Photometric Determination of Chromium as Perchromic Acid in Ethyl Acetate Solution ABRAHAM GLASNER
and
MENACHEM STEINBERG
Department o f lnorganic and Analytical Chemistry, The H e b r e w University, lerusalem, Israel
Chromium may be extracted with ethyl acetate and determined photometrically in the form of the blue perchromic acid. Large amounts of iron, manganese, vanadium, niolybdenum, and tungsten do not interfere.
B
ROOKSHIER and Freund ( 1 ) proposed the separation of chromium from vanadium by extraction of perchromic acid with ethyl acetate, t o be determined subsequently, by the diphenylcarbazide method (2). This procedure requires the re-extraction of the chromate with water, involving a number of manipulations that could be spared, if the perchromic acid viere estimated directl! in the ethyl acetate solution. The optimum conditions for the extraction of perchromic acid n-ere investigated hj- Broohshier and Freund, who found that the aqueous solution a t equilibrium should have a pH of 1.7 =!= 0.2, and that the ethyl acetate extractions are fairly stable up to 20' C. Their results were confirmed, and the conditions mentioned adhered to i n the work descl'ibed here.
850
the efficiency of the extraction is independent of the amount of dichromate and Beer-Lambert's law is observed. I n Table I are compared the results of a series of determinations made with known amounts of pure dichromate solutions: (1) by the direct photometric measuiement of the ethyl acetate extract, and (2) by the diphenylcarbazide method, in an aliquot of the solution from a single extraction.
Z
P
3
d 800
: 3 s!
X 400 W'
Y 2
5
s o
1
0.2
0.4
0.8
0.6
1 .o
KKRz0;ML.
Figure 2. Absorbance of perchromic acid in ethyl acetate extracts from aqueous solutions, with varying initial concentration of dichromate Composition of aqueous solution. x ml. 0.00512M KlCrzOi ml. 0.082M HzOs 1 ml. 0.1N acid f (3-2)ml. Hz0
+
I " " " ' 420
500
A.
600
680
mp
Figure 1. Absorption spectrum of a ethyl acetate solution of blue perchromic acid
Figure 1 shons the absorption spectrum of a perchromic acid solution in e t h j l awtate as measured by a Unicam diffraction grating spectlophotonleter (Model S.P. 350). There is a single absorption band in the visible region with a maximum a t X = 580 mp. (The absorption spectrum obtained in a Beckman DU spectrophotometer exhibited tTyo bands of lesser absorbance around 300 and 750 mp, but no resolution of the main band.) Figure 2 shorn the absorbance of two series of solutions (at X = 565 mp) obtained by extraction from hydrochloric or sulfuric acid solution, containing varying initial amounts of potassium dichromate. The absorbances are seen to be a linear function of the concentration of the dichromate, up t o and somewhat above the absorbance one, and independent of the mineral acid added. Hence, under the experimental conditions employed,
f1
The satisfactory results of the direct method are further proof of the good reproducibility of this method. The results obtained by the diphenyl-carbazide method are a measure of the amount of chromium actually extracted by the ethyl acetate. This is in complete agreement xith Brookshier and Freund's results; they obtained 96 to 9 i % of chromium in one extraction. Because of this, three extractions are advised for one quantitative analysis. hI.ATERIALS
The folloxing solutions were used. Potassium dichromate 1.5070 grams, Baker's Analyzed, dried at 160" C. for 2 hours, and made up to 1000 ml.
Table I. Comparison of Determinations Made by Direct and Diphenylcarbazide Methods Initial Amount of C r , hlg. 0.05320 0.206j 0.1598 0.2180
0.2663
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Chromium h y Direct Method
Ire.
so
0.05334 0.1060.161; 0.212~ 0,2647
100.2 84 2
100.8 99.9 99.4
Chromium b s Diphenylcarbazide Method llg. % 0.05ljq 98.8 96.3 0.1025 97.1 0,1552 96.6 0.205~ 0.2554 95.9
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V O L U M E 2 7 , NO. 1 2 , D E C E M B E R 1 9 5 5 Hydrogen peroxide, 100 volume 11.& B. “for analysis,” diluted as required and determined daily by titration with standard 0.1N potassium permanganate. Ethyl acetate, Baker’s Analyzed. PROCEDURE
The solutions were measured out. from microburets into 50-ml. stoppered flasks. For each 5 ml. of the total aqueous solution, 6 nil. of ethyl acetate were added. The flasks were cooled to about 10” C., then hydrogen peroxide was introduced last, and the solutions were well mixed for 30 seconds. The liquid layers were allowed to separate for 2 minutes, whereupon an aliquot of the blue perchromate solution was transferred by pipetting into the spectrophotometer tube, and its absorbance was compared with that of the pure solvent. One milliliter of the same sample solution was simultaneously taken for the diphenylcarbazide determination, decomposed by potassium hydroxide, extracted with water, and further treated as in ( 2 ) . Lastly, the direct method was tested by two analyses of alloy steel from Bureau of Analyzed Samples, Ltd. The samples n-ere certified to have the percentage composition:
C Si S P Mn Ni 17 hlo 0.34 0 . 2 5 0.036 0.014 0.64 2.59 0 43 0.80 .. .. . . 1:k7 4:11
I
..
I1
..
W 5:66
Cr 0.75
4.40
The percentage of chromium found by the photometric estimation of the blue perchromic acid was: I
I1 4.24 =t0.04%
0 . 7 1 2 f 0.008%
Identical results were obtained whether or not the iron v a s precipitated by ammonia before extraction. LITERATURE CITED
(1) Brookshier, R. K., and Freund, H., ASAL. CHEW, 23, 1110 (1951). (2) Sandell, K. B., “Colorimetric Determination of Traces of Metals,” pp. 191-5, Interscience, New York, 1944. RECEIVED for review March 22, 1955. Accepted August 5 , 1955.
Radiochemical Separation of Actinium and Its Daughters by Means of lead Sulfate N. E. ROGERS and R. M. WATROUS M o u n d Laboratory, Monsanto Chemical Co., Miamisburg, O h i o
A radiochemical separation of actinium-227 and its daughter elements from rat urine was required as an analytical method in the determination of these radioactive elements. A method is described in which actinium and its decay products are separated from urine salts on precipitated lead sulfate. Approximately 90% of the radioactivity in rat urine can be recovered by coprecipitation on lead sulfate in hot solutions. The lead is removed by precipitation with hydrogen sulfide, leaving the activity in solution. The actinium fraction is free of interfering salts except for a small quantity of urine salts coprecipitated with the lead sulfate. The method requires about 8 hours.
on a controlled diet, the chemical composition of the urine remained relatively stable during the course of this work. Composition of a sample of typical rat urine is presented in Table I.
Table I.
a
I
N T H E study of the effect of actinium-227 and its daughter elements on rats, a reliable radiochemical separation of these elements from rat urine was needed. Data obtained from such an analytical procedure lvould be of material assistance in ascertaining the ratio of radioactivity administered into a rat to that excreted daily through the kidneys. Lead sulfate has been used as a carrier for radium in solution. Ames and others ( I ) described a method for the determination of small amounts of radium in uranium ore8 by the separation of the radioactivity on precipitated lead sulfate. This method was later adopted, with slight modifications, for the separation of radium from large volumes of human urine ( 7 ) . The technique of using lead sulfate as a carrier for actinium-227 was reported by McLane and Peterson (6). They stated that over 98% of a carrier-free actinium tracer is precipitated -with 1 gram of lead per liter of 6 M sulfuric acid. Lead sulfate has been used as a carrier for actinium-227 onlv in relativelv salt-free solutions. The presence of concentrated rat urine salts tends to contaminate the lead sulfate precipitate and makes it difficult to obtain quantitative separations. The major components found in digested rat urine are sodium, potassium, and magnesium chlorides. As the rats are maintained
Composition of Digested Rat Urine Salts % 38.0 7.5
Element Potassiuma Sodiuma Chlorine
27.5 1.5 0.5
Magnesium Iron Calcium Phosphates Sulfates Flame photometer.
0.1 6.5 1.6
Silicon and boron, in moderate amounts, are also present in digested rat urine. These ti?-o elements are probably leached from the glassrare during the digestion period. PROCEDURE
To a 3-day sample of rat urine (approximately 120 ml.) in a 250-ml. boiling flask, add 60 ml. of concentrated nitric acid. Enclose the flask in an electric heating mantle and evaporate the contents nearly to dryness. Add small portions of nitric acid, with repeated evaporations, until the solution is clear and the residual urine salts are white. Finally, evaporate the contents of the flask to dryness. Dissolve the dried urine salts in the flaqk with 10 to 15 ml. of dilute nitric acid and transfer the contents quantitatively to a clean 150-ml. beaker. Evaporate the salts to dryness and take up the residue in 100 ml. of O . 1 N nitric acid. Slowly add 3 ml. of concentrated sulfuric acid to the solution. Heat the solution to 75” C. on a hot plate with mechanical stirring, and add dropwise 100 mg. of leadnitrate solution (0.5 ml. of a solution prepared by dissolving 32 grams of lead nitrate in 100 ml. of distilled water). Stir the lead sulfate precipitate for 30 minutes with the temperature maintained a t 75’ C. Rinse the stirring rod and thermometer with sufficient mater to bring the volume of the
