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Acid value (acid number or acidity) is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance. The acid value is a measure of the number of carboxylic acid groups in a chemical compound.
Acid value can be tested by the methods of titration, colorimetric, chromatography and near infrared spectroscopy. While the commonly used are titration method and colorimetric method.
The indicator electrode and the reference electrode are inserted into the sample solution to form a galvanic cell, and the electromotive force is related to the concentration of hydrogen ions in the solution.
Titrate with the standard alkali solution, recording the burette and pH meter readings. Determine the inflection point (point of maximum change in pH per milliliter of alkali solution) by plotting the pH readings against the milliliters of alkali used. For greater accuracy, a plot may be made of the change in pH per milliliter of alkali, against the pH. The peak of this curve will indicate the exact inflection point, which is considered the end point of the titration, and then the acid value can be calculated.
In this method, organic solvent, surfactant, and a small amount of water are mixed in a certain proportion to form an optically transparent stable reverse micelle system. Phenol red is dissolved in the alkaline aqueous phase of the reverse micelle. When pK1 of phenol red is equal to 7.8, it appears red, and the aqueous solution has the maximum absorption at 558nm. When the pH of the aqueous solution decreases, the color turns to yellow and the maximum absorption peak will change.
The sample to be tested is added to the aqueous solution that causes the acid-base neutralization reaction, so that the pH decreases, color changes. Then a colorimetric assay can be established to calculate the acid value.
ASTM D974. ASTM D664. DIN 51558. EN 14104
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