Water, Soap, Alkali, and Acids

The test methods discussed in this context are broad in scope and relate to a variety of applications. In some such tests, a pigment powder is extracted with water and its contents analyzed other procedures reveal the stability of a pigmented system with regard to water, acids, or bases. A number of these tests have developed into industrial standards. 

A variety of techniques are used to determine the water-soluble content of a pigment. 

The cold extraction method [15] involves moistening defined amounts of pigment powder (between 2 and 20 g) with small amounts of water, alcohol, or a suitable wetting agent. 200 ml of freshly distilled or completely deionized water are then added and the sample allowed to remain in this solvent for 1 h at ambient temperature. After thorough shaking, the liquid is removed by filtration. 

The hot extraction method is a variation of this procedure. The aqueous pigment suspension, prepared as above, is refluxed for a certain time, usually 5 minutes, cooled rapidly, and filtered. A known amount of extract is then dried by evaporation and the weight of the residue determined, or the extracted pigment weighed and the dissolved portion determined by calculating the difference. 

The acid or alkali number of a pigment is determined by titrating the aqueous extract of 100 g of pigment with a 0.1 N alkali (acid) solution [16]. 

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