The 'USE of STANDARD SODIUM HYDROXIDE SOLUTION for the STANDARDIZATION Jof PO-
TASSIUM PERMANG-ANATE D. L. SCOLES AND JOSEPH ROTHSTEIN Long Island University, Brooklyn, New York
A
STUDY of the purpose of teaching elementary quantitative analysis brings the instructor to the realization of the fact that of those who enrol in this course, only a small number will do their major college work in chemistry, and a far smaller number will become practicing chemists. The primary thought, then, should be not to teach analysis for itself alone, hut rather to teach chemistry through the medium of the approach which quantitative analysis offers. With this thought in mind, the senior author has for the past eight years had his classes in beginning qnantitative analysis standardize permanganate against standard sodium hydroxide solution, using oxalic acid as the "bridge" compound to pass from acid-base to oxidation-reduction standard solutions. The analyses reported by students who had standardized the permanganate in this manner were quite as acceptable as were the analyses of those whose permanganate had been standardized against iron wire. This being contrary to the dictum that potassium permanganate, which is to be used for the determination of iron, should be standardized against iron of known purity, i t seemed advisable to institute a study of the procedure with a view to determining details by which i t could best he accomplished, the saving of time which might he effected, and the dependability of the results obtained. EXPERIMENTAL
Apparatus and chemicals: Weights, B. S. Certificate B7582.
Buret, B. S. Certificate 1240. Benwic acid, B. S.Certificate 39e. Potassium acid phthalate, B. S. Certificate 84. Sodium oxalate, B. S. Certificate 406. Six different samples of oxalic acid: 1. Purified (1). 2. Commercial, a very poor grade, so contaminated that it was necessary to filter it before use. 3. United Drug, technical grade. 4. Exeller, a commercial brand. 5. Merck, reagent grade. 6. Eimer and Amend, T. P. Ten approximately 0.1 N solutions of potassium permanganate, prepared by the method of Takio Kato (2). A saturated aqueous solution of phenolphthalein. Baker and Adamson's C.P.sulfuric acid.
Procedure: 1. A carbonate-free solution of sodium hydroxide
was prepared and used by the method of Foulk (3) and preserved by the methodof Treadwell and Hall (4). This solution was standardized against B. S. potassium acid phthalate and B. S. henzoic acid, and found to be 0.2613 normal. 2. Each permanganate solution was standardized against B. S. sodium oxalate. 3. Each permanganate solution was standardized against the standard sodium hydroxide by using each of the six oxalic acid samples as a "bridge," using the following procedure.
A small amount (about 0.25 g., not weighed) of oxalic acid was dissolved in 200 cc. of water, 5.0 cc. of aqueous phenolphthalein was added (an aqueous solution was used because this in no way interfered with the permanganate titration later), and the solution was titrated with the standard sodium hydroxide. This solution, to which had been added 10.0 cc. of (1:l) sulfuric acid, was heated to 85 degrees, and then titrated with permanganate according to the method of McBride (5).
Lest the student's error in standardization of sodium hydroxide appear in the potassium permanganate standardization, the instructor titrates each student's sodium hydroxide, giving him the corrected normality of the same, basing the student's grade upon the accuracy of the submitted value. This corrected value is then used by the student for further work. Thus, with the standard sodium hydroxide a t hand, the method here outlined is no longer "a very roundabout affair" that "should be employed only as a matter of expediency" (7), but rapid and accurate and also serves to
Samsk of Oxdic Acid P.,ilird
commrcid fii1rd
Lha
Edlcr Mwck E i m n and Amend
the six Sam& - 0.1051 Normality by the B. S. Standard 0.1023 Deviation of Mean from B. S. Stsndard 0.0028 Mcan Deviation 0.00250
-
0.09831 0.09858 -0.W027 O.NO44
0.09596
0.1089
0.09690
0.1068
-0.00094 0.0021 0.00132 0.00250
* Subsequent examination showed the presence of
0.09809
0.1163
0.09700 0.1151 0.0011 O.tXQ1 0.00110 0.01190
0.1033
0.1047
0.W127
0.1057
0.1033
0.08965 0.08542
0.08601 0,10069 0.1W08
-0.0024 0.0014 0.0016 @.LUX3 O.WO61 0,00528 0.00140' 0.00161 O.W059 O.CO274
..... ... . .
.....
.....
manganese dioxide in this solution.
This, of course, gives a direct relationship between the standard sodium hydroxide and the permanganate. The results of the experimental work are given in Table 1. Discusswn.Since in a beginning course in volumetric analysis the first standardization is of hydrochloric acid and sodium hydroxide, the latter is then immediately available for the potassium permanganate standardization without further ado. If the instructor feels that i t is not desirable to have the student prepare carbonate-free standard sodium hydroxide, he may substitute standard sulfuric acid for the hydrochloric, and then for standardization of the potassium permanganate solution titrate the oxalic acid with the carbonate-containing sodium hydroxide by boiling out the carbon dioxide by the method of Smith (6).
emphasize the two functions of the oxalic acid-as an acid and as a reductant. Conclusions.-This method of standardizing potassium permanganate possesses the following advantages: 1. A standard carbonate-free solution of sodium hydroxide being available, the permanganate standardization can be completed in a few minutes. 2. Any good grade of oxalic acid is satisfactory as the acid samples are free from oxalate salts and other acid ions. 3. The results are quite as dependable as any other, the deviations being well within the experimental error, bearing in mind that the same experimental errors of the operator occur in the standardization by B. S. materials as in the others. (The purity of the B. S. sodium oxalate could not be assumed to be more than 1 part per 1000, according to the certificate.)
LITERATURE CITED
(1) KOLTEOFP, I. M. AND E. B. SANDELL, "Textbook of quantitative inoreanic analvsis." The Macmillan Co.. New York City, i936. D. 525
p. 122.
(4)
TUBADWELL, F. P. AND W. P. HALL, "Analytical chemistry,"
7th ed., John Wiley & Sons, Inc., New York City, Vol. 2, 1930, p. 481. MCBRIDE, R. S., J. Am. Chen. Soc., 34, 393416 (1912). SMITE, G. M., "A course of instruction in quantitative analysis for beginning students," 3rd ed., The Macmillan Co., New York City, p. 114. FALES, H. A., "Inorganic quantitative analysis," The Century Co., New York City, 1925, p. 322.
