Vapor pressures, osmotic and activity coefficients for (LiBr + acetonitrile) between the temperatures (298.15 and 343.15) K

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Abstract

Precise vapor pressure data for pure acetonitrile and (LiBr + acetonitrile) are given for temperatures ranging from T=(298.15 to 343.15) K. The molality range is from m=(0.0579 to 0.8298) mol · kg−1. The osmotic coefficients are calculated by taking into account the second virial coefficient of acetonitrile. The parameters of the extended Pitzer ion interaction model of Archer and the mole fraction-based thermodynamic model of Clegg–Pitzer are evaluated. These models accurately reproduce the available osmotic coefficients. The parameters of the extended Pitzer ion interaction model of Archer are used to calculate the mean molal activity coefficients.

Introduction

Electrolyte solutions exhibit considerable deviations from ideal behavior, especially at high solute concentrations and temperatures. Many practical applications involve such systems, for example desalination processes and process design. Thus the knowledge of thermodynamic properties is indispensable for practical and theoretical purposes.

Acetonitrile is the most commonly used solvent for background electrolytes in non-aqueous capillary electrophoresis. Although toxic, this solvent is inexpensive and easily available at high purity. The (LiBr + acetonitrile) are of particular interest to engineers and chemists and they are commonly used in modern electrochemical technologies [1].

For (LiBr + acetonitrile), osmotic coefficients from vapor pressure measurements [1] can be found in the literature at T=298.15 K, whereas data on the thermodynamic activity of acetonitrile salt solutions and the corresponding osmotic and activity coefficients at temperature other than T=298.15 K are very limited.

Therefore, a systematic study has been undertaken to determine the vapor pressure of (LiBr + acetonitrile) with high precision in the temperature range from T=(298.15 to 343.15) K and for concentrations ranging from m=(0.0579to0.8298) mol · kg−1. These data may be used as reference data for indirect measuring methods (isopiestic method). On the other hand, they yield precise osmotic and activity coefficients needed for examination of theoretical developments claiming applicability to highly concentrated electrolyte solutions of associating electrolytes up to high temperatures.

The Clegg–Pitzer [2] and extended Pitzer ion interaction model of Archer [3], [4] were successfully applied to reproduce the experimental osmotic coefficients. We use our results to evaluate the parameters of both an Archer-type modified Pitzer model [3], [4] and a similar model using mole fraction compositions [2]. The mole fraction concentration model is able to represent the data accurately for the studied temperature and concentration regions. A similar good data description obtained with the modified Pitzer model [3], [4]. This extended equation has an additional, adjustable, third virial parameter and is capable of representing experimental value of the osmotic and activity coefficients of most aqueous and non-aqueous electrolytes more accurately over a wide range of ionic strengths than the original model of Pitzer [5].

Section snippets

Materials

Lithium bromide (Merck, suprapur) was dried in an electrical oven at about 393 K for 24 h prior to use. Then, this salt was degassed for several hours.

Acetonitrile (Merck, >0.9999 mass fraction purity) was dried with molecular sieve and was checked with gas chromatography (HP-6890) with a flame ionization detector (FID) and a mass fraction purity of 0.99997 was found. No water could be detected by Karl–Fischer titrator (mci, model CA-02). The salt and acetonitrile were used without further

Experimental results

The temperature-dependent vapor pressure of pure acetonitrile was measured from T=(298.15to343.15) K. In the literature, there are numerous vapor pressure data for acetonitrile [7], [8], [9], [10], [11], [12], [13], [14]. A comparison of various values for acetonitrile vapor pressure from T=(298.15to343.15) K is shown in figure 1.

The vapor pressures of (LiBr + acetonitrile) were measured from T=(298.15to343.15) K in 5 K intervals for molalities between m=(0.0579and0.8298) mol · kg−1. The salt is

Conclusions

Precise experimental osmotic coefficients were reported for (LiBr + acetonitrile) for molalities between m=(0.0579and0.8298) mol · kg−1 and between T=(298.15and343.15) K. Experimental data of the investigated system were satisfactorily correlated using the extended Pitzer model of Archer and the Clegg–Pitzer model.

The application of the Pitzer model with Archer extension has been shown to correlate the experimental osmotic coefficient data with very good accuracy, especially using the Pitzer model

Acknowledgements

K.N. is grateful to the Iranian Ministry of Science, Research and Technology for a grant. We thank also the German Arbeitsgemeinschaft industrieller Forschung AiF “Otto von Guericke” e. V.(AiF) for financial support.

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