acid number, acid value definition

An acid number is a lab measurement taken in order to find the degree of free fatty acid in a vegetable drying oil.  This amount is valuable to know because it assists in the selection of an oil for whatever purpose it is being used for.  The milligrams of potassium hydroxide needed to neutralize the free fatty acids contained in a gram of oil is represented by the acid number.  

An acid number between 5-10 indicates that an oil is best employed for grinding paints.  And an acid number between 1-3 indicate that an oil is best employed in varnishes and other kinds of clear coatings.  Often low-numbered oils are used for paint oil colors due to the nonyellowing characteristics and better color stability.  These factors are more important than ease of grinding considerations.