Refractometric Determination of Mutual Solubility as a Function of Temperature Tributyl Phosphine Oxide and Water CECIL E. HlGGlNS and WlLLlS H. BALDWIN Chemistry Division, Oak Ridge Nafional laboratory, Oak Ridge, lenn.
b This work was undertaken to see if the solubility of one component in another could b e determined refractometrically over a wide temperature range, given refractive indices and densities of standard solutions only a t 25" C. The solubilities of tributyl phosphine oxide (TBPO) in water and water in TBPO have been determined over the temperature range 13" to 99" C. by measuring refractive indices a t 25" C. of water solutions or dilutions and interpolating on the linear plot of ng5 vs. known TBPO concentration in water. Both solubilities are inversely temperature-dependent, increasing radically as f drops from 25" to 13" C. The refractometric procedure proved fruitful for the measurement of TBPO distribution between carbon tetrachloride and water a t 25" C. The relative hydration of the TBPO increased with the concentration of TBPO in the carbon tetrachloride phase.
T
method for determination of the composition of binary mixtures is partly dictated by the concentration ratio of the two components. Thus organophosphorus compounds which are only slightly soluble in water and process solutions are best determined by the use of phosphorus-32--labeled compounds and a solution counter ( I ) . When the substance is sufficiently soluble in the second component, other means may be employed. The common refractometric procedure for measuring the concentration of one component in another is that of determining the refractive index of the unknon-n binary mixture and interpolating on a curve of refractive index against known composition (7). The solubility of tributgl phosphine oxide (TBPO) in 17-ater at 25' C. ( I , 5 ) was great enough to suggest that a refractometric technique could be applied to a determination of the solubility as a function of the temperature. Qualitative measurements (cloud point) indicated rather drastic changes in the mutual solubility of TBPO and water HE
as a function of temperature. Quantitative measurements of these solubilities over a wide temperature range were made by using a variation (described below) of the standard refractometric technique. Standard solutions of known concentration of TBPO in TI ater were prepared a t 25" C. and the refractive indices of these standards lvere recorded. Because TBPO became less soluble as the temperature rose, the composition of water solutions of TBPO above 25' C. was determined by sampling the solutions a t the temperature of the test, cooling to room temperature, observing the refractive index of the sample a t 25" C., and interpolating on the linear plot of n',5 us. knonn TBPO composition in the standard solutions. Below 25" C. known weight samples of the solutions of TBPO in n ater \I cre diluted to a known volume a t 25" C. The concentration of the solutions saturated with TBPO belon 25" C. nas calculated from the amount of TBPO found in these dilutions. The solubility of water in TBPO was measured by difference. The weight of TBPO in a sample of kno\\n weight was determined refractometrically and subtracted from the sample rveight. A logical extension of this solubility work was the measuring of the distribution of TBPO between organic and water phases refractometrically. Burger (3, 4) has already developed and used effectively a refractometric method for measuring distribution coefficients of many substances, including inorganic nitrates and nitric acid, in organicaqueous systems. The distribution of TBPO between the immiscible solvents carbon tetrachloride and water as a function of TBPO concentration is reported here. EXPERIMENTAL
Chemicals. Distilled water and carbon tetrachloride were used as the solvents. Tributyl phosphine oxide (TBPO) was prepared by the reaction between butplmagnesium bromide and phosphorus oxychloride (6). The product C.P.
was distilled a t 136" to 138" C. a t a pressure of 2 to 3 mm. of mercury. Titration of the product (white solid a t room temperature) revealed
