LETTERS To the Editor:
which was printed in the Journal of Chemical EducaIn the January issue of the JOURNAL OF CHEMICALtion rather than in the Journal of Physical Chemistry EDUCATION there appears a statement by Professor in order to reach more teachers of qualitatiue analysis. William T. Hall criticizing the conclusions of an Somefewyears ago, the top ranking student ofhisclass article on the absence of coprecipitation of harium atWestPointwassuhsequentlysentto theMassachusetts ion with the sulfides of Group I1 [J.Phys. Chem., 47, Institute of Technology to he trained as a chemical 264 (1943)l. Professor Hall points out several ways engineer. At the Institute, as a t West Point, he in which harium may he lost during the course of a proved the brightest and most painstaking man of qualitative analysis. However, the conclusions of the the class. He was given a solution to analyze which article in question are: "No coprecipitation (adsorp- contained 3 mg. of Ba++ and somewhat more As04'. tion, occlusion, or postprecipitation) of barium ion When he made his report, the instructor told him that takes place wheu the sulfides of the copper and tin he had missed Ba++. I t was easy to prove by direct groups are precipitated in the presence of ammonium test that the Ba++ was present but this did not satisfy ion," and "No coprecipitation of barium ion takes the lieutenant and he worked assiduously for over 10 place wheu the chlorides of the silver group are pre- hours trying to find where he lost the Ba++ and the cipitated." While Professor Hall's statements re- instructor finally came to me for advice. I talked garding the loss of barium by the formation of known with the student and found that he had followed very insoluble barium compounds may he correct, they carefully the excellent text of the late Arthur A. Noyes have no significance with respect to the conclusions who recommended making the solution acidic with quoted above. HNOa before introducing H2Sand subsequently heating The conclusions of the article under discussion do not to see if there was any indication of arsenate's being state that barium is never precipitated with the present. Subsequent study showed that every time copper and tin groups. Professor Hall has read this he did this, the Ba++ was precipitated together with meaning into them. Therefore, his statement about sulfides of the copper-tin group. Now the question is the conclusions is incorrect. whether the precipitation of Ba++ in Group I1 of the LEOLEHRMAN qualitative scheme of systematic analysis can he attributed to what is called coprecipitation. Tne COLLBCE OF THE CITYOF NEWYORK NEW YORKCITY The word in question is used by Dr. I. M. Kolthoff and his students and I find in my copy of KolthoffSandell on page 103, "The contamination of a preTo the Editor: In the letter of Professor Leo Lehrman he protests cipitate by substances that are normally soluble under that I cannot understand the English language as he the conditions of the precipitation is called coprecipiwrites it. The trouble all lies with respect to the word tation." Since Ba+f does not normally precipitate coprecipitation which is a recently coined word and is with the copper-tin group and will not be precipitated not in my dictionaries. by H2S unless the solution contains arsenic (and the The paper of Lehrman and Mandel was published in solution is evaporated with HCI until fairly strong an excellent journal, which, unfortunately, few analyti- aqua regia is formed), this would appear to be a case of cal chemists read regularly. It does not, as a rule, coprecipitation. To my mind, i t is just as much copreconcern itself with the ordinary problems of the cipitation as when Ba++ is precipitated by NH40H analytical chemist nor with those of instructors in when Cr+++ is present. In this case barium chromite elementary qualitative analysis. It seemed, however, is probably formed which, to be sure, is not a "suhto the editors of Chemical AGstracts that the paper was stance normally soluble under the conditions of the really concerned with qualitative analysis so it was precipitation." A careful reading of the chapter in sent to me to abstract. The impression that the Kolthoff-Sandell makes me think that my esteemed paper made upon me, which may be entirely wrong, friend Kolthoff would probably agree with Professor as I haven't seen it for about eight months, was that it Lehrman in the case under discussion. Much as I was written as the first part of a careful research origi- admire Kolthoff I do not always agree with him. Thus nated with the purpose of finding how and where be- he distinguishes between iodometry and ipdimetry ginners often miss Ba++ in qualitative analysis. The although he seems to be one o'f a very few chemists conclusion, as I read it, seemed to show that there was making such a distinction. no danger of losing Ba++ by precipitation of the silver Professor Lehrman's statement that "no precipitation group with CI- (to which I agree) or of the copper-tin of barium ion takes place when the sulfides of the group with HzS. This last is contrary to my own ex- copper and tin groups are precipitated in the presence perience. Now i t is a rigid rule of Chemical Abstracts of ammonium ion" appears to be correct because in that abstractors must give the content of papers with- the heating and evaporating of a nitric acid solution out any criticism or correction on the part of the ah- the NH,+ undoubtedly is oxidized to N1 before the stractor and that is my excuse for writing my note Aspss or As& is dissolved.
