lon-Exchange Resins-A
Simple Apparatus
A very old and general procedure for the determination of total salts concentration in water is the use of ian-exchange resins.' This is a very simple experiment that freshmen students with a poor laboratory background can perform with' good profit. The simplicity of the technique frees the students to see the correlation between the abstract work m the laboratory and the real problems that everyone is facing in everyday life, i.e., the water hardness, the water pollution by ionic detergents as polyphosphates, etc. In our teaching experience, we find that if this experiment is performed in the usual way, about two hours are necessary to regenerate the resin, to rinse the column, to percolate and to elute the sample under test. Therefore, the students have spent too much time, that could be used in a better way, watching drops solutions slowly coming out the ion-exchange column.
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I
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Traditional and Modified Exchanger Apparatus: Comparative Results
Procedure
Number Testa
Time Required for Each Test
traditional modified
20 20
2 hr 15 min
of
Mean Erm. in 20 Deter-
minations
1% 0 8%
Mean Error for the Five Best Students
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0.1% 0.1%
To speed up the procedure, without loss in precision, we assembled the simple modified exchanger apparatus shown in the figure. The apparatus consists of 1) the column of resin; 2) a graduated pipet in which the eluent fractions are collected; 3) a pipet filling bulb by which a slight suction is applied to the column; and 4) a Y junction that, together with the Hoffman screw clip shown below allows the collected fractions of eluent to be discarded. The experiment performed by our students using this apparatus consisted of passing an unknown solution of potassium chloride through a strongly acidic catian-exchange resin (Dowex 50 w). This solution was converted into the corresponding mineral acid, and the effluent was titreted with a standard 0.1 NNaOH using methyl orange as indicator. The results that we were able to get with this apparatus are shown in the table. It is clear that the proposed procedure is about eight times less time consuming and gives results of the same precision as the usual one.
This work was partially supported by the "Centro di Studio sulla Chimiea Analitica Strumentale, C.N.R., Bari (Italy). "Ion Exchange Resins", 5th edm, The British D N Houses ~ LTD, B.D.H. Laboratory Chemicals Division, Pool England, 1965, p. 52. University of B a r i Via Amendola 173 70126 Bari, Italy
572 / Journal of Chemical Education
Basilio Morelli Leonardo Larnpugnani
