The (water + acetonitrile) mixture revisited: A new approach for calculating partial molar volumes

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Abstract

Density and viscosity of (water + acetonitrile) mixtures were measured over the whole composition range at the temperatures: (298.15, 303.15, 308.15, 313.15, and 318.15) K. A new mathematical approach was developed which allows the calculation of the derivatives of density with respect to composition avoiding the appearance of local discontinuities. Thus, reliable partial molar volumes and thermal expansion coefficients were obtained.

Introduction

Acetonitrile (AN) is employed in a large number of industrial processes that take place in the presence of water. By the other hand, (water + AN) mixtures are of interest in the study of solutions physical chemistry. Thus, the availability of accurate data for this system is of interest both for theoretical and practical applications. For these reasons, a significant number of papers have been published dealing with density and viscosity measurements of this binary.

A review of the literature shows that density of the (water + AN) system has been measured by several authors [1], [2], [3], [4], [5], [6], [7], [8], [9]. However, reported data for mixtures with very low water content show differences that are well beyond experimental error and values for derived quantities, such as partial molar volumes and thermal expansion coefficients calculated by different authors are not in good agreement.

By the other hand, viscosity measurements have been reported only in two of these papers [2], [9]. In the first one data with an absolute error of ±0.002 mPa · s are reported in the 288.15 K to 323.15 K temperature range, while in the work of Hickey and Waghorne precise data are given only at 298.15 K.

In the present work, we report precise density and viscosity measurements performed on (water + AN) solutions which extend the temperature range found in the literature and, on the basis of these measurements and of those previously obtained by other authors, discrepancies found in the literature are discussed.

Section snippets

Experimental

Spectroscopic grade AN (Aldrich, purity > 0.999 mass fraction) and bidistilled deionized water with specific conductance below 1 μS · cm−1 were employed throughout the experiments. Chromatoghraphic analysis of the provided AN showed no detectable impurities and water content in the solvent was determined as in previous work [10]; water molar fraction was found to be 0.0010 ± 0.0002. All samples were prepared by weight employing a Mettler H20-332131 balance with an uncertainty of 10−5 g.

Density

Density measurements and derived quantities

Density was measured at (298.15, 303.15, 308.15, 313.15, and 318.15) K. The obtained values are reported in table 1 where indexes 1 and 2 refer, respectively, to water and AN. As shown in figure 1(a) the results obtained at 298.15 K agree very well with previously published data [3], [8].

A first point arises here, regarding the real value of AN density. In fact, and as seen from table 2, the literature data reported by several authors at 298.15 K [1], [2], [3], [4], [5], [6], [7], [8], [9] show

Conclusions

The temperature range of experimental data for density and viscosity values of (water + AN) mixtures has been extended and a new approach for describing composition dependence of density is given. This mathematical approach eliminates previous inconsistencies observed when quantities requiring derivatives of density with respect to composition are calculated. Thus, reliable values for partial molar volumes and coefficients of thermal expansion can be obtained.

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