1050
ANALYTICAL CHEMISTRY
difference bettveen the actual and theoretical standard devisitions ma!' he considered negligible. ACKNOWLEDGMENT
The authors Fvish to express their deep gratitude to Willard S. Clewell, Sr., a-hose superb craftsmanship in the construction of most' of the parts of the analyzer contributed greatly t,o the work described here; to Beckman Instruments, Inc., South Pasadena, Calif., for its financial support of thex-rayphotoelectronspectrometer program; and to Earl J. Serfass for his generous interest :itit1 valuable help. LITERATURE CITED (1) (1))
Backus, J., Phys. Rers., 68, 59 (1945). Compton, 8 . H., and Allison, S. K.. "X-Rays i n Theory and Experiment," pp. 784-5, 788, Yan Sostrand, Sew York, 19:J.j.
(3) Ibid.. pp. 642-6.
(4) Felt, G. L., Harris, J. S . , and Dullond, J . IT.M . , P h y s . Rers., 92, 1160 (1953). (5) Iliginhotham, W., Rez.. Sci. Instr., 2 2 , 429 (1951). (6) Hill, R. D., Church, E. L.. and Mihelich, J . W., Ibid., 23, 523 (1952). (7) Hughes, A. L., and Ilojansky, V., P h y s . Recs., 34, 284 (1929). (8) Persico, E.. and Geoffrion, C., Rev. Sci. Tnstr., 21, 945 (1950). 19) Rogers, F. T., Jr., Ibid., 8, 22 (193T). (10) Steinhardt, 11. G., J r . , and Serfass, E. J . , Asar.. CHEM..2 3 , 1585 (19.51). - --,\-
(11) Ibid., 25, 697 (1953). (12) Wilkinson, D. H.. "Ionization Chamhers and Counters," p. 224, Cambridge University Press. 1950 RECEIVED for review August 20, 1954. Accepted Februaryl23, 1953. Presented in part a t the Pittsburgh Conference on Analytical Chemistry and Applied Sprrtroscopy, 3Iarch 1954.
Spectrophotometric Determination of Cerium After Oxidation to Cerium(lV) with lead Dioxide LOUIS GORDON and A. M. FEIBUSH D e p a r t m e n t o f Chemistry, Syracuse University, Syracuse,
The instability of dilute solutions of ceriuni(1V) presents a limitation on spectrophotometric methods based on the cerium(1V) color. Other limitations arise because of interfering substances which absorb in the same spectral region as does cerium(1V). Such substances ma! also include the excess oxidant used to convert cerium to the quadrivalent state and are often difficult to remove. In the present method cerium(II1) is oxidized to the quadrivalent state by lead dioxide in sulfuric acid medium. Excess lead dioxide is easily separated from the unstable ceric solution, which is immediately reacted with excess ferrous ammonium sulfate. The latter is subsequently treated with an excess of o-phenanthroline and compared to a cerium-free blank prepared in the same manner. From 20 to 1000 y of cerium can easily be determined. The method is simple, rapid, and accurate. It permits the determination of cerium in the presence of such interferences as thorium and the rare earths. The general procedure should be applicable to other elements.
I
S T H E course of a coprecipitation study it was found desirable to have a method for the determination of small quantities of cerium. Many of the methods which have been proposed ( 3 , 5 , '7, 10) are based on the measurement of the cerium(1V) color intensity in the ultraviolet region of the spectrum. However, dilute solutions of cerium(1V) are unstable ( 1 2 ) . Furthermore, excess reagent used t o oxidize cerium(II1) is often difficult to remove and may also absorb in the ultraviolet region, as do thorium, the rare earths, and many other common ions. Methods based on the measurement of ceric perhydroxide ( 1 , 10, IS) suffer from the same disadvantages as above and because of the many interferences require preliminary separations. A number of other reagents such as brucine ( I O ) , and gallic acid ( I I ) , have been suggested for this determination, but have not found wide application. I n the present method, cerium(II1) is oxidized to cerium(1V) with lead dioxide in sulfuric acid media. T h e cerium(1V) is then
N. Y.
made to react with excess iron(I1j immediately after e x w w osidant is removed by filtration. The residual iron(I1) is determined spectrophotometrically with o-phenanthroline, and coinpared to a cerium-free blank. EXPERIM EiYTA L
Reagents and Apparatus. Ammonium hesanitrocerate (standard of reference purity), G. F. Smith Chemical Co. This salt was converted to cerous sulfate or cerous perchlorate. It \vas first treated with hydrochloric acid to reduce the cerium to the trivalent state. Sulfuric or perchloric acid \vas then added and the solution was evaporated to dryness to remove ammonium, nitrate, or chloride ions. The residue was dissolved in 0.5S sulfuric acid. Solutions were standardized gravimetrically by the oxalate procedure and volumetrically by titration with standard ferrous sulfate after oxidation with persulfate. Lese concsentrated solutions were prepared by dilution. Ferrous ammonium sulfate (reagent grade) Baker and hdpmson. A solution containing 4.2 grams of ferrous :tmmonium sulfate per liter of 0 . 5 5 sulfuric acid was prepared; dilute solutions were freshly prepared as needed. For 0 to I50 -!of cerium, 5 ml. of this solution were diluted t o 100 ml. For larger quantities of cerium, up to 20 ml. were diluted. o-Phenanthroline monohydrate, G. F. Smith Chemical Co. One gram was dissolved in water a t 80" and the solution diluted to 1 liter. Lead dioxide (manganese- and chloride-free) Baker's analyzed. Thorium nitrate, Code 103 (cerium maximum 0.0001yo),Lindsay Chemical Co. This was converted to the perchlorate and dissolved in water. Yttrium, neodymium, lanthanum, praseodymium, and samarium oxides (>99.9% pure). These were dissolved in perchloric acid. Beckman spectrophotometers, Models B and DE, with 1-cm. silica and borosilicate glass cuvettes. Sintered-glass fiber filtering disks, Hurlbut Paper Co.
Choice of Oxidizing Agent. Several oxidants are used for the oxidation of cerium(II1). Sodium bismuthate ( 1 0 ) is not satisfactory because bismuth interferes in the iron o-phenanthroline method. Anodic oxidation proved difficult to control. Perchloric-sulfuric acid media will oxidize cerium( 111), but the time interval required for cooling and dilut,ion before addition of iron(I1) is sufficient for some reduction of cerium(I1') to occur. From a consideration of the electrode potenti:h (4)of the half reactions :
V O L U M E 2 7 , N O . 7, J U L Y 1 9 5 5 C e + + ++ Ce’--+ + e PbS04 2 H 2 0 + PbO?
+
+ 4H+ + SO,-- + 2e
1051 Ea = -1.44
EO = -1.67
and other factors, it appeared t h a t lead dioxide would serve as a useful oxidation agent. It is insoluble in the media used as is the lead sulfate formed so that both may easily be removed by filtration. Job (6) previously suggested the use of lead dioxide and reported t.he oxidation t o be instantaneous and complete in strong nitric acid. However, because concentrated nitric acid will oxidize iron(I1) to iron(II1) in the present work sulfuric acid n-as substituted with excellent results. From 20 to 1000 y of cerium were treated with 0.3 t o 0.5 gram of lead dioside in solutions 0.5 to 6S in sulfuric acid. The reaction was found to be quantitative in this acid range. One t o 2 5 acid was subsequently used, so that excessive amounts of ammonia would not be required for later neutralization. Batch treatment of Cerous solutions with lead dioxide was found to be effective. Satisfactory separation of t,he solution and solid phases was achieved with the use of a sintered-glass fiber filtering disk in conjunction ivith a Royal Berlin filtering crucible, porosity .-\-3. The use of a filter disk permitted the same crucible to be used for eight t o ten successive filtrations without clogging. Color Reaction. Freshly oxidized samples of cerium were found t o decrease in absorbancy rapidly. If kept in the dark, the solutions were more stable but decomposition still occurs. Introduction of large amounts of sulfate t o complex the cerium and thus stabilize the solution was found t o be ineffective. As cerium(1V) reacts quantitatively with iron(I1) and the latter is easily measured by the o-phenanthroline method ( 2 ) ,this system was therefore employed as a measure of cerium. Comparison of the absorption spectra of solutions containing iron o-phenanthroline and cerium( 111) with an equivalent amount of ferric ion showed the same peak in the 505-mp region as is shown tly the iron o-phenanthroline system. Although the color intensity of the iron o-phenanthroline is independent of p H in the range 2 to 9 (Z), solutions were adjusted to p H 2.5 to 2.8 with ammonia or sulfuric acid in order t o prevent precipitation of the hydroxides of cerium, other rare earths, or thorium. Color development is complete in 30 minutes a t this p H (9). The color is st,able for :it least 72 hours beyond this. The solutions of oxidized cerium were filtered directly into a measured excess of ferrous ammonium sulfate w-hich was subsequently treated with an excess of o-phenanthroline. T h e p H of the solutions was adjusted and the latter were diluted to 100 ml. in volumetric: flasks. A similarly treated solution without cerium \vas also prepared. The difference in intensity of t,he two solutions is directly proportional to the amount of cerium present. Untreated blanks containing cerium were identical to cerium-free 1)lanks Fvhicah had been treated by the oxidation procedure. However, it is recommended that this be verified for each batch of lead dioxide. Samples containing highly colored substances may thus be determined by using the sample itself as a blank. RECOMMENDED PROCEDURE
T o 20 to 1000 y of cerium, chloride-free, add 2 to 4 ml. of concentrated sulfuric acid and adj’ust the volume to 10 to 25 ml. Add 0.3 gram of lead dioxide. Stir occasionally. After about 5 minutes filter through a filtering crucible fitted with a sint,eredglass disk into 10.00 ml. of the dilute ferrous sulfate solution. -4dd 10 ml. of the 0.1% o-phenanthroline solution and sufficient ammonia to turn the solution red. Cool, then adjust the p H t o 2.5 to 2.8, and dilute the solution to 100 ml. Prepare a ceriumfree blank in the same manner. The difference in absorbancies measured a t 505 mp is proportional to the cerium content. Using 1-cm. borosilicate glass cells and a standard cerium solution, a calibration curve was prepared following the recommended procedure. The change in absorbance was plotted against micrograms of cerium. RESULTS AND DISCUSSION
The met’hod was found t o follow Beer’s law over the range of cerium conccntrations studied. T h e molar extinction coefficient
was found to be the same as for the determination of iron u i t h o-phenanthroline-Le., 1.0 x 104. The effects of thorium and some rare earths Fere studied n i t h the results shown in Table I . A factor which must be considered when working with thorium is the limited solubility of thorium sulfate in sulfuric acid. Chloride must be absent, as xell as ions which interfere in the iron o-phenanthroline method (IO). Iranganese and vanadium would also interfere.
Table I.
Effect of Interferences
(Cerium taken = 92 y) Dii-erse Iona
La
Ratio:
y
Direrse Ion y Cerium 4.54 228 196 217
Crriuin 1;ound Difference ?
io,
I Nd -0, +l. + I Pr -0, - 1 SI11 +I, +l Y 44 +I, f l Th 543 +2. + 2 , tl,+ I “ All taken as perchlorates; thorium used as nitrate, sulfate, and ~ ( T I I I o rate with identiral result-.
On the basis of the spectra of europium, gadolinium, erbium, thulium, and ytterbium, as reported by Moeller and Brantly ( 8 ) , and terbium, dysprosium, and holmium as reported by Yost el 01. ( I d ) , the rare earths would not be expected to interfere. The method presented is simple, fast, and accurate. It is considerably more sensitive than other methods for cerium and i c applicable in the presence of thorium and the rare earths. Its advantage over the persulfate and peroxide procedures lies in the ease of removal of excess oxidant and the greater stability of the colored substance measured, because the cerium(1V) T\ hirh is formed is immediately reduced. Preliminary experiments in this laboratory indicate the frasibility of oxidizing larger quantities of cerium by column treatment with lead dioxide. Subsequent determination can be completed by titration ith standard ferrous sulfate solution. Furthermore, the same general procedure may be applied to the oxidation and determination of other elements-for example, manganese could be ovidized with lead dioxide and determined with iron o-phenanthroline, in a similar manner 4CKNOWLEDGMEh-T
The authors wish t o thank the Atomic Energy Commission for its support of this investigation under contract .-\T(30-1)-1213. LITERATURE CITED
Edwards, R. E., Ayers, A. S.,and Banks, C. V., U. S.Atoniic Energy Commission, Rept. ISC 165 (8ugust 1951). Fortune, W. B., and Mellon, 11.G., ANAL.CHEM.,10, 60 (1938). Freedman, A. J., and Hume, D. S . ,Ibid., 22, 932 (1960). Hodgman, C. D., ed., “Handbook of Chemistry and Physics.” 34th ed., p. 1554, Chemical Rubber Publishing Co., Cleveland, Ohio, 1952. Hure, J., and Saint James-Schonberg, R., Anal. C h i m . Acta, 9, 415 (1953). Job, A , Compt. rend., 128, 101 (1899). lledalia, -4.I., and Byrne, B. J., ANAL.CHEM.,23,453 (1951). hloeller. T., and Brantly, J. C., Ibid., 22, 433 (1950). Ryan, J. A., and Botham, G. H., Ibid., 21, 1.521 (1949). Sandell, E. B., “Colorimetric Determination of Traces of Metals,” Interscience, Xew York, 1944. Shemyakin, F. AI., Zauodskaya Lab., 3, 1090 (1934). Smith, G. F., “Cerate Oxidimetry,” pp. 14-19, G. F. Smith Chemical Co., Columbus, Ohio, 1942. Telep, G., and Bolts, D. F., .IXAL. CHEY.,25, 971 (1953). Yost, D. M.,Russell, H., Jr., and Garner, C. S.,“Rare Earth Elements and Their Compounds,” pp. 22-3, Wiley, Sew York, 1947. RECEIVEDfor review January 22, 19.55. Accepted February 26, l!ldJ. h e sented before t h e Pittsburgh Conference on Analytical Chemistry arid Applied Spectroscopy, February 1956.
