INSOLUBLE MATTER.
The insoluble matter in soap may consist of organic or inorganic substances. Among the organic substances which are usually present in soap are oat meal, bran, sawdust, etc., while among the common inorganic or mineral compounds are pumice, silex, clay, talc, zinc oxide, infusorial earth, sand or other material used as fillers.
To determine insoluble matter, 5 grams of soap are dissolved in 75 cubic centimeters of hot water. The solution is filtered through a weighed gooch crucible or filter paper. The residue remaining on the filter is washed with hot water until all the soap is removed, is then dried to constant weight at 105 degrees C. and weighed. From the difference in weight of the gooch or filter paper and the dried residue remaining thereon after filtering and drying, the total percentage of insoluble matter may easily be calculated. By igniting the residue and reweighing the amount of insoluble mineral matter can be readily determined.