Free acldlty In uranlum(V1) and plutonium( IV) solutions has been determlned with sodium sulfate as complexant and sodlum carbonate as tltrant. The described method ls simple, accurate, and appllcable to all ranges of nHric acld and heavy metal concentratlons relevant to Purex process. The method Is adaptable for remote Operation. The overall recovery of nitrlc acid Is 99.5 % with 0.7 % relative standard deviation.Uranlum content has also been determined In the same allquot wlth a recovery of 98.5% and 1.2% relative standard deviation after the determlnatlon of acldlty.Analytical procedures for the determination of free acidity in uranium(V1) and plutonium(1V) solutions are required for nuclear fuel reprocessing. Knowledge of distribution ratios of actinides and nitric acid in extraction systems involving tri-n-butyl phosphate (TBP) and an alkane diluent is of great importance for modeling of the extraction or reextraction step of the Purex process. Accurate determination of free nitric acid is necessary for solvent extraction, ion exchange, and precipitation operations that separate and purify the actinide products. Free acid content must be established prior to neutralization of nitric acid in waste solutions generated from these operations. The knowledge of free nitric acid concentration is also required in the studies of third phase formation and complex formation. However, the acidity produced by the hydrolysis of uranium and plutonium due to their large ionic potentials contributes to the initial acidity of the solution and poses problem in the accurate determination of free acidity. To remove hydrolytic interferences of uranium and plutonium and to prevent polymerization and disproportionation reactions of plutonium, these species are chemically removed by adding complexing agents. A survey of literature reveals that the most commonly used complexing agents are oxalate (1, 2), citrate (3), fluoride (4,5), EDTA (6), or a mixture of fluoride and oxalate (7,8). The use of neutral potassium oxalate as complexing agent for uranium has been reported to give biased results with the magnitude of bias depending on the concentration and species of hydrolyzable ions present and with the amount of oxalate salt used. The potassium oxalate method is not reliable a t high uranium to nitric acid ratios. In the case of plutonium, complexation with oxalate and subsequent titration to a preselected end point (pH 5.55) have been reported to result in an acceptable bias over a restricted acid range centered around 1 mequiv of acid. Therefore, the neutral potassium oxalate method serves only as a relative index for plant acidity control and cannot be accepted as an accurate method. Other complexing agents have also been used with varying degrees of success with neutralization titrations. Other methods based on precipitation and separation of heavy metals followed by alkalimetric determination (9) and methods invovling the partial neutralization of acids by standard addition of base and the calculation of end point by Gran plot ...