic anhydride is converted to acetic acid
, and measured by titration with potassium hydroxide. Other functional groups, such as primary or secondary amines
, will be reported as hydroxyl. It is expressed as the mass of potassium hydroxide
(KOH) in milligrams equivalent to the hydroxyl content of one gram of the chemical substance
, corrected for carboxyl hydroxyl groups by titration of an unacetylated sample of the same material.
Where VB is the amount of titrant
(ml) consumed by the blank, at the equivalent point, Wacet is the weight of sample (in grams) used for acetylation, Vav is the amount of titrant used for determining the acid value, Wav is the weight of sample used for determining the acid value, Vacet is the amount of titrant used for the acetylated sample, N is the normality of the titrant, and 56.1 is the molecular weight of KOH
.
The standard method for determining the hydroxyl value is ASTM D 1957.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, the hydroxyl value is a measure of the content of free hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
groups in a compound, typically a fat, oil, or natural or synthetic ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
. This value is a useful measure of the degree of esterification in ester synthesis. The method involves acetylation
Acetylation
Acetylation describes a reaction that introduces an acetyl functional group into a chemical compound...
of the free hydroxyl groups of the compound with acetic anhydride
Acetic anhydride
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula 2O. Commonly abbreviated Ac2O, it is the simplest isolatable acid anhydride and is a widely used reagent in organic synthesis...
in pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
solvent http://www.astm.org/Standards/D1957.htm. After completion of the reaction, water is added, the remaining acetic anhydride is converted to acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
, and measured by titration with potassium hydroxide. Other functional groups, such as primary or secondary amines
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
, will be reported as hydroxyl. It is expressed as the mass of potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
(KOH) in milligrams equivalent to the hydroxyl content of one gram of the chemical substance
Chemical substance
In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.Chemical substances are...
, corrected for carboxyl hydroxyl groups by titration of an unacetylated sample of the same material.
Where VB is the amount of titrant
Titration
Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...
(ml) consumed by the blank, at the equivalent point, Wacet is the weight of sample (in grams) used for acetylation, Vav is the amount of titrant used for determining the acid value, Wav is the weight of sample used for determining the acid value, Vacet is the amount of titrant used for the acetylated sample, N is the normality of the titrant, and 56.1 is the molecular weight of KOH
KOH
-Chemistry:*Potassium hydroxide, formula "KOH"** KOH test, a procedure in which potassium hydroxide is used to dissolve skin and reveal fungal cells under the microscope** KOH titration, a procedure for determining a lubricant's acid level or TBN-Person's name:...
.
The standard method for determining the hydroxyl value is ASTM D 1957.
See also
- Saponification valueSaponification valueSaponification value represents the number of milligrams of potassium hydroxide required to saponify 1g of fat under the conditions specified. It is a measure of the average molecular weight of all the fatty acids present...
- Iodine value
- Acid value