1. The effect of phosphate on the oxidation of glyceric aldehyde by methylene blue, 1-naphthol 2-sulfonate indophenol, and phenol-indophenol has been studied.

2. At pH 4.77 in a phthalate-buffered medium phosphate does not catalyze the reaction.

3. At pH 7.9 in solutions buffered with borate, carbonate, or phenylalanine marked catalysis by phosphate is observed. The effect is most pronounced in borate.

4. Phosphate catalysis, within the limits studied, is strictly a linear function of the phosphate concentration.

5. The high concentration of HPO4= and the low concentration of PO4 relative to that of the substrate virtually demand the conclusion that the PO4 ion is the active catalytic species.

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