A N A L Y T I C A L CHEMISTRY
1426 Table 111. Proportionality between True Diffusion Current and Concentration of Cobalt (Csina a siinulated 0.45-K. .41 alloy sample containing chromium. vanadium, a n d manganese BS well as o t h e r metals) A4pparent - True Diffusion Current,& Final Diffusion Corrd. for Per millimolar % Error Cobalt Molar Current dilution concn. corrd. Taken, Concn. of a t -0.1 V.. and deoomp. for dilution Decamp. Decomp. Mg. Cohalt ra. * i n5 min. and decomp. not corrd.‘ corrd. 2.95 5 X 10-4 0.90 0.91 1.82 +0.9 +0.9 2.95 0.90 0.91 1.82 +0.9 +0.9 +o 4 -1.2 5.89 1.76 1.81, 1.82r 3.49 3.59, 1,791 11.79 2 X 10-2 I -0.4 29.47 5 X 10-8 8.90 9.05 1.81 f0.3 -0.1 Using value 1.804 microa‘mperes per inillimolar concentration in presence of 0.6 g. of nickel.
P 2 3’;
a
half the filtrate had been collected. The error due to decomposition of cobalt(II1) amounted to less than 2%. .4 preliminary- separation of chro. mium, vanadium, and cerium from cobalt and manganese with pyridine as described by the authors (6) was also studied. No cobalt was lost and only sufficient reducing agent to reduce the manganese was required. SUMMARY
-
Cobalt. is oxidized to the emerald ereen -. ______ trioxalatocobaltate( 1II)with lead dioxide in slightly acidic oxalate solution. The found that about 5% of the chromium had been oxidized during polarographic wave for the reduction of the complex is obtained the lead dioxide oxidation procedure. When 1 drop of hydroxylimmediately following the anodic dissolution potential of merammonium chloride was added for each 6 mg. of chromium finally cury. Copper, nickel, tin, zinc, and iron which form soluble as dichromate, the current was immediately decreased to 0.00 pa. oxalate complexes are rdduced after the cobalt and do not interthe value obtained for a blank. fere. Uranium and molybdenum form anions which yield no inIn analogous experiments using samples containing vanadyl terferhg waves. However, manganese which is oxidized to the ion, the blue color of vanadyl ion was observed to change to the cherry red trioxalatomanganate(III), vanadium which is oxidized yellow of vanadate during the oxidation with lead dioxide. Polaroto vanadate, and chromium which is partially oxidized to chrographic experiments showed that vanadyl ion is quantitatively mate, yield waves in the same potential range. Their interference oxidized to vanadate. Thus vanadium interferes seriously. One is eliminated through reduction by hydroxylammonium ion which drop of hydroxylammonium chloride solution was sufficient to redoes not appreciably reduce the green cobalt(II1) complex. The duce each 6 mg. of vanadium in about 2 to 3 minutes, and to method is applicable to the determination of cobalt in a large eliminate the interference. variety of alloys. Molyhdmum and uranium do not interfere. Calcium preripitatcs as the oxalate and appreciable amounts of cobalt are coLITERATURE CITED precipitated. Iodide interferes as a result of its oxidation to (1) Britton, H. T. S., a‘nd Jarrett, E. D., J. Chem. SOC.,1936, 1489iodine during the procedure and of its anodic depolarization wave 93. a t fairly negative potentials. Bromide is not oxidized by the lead and Nichole, P. M., IND.ENQ,CHEM., ANAL. (2) Cartledge, G.H., ED.,13, 20 (1942). dioxide, but its depolarization wave interferes below -0.1 volt (3) Kolthoff,I. M.,and Lingane, J. J., “Polarography,” New Pork, (S.C. E.). Chloride, as a result of its depolarizing effect on merInterscience Publishers, 1941. cury, shifts the cobalt(II1) wave to more negative potentials. (4) Kolthoff,I. M., and Watters, J. I., IND.ENG.CHEM., ANAL.ED., Therefore, large concentrations of chloride should be avoided, 15,8 (1943). especially in the presence of much iron. Chloride ion, in concen; (5)Zbid., 16, 187 (1944). (6) Sorensen, S. P. L., 2.anoia. Chem., 11, 1 (1896). trations of 10 millimolar, or less, does not interfere, because it (7) Souchay, P., and Faucherre, J., Anal. Chim. Acta, 3, 252 (1949). produces only a negligible decrease in the residual current a t (8) Vosburgh, W.C., and Reckman, J. F., J. Am. Chem. SOC.,62, -0.05 volt (S. C. E.), 1028 (1940). In Table I11 are given the results using simulated samples (9) Vosburgh, W. C‘., Israel, K., and Birch, 0. G., Zbid., 58, 2282 corresponding to 0.45 gram of a composite alloy containing about (1936). llyoiron, 22y0 nickel, 25% cadmium, 14% copper, 6% man(10) Vranek, J., 2. Elektrochsnr., 23,336 (1917). (11) Watters, J. I., and Kolthoff, I. M., ANAL. CHEM.,21, 1467 ganese, 6% chromium, 2% vanadium, and 0.6 to 6% cobalt. (1949). Seven drops of hydroxylammonium chloride were put in the cell RECEIVEDMay 10, 1950. in which the filtrate was collected and the timer was started when
Oxidation of Chromium(lll) with Potassium FerratdVI) J. M. SCHREYER, G. W. THOMPSON’,
A N D L. T. OCKERMAN2, University of Kentucky, Lexington, K y .
T
HIS paper reports results of a continuation in the development of methods for the analysis of samples of potassium ferrate(V1). The authors’ previous paper (1 ) presented data relative to methods of analysis based on the oxidation of arsenite with the ferrate(V1) ion: A method involving an estimation of the total iron present in the compound (potassium ferrate) was reported also. The arsenite-cerate method was not recommended for the analysis of solutions of potassium ferrate(V1) which are highly decomposed and contain large quantities of hydrous ferric oxide, be1
*
Present address, Richfield Oil Corporation, Box 22, Cuyama, Calif. Deceased April 11, 1950.
cause the o-phenanthroline end point is obscured by the color of excess ferric ions. The total iron method appears unsuited for the general analysis of solutions because of its restriction to highly alkaline solutions during the removal of the hydrous ferric oxide. The chromite method, as de\;eloped in this laboratory, is based upon the oxidation of chromite in strongly alkaline solution with the ferrate(V1) ion as shown in the following equation. Cr(OIl)r-
+ FeO,-- + 3HJO
----f
+
Fe(OH)a(H20)s G O 4 - -
+ OH-
This method, in contrast to the previously reported arsenite
1427
V O L U M E 2 2 , N O . 11, N O V E M B E R 1 9 5 0 A n additional method of analysis of potassium ferrate(V1) is described, based on the oxidizing property of the ferrate(V1) ion in alkaline solution.
cerate method, is applicable to the analysis of solutions containing low concentrat,ion of the ferrate(V1) ion. A weighed sample of potassium ferrate, or an aliquot of 5. sohtion containing the ferrate(V1) ion, is added to an excess of alkaline chromite solution. The chromate(VI) solution produced by the oxidation is acidified and the resulting dichromate is titrated with a standard solution of ferrous ions.
( 1 J which has been shown to give values in good agreement with an independent method of analysis based on the amount of iron present in the compound potassium ferrate. SOLUTIONS REQUIRED
Chromic Chloride Stock Solution. Add 25 grams of chromic chloride hexahydrate to 150 mi. of distilled water. Saturated Sodium Hydroxide Solution, Free of Reducing Agent. Saturate 500 ml. of distilled water with sodium hydroxide and add 0.05 gram of potassium ferrate. Destroy the excess pot@ sium ferrate by boiling. Sulfuric acid solution, 1 to 5 sulfuric acid. Sulfuric Acid-Phosphoric Acid Mixture. To 240 ml. of distilled water, add 60 ml. of concentrated sulfuric acid and 150 ml. 85% phosphoric acid. Standard dichromate solution, approximately 0.085 N . Ferrous ammonium sulfate solution, approximately 0.085 N . Sodium Diphenylamine Sulfonate Solution. Dissolve 0.32 gram of barium di henylamine sulfonate in 100 ml. of water and add 0.5 gram of s o i u m sulfate.
Table I. Effect of Alkalinity of Chromite Solutions on Determination of Percentage of Potassium Ferrate Sample
NO. 1
2
Average Yo KzFeOd 49.70 49.93 50.58 50.71 50.69 69.69 69.53 69.74 70.29 70.96 70.97 70.99
Alkalinjty
[OH-] 3.8 4.5 5.5 6.3 7.0 4.2 5.0 5.9 6.0 6.5 7.4 8.9
Table 11. Comparison of Methods for Analysis of Potassium Ferrate Potassium Ferrate, % Arsenite-cerate method Chromite method 73 76 73.94 73,80 73.96 2 80.33 80,38 80,38 80,36 3 86.20 86.25 86.26 86.26 ... 86.19 94.40 94.41 94.45 94.49 94.50 94.55 94.4l0 94.56 94.29 94.61 94.55 ... Values previously reported ( I ) . Sample No. 1
a
DEVELOPMENT OF METHODS
Investigation to determine the optimum alkalinity of the chromite solution used in the determination of potassium ferrate(V1) showed higher percentages of potassium ferrate in those determinations conducted in the more alkkline solutions. The analyses reported in Table I were carried out by the chromite method described in this paper. For samples of potassium ferrate in a.state of high purity, erratic results were sometimes noted in cases where chromite solutions 5 to 8 molar in hydroxyl ions were used. Consistent results were obtained by using chromite solutions 11 to 12 molar in hydroxyl ions. The greater the alkalinity of the chromite solution, the longer the time necessary for the oxidation by the ferrate(V1) because of the lessened solubility of solid potassium ferrate. Sodium hydroxide, instead of potassium hydroxide, is used because of the greater solubility of potassium ferrate in alkaline solution prepared from sodium hydroxide. This difficulty is not encountered in the case of analysis of solutions of the ferrate(V1) ion. As a method of reference for the chroihite method, potassium ferrate samples used were analyzed by the arsenite-cerate method
ANALYTICAL PROCEDURE
Procedure for Chromite Method. Add 20 ml. of saturated sodium hydroxide solution, free from reducing agents, to 3 to 5 ml. of chromic chloride solution and 5 ml. of distilled water. Cool to room temperature. Prepare a fresh solution immediately prior to each analysis. ANALYSISOF SOLID SAMPLES. Weigh a sample, containing approximately 0.15 to 0.20 gram of potassium ferrate into a flask containing the alkaline chromite solution. Add the potassium ferrate sam le carefully and do not allow it to strike the sides of the flask. &irl the liquid rapidly until dissdlution of the potassium ferrate is complete. Complete dissolution may require considerable shaking, depending on particle size, and premature addition of acid will give erratic results. Add 150 ml. of distilled water, and acidify with 60 to 70 ml. of 1 to 5 sulfuric acid and 15 ml. of sulfuric-phosphoric acid mixture. Titrate immediately with standard ferrous solution using 5 to 6 drops of sodium dlphen lamine sulfonate indicator. The ferrous solution must be stanLrdized against the standard dichromate solution immediately prior to use. The end point is marked by a change from purple to light green. If the end point is overstep ed, a known volume of the standard dichromate solution may {e added and the cnd point again approached cautiously. A correction should then be made to account for the fcrrous solution equivalent to the added dichromate. From the known titer of the ferrous solution and the volume used, calculate the per cent potassium frrrate as follovs: Per cent KiFe04 =
(ml. of F e + + X I\; Fe++) X K2Fe0, 3000 X wt. of sample
ANALYSISOF SOLUTIONS.Using a 5-ml. pipet, introduce a 5ml. aliquot of a solution containing the ferrate(V1) ion into a flask containing the alkaline chromite solution. Add the aliquot carefully and do not allow it to strike the sides of the flask. Swirl the liquid in the flask. Analyze in the manner described above. From the known titer of the ferrous solution and the volume used, calculate the concentrfition of potassium ferrate as follows: K2FeOc (mole/liter) =
(ml.of F e + +
+ NFe++)(1000)
(3000x5)
DISCUSSION OF RESULTS
The results of analyses made by the chromite method were ill good agreement with those obtained by the arsenite-cerate method. The validity of the arsenite-cerate method has previously been demonstrated by comparison with data obtained by an independent method of analysis. The chromite method is particularly suitable for the analysis of dilute solutions of ferrate(V1) ion. This direct method is also recammended for routine analyses twcause of ease of nianipulntion. LITERATURE CITED
(i)Schreyer,
J. M., Thompson, G. W., and Ookerman, L. T., ANAL.CHEM.,22,691 (1950).
RECEIVED June 10, 1950.
