have some of the instability problems associated m-ith the methods in which cerium(1V) or a cerium(1V) complex is the basis of the absorbance measurement. LITERATURE CITED
(1) Conca, S . , Pllerritt, C., Jr., A Y U . CHEJI.28, 1264 (1956). (2) Edwards, R. E., ilyers, A. S., Banks, C. V., Ames Laboratory Rept. ISC-165 (1951).
(3) Freedman, A. J., Hume, D. S., Ar.4~.CHEJI.22 , 932 (1950). (4) Greenhaus, H. L., M.S. thesis, Svracuse University, 1957. ( 5 ) RIedalia, A. I., Byrne, B. I., ANAL. CHEM.23, 453 (1951). (6) Sem-ton, T. IT., Arcand, G. RI., J . i l m . Chem. SOC.75,2449 (1953). ( 7 ) Plank, J., 2. anal. Chevz. 116, 312 (1939). (8) Ringbom, iZ., Ibid., 115, 332 (1939). (9) Sandell, E. B., “Colorimetric Determination of Trace Metals,” Interscience, Xew York, 1944. (10) Smith, G. F., “Cerate Oxidimetry,”
G. F. Smith Chemical Co., Colunibus, Ohio, 1942. (11) Stenger, V. A , Don- Chemical Co., Midland, JIich., private communication. (12) Stewart, D. C., University of California Radiation Laboratory. Rept. AECD-2389 (declassified 1948). (13) Telep, G., Boltz, D. F., ASAL. CHEX 25, 971 (1953). RECEIVEDfor reviepv RIarch 27, 1957, LVork supported in part by the U. S. Atomic Energy Commission under Contract dT(30-1)-1213.
Stabilization of Ferric Thiocyanate Color in Aqueous Solution Spectrophotometric Method Using Methyl Ethyl Ketone PAUL BAILY Transvaal and Orange Free State Chamber of Mines Research Laboratory, Richmond, Johannesburg, South Africa
b The ferric thiocyanate color can be stabilized in aqueous solutions by the addition of methyl ethyl ketoneacetone. The color is stable for at least 1 hour or more and is not affected by exposure to light during this period. The optimum quantity of ferric ion is 0.02 mg. per 100 ml. of solution, using a 5-cm. glass cell at 490 mp.
T
for determining iron is one of the most convenient and is generally accepted as official in analytical practice. The distinctive color reaction between ferric and thiocyanate ions was made use of by Berzelius in 1826 ( 2 ) and was again proposed by Ossian in 1837 (6). However, without the aid of a stabilizer, the color tends to fade rapidly on exposure to light. Stokes and Cain (IO) suggested thiocyanic acid reagent stabilized IT ith mercuric thiocyanate. Potassium persulfate m-as added to oxidize the iron. Hydrogen peroxide and potassium permanganate have been used for the same purpose (7’). The extraction of ferric thiocyanate by a solvent immiscible nith water was suggested by Bernhard and Drekter ( 1 ) . They used a mixture of the monobutyl ether of ethylene glycol arid ethyl ether. The extract had a more intense color and did not fade for 24 hours. Narriott and Kolf ( 5 ) showed that the addition of acetone increased the sensitivity of the reagent. Again, although acetone increased the stability of the ferric thiocyanate color, it faded rapidly after 10 minutes. Winsor ( I d ) , n 110 usecl 2-niethosyethanol for stabilizing the color, found that this chemical HE THIOCYANATE XETHOD
1534
ANALYTICAL CHEMISTRY
was subject to photochemical change and formed a yellow color on exposure to light. Rakestraw, Nahncke, and Beach ( 8 ) used small amounts of ethylene glycol monobutyl ether to stabilize the color reaction and retard fading long enough to permit the colorimetric determination in aqueous solution. Lately, Lister and Rivington (4) carried out a spectrophotometric study of the ferric thiocyanate system. They found that benzyl alcohol was able to inhibit fading in many solutions even at 45” C. However, it was found that under the conditions used in this laboratory, 30 minutes elapsed before the color became stable. Walden (If) and Winsor (f2)studied the dielectric constants of organic liquids and their effect on inhibiting the fading of ferric thiocyanate color. As a result of their studies, the method presented here was developed. It uses a combination of acetone and methyl ethyl ketone, which proved suitable in preventing the fading of the ferric thiocyanate color so that spectrophotometric determinations could be carried out on several samples a t once. Maximum color development was practically instantaneous. REAGENTS
Ferrous ammonium sulfate solution. Prepare according to Snell and Snell (9) by dissolving 0.i022 gram of ferrous ammonium sulfate in 100 ml. of distilled water. Add 10 nil. of 1 to 1 sulfuric acid. Warm the solution and oxidize with approxiniately 0.1% .POtassium permanganate until the iron solution remains faintly pink. Cool and dilute to 1 liter. Carefully dilute a n aliquot of this solution 1 to 10 and
use as the standard solution; 1 nil. equals 0.01 mg. of ferric iron. Potassium thiocyanate solution. Add 40 grams of potassium thiocyanate (Merck I%Co., Inc.) to 100 ml. of distilled n-ater. Hydrochloric acid, reagent grade. 2N. British Drug Houses reagent was used. Acetone, British Drug Houses Analar reagent. Methyl ethyl ketone, British Drug Houses laboratory reagent. APPARATUS
All spectrophotometric measurements were made with a Beckman Model B spectrophotometer. Glass cells of 1.0 cm. and 50-cm. light paths fitted with glass stoppers M-ere used. A Beckman Xodel G p H meter was used for p H measurements. PROCEDURE
Measure a n aliquot of the iron solution (1 to 20 ml. containing 10 t o 200 p.p.m. of iron) into a 100-ml. volumetric flask. Dilute to almost 30 ml. with distilled water. Add 2 ml. of 2N hydrochloric acid, so that the p H of the resulting solution is approximately 1.5. Measure 20 ml. of acetone, folion-ed by 40 ml. of methyl ethyl ketone. into the flask and mix the content;. Finally, add 5 ml. of the potassium thiocyanate solution from a pipet. Make up to volume with distilled nater and mix the contents of the flash welC Measure the transmittance a t 490 m u . DISCUSSION
Optimum Wave Length. Figure 1 s h o w that the optimum wave length for this solution is a t 490 nip. A satisfactory u-orkable concentration range
1001-
I /49o my I
WAVE
LENGTH
I N rnp
Figure 1. Optimum wave length and concentration range for 1.0-cm. glass cell A. 0.0003 mg. per ml. C. 3.001 7 mg. per ml. B. 0.001 1 rng. per ml. D. 0.0020 mg. per ml.
I
50
401 0
Figure 2.
I
I
10
20
I
30
,
I
1
40
50
TIME
IN
60
70
I
L-1
80
90
100
MINUTES
Change in transmittance with time using different stabilizers
Solutions for curves 1 , 2, and 3 contain 0.0003 mg. per ml.; solution for curve 4 contains 0.0002 mg. per ml. 1. No stabilizer 2. Benzyl alcohol 3. Acetone 4. Methyl ethyl ketone-acetone
--
2ol IO
0
I
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using a 5.0-cm. glass cell is 0.10 to 0.50 p.p.m. of ferric iron. With 1.0c n ~ .glass cell, the concentration range can be increased from 0.10 to 2.00 p .p.m. Calibration Curve. A plot of the logarithm of the transmittance a t 490 mp against concentration s h o w that the Beer's lan- holds a t least up to 200 p.p.m. Stabilization of Color. A time study was made of the ferric thiocyanate complex by preparing solutions containing 0.30 p.p.m. of iron (in curve 4 the solution contains 0.2 p.p.m. of iron) and noting the effect of stabilizers. Figure 2 s h o w t h a t when no stabilizer is used, the loss in intensity of the ferric thiocyanate color is 6% within 10 minutes and 30Oj, within 90 minutes. The combined acetone and methyl ethyl ketone, with dielectric constants (3) of 21.45 and 18.51, respectively, a t 20" C., had the effect of inhibiting the fading of the color for at least 90 minutes. -4sample of the methyl ethyl ketone mas redistilled and the effect on the color stability TTas noted. Yo change was noticed, thus ruling out the possibility that some impurity in the methyl ethyl ketone might be responsible for the color stabilization. Intensification of Color. The addition of the acetone-methyl ethyl ketone mixture increased the sensitivity of the method. The intensity of the color a t a concentration of 0.30 p.p.m. was increased by 90%. Effect of pH. The pH of the final solution was varied from 1.1 t o 1.9 by adding different amounts of 2 N hydrochloric acid. The intensity of the thiocyanate color did not change. Effect of Potassium Thiocyanate. Figure 3 shows the effect of reagent concentration on the transmittance. A final reagent concentration of 5 ml. of potassium thiocyanate per 100 ml. of solution was chosen. This concentration was selected because the change in transmittance with increasing reagent Concentration hereafter is small. Effect of Interfering Ions. Six cations were tested, including sodium potassium, manganese, magnesium, calcium, and uranyl. When these were present in concentrations 100 times t h a t of iron, all except uranyl gave a change in transmittance of less than 2%. The uranyl ion interferes because it forms a yellow complex with the thiocyanate; a change in color was noticed. The seven anions tested included chloride, acetate, sulfate, nitric, silicate, carbonate, and pyrophosphate. When present in concentrations 100 times that of iron, a change of less than 2% in the transmittance was noted for all the cations except pyrophosphate. VOL. 2 9 , NO. 10, OCTOBER 1 9 5 7
1535
The change in transmittance with pyrophosphate was 57%; this ion forms a colorless complex with iron.
(7) Peters, C. A,, MacMasters, bl. XI.,
State Chamber of Mines for their permission to publish this paper.
French, c. L., IND. ENG. CHEM., ANAL.Ed. 11,502 (1939). (8) Rakestraw, N. W., Mahncke, H. E., Beach, E. F., Ibid., 8, 136 (1936). (9) Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” 3rd ed., Vol. 11, p. 306, Van Nostrand. York. 1949. Nostrand, New York, (10) Stokes, H,-N., H, N., Cain, J.-R., J. R., J . Am. Chem. SOC.29,409 (1907). (11) Walden, P., Z. physik. Chem. 147A,
LITERATURE CITED ACKNOWLEDGMENT
(1) Bernhard, A,, Drekter, I . J., Science 75. 517 (1932). .. (2) Berdlius, J., “Lehrbuch der Chemie,” Vol. 11, p. 771, 1826. (3) . . Cole, R. H., J . Chem. Phw. . 9,. 251 (1941). (4) Lister, M., Rivington, D., Can. J. Chem. 33,1572 (1955). \ - - -
The author wishes to acknowledge the assistance of N. G. Harvey, Morag Mullins, and Fay Pein in the experimental determinations and preparation of the graphs. Appreciation is expressed to members of the st& of this laboratory for helpful suggestions and to the Transvaal and Orange Free
I .
’
1 (1930). (12) Winsor, H. W., IND.ENQ. CHEX., ANAL.ED.9,453 (1937).
(5) Marriott, W. M., Wolf, C. G. L., J . Bid. Chem. 1,451 (1906). (6) Ossian, P., Pharm. Zentr. 13, 205
RECEIVED for review December 3, 1956. Accepted April 27, 1957.
(1837).
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n
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Spectrographic Evaluation ot beparation ot Platinum from Palladium, Rhodium, and Iridium GILBERT H. AYRES and HERBERT J. BELKNAP’ Department of Chemistry, The University o f Texas, Ausfin, rex. F A spectrographic method was developed to evaluate the sharpness of the conventional separation of platinum from palladium, rhodium, and iridium by double precipitation of the latter elements as their hydrous oxides. Solutions of the different fractions were evaporated to dryness with a weighed amount of pure powdered graphite, aliquots of which, in a shallow-cup electrode, were burned with a direct current arc; cobalt, added as cobalt(l1) chloride, was used as the internal standard. The relative analysis error was about 5%. The separation was found to be 99.94% complete, except in samples in which only small amounts (20 mg. or less) of the elements were present.
T
HE S E A R P N E S S of the separations of the platinum group elements by the method of Gilchrist and Wichers (4) has been under study in this laboratory for some time. Evaluation of the separations has taken advantage of the specificity of emission spectrographic methods for determining the amounts of other platinum elements coprecipitated with the desired constituent, and the amount of the desired constituent remaining in solution. Ayres and Berg reported the application of the spark, porous-cup electrode technique to the spectrographic determination of palladium, platinum, iridium, and rhodium (I), and also evaluated the separation of palladium from the other three elements by precipitation as palladium dimethylglyoximate 1 Present address, E. I. du Pont de Nemours & Go., Inc., Memphis, Tenn.
1536
ANALYTICAL CHEMISTRY
(2). Ayres and Maddin (3) studied the separation of rhodium (111) from iridium (IV) by reduction of the former to the metal with titanium (111). I n the previous work it was shown that both of these separations were a t least 99.7’70 effective; there was evidence of slight compensation of errors in the gravimetric separations. The present work concerns a similar study of another separation in the GilChrist and Wichers scheme-namely, the separation of palladium, rhodium, and iridium by hydrolytic precipitation from bromate solution buffered with sodium bicarbonate, leaving platinum in solution. The required sensitivities were obtained by the use of a modified direct current arc technique and by careful selection of line pairs. APPARATUS
The spectrographic equipment, film developing procedures, and film calibration methods were the same as used in the previous studies (1). I n the present work, however. direct current arc excitation n‘as used, instead of the high voltage spark; the power source could supply up t o 15 amperes of current through a 4-mm. electrode gap maintained a t approximately 50 volts across the gap. High purity A’ational graphite 0.25-inch spectrographic electrodes were used. The sample was contained in the bottom electrode (anode) in a cup 2 mm. deep, made by a special mandrel, the counter-electrode was pointed with a pencil sharpener. REAGENTS
The platinum metals and their compounds, obtained from A. D. Mackay,
Inc., were stated by the supplier to be of All samples were subjected to direct current arc analysis to detect any impurities of other platinum elements. No other platinum elements could be detected in the metallic platinum. Rhodium(II1) chloride showed traces of platinum and palladium, but no iridium. Palladium showed only faint traces of platinum by its most sensitive lines, and iridium chloride contained faint traces of rhodium and palladium, and platinum estimated to be about 0.03%. Solutions of palladium, platinum, and rhodium were the same stock solutions prepared by Ayres and Berg (1); the iridium stock solution was prepared according to Ayres and Maddin (3) by removing traces of rhodium and platinum from the hydrated iridium(1V) chloride source material. Cobalt(I1) chloride solution, containing 8.00 grams of cobalt per liter, was used as the stock solution for the internal standard. Potassium bromate, sodium bicarbonate, and other required reagents were of C.P. or ACS specifications quality.
99.5% purity or better.
EXPERIMENTAL
Spectrographic Procedure. Because of the relatively low spectrographic sensitivity of some of the platinum group elements. only the most sensitive lines are generally useful, Using high voltage spark excitation and the porous-cup electrode technique, Ayres and Berg ( 2 ) reported the lower limit of detection t o be palladium, 5 p.p.m.; platinum. 20 p.p.ni.; rhodium, 10 p.p.m.; and iridium, 25 p.p.m. It was anticipated that the present work would require greater sensitivity for platinum and iridium than that given above. Direct current arc excitation was used for increased
