a treatment tvhich simultaneously strips barium from rhodizonate. Under these conditions a homogeneous distribution within the solution occurs between the tracer and carrier introduced at this stage and the analysis then proceeds as usual. The crystallization step of the analysis does not decontaminate certain gammaemitting fission products from the barium tracer. On the other hand, some of the radioactivities cocrystallize quantitatively, or at least very nearly so, and this technique, which in essence is carrier-free, may be useful in their separation as well. The cocrystallizationof tracer amounts
of the elements with a variety of organic compounds will be studied to provide for wider application of the general met hod. ACKNOWLEDGMENT
The authors Jack Quan of this laboratory FI ho performed the x-ray diffraction analyses, the Paul B. Elder Co., Bryan, Ohio, for its contribution of potassium rhodizonate, and the Mound Laboratory, Miamisburg, Ohio, which kindly furnished actinium-227. LITERATURE CITED
(1) Fajans, K., Berr, P., Ber. deut. chem. Ges. 46, 3486 (1913).
( 2 ) Feigl, F., Xikrochemze 2, 187 (1924) Chem. 5’OC. 72, (3) Hagemann, F., J . 768 (1950). (4)Hahn, 0 , ttAippliedRadiochemistry,l, Cornel1 University Press, Ithaca, N. Y., 1936. (5) Kuanetsov, V. I., Zhur. Anal. Khim. 9,199 (1954) (6) Sunderman, D, S , , hIeinke, W, R,, ANAL.CHEW29, 1578 (1957). (7) Svedrup, H. V., Johnson, 11. W., Fleming, R. H., “The Oceans,” Prentice Hall, Englewood, X. J., 1942. (8) Jvahl, A. c., B ~ K. A.,~ ctRadio~ ~ activity Applied to Chemistry,” Table 6B, Wiley, New York, 1951. (9) Weiss, H. V., Shipman, W.H., ANAL. CHEM.29,1764 (1957). for review September 8, 1959. RECEIVED Accepted January 7, 1960.
Si mu1ta neous Extraction and Spectrophotometric Determination of Cerium with 2-Thenoylfluoroacetone SHRIPAD M. KHOPKAR and ANlL K. DE Department o f Chemistry, Jadavpur University, Calcutta A colorimetric method has been developed for milligram amounts of cerium(lV) on the basis of color reaction with 2-thenoyltrifluoroacetone in benzene. The bright orange-red cerium(lV)-TTA chelate solution (in benzene) follows Beer’s law at 450 mp over the range of 3 to 40 y of cerium (IV) per ml. At pH 4.0 to 6.0 80% or more of cerium(lV) i s removed from an aqueous solution of TTA-benzene in a single extraction. The colored system is stable for 2.5 hours. It can tolerate equal amounts of silver, manganese, copper, and cobalt, whereas nickel, bismuth, aluminum, thorium, uranium(VI), citrate, tartrate, and EDTA seriously interfere. The method i s accurate and reproducible to within f2% and offers a simple procedure for simultaneous extraction and determination of cerium(1V).
T
HE chelating agent, 2-thenoyltrifluoroacetone, commonly known as TTA, has been used in this laboratory (1-3) for extraction and colorimetric determination of metals such as iron (111), copper(II), and uranium(V1). From a slightly acidic solution cerium (IV) gives a deep orange-red chelate with TTA, which is extractable into solvents such as benzene, giving a brilliant orange-red solution. I n its simplest form, the over-all chelate .formation can be expressed as:
Ce.’
4
+ 4 HT, e CeT,* + 4 H.C
where HT is the enol form of TTA, and subscripts a and b refer to aqueous and
478
ANALYTICAL CHEMISTRY
32, India
benzene phases, respectively. T T A and cerium chelate have very little solubility in aqueous acid solutions but are soluble in benzene. Smith and Moore (6) described a method for fast extraction of radiocerium (with or without carrier) with TTB. Cerium(II1) is extracted into 0.5X TTA-xylene from a 1N sulfuric acid solution containing potassium dichromate and sodium bromate, with about 80% yield and thus separated from many other elements. They observed a dark reddish brown color in the organic phase, but so far no spectrophotometric studies have been reported. I n this paper systematic investigations on the liquid-liquid extraction behavior of cerium(1V)-TTA chelate a t different pH’s and spectrophotometric studies of the chelate are described. This offers a simple and rapid procedure for simultaneous extraction and spectrophotometric determination of cerium(1V) a t the milligram level. The method requires simple equipment and only moderate amounts of time, is suitable for small amounts of samples, and is adaptable to automatic or semiautomatic manipulation with the minimal introduction of chemical reagents and solvents. APPARATUS
Absorbance measurements were carried out with a Hilger quartz spectrophotometer, using matched 1-cm. quartz cells, and p H measurements with a Cambridge p H indicator. All chemicals were chemically pure or reagent grade materials, unless otherwise mentioned.
TTA (Columbia Organic Chemicals, Columbia, S. C.) solutions in benzene, about 0.15M, were used. A stock solution of ceric sulfate was prepared by dissolving about 3.5 grams of ceric oxide in 1 liter of 1N sulfuric acid. The solution, standardized gravimetrically as ceric oxide after oxalate precipitation and ignition, contained 2.79 mg. of cerium per ml. For spectrophotometric studies the stock solution was diluted tenfold (1% in sulfuric acid), so that the cerium content was 279 y of cerium per ml. A buffer solution of p H 5.4 was prepared by dissolving 77 grams of ammonium acetate in water, acidifying with acetic acid t o p H 5.4, and diluting to 1liter. GENERAL PROCEDURE
i i n aliquot (2 ml.) of the ceric sulfate solution, containing 279 y of eerium(1V) per ml., was adlusted to the desired p H with 0.01N sulfuric acid and 0.01N sodium hydroxide, using a pH-meter. Hydrochloric acid was avoided for p H adjustment, because chloride ion is known to accelerate the reduction of cerium(1V) (6). For the study of diverse ions, the solution containing the ion under investigation was added prior to pH control. The aqueous solution, unless otherwise mentioned, was adjusted to p H -5.4, treated with 10 ml. of buffer solution (pH 5.4), and made u p t o 25 ml. with water. It was introduced into a separatory funnel (250 ml.) and shaken for 15 minutes with 10 ml. of 0.15M TTA-benzene. The lower aqueous layer was transferred to a beaker and the upper benzene layer to a 25-ml. volumetric flask. The aqueous layer was rinsed once with 5 ml. of benzene and the benzene solution
~
1
I
I
9\ 01 3 60
h. -2- - * 8 , - . . - 4 60
WAVE
Figure 1. A.
B.
-0-,
--
560 LENG1H.m
660
r
Figure 2. Extraction of cerium(lV)-TTA chelate with benzene as a function of p H
Absorption spectrum
Cerium(lV)-TTA chelate in benzene against reagent blank. Ce(lV) 16 X 1 O-sM, p H 5.4 Reagent blank against benzene. TTA 15 X 10-2M, pH 5.4
drained as before into the same flask. The benzene extracts were diluted t o 25 nil. with benzene and the absorbance was read nithin 2 hours a t 450 mp against a reagent blank. The amount of cerium extracted was directly obtained from the calibration curve. The aqueous layers a t the end of extraction were saved for p H measurement. RESULTS AND DISCUSSION
Absorption Curve. T h e absorption spectrum of a solution of cerium(1V)TTAcomplex (cerium = 16 X 10-siV), extracted a t p H 5.4, is illustrated in Figure 1, the reagent blank being used as reference solution. The spectrum of the reagent hlank against benzene is also given. The orange-red cerium(1V) chelate solution shows maximum absorption a t 410 mp. Then the curve steadily falls and the absorbance becomes negligible beyond i o 0 mp. The reagent hlank itself has strong absorption up to 410 mp, which becomes insignificant, compared to the chelate solution, from 430 [n,u onward. All absorbance measurernents were performed at 450 mp. The absorptivity at 430 nut is 2486 =t46 (calculated on the basis of cerium content). Effect of pH. T h e p H range 0.5 to 10.0 was studied t o investigate the liquid-liquid extraction behavior of the cerium (IV)-TTA system. The distribution ratios, D, were calculated (4) from the extraction curve (Figure 2 and Table I).
where V = volume of aqueous phase, V’ = volume of organic phase, and z = per cent cerium extracted. The
extraction starts after p H 1.0 and becomes 100% at pH 5.4. In the p H range 4.0 to 6.0 extraction occurs to the extent of 80% or more. The extraction curve drops sharply beyond p H 6.0 and terminates at p H 10.0, corresponding to
Table 1. Distribution Ratios of Cerium(IV)-Thenoyltrifluoroacetonate between Benzene and Aqueous Solution as a Function of pH
% Cerium
pH 1.3
Extracted into Benzene
I> 0.05 0.10 0.22 0.48 0.83 1.29 1.96 3.18 5.83 13.12 32.21
1.8
1.4 1.8 2.2 2.6
4
8
16 25 34 44 56
3.0
3.4 3.8 4.2 4.6 5.0 5.4 5.8 6.0 6.5
84 93 100 94
10.0
0
70
a
39.15 18.33 4.08
88 62
0
Table
II.
Adherence to Beer’s Law
Cerium( IV)
Taken, 977 698 558 419
y
420mp
0.91 0.63 0.44 0.35
zero extraction. Ceric hydroxide precipitate formed during p H adjustment in weakly acidic and alkaline solutions (cerium -558 y) is extracted without any trouble by TTA-benzene, leaving behind a clear aqueous phase. Around p H 10.0 the hydroxide precipitate persists and inhibits the extraction. Repeated efforts to extract cerium (IV) from 1 N sulfuric acid by 0.5M T T A (6) have not been successful from the viewpoint of colorimetric measurement. The benzene layer s h o w no color under this condition. Presumably the optimum conditions for the radiochemical method of Smith and Moore (6) are not the same as for the colorimetric method. Calibration Curve. Different amounts of cerium(1V) Fere taken and extracted as above a t p H 5.4, and the corresponding absorbances were measured against reagent blank at various wave lengths-420, 430, 450, 460, and 480 m,u-to observe the adherence of the colored system to Beer’s lam. I n each case the solution of cerium(1V) \vas exhaustively extracted for 20 minutes and the aqueous phase was clear and colorless. Table I1 indicates that the cerium (IV)-TTL4 system closely obeys Beer’s law a t 450 mp over the concentration range 3 to 40 y of cerium per ml. Stability of Color. The absorbance of a benzene solution of cerium(1T’)T T A complex, prepared according to
430mp 0.86 0.585 0.405 0.32
Absorbance 450m~ 0.69 0.515 0.390 0.28
VOL. 32,
460mp 0.67 0.475 0.335 0.26
NO. 4,
480mp 0.60 0.395 0.285 0.15
APRIL 1960
479
the general procedure at p H 6.0, was measured at elapsed times of 5, 30, 45, 75, 105, 150, 165, 195, 225, 300, and 1440 minutes; the corresponding absorbances were 0.350, 0.350, 0.350, 0.355, 0.355, 0.360, 0.360, 0.376, 0.390, 0.390, and 0.440. It is evident that the color is stable up to 2.5 hours-at the end of 5 hours the absorbance increases by about 12% and, after 24 hours, by about 25y0. Hence i t is suggested that the color be measured within 2 hours. Reagent Concentration. Kith other factors constant, t h e concentration of TTA was varied from 0.008 to 0.45-Tf. The results in Table I11 prove that the optimum reagent concentration is 0.15X; the absorbance is essentially constant with higher reagent concentrations-e.g., 0.4521. With dilute solutions of the reagente.g., 0.008 or O.Ol521-the extraction is incomplete. Diverse Ions. The folloning ion. Table 111.
Effect of Reagent Concentration
[558 y of cerium(1V) after extraction by TTA4-benzeneat pH 5.4 gives an absorbance of 0.396 f 0.006 in 25 ml. of benzene] TTA Added, Absorbance TTA at 450 M,u Concn., M M1. 10 0.04 0.008 0.015 10 0.14 2 0.36 0.15 5 0.36 0.15 0.15 10 0.39 0.15 20 0.39 0.45 10 0.40
Foreign Ion None
(-0.5 or 5 mg.) were carried through the procedure, p H being maintained a t about 4.7: Ag+, Mn+Z, C O + ~Ni+2, , Bi+3, AIf3, Fe+3, Th+*, U+6,citrate, and tartrate. The results in Table I V show that equal amounts (0.5 mg.) of copper(I1) silver, and manganese(I1) do not interfere. Cerium(1V) can tolerate ten times its amount of cobalt(I1). Iron (111) and uranium give color reactions. Nickel gives rise to precipitation of a green chelate with TTA, irhich is filtered off before measurement. I n case of aluminum the hydroxide precipitate remains almost unextracted by TTAL Citrate and tartrate form strong coniplexes. (Ethylenedinitri1o)-tetraacetic acid (EDT-1) seriously interferes by masking cerium(II1 and IT-). Cyanide also interferes. Because the extractions of ceriuni(1V) are carried out from weakly acidic solutions, metals that yield chelates with T T A under this condition niay be espected to interfere. However. the interferences of metals such as thorium and zirconium can be eliminated by extracting them first with TTA from fairly acidic solutions (pH < 1.0) (61 and finally cerium(1V) can be extracted from the residual aqueous phase a t higher pH (-4.6). Interferences of large amounts of qilver, cobalt, nickel, manganese, copper, and iron map be eliminated by converting them into cyanide complexes (avoiding large excess of cyanide) and passing through a column of anion exchange resin (sulfate form);
Table IV. Diverse Ions [Cerium(IV) 558 y. pH 4.71 Ion Concn., hlg./Ml. Bdded as
gj
r2
hln +2
co +z
S i +2 Bi + 3
*..
0.5 0.5 0.5 0.5 4.3 2.0 5.0
A1 +3 Fe + 3 T h +I U +2
,24:HZ0
0.1 4.5 4.9
5 Cit-3 5 Tart -3 20 (EDTA)-4 20 CIi Precipitation of chelate. b Hydroxide precipitate interfered. c Color reaction.
1
.4bsorbance at 450 1Ip 0 330 f 0 005 0 325 0 340 0 335 0 340 0 115" 0 110 0 04h 0 47" 0 215 1 13" 0 056 0.060 0.0 0.170
these anionic complexes are retained in the resin bed and cerium(1V) alone passes through. Cerium(1V) may then be extracted as usual after careful removal of excess cyanide with silver sulfate solution. Recommended Procedure. Take a n aqueous solution containing 0.1 to 1.0 mg. of ceriuni(1V) and adjust the p H with 0.01.V sulfuric acid and 0.01.1sodium hydroxide, using a p H meter so that the final pH after extraction is -5.4. Add 10 ml. of buffer solution a t pH 6.4 and introduce the solution into a 250-ml. separatory funnel. Shake for 15 minutes nith 10 nil. of 0.15M TT.1-benzene solution. A\llow the layer? to settle. and transfer the aqueous layer into a beaker and the benzene layer into a 25-ml. volumetric flask. Pour the aqueous layer back into the separatory funnel and rinse once with 5 nil. of benzene. Withdraiv the benzene phase as before into the volumetric flask. Dilute these benzene estracts to 25 nil. 111th benzene and within 2 hours measure the absorbance at G O nip against a rengcnt blank. Frequently the anal? tical chemist is confronted with ceriuni(II1). I n this case make the solution of cerium (111)0.1S in potassium dichromate and 0 . 2 5 in potassium bromate and then adjust pH, etc.. as above. Such an oxidation procedure does not interfere with the colorimetric method. I n siu runs with 558 y of cerium(1V) , the absorbance found was 0.396 =k 0.006. The standard deviation was &1.5%. Different amounts of cerium(lV) were taken and determined by the recommended procedure. The results (Table V) are accurate to within k11.8%. Thus the proposed mpthod enjoys fairly good precision and accuracy to within =t2%. The total operation for each run requires only 25 to 30 minutes. As little as 2 y of cerium(1V) per ml. can be detected. ACKNOWLEDGMENT
The authors thank Columbia Organic Chemicals. Inc., Columbia, S. C., for the gift sample of TT.1 and the Council of Scientific and Industrial Research, India, for anarding a fellon-ship to one of them (S.11.K.).
5
Table V.
Cerium( IV) Taken, y 837 781 558 502 279 233 140
480
0
LITERATURE CITED
( 1 ) Khopkar, S. SI., De, A. K., Bnal. Chim. Acta, in press; 2. anal. Chem. in
ANALYTICAL CHEMISTRY
press.
Accuracy of Method
Absorbance a t 450 h'Ip 0.585 0,560 0.395 0.365 0.190 0.170 0,095
Cerium( IV) Found, y 830 800 562 509 275 238 135
yo Error -0.8 +2.1
+0.7 +l.4
-1.5 +2.2 -3.6
Av. 1 1 . 8
( 2 ) Khopkar, S. M., De, A. K , Chem. & Ind. (London) 1959, 291. (3) Ibid., p. 854. ( 4 ) Morrison, G. F., Freiser, H., "Solvent
Extraction in ilnalytical Chemistry," p. 12, Xiley, Yew York, 1957. (5) I b i d . , p. 160. (6) Smith, G. W., hloore, F. L., A N A L . CHEX 29,418 (1957). RECEIVEDfor review August 3, 1959. Accepted December 11, 1959.
