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S. Ren. o,a n a t h a n Research and Development Cell ~
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Bombay, lndia Bharati J. Mehta CSMCRI IAEC Bhavnagar, lndia
A Simple and Impressive Laboratory Exercise in lon-Exchange
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Silver chloride obtained as a precipitate i n argentimetric determinations of C1- is usually thrown off a s a waste. A simple laboratory exercise involving ion-exchange t o recover t h e silver from t h e precipitate can he given t o students of undergraduate level. T h e experiment throws light on t h e usefulness of ion-exchange a s a tool for recovery of valuable materials from waste', and further the whole process imparts a sound training t o t h e students in the fundamentals of ionexchange technology. Principle Silver chloride dissolves in aqueous ammonia forming a complex according t o t h e equation AgCl + 2NH1- Ag(NHht C1When this solution is passed through a cation-exchange resin in ammonium form, t h e Ag(NH&+ ions exchange for t h e ammonium ions of t h e resin and t h e resin can then he advantageously eluted with dilute nitric acid. T h e elution gives a mixture of silver nitrate and ammonium nitrate. Scheme NHI
AgCl -Ag(NHsh+
C1-
passed through a eation-exchange resin in ammonium form
Resin loaded with Ag(NHht cations Elution
4
+ ammonium chloride effluent
dil. HN08
silver nitrate and ammonium nitrate Procedure A known amount of silver chloride (say 3 g) is dissolved in a minimum amount of 2 N ammonia and thesolution is made up to 250 ml with deionized water. This is preserved, suitably protected from light. (The precipitate obtained from Mohr's titrations should be used only after washing it free of silver chromate with dilute nitrie acid followed
a stopcock. The resin is converted to the ammonium farm bypassing a sufficient excess of 2 N ammonia.' It is then washed free of alkali,
hack washed, allowed to settle, and kept ready. Exhaustion Cycle The Ag(NH&+ C1- solution is now passed through the resin bed a t s flow rate of 5 mllrnin. The void volume is rejected and fractions
of 25 ml are collected. The silver content in the various fractions that mieht have leaked through is estimated bv the standard eravimetric
Elution Cycle Dilute nitric acid of known concentration is passed through the exhausted resin bed at a flow rate of 5 mllmin. Fractions of 25 ml are collected and the silver elutedas silver nitrate is estimated as above. The elution is discontinued once the effluent is free of silver, which can be qualitatively tested with a small amount of the sample. (This test is safely conducted after collecting ahout 5 bed volumes of effluent ..... ...i,
A mdt~rlnlhalance fc>rthe kilwr loaded nnd r l u d i i al,~,d r r i \ c d at. 'Tiw elu~cds ~ ~ l u t ~aonta~nq n 3 rnutuw or >4ver nitvaleand a m mc,nnlrnnltmre.Th~scnnbe uiedai 2rea:cnt in borhqoalit.et:\,?dn
