The mutual solubility of TBP and water has been measured in the presence of nitric acid and of various inert diluents for the TBP. Nitric acid partitions have also been studied. It is concluded that the most important compound formed is HNo3.TBP, and that this m a y be partly hydrated. Several new effects occur above ca. 7 M aqueous acidity. Ideal thermodynamic behaviour is observed only at rather low TBP concentrations ; at high TBP concentrations the deviations from ideality are considerable.

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Thermodynamic equations have been derived which permit, for the first time, the exact calculation of the activity coefficients of involatile solutes in mixed solutions from vapour pressures. One of these is especially convenient for application to isopiestic experiments. The other is applied to the results of Owen and Cooke on mixed aqueous solutions of lithium and potassium chlorides.

Evidence is presented pointing to the conclusion that nitrates are extracted by TBP in the neutral, unionized form, always solvated by a definite number of TBP molecules, e.g. Ce(N03)3 .3TBP, UO2(NO3)2 .2TBP ; the compounds formed are sometimes hydrated, sometimes almost unhydrated. The evidence is much fuller for uranyl than for any other nitrate, but all the data support this general conclusion.The nature of the species present in the TBP phase must be investigated before the distribution data for water + TBP systems can be properly understood, The present paper is concerned with experiments bearing on this question.

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