Use of the computer program EQUIL to estimate pH in model solutions and human urine

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Abstract
Summary The computer program EQUIL was designed to calculate relative supersaturations of solute components of common urinary stones. In an extended software version, quantitative consideration of charge balance for a priori or a posteriori pH estimation was added. The reliability of this computation was tested with hydrogen ion titration of buffer solutions containing HEPES [N-(2-hydroxyethyl)piperaizine-N′-ethanesulfonic acid] as well as samples of normal human urine. In the model solutions with HEPES, the difference between calculated pH values and the measured pH was smaller than 1.2% for any titration step within the buffer zone (pH 8.5-6.8). The pH values calculated for whole urine differed from the measured pH by 7% to 53%, and the calculated charge inbalance ranged from 2.6 to 9.6 mM. This net cation inbalance indicates that there is a need to account for other anionic components, including hippurate, amino acids, and isocitrate. In experimental solutions, charge balance calculations with EQUIL can be of great utility because they permit a priori estimation of pH or computation of the composition at a desired pH.