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Correspondence THE PERCHLORATE METHOD FOR POTASSIUM-
A CRITICISM
I would like to call your attention to some of the regrettable features of the article by J. P. Mehlig, in the December edition of the JOURNAL OF CHEMICAL EDUCATION, entitled "The Perchlorate Method for Potassium." The paper, according to the author, "was undertaken with the idea in mind of showing the practicability of the perchlorate method as a suitable substitute, especially in the hands of students, for the chlorplatinate method." The perchlorate has been previously shown to be not only a satisfactory method to substitute for the chloroplatinate method but is now recognized as a method superior in accuracy to the platinum process. In this connection, allow me to quote Professor H. H. Willard, Head of the Division of Analytical Chemistry in the University of Michigan in a book review of Mahin's "Quantitative Analysis," 1924 edition, to he found in the J. Am. Chem. SOC., 49, 2950 (1927) of which Professor Willard is associate editor. Referring t o p . 84 of the Mahin text he says: "On page 84, from a comparison between the perchlorate and chloroplatinate method for potassium, one would suppose that the former was less accurate and convenient whereas such is not the case. The statement made here and on p. 91 that the preparation of aqueous perchloric acid is dangerous is absurd. It is, however, quite unnecessary t o prepare it because C.P. 60 per cent perchloric acid has been on the market for years and is no more dangerous than other common acids. The use of 20 per cent acid is a disadvantage and not an advantage." The directly opposite position for all these points has been taken by Mehlig in the material of p. 1537. In addition the statement is made that: "Sodium perchlorate is easily soluble in alcohol but no definite data are available" is as with the rest of the material of this page not the result of very extensive search outside Mahin's text. Two references to such data are given by the following: "The Perchlorates of the Alkali and Alkaline Earth Metals and Ammonium, Their Solubility in Water and Other Solvents." By H. H. Willard and G. Frederick Smith, J. Am. Chew. Soc., 45, 286 (1923). "The Separation and Determination of the Alkali Metals Using Perchloric Acid. I. Solubilities of the Perchlorates of the Alkali Metals in Mixed Organic Solvents." By G. Frederick Smith, J. Am. Chem. Soc., 47, 762 (1925). The practice of using perchloric acid-alcohol mixtures in this separa-
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tion such as is advised by Mehlig on p. 1537 is neither necessary nor desirable. See G. Frederick Smith and J. F. Ross, 1. Am. Chem. Soc., 47, 774 (1925). Also in this connection the temperature of dxying potassium perchlorate should he 300-350°C. and not 130" as Mehlig recommends. On p. 1540 in testing a method for the separation of potassium and sodium by analyzing 9 pure potassium salts is hardly a fair test of a separation from sodium. A little sodium salt could be added here easily and need not he accurately weighed. Note the following four analyses out of seven attempts by Mehlig: Salt
KI KsSO1 KCIOs KL2rsOl
Theory
Amount found
23.55 44.88 31.91 26.59
23.54 44.88 31.93 26.64 -
K
K
23.53 44.84 31.91 26.59
Error
-0.01 and +0.00 and +0.02and +0.05and
-0.02 -0.04 ~0.00 +0.00
If these materials analyzed were pure and anhydrous, then as a result of the analyses ohtained potassium perchlorate must be quantitatively insoluble in 97 per cent ethyl alcohol containing 0.2 per cent of perchloric acid. Such a series of results might be well ohtained by analysis because there are several compensating errors following the directions as given in this paper by Mehlig. The miter is quite aware of these errors. Would not a careful observer wonder about the fact that, according to these analyses above given, potassium perchlorate is indicated to be insoluble? As a result of the criticisms so far made I believe you can see clearly that the paper of Mehlig can hardly aid materially in causing the perchlorate method to be adopted more readily than it would if this had never appeared. G. FREDERICK SMITH, UNIVERSITY OF ILLINOIS, URBANA.ILLINOIS Associate in Chemistry
IN REPLY It is quite evident that Prof. Smith has misinterpreted the purpose of my paper on "The Perchlorate Method for Potassium," in his summary of the regrettable features as he chooses to call them. The work described in the paper was done to show the practicability of the perchlorate method as a suitable substitute for the chlorplatinate method, especially in the hands of students. It should he evident to every fair-minded observer that the paper proves exactly what i t set out to do. No claim for any original research was put forward by the writer. The work was simply a tryout by students of the perchlorate method as given in standard texts of recent issue, such as Mahin's "Quantitative Analysis" and Scott's "Inorganic Quantitative Chemical Analysis."
The perchlorate method as used is not out of date as the dates of issue (1924 and 1926) of these texts show. Nor is the assumption by Smith that the method is widely known true. Many chemists and teachers are not familiar with i t and it was the purpose of the writer t o show to such that it is a satisfactory method. Griffin in his "Technical Methods of Analysis," 1925 edition, does not even mention the method. The writer did not a t any time take the position in opposition to Willard that the perchlorate method is less accurate and less convenient than is the chlorplatinate method, nor did he claim that the preparation of aqueous perchloric acid is dangerous. The statement made on p. 1537 is, "this solution (20'3&perchloric acid) keeps well and there is little or no danger of accident in handling it as there may be in handling the pure acid." Scott in his "Inorganic Quantitative Chemical Analysis," 1926, p. 86, says. "Perchloric acid often explodes spontaneously. The pure acid explodes when heated to 92O." Sixty per cent perchloric acid might well be substituted for the 20%. This is immaterial. It seems that the most grievous error that the writer made was in not referring to the work of Smith on the solubility of sodium perchlorate in alcohol and in not following the method as given by Smith in 1.Am. Chem. SOG., 47, 762 (1925), and by Smith and Ross, ibid., p. 774. The work was not a test of Smith's modifications, but simply a test t o show the feasibility of the method as given in such standard texts as those of Mahin and Scott. The latter was published in 1926 after Smith's article was in print. Even if Smith did find it necessary to dry potassium perchlorate a t 300-350' to remove the last traces of moisture, the drying a t 120-130' is sufficiently accurate for commercial analyses and gives satisfactory results. The writer's results given on p. 1540 are not intended as a test of the separation of potassium from sodium. Certainly nothing is said in the article to give that impression. The test for the separation is shown in the results of the soil analyses on p. 1543. I n this set of analyses very close agreement between the chlorplatinate and perchlorate results was obtained, sufficiently close to satisfy Prof. J. S. Jones, Chief Chemist of the Oregon State Experiment Station, that the method is a good substitute for the chlorplatinate method. The citing of four analyses of pure potassium salts made by the writer, which Smith claims prove erroneously that potassium perchlorate is quantitatively insoluble in 97% alcohol, is obviously unfair. All the results or none should be considered. There are nine other analyses given in the article to say nothing of some twenty-seven student analyses that will ease the mind of the "careful obsemer" and cause him to cease his wanderings, if any. J. P. MEHLIG OREGON STATE AGRICULTURAL COLLEGE, CORVALLIS, OREOON
