Acid number

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A measurement of the free acids (protons) present in a substance. The acid number corresponds to the weight in milligrams of Potassium hydroxide, required to neutralize the free acids present in one gram of the test material, such as as Oil, Natural resin, or Wax. For testing, the substance is dissolved in an alcohol solution, then titrated with the alkali solution using phenolphthalein to indicate the end point change. Oils with high acid numbers (5-10) tend to disperse pigments better but they also tend to become more yellow with age. Oils and varnishes with low acid numbers (1-3) exhibit greater color stability. The acid numbers for linseed and poppy seed oils can range from 0 -10 while Walnut oil is usually about 2.5 (Gettens and Stout 1966).

Synonyms and Related Terms

Säurezahl (Deut.); acid value; hydroxyl number

Resources and Citations

  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971
  • ASTM, "Standard Terminology Relating to Paint, Varnish, Lacquer and Related Products", Annual Book of ASTM Standards, Section 6, Paints, Related Coatings and Aromatics, ASTM, D16, 7-Jan, Jul-96
  • R. J. Gettens, G.L. Stout, Painting Materials, A Short Encyclopaedia, Dover Publications, New York, 1966
  • Ralph Mayer, A Dictionary of Art Terms and Techniques, Harper and Row Publishers, New York, 1969 (also 1945 printing)