Comparatively Speaking--Analytical Methods: The mg KOH/gm System

Cosmetic raw materials commonly are compositions, i.e., complex mixtures of compounds that are the result of an industrial process. The quantification of each component is difficult and often beyond the scope of quality control laboratories. Therefore, the composition typically is analyzed using common wet methods to examine the entire composition rather than each component. This type of analysis looks at the colligative properties of the raw material to see the composition as a whole rather than each compound in it.

This is the first of several columns that will look at the key analytical methods used in quality control laboratories to discuss their applications to raw materials. The most commonly used system for the analysis of surfactants is the mg KOH/gm system. This system originally was developed for use by chemists in the soap business. Analyses conducted by this system are expressed in units of mg KOH/gram of sample tested. For surfactants, this system commonly analyzes acid value, alkali value, saponification value and hydroxyl value.

In the mg KOH/gm system, each compound evaluated is compared to KOH. KOH has a molecular weight of 56.11. By definition, 100% KOH has 1,000 mg of KOH per gram of KOH. Pure NaOH, which has a molecular weight of 40, is equivalent to (56.11/40)(1,000 mg KOH/gm) or 1,402.7 mg KOH/gram.

The alkali value of 1 gram of 100% NaOH is 1,402.7 mg KOH/gm. This system is significant in that the number relates to molar values, not weight values. Since all chemistry work is performed and expressed in molar quantities, this number is far more significant than the weight of compound added.

Looking at NaOH vs. KOH, 1 gram of the former provides 140% of the combining alkalinity as 1 gram of KOH. Therefore, to neutralize 1 mg KOH/gm of acid determined by titration, one would need 1 gram of alkalinity. To accomplish this task, one would need 1 gram of 100% KOH or 0.71 grams of 100% NaOH.

The relationship between the mg KOH/gm value and molecular weight is an important formula. The apparent molecular weight (AMW) of any material can be determined by the following formula:

AMW = 56,110/mg KOH/gm

Since the number of mg KOH per gram can be determined by titration, this value is independent of the type or concentration of the pure compound. If one had two different concentrations of a fatty acid in isopropanol and water and wanted to make a salt, an acid value titration could be run on both samples.

From the observed acid value, a required amount of base can be determined to exactly neutralize the acidity present. The alkali value of the base used could be used to determine the exact quantity of each type of base used. The values provided by this system are colligative; that is, not related to the specific species or combination of species that contribute the acidity.

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