Anions in wastewaters - Determination of Anions in Natural and Treated Waters - page 563

8.20.2 Column coupling isotachoelectrophoresis
The application of this technique is discussed under multianion analysis in section
14.6.1.1.
8.20.3 Ion selective electrodes
Ion selective electrodes are suitable sensors for activity measurements in process control,
continuous ion flow analysis and in industrial wastes. Inorganic gels possessing ion
exchange properties may be developed as membrane electrodes selective to various ions.
There have been several attempts to construct electrodes selective to sulphate ions.
Fischer and Babcock [93] discussed a heterogeneous sulphate membrane consisting of
powdered barium sulphate dispersed in paraffin. Rechnitz et al. [94], Rechnitz and
Mohan [95], Ross [96] and Nagy and Fjeldly [97] prepared precipitate-based membranes
from lead sulphate, lead sulphide and silver sulphide etc. All these are susceptible to
interference by sulphide and sulphite etc. anions and a number of cations. Numerous
other investigations made in this direction are summarised by Buck et al. [98] in the
literature they have compiled for membrane electrodes.
Srivastava and Jain [99] investigated the performance of a sulphate ion selective solid
state membrane electrode prepared from hydrous thorium oxide gel with polystyrene as
binder. The membrane showed good selectivity for sulphate ions in the range 0.1-100µ
mol L −1 and there was practically no interference from a large number of anions and
cations. The electrode could be used at pH 6-10 and could be used in partially non-
aqueous systems. This electrode was used to determine sulphate in waste waters from the
pulp and paper and leather processing industries.
The electrode is non-Nernstian in nature. The useful pH range for this assembly is 6-
10. The membrane electrode shows good selectivity to sulphate ions and there is
practically no interference from a large number of anions and cations.
The electrode has also been used as an end-point indicator in potentiometric titrations
involving sulphate ions. The titration of potassium sulphate (20ml, 10 −3 mol L −1 ) was
performed with barium chloride solution (5×10 −3 mol L −1 ). The break in the curve is
quite sharp and represents a perfectly stoichiometric end-point (Fig. 8.5). Compared with
direct potentiometric methods (based on Nernst equation) potentiometric titrations offer
substantial improvement in accuracy and precision. The standard deviation (10 replicates)
of this particular titration was 0.01ml.
Kotek [100] used barium chloride to determine sulphate in wastewaters at
concentrations in excess of 0.5g SO 4 L −1 by titration with barium chloride using a barium-
selective electrode. This method was more accurate than