In view of the experimental work described above, it appears that changes in viscosity of zinc oxide-compounded latex are due largely to the presence of ammonium salts of naturally occurring acids. These salts enable zinc to pass into solution, and to be absorbed by the latex globules, thus giving rise to the thickening commonly experienced. The amount of zinc passing into solution is directly proportional to the amount of acid present, and it is therefore suggested that determination of the acids by either of the methods described (viz., titration to pH 11 with caustic alkali, or from pH 7 to the end-point with hydrochloric acid), would form a sound basis for the classification of latices in regard to their behavior on compounding with zinc oxide. The investigations thus indicate that the variability of latex is due, in part at least, to the presence of acidic substances and may, therefore, be controlled by preventing the formation of the acids, or by their removal. Summary.—It is demonstrated that naturally occurring acidic substances control the solubility of zinc oxide in ammonia-preserved latex, and it is concluded that they are probably the controlling factor in determining the increase in viscosity which occurs in latex compounded with zinc oxide. In support of this view, it has been shown that the addition of various types of acid to ammonia-preserved latex markedly increases thickening after mixing with zinc oxide, and that the removal of naturally occurring acids reduces the thickening. It is suggested that titration of the acids in latex by the methods described forms a sound basis for the classification of latices in regard to their behavior on compounding with zinc oxide.

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