Determination of pH in human erythrocytes. Sources of systematic error.

The results of a simultaneous determination of the pH value in erythrocytes with the potentiometric measuring method and the 14C-labelled 5,5-dimethyl-2,4-oxazolidinedione (DMO) method showed a mean method difference of 0.026 pH units. The cause of this discrepancy was assumed to be matrix-inherent liquid-junction potentials in the potentiometric measurement. Taking these into account in the calculation leads to consistent values for the methods investigated. The DMO method proved to be free of systematic errors. Another indication of this is that its mean ratio of extracellular to intracellular H+ ion concentration (H+e/H+i) substantially agreed with the distribution ratios of other freely diffusible ions and their pH dependence.