Employment of Potassium Ferrocyanide in Standardization of Dilute Potassium Permanganate EDWINJ. DEBEERAND AXEL M. HJORT Experimental Research Laboratories, Burroughs Wellcome and Company, Tuckahoe,
I
N T H E process of applying permanganimetry to micro-
N. Y,
oxalate. The former method has been questioned by Halverson and Bergeiin (2) and the latter method is laborious and time-consuming. An investigation into the conditions of standardization against dilute potassium ferrocyanide, using erioglaucine as an indicator, confirmed Knop's observations as to the excellence of the end point. The change from apple-green to amber is sharp and unmistakable even in artificial light. One hundred routine titrations, selected a t random, showed a standard deviation of +0.0017 cc. when carried out under the following conditions:
chemical analysis, a more convenient standard than sodium oxalate seems desirable. When employing 0.01 N potassium permanganate in analytical procedures, frequent standardizations are essential. Although sodium oxalate has proved to be accurate for the purpose, it has the disadvantage of having to be freshly prepared. Potassium ferrocyanide is a good reducing agent, which is very stable in properly prepared solutions; is obtainable in a high degree of purity; has a high equivalent weight (an error of 37 mg. in weighing one equivalent of the anhydrous salt would cause an error Of Only 0*01per cent) ; and does not require heat for titration against potassium permanganate.
The sulfuric acid concentration at the end of the titration should not be less than 0.1 N or than 1.0 N . The final velume should not be greater than 50 cc. The temperature should not exceed 40' C. Titrations should be carried out rapidly with TABLEI. COMPARATIVE STANDARDIZATION OF POTASSIUM continuous agitation. PERMANGANATE That such titration figures are quite valid is shown by comNORXALITY OF KhhOa Against AVERAQE Against parative standardization of dilute permanganate against N a z C ~ 0 4 ~ DIFFERENCB KaFe(CN)e both sodium oxalate and potassium ferrocyanide (Table I). % Weighing 0.009741 0,009723 An indicator correction must be applied to the potassium buret 0.009736 0.009719 ferrocyanide titer. The work of Kolthoff and Pearson (6) 0.009739 0.009750 0.08 0.010557b 0.010566b 0.09 with regard to the stability of solutions of potassium ferro10-cc. buret 0.01274 0.01275 cyanide has been confirmed and extended (Table 11). Be0.01275 0.01275 0.02 cause of the stability of the standard and the convenience of 0.01275 0.01275 0.01052b 0.01051b 0.10 the titration, this method offers a considerable saving of time 0.01280b 0.00 0.01280b and labor over the commonly used oxalate method for the 5-cc. buret 0.01051 0.01050 routine standardization of potassium permanganate. 0.01050 0.01051 0.01051 0.01285b 0.01041b
0,01054 0.01286b 0,01043b
0.10
0.08
TABLE11. STABILITY OF DILUTEPOTASSIUM FERROCYANIDE SOLUTIONS
0.19
a Sodium oxalate titrations were carried out in the concentrations recommended by MpBride 8) Good results were also obtained with arsenic trioxide according to tke'method of Lang (7). 6 Average of three or more titrations.
The employment of potassium ferrocyanide as .a primary standard for potassium permanganate was first suggested by de Haen (1) in 1854. Since then it has been suggested repeatedly for this purpose by many investigators, but has not gained popularity because of the difficult end point. Potassium ferrocyanide is a component of a reversible oxidationreduction system which lends itself well to Potentiometric titration with potassium permanganate (5, 9 ) and to titration with permanganate in the presence Of suitable oxidationreduction indicators (5, 4), The latter articles by Knop and his eo-workers deal with the use of the dye, erioglaucine, as such an indicator. Although Kolthoff (6),using potentiometric methods, had shown that the reaction between ferrocyanide and permanganate was quantitative, even in very dilute solutions, KnoP and Kubelkova (4,in attempting to use Potassium ferrocyanide with erioglaucine as the indicator for the standardization of 0.005 N permanganate, obtained unsatisfactory results which they ascribed to the action of light and lack of knowledge as to the exact conditions of the titration. Dilute solutions of potassium permanganate (o.ol N ) are usually prepared by the dilution of more concentrated Standard solutions or by standardization against 0.01 N sodium
(Solutions were prepared from water distilled from permanganate, stored in brown bottles, and kept in the dark. Room temperature fluctuated from 20° to 35" C. Sodium carbonate concentration was 0.2 per cent in all cases.) ORIQINAL CHANGZ IN NORMALITY AGB NORMALITY CHANQE
%
Days
0.010878 0,008744 0.010222 0.010000
22 27 176 216
-0.000003 -0.000009 -0.000009 -0,000012
0.03 0.10 0.09 0.12
EXPERIMENTAL All volumetric and gravimetric apparatus had been recently calibrated. A 5-ml. buret, graduated in 0.01-ml. divisions (which could be estimated to 0.001 ml, with the aid of a reading glass) and equipped with & platinum-iridium tip, as described by &oh1 (io),was used for the determination of the end-point correction and routine standardization. For larger volumes a IO-mI. buret and a weighing buret were used. SOdium oxalate was obtained from the U. Bureau of Standards. Potassium ferrocyanide, purified by recrystallizatioll from hot water, or Kahlbaum's "guaranteed" grade was used. Potassium permanganate was prepared by the ~~l~~~~~~ and ~~~~~i~ procedure ( 2 ) .
s,
PREPARATION OF POTASSIUM FERROCYANIDE SOLUTIONS. Heat, powdered K4Fe(CNh.3HzO in a tared weighing bottle to constant weight at 106' C. and make up to volume in 0.2 per cent sodium carbonate solution. The equivalent weight of the anhydrous salt is 368.3. Store in brown bottles and protect from light.
120
March 15, 1935
ANALYTICAL EDITION
METHOD O F STANDARDIZATION OF 0.01 N POTASSIUM PERMANTo each cc. of 0.01 N potassium ferrocyanide add 2 cc. of N sulfuric acid and titrate with 0.01 N potassium permanganate in the presence of 0.05 cc. of 0.1 per cent aqueous erioglaucine. Subtract an end-point correction of 0.012 cc. from the titer for each 0.05 cc. of erioglaucine. (The authors' samples of this dye showed no deterioration on long standing.) GANATE.
LITERATURE CITED (1) Haen, de, Ann., 90, 160 (1854). (2) Halverson and Bergeim, J. ISD. ENG.CHEV.,10, 119 (1918).
1.21
(3) K n o p , 2. anal. Chem., 77, 111 (1929).
(4) Knop and Kubelkova, Ibid.,85, 401 (1931). ( 5 ) Kolthoff, Rec. trav. chim., 41, 343 (1922). (6) Kolthoff and Pearson, IND.EKQ.CHEX, Anal. Ed., 3, 381 (1931). (7) Lang, 2. anorg. allgem. Chem., 152, 197 (1926). (8) McBride, J. Am. Chem. Soc., 34, 393 (1912). (9) Maller a n d Lauterbach, 2. anal. Chem., 61, 398 (1922). (IO) Shohl, J. Am. Chem. SOC.,50, 417 (1928). RBCEIVBXJ October 23, 1934.
Separation and Detection of Cyanide L. J. CURTMAN AND S. M. EDUONDS College of the City of New York, New York, 'N. Y.
H
YDROGEN cyanide is rapidly displaced when a n
inert gas is bubbled through its aqueous solution. Utilizing this fact, Chellel described a n apparatus in which the acid is separated by means of a stream of carbon dioxide-free air and caught in alkali; in the alkali solution the test for cyanide is subsequently made. This paper describes a simpler and much more convenient set-up, though employing the same principle, which is particularly useful for the detection of cyanide in highly insoluble cyanides, in solid mixtures containing carbonate, and in sodium carbonate solution. The apparatus (Figure 1) consists of two 15 X 180 mm. test tubes and a 100-cc. Erlenmeyer flask connected by rubber and glass tubing as indicated. Two-tenths gram of the substance to be analyzed for cyanide (or 3 cc. of a prepared solution, made by treating 3 grams of the substance with 50 cc. of 1.5 M sodium carbonate, boiling for 3 minutes, and filtering) is introduced into the flask. The first test tube contains 6 cc. of 3 M hydrochloric acid, and the second, 10 cc. of 6 fif sodium hydroxide. A plug of absorbent cotton in the neck of the flask prevents any of the liquid from being carried over into the alkali. By slowly turning on the compressed air, the acid in the first test tube is forced over into the flask. A slow stream of air is then allowed to bubble through the flask for 30 minutes. Any cyanide in the mixture is thus carried over into the second test tube, where it is absorbed by the alkali. It is detected in this solution by means of the Prussian blue reaction carried out as follows: To the alkaline solution are added a few drops of a freshly prepared 1 M ferrous sulfate solution, the mixture is heated almost to boiling and then thoroughly cooled. The solution is then carefully neutralized with 12 M hydrochloric acid and a few drops of 1 M ferric chloride solution are added. The formation of a blue precipitate or, with very small amounts of cyanide, of a blue or blue-green coloration indicates the presence of cyanide.
chloride. However, a satisfactory final test is obtained if the solution, after the addition of ferrous sulfate and acidification, is boiled for one minute to expel the greater part of the hydrogen sulfide and then treated with ferric chloride. I n this way a test for cyanide was obtained in a mixture of 1 mg. of cyanide as sodium cyanide and 0.2 gram of finely powdered ferrous sulfide. I n the presence of nitrite the separation and tests for cyanide fail, doubtless because of oxidation of the cyanide.
--+
i
FIGURE1
INTERFERING ACIDS. The Prussian blue test cannot be directly applied to mixtures containing ferrocyanide, ferricyanide, or thiocyanate. These acids, however, cause no interference in the proposed method, since excellent controls were obtained when 0.2 gram of potassium ferrocyanide, ferricyanide, and thiocyanate were separately analyzed. TABLE I. TESTFOR CYANIDE
EXPERIMENTAL Following the proposed method, 0.2 mg. of cyanide as sodium cyanide in 3 cc. of 1.5 M sodium carbonate yields a decided positive test. With large amounts of cyanide (10 to 100 mg.) the separation of the acid, although not quite complete in 30 minutes, is sufficient to yield a very large Prussian blue precipitate, proportional to the amount of cyanide present. EFFECT OF OTHERVOLATILE ACIDS. The presence of large amounts of carbonate as sodium carbonate (3 cc. of 1.5 M sodium carbonate containing 0.5 gram of the anhydrous salt) offers no interference. Similarly, sulfite and thiosulfate have no disturbing effect, for, with a mixture of 1 mg. of cyanide as sodium cyanide and 0.2 gram of ?;aaSO~ or Na2S20,.5Hz0, an excellent test for cyanide was obtained. In mixtures containing sulfide, the test is complicated by the precipitation of ferrous sulfide when ferrous sulfate is added and by the separation of sulfur upon the addition of ferric 1
Chelle, J . pharm. chim., 20, 166 (1919).
A I R DISPLACEYBNTPRUSSIAX BLUETEST SUBSTAXCE 0.2 gram of solid 3 cc. of ureuared soln. 3 eo. of ureuared soln. DOUbtfUlQ Faint b Doubtfulc 7 -
+ ++ ++
a
+ ++++
f -
Faint -d
Large bluish white precipitate, probably Zn(CN)z or ZnzFe(CK)s
b Indicating less than 1 mg. of cyanide.
Greenish white Drecioitate. urobablv NinFeiCNh. d Heavy brown precipitate, probablvmer&ry-formed by reduction Fith FeSOd. Mercuric cyanide also yields the same confusing result by direct test. Like the oxyoyanide, it yields a satisfactory cyanide test by the airdisplacement method. C
INSOLUBLE CYANIDES.The common insoluble cyanides (all commercial products) and their prepared solutions were tested for cyanide by the air-displacement method, with the results indicated in Table I. For comparison, the Prussian blue test was applied directly to the prepared solution of each of the substances, in accordance with the normal procedure if ferrocyanide, ferricyanide, and thiocyanate were known to be absent.
