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In their letters, Eby and Godfrey claim that Zn2+ will be tightly bound to citrate in saliva upon administration of lozenges containing the two ingredients (G. A. Eby, Letter, Antimicrob. Agents Chemother. 32:606-607, 1988; J. C. Godfrey, Letter, Antimicrob. Agents Chemother. 32:605606, 1988). The lozenges contained citric acid (6), however, and unless several equivalents of base per mole of citric acid are added, citrate will not be produced and tight citrate complexes cannot form. The limited buffer capacity of saliva (1) is insufficient to raise the pH to promote formation of tight complexes. Therefore, much of the zinc added with citric acid in a lozenge is likely to appear as free, unbound Zn2+ in saliva. If a 4.5-g lozenge containing 2% citric acid (6) generates 20 ml of saliva (Godfrey, Letter), the citric acid concentration becomes 23 mM and produces a pH of 2.3. For citric acid (molecular weight, 192) the successive acidity constant pKa values are 3.0, 4.4, and 5.8 (8). Addition of one equivalent or nearly 0.5 mmol of base raises the pH only to 3.7, and even addition of two equivalents of base or almost 1 mmol yields only a pH of 5.1. Thus, due to the acidity of citric acid and the low buffer capacity of saliva, much of the added Zn2+ is likely to remain as the free metal ion. Although ligands present in saliva will bind some Zn2+, the acidity created by adding citric acid reduces their chelating ability. Figure 1 shows the distribution of zinc-citrate species as mole fraction on a zinc basis versus pH for a citric acid-tototal-zinc mole ratio of 1.3, identical to that in the lozenges (6). Stability constants for the Zn2+ and citric acid system

pH as a variable in free zinc ion concentration from zinc-containing lozenges