DETERMINATION OF FREE ACID BY OXALATE COMPLEXATION

Scope

This method covers the determination of the free acid content of uranyl nitrate solutions that may contain a ratio of up to 5 moles of acid to 1 mole of uranium.

Summary of method

To a diluted solution of uranyl nitrate, solid, pulverized potassium oxalate is added until apH of about 4.7 is reached. Thesolution is then titrated with standard NaOH solution by the delta pH method to obtain the inflection point.

Equipments

• pH meter, with glass and calomel electrodes, or potentiometer with electrode

• Buret class A, 50 mL.

Reagens

• Nitric Acid (2.0) – Dilute of HNO3 (sp gr 1.42) to 1 L with water. Standardize with sodium hydroxide solution.

• Potassium Oxalate (K₂C₂O₄.H₂O), crystals.

• Sodium Hydroxide Solution (0.3 N) – Dissolve 12.0 g of NaOH in 1 L of water. Standardize with acid potassium phthalate.

Procedure

• Transfer a 5-mL sample aliquot into a 250 mL beaker.

• Add 100 mL of distilled water or such volume that the uranium concentration will be between 7 and 50 g/L.

• Add a spike of sufficient 2.0 N standard HNO3 to make definitely acid if the sample is neutral or acid deficient.

• Add pulverized K2C2O4.H2O slowly and with constant sterring until a pH of 4.7 to 4.9 is reached.

• Immerse the titration beaker in an ice bath. Titration made at room temperature are possible but are less sharp.

• Titrate with 0.3 N NaOH using 0.20 mL increments and determine the inflection point by the delta pH an “analytical” method.

Calculation

Calculate the free acid normality, N, as follows

N = (A x Nb – C x Na) / 5

Where
A = NaOH solution used in the titration, mL
Nb = normality of the NaOH solution,
C = HNO₃ solution used in the spike, mL
Na = normality of HNO₃ solution.

Precision

The limit of error at the 95% confidence level for a single determination is ± 3%

Literature
ASTM-C799, Standard Methods for “Chemical, Mass Spectrometric, Spectrochemical, Nuclear, and Radiochemical analysis of Nuclear-Grade Uranyl Nitrate solution”