H+-mg2+ And H+-ca2+ Exchange Onto Carboxylic Ionites In Terms Of The Exchange Equilibrium Theory
H+-Mg2+ AND H+-Ca2+ EXCHANGE ONTO CARBOXYLIC IONITES IN TERMS OF THE EXCHANGE EQUILIBRIUM THEORY
A. Mamchenko, Y. Valuiskaya, H. Klimova
Dumansky Institute of Colloid and Water Chemistry of Ukrainian National Academy of Sciences, Kiev, Ukraine
Magnesium and calcium ion sorption by H-type carboxylic ionite Lewatit CNP 80 while as the magnesium and calcium sulphate concentration in the “outer” solution was varied by two orders was considered from point of view of the ion exchange equilibrium theory. The practical significance of investigation of this sistem is defined first of all by widely use of H-type carboxylic cationites for partial desalination and softening of water.
The exchange of one-charged hydroxonium ions into two-charged cations was considered from point of two view which were based on various geometry of fixed exchange centres disposition and leading to the following equations of exchange equilibrium constants:
2pH – pMе = pК1 + n lg{a (2 - a) /[4(1 - a)2]},
2pH – pMе = pК2 + n lg{a /(1 - a)}, where
α – average degree of exchange centre filling by second component;
pН и pMе – negative logarithms of activity of hydroxonium and two-charged metal ions in «outer» solution;
n – empirical constant.
The experimental data show, the solution of resinates in gel phase as in case of magnesium ion sorption as calcium ion sorption is far from an ideal behaviour (n1). Deviations are connected only with the influence of three or four nearest pairs of exchange centres. The theory allows to give quantitative estimation of this ifluence onto value of effective equilibrium constant. A general equation of exchange isotherm for one- and two-charged ion exchange can be represented in such form:
, where
i – a number of the nearest fixed ions;
j – a number of the nearest fixed ions bounded with counter-ion;
K(j) – the local thermodinamic equilibrium constant;
C – the relation of activities of displaced (H+) and sorbting (Me2+) ions in the external solution.
The equation has general analytical resolutions for values i not more than 4. The results of approximation by two- or three- parametric equations of ion exchange equilibrium parameters are characterized by calculation error of 2pH – pMe values not more than 10%. Maximum precision is provided by the model K(0
The equilibrium constants in the same salt composition of external solution approximately half as big in the case Ca2+- exchange, than in the case Na+- exchange, that is connected apparently with the lesser influence of solution composition onto ionite swelling. It was determined, that selectivites of calcium and magnium ion sorption by the polyacrylic cationite in H – type are close. An explanation of this phenomenon was proposed.
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