Abstract
The index of refraction of a mixed solution of gelatin and of hydrochloric acid as measured by an immersion refractometer has been stated to be an additive function of the indices of refraction of the individual solutions. 1 Using the Zeiss-Löwe interferometer, which is a differential refractometer capable of measuring small changes in refractive index to which an immersion refractometer is insensitive, the authors have found that the refractive indices of mixed solutions of gelatin and of hydrochloric acid are not additive.
However, the refraction of mixed solutions of gelatin chloride and of hydrochloric acid are additive. Consequently, when a solution of gelatin is titrated with an acid, an abrupt change in slope of the curve (refraction as function of acid added) is noted at the stoichiometric point. The authors have found this to be the case also when an amino acid is similarly titrated, and recently the interferometer method of following simple acid-base neutralizations has been reported elsewhere. 2
The data for a gelatin titration are given in Table I and are shown graphically in Figure I. The gelatin used was a pure specimen obtained from the Eastman Kodak Company. Its ash content was 0.03%.
The 2 branches of the curve intersect at the point equal to about 7.9 cc. of acid. Consequently the equivalent weight of this specimen of gelatin is about 1111. For the benefit of the reader interested in a description of the instrument used by the authors, the excellent paper of Adams 3 is recommended.
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