The colorimetric estimation of the hydrogen ion concentration of urine.

Abstract

A year ago a simple technique of estimating the pH of urine was suggested, in which application was made of the bicolorimeter and the phthalein dyes, phenol red, brom cresol purple and brom cresol green. At that time we concluded that as far as the matching of colors went, the method had a probable error of pH ± 0.02 to 0.04, but we realized that the factors of temperature and dilution must exert some influence on the true pH. Obviously one desires to know the pH of the undiluted urine at body temperature. During the past year we have been trying to ascertain how far colorimetric determinations on diluted urine at room temperature differ from electrometric determinations on undiluted urine at 38°. In connection with an acid-base balance study we felt that we should know just how far the present colorimetric determinations might deviate from the true pH values and if a correction factor might be applied which would hold for all urines. Since our preliminary report was made and the present study begun, Hastings, Sendroy and Robson have reported a study of this question. They carry out their determinations at 38° diluting the urine 1-5. They state that the dilution error is the important error and amounts to about 0.1 pH, which must be subtracted from the colorimetric result. They believe that with this correction their results fall within 0.1 of the actual pH. In routine work it is quite troublesome to carry out colorimetric estimation at 38°. In the present study it has been found that the factor necessary to correct for the determination being made on the diluted urine at room temperature amounts to roughly 0.2 pH, about half of which is a temperature correction and half a dilution (salt and buffer) correction.