Coprecipitation of Barium in Group I1 of the Qualitative Procedure WII.LIAI\I T. HALL Rochester, Massachusetts
N A PAPER by Leo Lebnnan and Irwin Mandel,'
1considerable experimental evidence is given to show
that when a student of qualitative analysis misses barium it is not during the testing for the silver or coppertin groups. With respect to the silver group, some texts state that barium chloride may be precipitated if too much hydrochloric acid is present. I have seen students dissolve a sample in concentrated hydrochloric acid and get a residue of barium chloride. They have mistaken i t for a member of the first group, but since the precipitate is soluble in water and gives a yellow precipitate with chromate ions, lead is the only cation for which the barium can be mistaken, and this error only rarely accounts for missing the barium. With respect to Group 11, there is grave danger of carrying down barium sulfate with thc sulfides if the solution is hot and contains an oxidizing agent. I have known an honor student to miss barium in this way and spend about 12 hours trying to find the barium when the instructor told him that it was present in the solution given out for analysis. . The older chemistry tests used to require the removal of nitric acid before the introduction of hydrogen sulfide to precipitate Group 11. A. A. Noyes in his excellent text recommends the use of nitric acid for mahing the solution 0.3 N in mineral acid before precipitating the coppertin group, but he recognizes the fact that a chloride solution is better if hydrogen sulfide is to be passed into a hot solution to precipitate arsenic sulfide. Therefore he recommends diluting the solution, heating nearly to boiling, and introducing hydrogen sulfide to the filtrate from the first sulfide precipitation. If a yellow precipitate is formed, suggesting arsenic, the directions call for adding hydrochloric acid and evaporating until all the nitric acid is removed. If the solution contains only hydrogen sulfide, most of it is boiled off without being oxidized, but if a precipitate of arsenic sulfide is present this will dissolve eventually in the aqua regia, with the formation of arsenic and sulfuric acids. Then, if a little barium is present, a fine precipitate of barium -
' 3. I'hys. Chetn., 47, 264-6 (1943). ' 2. anal. Chcm.. 124, 391429 (1942)
sulfate will form which is likely to be mistaken fbr finely divided sulfur. The conclusion of Lehrman and Mandel that barium is never precipitated with the copper-tin group is, therefore, wrong. One gets the idea from the above-mentioned paper that the loss of barium is likely to be the result of physical and not chemical causes, though, of course, no one can define sharply just where physics ends and chemistry begins. In a recent paper by Shulek and Boldizar2 the opinion is adv;mced that many cases which have been attributed to ndsorption, occlusi,on, etc., are really due to the formation of insoluble precipi-. tates. As an example they cite the difficulty in getting pure precipitates of barium sulfate, but it seems to one who has read most of the evidence that i t is somewhat doubLful whether such precipitates as (BaC1)2S04really exist. However, in the case of barium there are many insoluble comp,ounds which. may lorm in Group I11 of qualitative analysis. Thus, i[ the solution is allowed to stand very long after the addition of ammonia, carbon dioxide will be absorbed from the air and barium carbonate will be precipitated. Then, too, barium may precipitate as oxalate, phosphate, tartrate, fluoride, chromate, chromite, etc. Some texts on qualitative analysis pay very little attention to any of these barium compounds, and only very few texts realize t h a ~barium, strontium, and calcium, like zinc, are likely to precipitate as insoluble chromiles when trivalent chromium is present, although the fact thxt zinc precipitates with chromium has been much emphasized. If a n old solution of ammonium sulfide is used to precipitate ihe sulfides of iron, nickel, cobalt, manganese, and zinc, there is danger of the precipitation of some barium sulfate. All things considered, the most srrious cause for the missing of bar-. ium by a student is unquestionably the formation of barium sulfate, because unless the student knows barium is present he will easily overlook the fine precipitate. The precipitate is easy to handle because hot sodium carbonate solution converts it quite readily into water-soluble sodium sulfate and acid-soluble barium carbonate, but very few texts provide definitely for its possible formation.
