Hydroxyl value

{{short description|Mass of KOH needed to neutralize 1 gram of acetylized substance}}

In analytical chemistry, the hydroxyl value is defined as the number of milligrams of potassium hydroxide (KOH) required to neutralize the acetic acid taken up on acetylation of one gram of a chemical substance that contains free hydroxyl groups. The analytical method used to determine hydroxyl value traditionally involves acetylation of the free hydroxyl groups of the substance with acetic anhydride in pyridine solvent. After completion of the reaction, water is added, and the remaining unreacted acetic anhydride is converted to acetic acid and measured by titration with potassium hydroxide.

The hydroxyl value can be calculated using the following equation. Note that a chemical substance may also have a measurable acid value affecting the measured endpoint of the titration. The acid value ({{math|AV}}) of the substance, determined in a separate experiment, enters into this equation as a correction factor in the calculation of the hydroxyl value ({{math|HV}}):

::$\backslash mathrm\{HV\}\; =\; \backslash frac\{56.1\; \backslash times\; N\; \backslash times\; (V\_\backslash text\{B\}\; -\; V\_\backslash text\{acet\})\}\{W\_\backslash text\{acet\}\}\; +\; \backslash mathrm\{AV\}$

Where {{math|HV}} is the hydroxyl value; {{math|V{{sub|B}}}} is the amount (ml) potassium hydroxide solution required for the titration of the blank; {{math|V{{sub|acet}}}} is the amount (ml) of potassium hydroxide solution required for the titration of the acetylated sample; {{math|W{{sub|acet}}}} is the weight of the sample (in grams) used for acetylation; {{mvar|N}} is the normality of the titrant; 56.1 is the molecular weight of potassium hydroxide (g/mol); {{math|AV}} is a separately determined acid value of the chemical substance.

The content of free hydroxyl groups in a substance can also be determined by methods other than acetylation.{{cite journal|title= Quantification of hydroxyl group in polymers containing trace water by 19F NMR spectroscopy| doi=10.1016/j.eurpolymj.2012.09.019 | volume=49|journal=European Polymer Journal|pages=228–234|year= 2013|last1= Moghimi|first1= Abolghasem|last2= Omrani|first2= Ismail|last3= Nabid|first3= Mohammad Reza|last4= Mahmoodi|first4= Mehrdad}} Determinations of hydroxyl content by other methods may instead be expressed as a weight percentage (wt. %) of hydroxyl groups in units of the mass of hydroxide functional groups in grams per 100 grams of substance. The conversion between hydroxyl value and other hydroxyl content measurements is obtained by multiplying the hydroxyl value by the factor 17/560.{{cite web|url=https://www.iupac.org/publications/pac/pdf/1973/pdf/3302x0411.pdf|title=Recommended methods for the analysis of alkyd resins|last=|first=|date=|website=|publisher=International Union of Pure and Applied Chemistry Applied Chemistry Division, Organic Coatings Section, Subcommittee on Analytical Methods, 1973 33(2) pp. 418-421.|access-date=}} The chemical substance may be a fat, oil, natural or synthetic ester, or other polyol.{{cite web|url=https://www.who.int/phint/en/d/Jb.7.4.7/|title=Determination of hydroxyl value|publisher=WHO Pharmacopœia Library|accessdate=4 June 2013}}{{dead link|date=December 2021|bot=medic}}{{cbignore|bot=medic}}

ASTM D 1957 {{cite web|url=http://www.astm.org/Standards/D1957.htm|title=ASTM D1957-86(2001) Standard Test Method for Hydroxyl Value of Fatty Oils and Acids (Withdrawn 2007)|publisher=ASTM International|accessdate=4 June 2013}} and ASTM E222-10 {{cite web|url=http://www.astm.org/Standards/E222.htm|title=ASTM E222-10, Standard Test Methods for Hydroxyl Groups Using Acetic Anhydride Acetylation}} describe several versions of this method of determining hydroxyl value.