Meas ureme nts are rf' ported for t be ionizat ion constant and for r elate d thermodynamic qua ntities from 0° to 60° C for 5-5 '-diethylbarbituric acid . Two se ries of solutions corres ponding to molal ratios of barbituric acid:sodium barbiturate :sodium chloride (or iodide) of 1:0.5 :l.667, 1 :1 :l.667, and 1: 2 :l.667 were invest igated. The values obtained for the ionization constant of the acid using t hese t hree ratios of acid to sodium salt and two ty pes of refere nce e le ctrodes

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... e found to agree ' within the experimental error of t he mea,s urements. Two other series of solutions in which both the ratio and t he conce ntration of t he buffer materials were maintained a~ a fixed value and t hat of t he halide ion was diminished were also St udie d. The e ffect of sodium iodide on the pJ{' of t he buffers was normal, whe rea s t hat for t he sodium chloride was anomalOUS but co uld be explained in terms of a, de monstrated interaction at low co ncentratio ns of sodium chloride, betwee n the soluble ba.rbiturate ion and the silver-silver-chloride electrode used in the cell. The variation of pK for 5-5 ' -diethy lbarbituric acid with the absolute temperature T , where T = 273 .16 + tOC, can be expressed by t he equati on pI( = 2324.4 7 / T + 0.0118562 T -3.3491 over the ran ge 0° to 60° C.