GUESTAUTHOR
Textbook Errors, 60
Robert E. McCoy1
Francis T. Nicholls State College Thibodaux, Louisiana
Qualitative Tests for Kt
Laboratory textsa for qualitative analysis commonly recommend the use of sodium cobaltinitrite as a precipitant for potassium ion. The interference of ammonium ion, which also precipitates, is usually recognized and a procedure for elimination of interfering ions is often given. It is also recognized that students may fail to carry out this procedure properly, thus obtaining a spurious precipitate. Some texts therefore recommend a "confirmatory test," wetting or dissolving the precipitate with HC1 and testing in a flame, viewing through two layers of blue glass to tliminate the interference of sodium. I n the author's experience with qualitative analysis laboratory, the erroneous reporting of potassium ion has been one of the most common student errors. The author usually demonstrates to the student that the original unknown gives no potassium flame, hence potassium ion cannot be present. The student invariably complains, "But I got the precipitate!" and usually that the precipitate gave a flame test for potassium. (Even if the text used does not mention this "confirmatory test," students usually do it unless specifically warned not to!) The regularity of this error led the author to suspect that cobalt gives a flame which is confused with the potassium flame, although none of the analytical texts or the handbooks mentions a cobalt flame test. One tenth molar solutions of cobalt salts indeed give no distinctive flame test, but solid cobalt chloride and solid or concentrated cobaltinitrites give a definite flame which is easily mistaken for potassium by an inexperienced observer when viewing through blue glasses. The author and several of his students have endeavored to determine whether this flame could be distinguished from the potassium flame when viewed through the blue glasses and whether a satisfactory Suggestions of material suitable for this column and guest columns suitable for publication directly should be sent withas many detaiils as possible, and particularly with references to modern textbooks, to Karol J. Mysels, Department of Chemistry, Iinivemity of Southern California, Los Angeles, California 90007. Present address: Kentucky Wesleyan College, Owemboro, Kentnckv. Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of erron discussed will not be cited. In order to be presented an error must occur in at lesst two independent recent standard books. ~~~
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description of the flame could be given which would make clear the distinction. The observers were unable to agree on the color of the cobalt flame, though some felt it could be distinguished from the potassium flame in a direct comparison with KCl. The distinction between these flames appears inadequate for a reliable test by the average student, and possibly even by a seasoned instructor. (Both flames are sufficiently short-lived to allow little time for second thoughts.) The flame spectra of potassium and cobalt3 indicate that visual distinction between them would present serious difficulty viewing through the blue glass necessary to eliminate interference of sodium. The principal feature of the potassium spectrum is the red doublet at 766 and 769 mp, and the next most intense line is the unresolved violet doublet at 404.4 mp. The blue glass strongly favors the latter. The emission of cobalt is weaker than that of potassium, so that it becomes a problem only when concentrated. The spectrum of cobalt contains 22 reported lines from 340.5 to 411.1 mp. The strongest lines are at 345.4 and 353.0 millimicrons but these are beyond the range of human vision. Only the lines a t 384.2, 387.3, 389.4, 399.5, and 411.1 mp are likely to faU within the visual range of a significant number of observers, even allowing for a possibility of some variation in visual range. Both the blue glass and the average human eye will favor the lines of longer wavelength in this series, and the observer will find the light viewed virtually impossible to distinguish from the 404.4 doublet of potassium. Enough red light will probably pass through the glass to permit some viewers to distinguish potassium from cobalt by direct visual comparison, but this cannot be recommended for general use. The so-called confirmatory test should definitely be avoided. and the nreciwitation arocedure is of ouestionable value. ~he'mos't reliable test for potassium ion be a flame test On the unknown, appears concentrated, and if necessary treated with ammonium carbonate to remove ions such as cobalt, lead, manganese, or arsenic which might interfere with identification of the potassium flame.
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Journal o f Chemical Education
DEAN,J. A,, "Flame Photometry," McGraw-Hill Book Ca., New York, 1960, pp. 154, 256.
