A unit used to measure the amount of potassium hydroxide (chemical formula KOH) needed to neutralize the free acids found in oils, waxes, or resins. A high degree of acidity is undesirable in many printing applications, and the addition of a high-pH substance (called a base) is needed to counteract the acidity of a substance. Acid number is measured in milligrams of KOH needed to neutralize one gram of oil, wax, or resin.
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