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In chromatography, if analytes are strong acid anions or strong alkali cations, the dependence of a logarithm of the adjusted retention value on the logarithm of the concentration of the eluent is given by log tR' = const - (j/i).log C, where j is the charge of the analyte ion, i is the charge of the eluent ion, const is a constant that depends on the column, eluent and analyte ion. But in weak acid anion chromatography, existing forms of the sample anion change with the change of the concentration of the eluent. So the study on the relationship between retention value of weak acid anion and concentration of the eluent is more difficult. Relationship between adjusted retention value of some weak acid anion and concentration of the eluent was studied with sodium hydroxide as eluent. The nonlinear fitting function has been set up. We transform it into multiple linear fitting to give the answer. The correlation coefficients for phosphate defined by the function were over 0.99. The relative errors with the use of function were below 5%.