Vicente Gentil and Alcider Caldas University of Brazil, Rio de Janeiro Translated by Ralph E. Oesper University of Cincinnati
1
It is frequently desirable in qualitative analysis to separate as an insoluble silver group precipitate the following anions: SCN-, Cl-, I-, Br-. The presence of large amounts of CNS-, and/or Imay produce interference with the usual subsequent individual tests as follows: ( a ) Large amounts of Imay interfere with the detection of SCN- in that I reacts with Fe+a to liberate free iodine which masks the iron-thiocyanate color. FetJ
+ I- rr Fe+l +
I.
(b) Large amounts of SCN- may interfere with the halide tests based on the differential oxidation of these anions to produce free halogen. The red color of the iron-thiocyante complex may mask the detection of small quantities of iodine which have been extracted into chloroform. The SCN- may also be oxidized to CN- which mav then react with iodine or bromine t3 produce volatile iodo- or bromocyanogen. since the decomposition products of sodium formate react with SCN- with the CN-and S-and also dissolve the silver halides, it was suggested that this reagent be incorporated into the group analysis scheme.' The chemical resctions are as follows: 35,545 (1958) C A ~ AA,,~w, o GENTIL,V., J. CHEM.EDUC.,
386
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Journol o f Chemical Education
Detection of Thiocyanate and Dissolution of Silver Halides -
The NaCN and Na& are partially hydrolyzed, and the HzS produced may be detected in the usual manner. To a sample solution in a. test tube or porcelain crucible from which S-', CN; Fe(CN).-5 Fe(CN),-: S.Oa-P, and SO3-%have been removed, add AgN08 to farm the group precipitate oontaining AgSCN, AgI, AgBr, and AgC1. Wash the precipitate thoroughly. Add approximately twice the precipitate's weight of #odiumformate, and heat slightly above 400°C. Places. piece of paper moistened with a solution of lead acetateover the mouth of the test tube or crucible. The formation of lead ~ulfideon the paper indicates the presence of thiooyanate in the sample solution. Continue heating until the formate is completely d e oom~osedand the volatile ~roductsare driven off. Cool the reaction vessel, add water, i n d filter the suspension or use a centrifuge. The clear solution contains the sodium salts of the halides. Make this solution acidic with 6 N acetic acid. Apply heat, and test again for complete removal of the hydrogen sulfide. Any of the accepted procedures for the halides may then be s p plied to this solution. This procedure will deteit the presence of 1 mg of I; Br-, CI-, or SCN- in any mixture containing up to 100 mg of each of the other ions.
