Temperature independence of solvation numbers of electrolytes determined by the adiabatic compression technique up to the complete solvation limit

The apparent volumes of the salts in the systems H 2 O-NH 4 Cl (298 K) and H 2 O-NH 4 NO 3 (273 K, 298 K, and 323 K) are reproduced with an accuracy of 0.03-0.01 cm 3 /mol by the equationwhere w 2 is the salt content (mass fractions). The study shows that there is a correspondence between the critical (for determining the hydration number) structural parameters-the intrinsic volume of the electrolyte and the volume of water in ion hydration shells-and the limiting (at w 2 = 1) partial molar volumes of the components. The hydration numbers at infinite dilution are 6.9 for NH 4 Cl at 298 K and 9.1, 6.7, and 6.4 for NH 4 NO 3 at 273 K, 298 K, and 323 K. The water volume in ion hydration shells decreases in the sequence: 3 NO , − Cl -, and 4 NH . + The hydration numbers decrease with increasing salt concentration. The study shows that within a simpler model φ = φ 0 + a 0.5 2 w , the hydration numbers are temperature independent. Ammonium salts play a special role in solution chemistry. Thus, the equality of characteristics of the 4 NH + and Clions was used as a basis to derive the standard partial volumes* [1] and heat capacities [2] of the separate ions in aqueous solution. A reference scale for the contribution of the separate ions in the chemical shift of water protons was suggested from the condition δ 0 ( 4 NH + ) = 0 [3]. According to the classification [4], the ammonium ion is a weak water structure breaker; however, it can be inferred from the data in [5] that it is rather a structure maker. Speaking about Cland 3 NO − , the classification [4] tells us that both anions are water structure breakers of approximately the same strength.Ammonium chloride and ammonium nitrate are important for atmospheric chemistry [6,7]. The solubility of NH 4 Cl and NH 4 NO 3 in water at 298 K is 28.4% (7.4m) and 68.2% (26.8m), respectively (m is molality). At the same time the concentration of NH 4 Cl in small aerosol drops can be as high as 60% (28m) [7]. Ammonium nitrate can exist under similar conditions in the form of a virtually anhydrous melt [7]. *The standard partial molar volume of NH 4 Cl in aqueous solution at 298 K is equal to two molar volumes of water at this temperature.

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Temperature independence of solvation numbers of electrolytes determined by the adiabatic compression technique up to the complete solvation limit

Abstract: The theory of the dependence of solvation numbers on the temperature is very popular today. However, according to our works, solvation numbers do not depend on temperature until the complete solvation limit is reached. The purpose of this paper is to address this apparent contradiction.

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References 9 publications

Concentration dependence of NaCl in a wide range of temperatures and pressures

Concentration dependence of NaCl in a wide range of temperatures and pressures

Amino acid hydration in aqueous magnesium and sodium sulfate solutions

Bulk properties and hydration numbers of ammonium nitrate and ammonium chloride in solution: A structural and thermodynamic analysis

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