Colorimetric Estimation of Hydrogen Ion Concentration of Blood.

Abstract

Recently Myers, Schmitz and Booher 1 described a simple microcolorimetric technique of estimating the hydrogen ion concentration of the blood plasma, based upon an adaptation of the colorimetric method of Cullen 2 to the Myers bicolorimeter. 3 The pH readings are obtained on diluted plasma at room temperature and must be corrected to the actual pH of the plasma at body temperature by a constant. This constant includes corrections for dilution, temperature and protein error. The use of Cullen's factor of 0.22 is made in the following formula:

This factor serves very well for normal human plasma. However, recently considerable doubt has arisen as to its applicability to abnormal samples.

Hastings and Sendroy 4 found that the correction used by Cullen for colorimetric determinations performed at room temperature is unnecessary when readings are made with the diluted plasma at body temperature. Austin, Stadie and Robinson 5 find that this is not consistently the case and report considerable variation between the true pH and the colorimetric value of the diluted plasma at 38°.

During the past year we have had the opportunity of studying the variation in the constants in miscellaneous hospital cases, many of whom were receiving alkali. Colorimetric values were obtained on separated plasma, both at room and body temperature, employing the method of Myers, Schmitz and Booher 1 with the introduction of certain modifications. Eltctrometric determinations on the undiluted separated plasma were made at 38°. Estimation of the CO₂ content was made in every case and wherever possible plasma chloride and total plasma protein figures were obtained.

In 81 samples of plasma the C correction (colorimetric pH at 20°—the electrometric pH at 38°) gives an average value of 0.226 pH. Assuming that 0.22 is correct, 50 per cent of the values are within ±0.02 pH of this value. In two cases there is a maximum deviation of 0.08 pH.