Since it was introduced a quarter-century ago, Stewart’s model of plasma acid-base balance has been the subject of unnecessary controversy. Although Stewart himself emphasized that his methods were unorthodox, the Stewart formalisms are in fact derived from standard thermodynamic principles. Indeed, the Stewart equation contains all of the information embodied in the three classic acid-base formulas: the Henderson-Hasselbalch equation (PCO2 vs. pH), the Van Slyke equation (titratable base vs. pH) and the Van Slyke buffer formula (buffer value vs. pH). To reduce the number of limiting assumptions, the Stewart equation utilizes the balance of mass, the balance of charge, and the equilibrium constants of all reactive species. The Stewart theory, entailing the Stewart equation and implementation, are conceptually separate from the Stewart “mechanism”.
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