Photometric Titration of Cerium(lll> THOMAS L. MARPLE, E. P. PRZYBYLOWICZ, and DAVID
N. HUME
Department o f Chemistry and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge 39, Mass.
A rapid and accurate method for the direct photometric titration of submilligram to decigram amounts of cerium(III), even in the presence of large amounts of cerium(IV), has been developed. The method is based on the oxidation of the cerium to the quadrivalent state with permanganate in a neutral pyrophosphate medium. Reducingagentssuch as mercury(II),vanadium(IV), arsenic(III), antimony(III), thallium(I), and iodide interfere, as do substances such as chromium(111) and fluoride, which form precipitates.
s
MALL amounts of cerium(111),particdarly in the presence
of large amounts of cerium(IV), are not easily determined. Tomirek ( 7 ) showed potentiometric titration of macro quantities with ferricyanide solution t o be feasible, and others (6, 8) have suggested oxidation in alkaline media. The solutions are highly susceptible to air oxidation, however, and the alkaline conditions result in precipitation of many metals. Goffart ( 2 ) has shown that the method of Lingane and Karplus ( 4 ) for the determination of manganese can be applied with slight modification to the titration of cerium. In it, the cerium is oxidized to the quadrivalent state in neutral sodium pyrophosphate medium by titration with standard permanganate. The visual end point is obscured by the color of the manganese(II1) pyrophosphate complex which is produced and the potentiometric end point is unsatisfactory because of sluggishness in the vicinity of the equivalence point. Goff art suggested amperometric end point detection but the present authors have found considerable advantage to photometric detection of the end point, using the absorption of the excess permanganate at 525 mp. The effects of potentially interfering substances, including large amoiints of quadrivalent cerium, have been studied and optimum conditions for good precision and accuracy determined. APPARATUS AKD MATERIALS
Titrations were performed with the aid of a Beckman Model B spectrophotometer modified for photometric titrations as described by Goddu and Hume (1). A 150-ml. beaker and 5.000ml. buret were used. Chemicals were of reagent grade unless indicated to the contrary, and a good grade of distilled water was used throughout. Stock tetrapotassium pyrophosphate reagent was prepared daily by dissolving 86 grams of the C.P. salt in 750 ml. of water and filtering through a Milipore (Love11 Chemical Co., Vatertown, Mass.) filter. Unfiltered solutions contained sufficient suspended matter to give erratic absorbance readings when stirred in the titration cell. Approximately 0.02M potassium permanganate was prepared and stabilized by the usual procedure of boiling and filtering. Repeated standardizations against sodium oxalate showed no detectable change over a period of a month. The 0.02M solution, which is 0.1N against oxalate in acid medium, is 0.08N against cerium in the neutral pyrophosphate. For titration of very small amounts of cerium, the stock permanganate was diluted and the dilutions used within a few hours. Cerous sulfate octahydrate was prepared from ceric sulfate by reduction with hydrogen peroxide in acid solution and boiling down to crystallization. Recrystallization once from 1M sulfuric acid yielded a satisfactory product. A stock cerium(II1) solution was prepared in 1M sulfuric acid and standardized by oxidation with persulfate and silver catalyst, reduction with a measured excess of ferrous ammonium sulfate, and titration of the excess with standard ceric sulfate to a ferrous-phenanthroline visual end point. Dilutions of the standardized cerium(II1) solutions were prepared for studies a t several levels of cerium concentration. PROCEDURE
To the titration beaker is added 75 ml. of the stock solution of
1892
tetrapotassium pyrophosphate. Unless the sample contains sufficient acid to bring the solution to between pH 5.5 and 7.0, sulfuric acid (IM) is added, approximately 10 ml. being required for neutral samples. The solution to be analyzed (1 to 10 ml.) is then added, the pH checked with universal indicator paper and adjusted with sulfuric acid or potassium hydroxide as necessary, and the cerium(II1) titrated photometrically with standard permanganate solution a t 525 mF. EFFECTS O F VARIABLES
The pyrophosphate medium serves three main functions: to keep the cerium and manganese, together with other metals which may be present, in solution as soluble pyrophosphate complexes; t o diminish the oxidizing power of the cerous-ceric couple to the point where oxidation with permanganate is quantitative; and to stabilize the reduced manganese a t a definite oxidation number. -4fairly high concentration of pyrophosphate is desirable and it was found more practical to use the readily soluble tetrapotassium salt in place of saturated solutions of the sodium salt as previously suggested. The pH of the titration solution was found to be important but not extremely critical, as long as it fell within the range of 5.5 to 7.0. At higher pH values there is danger of basic salts precipitating out, and in more acid solutions, the stoichiometry of the reaction is altered, om-ing to incomplete stabilization of the reduced manganese a t the trivalent state. Although it is clear that a manganese(II1) pyrophosphate complex is formed, the formula in neutral media such as these has never been determined. The reaction, under titration conditions, is rapid and equilibrium is established immediately. No effect due to temperature was observed. From Goffart's data, one can estimate the formal oxidation potential of the ce-
"-
L
Volume T i t r a n t (ml.)
Figure 1. Titration curves of ceriurn(II1) under conditions of procedure Curves normalized to give end qoints of 1.000 ml. for ease of comparison 1. 0.0345.M 3. 0.00689M 2 . 0,0172.W 4 . 0.00345M
V O L U M E 28, NO. 1 2 , D E C E M B E R 1 9 5 6 rous-ceric couple in p H 6.5 pyrophosphate medium to be about -0.65 volt 1s. the standard hydrogen electrode and using Latimer's (3)sign convention The range of titrant concentration used in this study represents the most convenient working range for the path length given by a small beaker. At higher permanganate concentrations, the slope of the line of volume of titrant zs. absorbance is very steep (Figure 1); hence a smaller microburet must be used to add smaller volumes of titrant in order to plot this portion of the curve. The lower limit of titrant concentration is determined by the angle of intersection of the two straight lines determining the equivalence point (Figure 2). Khen this angle becomes very obtuse, the accuracy v i t h which the equivalence point can be determined gets poorer. From these considerations it is evident that the permanganate concentration should be in the range of 0.04 to @.0023f for this titration.
0
02
04
06
08
IO
12
1893 Table I.
Effect of Various Substances on Cerium(II1) Titration Ce(III), hlg. RIg. Salt Added ddded Taken Found Deviation 11.84 11.87 200 +0.03 15.73 lL79 + O . 06a 39 11.84 11.92 $0 08 200 15.73 156 15.75 +0.02b 15.73 Interferes 210 15.73 17 15.93 +0:2p 11.84 200 11.84 +0.02 11.84 200 11.86 +0.OZb 11.84 Interferes 200 .. d 11.84 Interferes 200 11.84 200 11.93 +0:09 11.84 200 11.86 +0.02b 11.84 Interferes 200 .. b 11.84 Interferes 200 11.84 11.91 +0:09 200 11.84 11.92 200 +0.08, K l 11.84 Interferes 200 .. c 11.84 Interferes 200 15.73 15.79 100 +0:06 15.73 15.89 200 $0.16 15 73 15.71 300 -0.02 15 73 15,73 0.00 400 15.73 Interferes 193 .. b 200 285 15.73 +0.06 15,79 200 750 +0.06 15.73 15.78 200 CuCl?.2H20 c0.02 15.73 413 15 75 200 Ce(HSO4)d a Without deliberate attempt to remove ferrous ion by aeration. * Concurrent or prior oxidation of added constituent. c XIanganous ion oxidized before cerium but end point determinable. Error much larger if manganese-cerium ratio not unity. Determination not recommended in presence of manganese. d Turbid mixture. 6 Cerous fluoride precipitated. ~
14
ml K M n 0 4
Figure 2. Titration of 0.655 mg. of cerium (111) added to 0.2344-gram sample of ceric ammonium nitrate after pretitration of oxidizable impurities to preliminary end point A.
B.
Blank end point Samples added
C. Final end point
I t is apparent from the titration plot (Figure 1 ) that in addition to permanganate, another species is absorbing a t 525 mp. This is evidenced by the finite slope of the line prior to the equivalence point. The initial increase in absorbance was attributed to the formation of the pale violet pyrophosphate complex of manganese(II1). This was verified by titrating a solution of manganese( 11) under identical conditions wherein the initial slope increased fivefold. EFFECT OF OTHER IONS
The effect of the presence of a number of potentially interfering substances was surveyed and the results are summarized in Table I. Various amounts of the substances under examination were added to samples containing known amounts (usually 12 to 16 mg.) of cerium(II1) and the error in the titration was determined. As would be expected, strong reducing agents such as mercury(I), arsenic(II1j, antimony(II1 j, thalliuni(I), vanadium(IV), and iodide vitiated the results by being preferentially or simultaneously oxidized by the permanganate. Surprisingly enough, the presence of neither tin(I1) nor iron(I1) constituted a serious difficulty. The rate of reaction of stannous ions with permanganate was found to be sufficiently slow to allow an excess permanganate end point to be measured if the points after the cerium equivalence point were taken quickly. Tin(I1) does not react with ce-
rium(1V) in neutral pyrophosphate. Praseodymium and neodymium do not react with permanganate under the conditions of the titration. Iron(I1) was observed to react moderately rapidly and quantitatively with the oxygen of the air in neutral or alkaline pyrophosphate medium. When only a few milligrams of iron were present, usually no pretreatment was found necessary. Larger amounts required bubbling a stream of air through the sample prior to titration. Aeration in this manner was found t o oxidize 25 mg. of ferrous ammonium sulfate in 5 minutes and 100 mg. in 15 minutes. The oxidation takes place readily regardless of whether the sample is first brought t o pH 6.5 or allowed to remain a t the initial alkaline pH of the pyrophosphate reagent. Fluoride interferes by forming insoluble cerium(II1) fluoride. Chromium(II1) was found to yield a turbid solution which could not be titrated accurately. Other substances which formed precipitates or suspensions in this medium likewise interfered. The accurate potentiometric titration of the reduced form of a substance becomes increasingly difficult in the presence of larger and larger amounts of the oxidized form because of the nature of the titration curve. It was therefore of particular interest to establish whether traces of cerous-cerium could be determined in the presence of large amounts of ceric salts. The data in Table I shoned no disturbance in the determination of 15-mg. amounts by the presence of as much as 400 mg. of ceric hydrogen sulfate. Experiments were then run in which 0.23- to 0.27-gram samples of reagent quality ceric ammonium nitrate (approximately the solubility limit) were titi ated in neutral pyrophosphate medium to give a preliminary "blank" end point with 0.00716N permanganate. -4known amount (3.000 ml.) of 0.225 mg. per ml. of cerium( 111)solution was then added and the titration continued. Duplicate titrations gave values of 0.675 and 0.677, compared with the theoretical of 0.675 mg. of cerium, an average error of less than 2 parts per thousand with the ratio of cerium(IV1 to
ANALYTICAL CHEMISTRY
1894 Table 11. Precision and Accuracy of Cerium Determination Standard Ce(II1) Ce(II1) Deviation KMnOn Taken, Found,= Error, 11 Jlg. llg. % lk. % 0.03445 0.01723 0.00689 0.003445 (1
88.73 29.60 11.84 5.921
88.77 29.64 11.84 5,926
+0.045 +0.13 0 00 $0.086
0.21 0.13 0.05 0.023
0.24 0.44 0.42 0.39
Average of 10 determinations done in pairs on 5 different days.
bias from nithin-day trends. The levels of cerium concentration studied ranged from about 6 to 89 mg. per 85 ml. of solution, and for each level a permanganate solution about 25 times as concentrated as the cerium solution was used. The results, which are summarized in Table 11. shon-ed no significant differences in accuracy or precision betn-een levels or between days. The over-all precision of the method under the conditions studied is best expressed by the coefficient of variation (standard deviation in per cent), which is 0.47,. .lutomatic performance of the titration using the apparatus of JLarple and Hume (6) n-as foiind to be satisfactory. ACKIVOWLEDGMEYT
cerium(II1) about 100 to 1. The method can therefore be recommended for the determination of traces of cerium(II1) in cerium(1V) salts soluble in neutral pyrophosphate.
This n-01k n as supported in part by the United States htoniic Energy Commission. E. P. Przybylowicz nishes to thank the Eastman Kodak Co. for a fellon-ship.
PRECISION AND ACCURACY
LITERATURE CITED
I n recognition of the fact that the agreement within a group of replicate samples all run at one time under identical conditions does not give a realistic estimate of the actual precision of an analytical method, an experiment was designed to determine the reproducibility of the method over a period of time and for different amounts of cerium. Samples of stock standard cerium(II1) solutions at four levels of concentration were analyzed in duplicate each day for 5 successive days. The order of titration of the eight samples run on any given day was randomized to minimize
(1) Goddu, R. F., Hume, D. K’.,-43.4~.CHEM.26, 1740 (1954). (2) Goffart, G., Anal. Chim. Acta 2, 140 (1948). (3) Latimer, W. M., “Oxidation Potentials,” Prentice Hail, Sew York, 1952. (4) Lingane, J . J., Karpius, R., IND. ENG.C H E Y . , BXAL.ED. 18, 191 (1946). (5) JIarple, T. L., Hume, D. S . ,A 3 . 4 ~ CHEM. . 28, 1116 (1956). (6) hleyer, R. J., Schweitzer, A , 2. anorg. Chenz. 54, 104 (1907). (7) TomiEek, O., Rec. trau. chim. 44, 410 (1925). (8) Weiss, L., Sieger, H., Z . anal. Chem. 113, 305 (1938).
RECEIVED for review May 19, 1956. bccepted July 25, 1956.
Differential Spectrophotometric Determination of Neodymium in Neodymium-Yttrium Mixtures CHARLES V. BANKS, JOHN L. SPOONER, r n d JEROME W. O’LAUGHLIN Institute for Atomic Research and Department o f Chemistry, Iowa State College, Amos, Iowa A differential spectrophotometric method for the determination of macro amounts of neodymium in neodymium-yttrium mixtures permits determination of neodymium in the presence of yttrium with errors in concentration of only 2 to 3 parts per thousand. A theoretical treatment is given of the corrections made necessary by differences in the lengths and absorbances of the cells.
B
ECAUSE the similar chemical properties of neodymium and yttrium make the analysis of mixtures of the two elements by classical methods extremely difficult, differences in physical properties are used whenever possible for the determination of these elements. Small amounts of neodymium have been determined in the presence of lanthanum, cerium, and praseodymium by spectrographic techniques ( 2 ) . Larger amounts have been determined by epectrophotomet~icmethods (1, 5 , 6, 8, Q ) , which involve measuring the radiant eneigy absorbed by neodymium in dilute acid solutions at the wave length of one of its absorption bands. The absorption spectrum of neodymium has a series of very narrox- bands in the visible region of the spectrum. Yttrium shows no absorption in this region of the spectrum and, therefore, does not interfere Kith the spectrophotometric determination of neodymium. The extreme narronmess of the absorption bands of neodymium necessitates special techniques for making spectrophotometric measurements. Very small slit widths are necessary. Because of slight inaccuracies in setting the wave length dial, the region near the absorption peak must be scanned until the wave length is found a t which maximum absorbance occurs. These
difficulties, coupled with the Ion. molar absorptivity of neodgmium, make relative errors of about 1%in concentration about the lowest that can be expected by conventional spectrophotometric methods. I n connection with a careful determination of certain ion exchange constants, it became necessary to analyze a number of neodymium-yttrium oxide mixtures. A4more accurate neodymium analysis was needed than could be obtained by the usual spectrophotometric methods. Reilley and Cranford ( 7 ) recently reviewed the general principles of spectrophotometry and suggested tn-o new methods. Their Method Is’, which seemed theoretically capable of yielding the desired accuracies, involves setting the spectrophotometer to read 0 and 100 Kith t v o standard refeience solutions of concentrations C, and C1in the light path. This method theoretically beconies more accurate as the difference between Cz and C1 becomes smaller. As the photometric error of a spectrophotometer is a minimum in reading an absorbance of 0.43, the best choice of concentrations for the refeience solutions is such that the absorbance of “
is approximately 0.43. 2 I n the course of this work it n a s found necessary to make cell corrections. Several treatments of cell corrections in differential spectrophotometi y appear in the literature ( 3 , 4). These methods refer, honever, to the technique in which a single reference solution is used to set the spectrophotometer a t 100 and darkness is used to set the spectrophotometer at 0. The present report is concerned with the application of Method I V of Reilley and Craaford ( 7 ) to the determination of neodymium in neodymium-yttrium mixtures. A method by which cell ~
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