Isotopieeffekte in den Hydratstrukturen von festem und gelöstem Kupfersulfat / Isotope effects in the hydrated structure of solid and aqueous copper sulphate

Abstract

The fractionation of the oxygen isotopes of the water molecules in CuS04-5 H20 and in aqueous solutions of copper sulphate has been measured at 25 °C. In the total crystal water (G) three kinds of water can be distinguished by their binding: 1) The bisector of the two lone pairs directed to­wards the copper ion (Kj), 2) One of the lone pairs directed towards the copper ion (K2), 3) The two lone pairs directed towards two hydrogen atoms of water molecules coordinated with the cop­per ion (L). The water molecules in the copper sulphate solution are considered either hydration water (HW) or bulk water (FW). Defining a.\-B= (180/160)a/(180/180)b the following results have been obtained: There remains some doubt if the values attributed to ctKi-G and aK2-G have to be interchanged. The elementary cell of CuS04*5 H20 consists of two molecules. Four molecules of Kj-water are coordinated with one copper ion and four molecules of K2-water are coordinated with the second copper ion. The remaining two water molecules are of L-type. It is concluded from the results that on dehydration at temperatures below 50 °C first one copper ion loses it’s water and then the other. There is some doubt as to which group breaks up first. In addition, the results show that the L-type water becomes quantitatively the water of the monohydrate in agreement with the conclusions drawn from the results of the fractionation of the hydrogen isotopes in CuS04-5 H20. It is de­monstrated that, by considering only the overall fractionation of the oxygen or hydrogen isotopes between total crystal water and the saturated solution, wrong conclusions about the crystallization process could be drawn. In the case of CuCl2 solutions, it has been shown that the separation factor aHW-FW is sensi­tive towards changes in the structure of the solution.