Effect of background electrolyte concentration on the heat effects in the processes of formation of L-asparagine complexes with Cu2+ ions in aqueous solution

Abstract

A direct calorimetric method was used to measure the heat effects in the reactions of formation of Cu(II) complexes with L-asparagine in aqueous solutions at 298.15 K and ionic strength 0.5, 1.0, 1.5 (KNO 3 ). The standard thermodynamic characteristics (Δ r H 0 , Δ r G 0 , Δ r S 0 ) of the processes of complex formation in the system under study were calculated.Numerous publications concern the determination of the constants of reactions of the copper(II) asparaginate complexes in aqueous solution. We recalculated the stability constants of CuL + and CuL 2 , which were obtained by the different researchers in a variety of concentration conditions, to zero ionic strength according to Davis equation [1]. The most probable values of the thermodynamic stability constants of complexes of L-asparagine (L -) with the ion Cu 2+ were calculated as the mean values of those published in [2-5]: log β 0 (CuL + ) = 8.29 ± 0.01, log β 0 (CuL 2 ) = 15.09 ± 0.05. The recalculation of the constants to fixed values of ionic strength (I) was performed according to the equation proposed in [6]. Table 1 shows the thermodynamic characteristics of the formation of asparaginate Cu(II) complexes obtained by different researchers. It follows from Table 1 that the published data on the heat of complex formation in the system Cu 2+ -L-asparagine are contradictory. The difference in the determined heat effects of the CuL 2 species amounts to 14 kJ mol -1 . Effect of background electrolyte concentration on the thermodynamic characteristics of complex formation was not studied.The purpose of this paper is the determination by direct calorimetric method of the heat effects of the formation of copper(II) complexes with L-asparagine at 298.15 K and different values of ionic strength, as well as the calculation of standard thermodynamic characteristics of the complexation processes in the Cu 2+ -L-asparagine system.The heat effects of the interaction of solutions of copper(II) with solutions of L-asparagine accounting for all the processes occurring in the system can be calculated by formula (1).Δ mix H -Δ dil H = Δ r H(CuL + )α(CuL + ) + Δ r H(CuL 2 )α(CuL 2 ) + Δ dis H(HL ± )α(HL ± ) + Δ dis H(H 2 L + )α(H 2 L + ) + Δ r H(CuOH + )α(CuOH + ) + Δ r H w α(OH -).(1)Here Δ mix H is the heat effect of mixing the solutions of L-asparagine and Cu(NO 3 ) 2 ; Δ dil H is the heat effect of dilution of Cu(NO 3 ) 2 solution in the background electrolyte solution; Δ r H(CuL + ) and Δ r H(CuL 2 ) are heat effects of reactions of complexation of CuL + and CuL 2 respectively; Δ dis H(H 2 L + ), Δ dis H(HL ± ) are the enthalpies of the separate steps of L-asparagine dissociation [7]; Δ r H (CuOH + ) is the heat of formation of 1:1 hydroxo complex of Cu 2+ ; Δ r H w is the heat of formation of water in a solution of KNO 3 [8]; α(CuL + ), α(CuL 2 ), α(H 2 L + ), α(HL ± ), α(CuOH + ), and α(OH -) are the degrees of the occurrence of the corresponding reactions.The calculation of equilibrium composition of solutions before and after the calorimetric experiment