Most essential oils contain only small amounts of free acids. Consequently the acid content is usually reported as an acid number rather than as a percentage calculated as a specific acid.
The acid number of an oil is defined as the number of milligrams of potassium hydroxide required to neutralize the free acids in 1 g. of oil.
In determining the acid number, dilute alkali must be employed since many of the esters (e.g., the formates) normally present in essential oils are capable of saponification even in the cold in the presence of strong alkalies. Moreover, phenols will react with the alkali hydroxides, making it necessary to use special indicators (such as phenol red) for oils containing large amounts of phenolic bodies ; this is particularly true in the case of the salicylates.
The acid number of an oil often increases as the oil ages, especially if the oil is improperly stored; processes such as oxidation of aldehydes and hydrolysis of esters increase the acid number. Oils which have been thoroughly dried and which are protected from air and light show little change in the amount of free acids.
Procedure: Weigh accurately about 2.5 g. of the oil into a 100 cc. saponification flask. Add 15 cc. of neutral 95 per cent alcohol and 3 drops of a 1% phenolphthalein solution. Titrate the free acids with a standardized 0.1 N aqueous sodium hydroxide solution, adding the alkali dropvvise at a uniform rate of about 30 drops per min. The contents of the flask must be continually agitated. The first appearance of a red coloration that does not fade within 10 sec. is considered the end point.
If the determination requires more than 10 cc. of alkali, it should be repeated using a 1 g. sample of the oil ; if more than 10 cc. of alkali is still required, then a 1 g. sample is titrated with 0.5 N aqueous sodium hydroxide solution.
The acid number is calculated by means of the following formulas : 
The acid number is calculated by means of the following formulas

* All molecular weights have been calculated from the values of the International Atomic Weights adopted by ttye Committee on Atomic Weights in 1938.

For oils containing large amounts of free acid (e.g., orris oil), the free icid content may be axpressed as a percentage, calculated as a specific acid, n such cases it is well to use a 0.5 N alcoholic sodium hydroxide solution.
Free acid content = ma/20w
Free acid content = mb/20w
where : m = molecular weight of the acid ;
             a = number of cc. of 0.5 X alkali used for neutralization;
             b = number of cc. of 0.1 N alkali used for neutralization;
             w = weight of sample in grams.
If the acid is dibasic, the result must be divided by 2 ; if tribasic, by 3. In Table 4.9 arc listed the molecular weights of those acids frequently encountered by the essential oil chemist.

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