MODIFIED METHOD for CATION GROUP I RALPH W. GELBACH The State College of Washington, Pullman, Washington
THE conventional scheme of qualitative analysis provides for the determination of silver, lead, and mercurous ions in Group 1in the form of their chlorides. The modification presented in this paper eliminates some difficulties and affords the advantage of a shortened procedure. The sample for cation analysis is prepared for precipitation in the usual manner after which the copper and tin groups are precipitated by means of H2S. The resulting precipitate which may consist of sulfides of silver, lead, bismuth, copper, cadmium, mercury, arsenic, antimony, and tin, is then treated with "sodium sulfide reagent," as recommended by A. 'A. Noyes,' to effect the separation of the copper and tin goups. By this treatment any mercurous sulfide is oxidized forming NazHgS2 which makes its detection possible in the tin group. Silver and lead are left in the residue from the sodium sulfide treatment along with other members of the copper group. After the copper group precipitate is washed with approximately normal NaN08 solution, which serves primarily to prevent colloidal suspension of sulfides,
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L N o ~ sA., A,, "Qualitative chemical analysis," 9th ed., The Mamillan Co., New York City, 1932, p. 71.
i t is dissolved in boiling dilute HNOJ. To the hot solution, which has been diluted to about 50 cc., dilute HC1 is added to precipitate the silver, which if present is filtered and confirmed. The filtrate from the silver separation is then analyzed for the remaining members of the copper group in the usual manner. To summarize, it is to be observed that'all the lead is removed in one precipitation making it possible for the student better to estimate the quantity present. This point is also true in connection with mercury. In many instances mercurous mercury may be partially oxidized while with the ordinary scheme mercury would be divided between the silver and eopper-tin groups. Further, the separation of silver and mercury by means of the sodium sulfide procedure overcomes the difficulty o' the precipitation of metallic silver in the presence of large quantities of mercury. The possible disadvantage of not determining the presence of mercurous ion is not serious for in case i t is desirable to determine the valence a special procedure may be used.z The author has used this procedure for the past three years and has found it highly satisfactory.
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Lot,
cit., p. 131.
