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The colorimetric method of determining pH in aqueous solutions has proved to be of such value in routine control work that it is well worth while to review the corrective factors necessary to eliminate errors inherent in the method. The refinements in technique referred to in this paper do not deal with such gross inaccuracies as incorrect measurements, use of unequal depths of solution, readings taken beyond the range of an indicator, etc., but with the more serious difficulties in the application of the indicator method. Such difficulties may be due to inappropriate indicators, slightly buffered solutions, temperature effects, variable salt concentrations, proteins, high color and turbidity, and certain specific effects. This paper does not attempt to define pH in terms of hydrogen ion concentration or activity. The hydrogen electrode determination based on 0.05M potassium acid phthalate, National Bureau of Standards sample 84a, as pH 4.005 at 25 C. is taken as the standard value (1). Potentiometric measurements given here for purpose of illustration were made with hydrogen electrodes and commercial glass electrode meters. The saturated calomel electrode was used throughout and the value 0.2434 v. at 25 C. was assigned to it (2). High precision is not claimed for the values given. They are used primarily for illustration.
McCrumb, F. R.
Chief Chemist, W. A. Taylor & Co., Baltimore, Md.