Qualitative Analysis of the Hydrogen Sulfide Group without Ammonium Polysulfide RALPHW. HUFFERD,DePauw University, Greencastle, Ind.
T
HE common practice in qualitative analysis is to oxidize
the elements of the tin division of the hydrogen sulfide group to their higher valence with ammonium polysulfide. Because of the instability and variation in concentration of the polysulfide solution, it is not uncommon for stannous sulfide to resist the treatment and remain with the copper division precipitate. There is the added difficulty of having large quantities of the copper division sulfides, especially copper sulfide, carried over into the tin division. Polysulfide also introduces the nuisance of having a large bulk of sulfur mixed with the reprecipitated sulfides of the tin division, often leaving the analyst in doubt as to whether or not there is any sulfide there. These difficulties can be either avoided or reduced so considerably as to cause no trouble to inexperienced students by the simple expedient of oxidizing the elements with bromine before precipitating them as the sulfides, and then making the division separation with ammonium sulfide. The sulfide is prepared as needed by passing hydrogen sulfide into 6 N ammonium hydroxide. As a precaution against oxidation of hydrogen sulfide to sulfur during precipitation of the sulfides, nitrate ion is destroyed by repeated boiling with 12 N hydrochloric acid. Evaporate the filtrate from Group I to about 5 cc. but not t o dryness. Add 10 cc. of 12 N hydrochloric acid and again evaporate t o small volume. Repeat addition of hydrochloric acid and
evaporation once or twice. This procedure does not take much time, but by destroying nitrate ion prevents excessive oxidation of hydrogen sulfide during precipitation. Add 5 cc. of water and, if necessary, heat to dissolve any solid. It is sometimes necessary to add 2 cc. of 6 N hydrochloric acid. Add an excess of saturated bromine water, and boil off excess bromine. About 2 minutes' boiling will usually suffice. Starchiodide paper can be used if there is any question. In the writer's laboratory the entire preparation is carried out in the 250-cc. Erlenmeyer in which precipitation is to take place. Neutralize the solution with ammonia and 6 N hydrochloric acid. Adjust acidity and volume in the customary manner, and precipitate with hydrogen sulfide. Transfer the well-washed precipitate on the filter t o a small beaker and add 10 cc. of fresh ammonium sulfide solution, prepared by saturating 10 cc. of 6 N ammonium hydroxide with hydrogen sulfide. Heat the beaker in boiling water for 3 minutes, stirring the contents occasionally. Unless the filter paper seems clean, no attempt should be made to remove it, as it will not interfere with the analysis of the cop er division precipitate. This treatment will remove 100 mg. eaci of the three members of the tin division as sulfides. After filtering and washing, the precipitate is ready for analysis for comer division elements. Preciaitate the tin division sulfides from i6e filtrate in the customary way. ~
The writer has found that the average student of elementary qualitative analysis can be expected to find 10 mg. of any member of the group in the presence of 100 mg. each of several or all other members of the group. RECEIVED January 18, 1933.
Filtering Device E. B. KESTER,Pittsburgh Experiment Station, U. S. Bureau of Mines, Pittsburgh, Pa.
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ONSIDERABLE time is lost in the laboratory purifying obtain one in which the bore is fairly uniform. A plunger by recrystallization of small quantities of substances, par- of Bakelite, hard rubber, or other inert substance is turned ticularly derivatives of organic compounds such as oximes, out on a lathe to such dimensions that the plunger has only hydrazones, semi-carbazones, anilides, and the like. Not onlv sufficient clearance to enable it to slio easily UD and down the " tube. A number of longitudinal piiholes"(a6out 0.7 mm. in must a Biichner or Wirsch funnel and sucdiameter) are bored for escape of the liquid. tion flask be arranged and filter paper installed, When a crystallization has been completed, the filtration but precipitates must be transferred back and is accomplished by merely forcing the plunger to the bottom forth from funnel to flask. Losses are incurred of the tube. This ooeration should be started slowly to Derinevitably. Moreover, it frequently happens mit a cake to build-up that will prevent escape ofcrystals that a chemist is isolating a product which up the sides and through the orifices, but may be accelerated during the earlier stages of purification melts when such cake has formed. At the end, the plunger may be below room temperature, in which case he finds pressed down with considerable force so that the product it difficult if not impossible (without recourse is free of excess liquid when the mother liquors are poured off t o more elaborate preparations) to carry out a and the plunger has been withdrawn. It can then be washed, filtration before his product fuses. triturated, or recrystallized and filtered again in the same conThe filtering device illustrated eliminates the tainer and in the same way as before. Drying is effected by need of transfers of solids or solid-liquid mixevacuation of the tube or passing a current of air over the tures. Numerous successive recrystallizations cake. can be carried out in a surprisingly short time A soft rubber plunger similarly molded affixed to a glass with little loss of product save that which brings rod is even more satisfactory from the standpoint of cleanthe mother liquors up to saturation. Filtraness of filtration, but it can only be used in connection with tions of crystals obtained by chilling in refrigmild solvents. Benzenoid hydrocarbons, for example, rot erating media can be conducted in the media. soft rubber so rapidly there is serious danger of contamination I n c r e a s e d pressure is obtainable over that of product, to say nothing of ruination of the device itself secured in suction filtrations. A large Pyrex test tube, preferably of 75 to RBCEIVED August 2, 1933. Published by permission of the Director, U. 8 80 cc. capacity, is selected, care being taken to Bureau of Mines. I
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