Molecular and Atomic Absorption Spectrometric Methods for the Determination of Cerium Utilizing the Formation of Molybdocerophosphoric Acid H. N. Johnson, G.
F. Kirkbright, and R. J.
Whitehouse
Chemistry Department, lmperial College, London S. W. 7, U.K.
The formation of the heteropoly molybdocerophosphoric acid (MCPA) complex is shown to permit the determination of cerium by three methods. Cerium may be determined directly in aqueous solution by measurement of = 7,300). A the absorbance of MCPA at 318 nm (E318 more selective method is described in which the excess molybdophosphoric acid (MPA) formed simultaneously is selectively extracted into chloroform-butanol, the MCPA is decomposed by heating the solution, and the liberated phosphate determined by formation of MPA and extraction into isobutyl acetate for absorbance measurement at 318 nm. Between 25 and 250 pg Ce are readily determined by this method and only those ions which suppress the formation of MCPA interfere. In the third method developed, the molybdenum of the MPA extracted into isobutyl acetate for the ultraviolet absorptiometric method is determined by atomic absorption spectrometry (AAS) at 313.2 nm in a nitrous oxide-acetylene flame. This latter method provides for the selective determination of between 40 and 400 pg, Le., 1% absorption is produced by the presence of 0.093 pg m l - ' Ce in the initial aqueous solution, so that the sensitivity is several orders of magnitude greater than that attainable by direct AAS in the same flame.
,,,,,
Owing to the high thermal stability of its monoxide, CeO, the direct determination of cerium by atomic absorption spectrometry (AAS) is possible with only very poor sensitivity, even when the hot nitrous oxide-acetylene or oxy-acetylene flames are employed ( 2 , 2 ) . Shifrin et al. ( I ) , for example, obtained an AAS sensitivity (for 1% absorption) of 83 ~g nil-1 of cerium using a premixed nitrous oxide-acetylene flame and a heated expansion chamber. The elements niobium ( 3 ) ,thorium ( 4 ) , titanium (.5), and vanadium (6, 7). which may also be measured directly by .4AS with only poor sensitivity may be determined indirectly by AAS using amplification methods. The general procedure in these methods involves the formation of a ternary heteropoly complex between molybdate, phosphate, and the ion of the element to be determined. This is extracted into an organic solvent after the preliminary separation of the binary molybdophosphoric acid (MPA) complex which is formed simultaneously; the eleven molybdenum atoms associated with each atom of N. Shifrin. A . Hell, and J. Ramirez-Munoz, Appl. Spectrosc., 23, 365 (1969). P. E. Thomas, Resonance Lines, I , 519 (1970). G . F. Kirkbright. A. H. Smith and T. S. West, Analyst (London). 93, 292 (1968) G. F. Kirkbright, A . P. Rao, and T. S. West, Spectrosc. Lett., 2, 69 (1969) G. F. Kirkbright. A. H . Smith, T . S. West. and R. Wood, Analyst (London), 94, 754 ( 1969). R . J. Jakubiec and D. F. Boltz, Anal. Lett.. 1 , 347 (1968). H. N . Johnson, G . F. Kirkbright. and T. S. West, Analyst (London), 97, 696 (1972).
the analyte element in the ternary complex are then determined directly in the organic solvent phase by AAS a t 313.2 nm using a nitrous oxide-acetylene flame. The formation and stability of a similar ternary heteropoly complex of cerium, molybdocerophosphoric acid (MCPA) has been investigated by several workers (8-22), and a method for the determination of cerium by molecular absorptiometry in solution based on the formation of binary molybdoceric acid has also been described ( 2 2 ) . This paper reports the study 9f the formation and use of the ternary MCPA complex for determination of cerium by molecular absorptiometry in solution; the development of an indirect amplification procedure, similar to those reported for other elements, for the determination of cerium by atomic absorption spectrometry is also described.
EXPERIMENTAL Apparatus. Solution spectrophotometric measurements were made in matched 10-mm silica cells using a Beckman DB doublebeam spectrophotometer. Atomic absorption measurements were made using a n AA4 flame spectrometer (Varian-Techtron Pty., Melbourne, Australia) fitted with a 50-mm nitrous oxide-acetylene slot burner and a molybdenum hollow-cathode lamp. Reagents. Standard Cerium(1V) Solution (1000 pg m l - l Ce). Dissolve 1.957 grams of analytical reagent grade ammonium ceric nitrate, (NH&Ce(NO3)6, in 300 ml of distilled water, add 125 ml of concentrated analytical reagent grade nitric acid, and dilute to 500 ml with distilled water. Store in a polyethylene container. The solution was standardized with potassium ferrocyanide ( 2 3 ) , and working solutions were made by appropriate dilutions while maintaining a n acidity of 1M in nitric acid. A ten-fold molar excess of sodium sulfate was added t o t h e working solutions to improve their stability ( I O ) . Phosphate Solution (62 pg ml-I of phosphorus). Dissolve 0.1361 gram of analytical reagent grade potassium dihydrogen phosphate in distilled water and dilute to 1 liter. Store in a polyethylene container. Molybdate Solutions. a ) Dissolve 25.2 grams of analytical reagent grade sodium molybdate in distilled water and dilute to 1 liter. Store in a polyethylene container. b) Dissolve 11.68 grams of analytical reagent grade sodium molybdate in distilled water and dilute to 100 ml. Store in a polyethylene container. Nitric Acid Solution (4M) T h e solution was made from analytical reagent grade acid and stored in a polyethylene container. Acid Wash Solution. Add 177 ml of analytical reagent grade hydrochloric acid to 500 ml of distilled water and dilute to 1 liter with distilled water. Store in a polyethylene container. n-Butanol: Chloroform Extractant. Mix one volume of n-butano1 (G.P.R. grade) with four volumes of chloroform (G.P.R. grade) and store in a brown glass bottle. (8) Yu. F. Shkaravskii, U k r . Khirn. Zh.. 30, 241 (1964). (9) N . F. Barkovskii and T. V. Velikanova, Russ. J. Inorg. Chem., 11, 1230 (1966). (10) N . F. Barkovskii and T. V. Velikanova, Russ. J. Inorg. Chem., 15, 385 (1970). (11) N. F. Barkovskii and T. V. Velikanova, Russ. J. Inorg. Chem., 15, 879 (1970). (12) 2. F. Shakhova and S. A. Gavrilova, Zh. Anal. Khirn., 13, 211 (1958). (13) A. I . Vogel, "A Text-Book of Quantitative Inorganic Analysis," 3rd ed., Longmans, London, 1962.
ANALYTICAL CHEMISTRY, VOL. 45,
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WAVELENGTH
(nd
Figure 1. Spectral characteristics of molybdocerophosphoric acid in aqueous solution
Reference solutions: sodium molybdate: 0.4N nitric acid (1 and 2):water ( 3 ) . Absorption spectra of: ( 1 ) molybdophosphoric acid soln., ( 2 ) molybdocerophosphoric acid, formed from 200 p g Ce, (3) ammonium ceric nitrate solution
0 0 0
0.2 1.3
0.4
0.6
2.6
3.9
4. I
8.3
12.5
06 M
(AI 5.2 1 6 ~ u (e) 16-6 x i e 3 M (c)
CONCENTRATION Figure 2. Curve A: Effect on absorbance for 300 pg of cerium of variation in molarity of nitric acid (solution 1 0 - 2 M in molybdate and 1.2 x in phosphate). Curve B: Effect on absorbance for 144 pg of cerium of variation in phosphate reagent concentration (solution 0.4M in nitric acid, 4.16 X 10-3M in molybdate). Curve C: Effect on absorbance for 81 p g of cerium of variation in molybdate reagent concentration (solution 0.4M in nitric acid, 8 X 1 0 - 5 M in phosphate)
Procedures. I. Procedure for Direct C7trai’iolet Spectrophotometric Determination of Cerium in Aqueous Solution. Add the sample solution, containing not more than 250 pg of cerium(IV), to a 25-ml volumetric flask. Add 1 ml of phosphate solution, 1 ml of molybdate solution ( a ) , and sufficient 4M nitric acid to make the solution 0.4M in nitric acid when the final solution volume is 25 ml. Dilute to 25 ml with distilled water and mix thoroughly. Allow to stand for 15 minutes before measuring the absorbance a t 318 nm against a reference solution prepared in a similar manner except for the addition of t h e cerium sample solution. ZI. I’rocedure for Indirect Z ‘ltraciolet Spectrophotometric Determination of Cerium in Isobutyi Acetate. T o six 100-ml separating funnels, add 0. 0.2, 0.4, 0.6, 0.8 and 1.0 ml of cerium(1V) solution (250 pg mIk1). Add to each funnel, 1 ml of phosphate solution, 1 ml of molybdate solution ( a ) , and sufficient 4M nitric acid to make the solutions 0.4M in nitric acid when the final solution volume is 25 ml. Dilute to 25 ml with distilled water. Mix the solutions and allow to stand for 15 minutes. Add 25 ml of the n-butano1:chloroform mixture to each funnel, shake for 1 minute and discard the lower organic layer. Repeat the procedure twice. Heat the aqueous phases a t ca. 55 “C for 5 minutes. Transfer to 100-ml separating funnels, add 1 ml of molybdate solution ( b ) and 1 ml of concentrated nitric acid. Add 10 mi of isobutyl acetate to each funnel and shake for one minute. Separate the organic layer in each funnel and wash each rapidly with a 10-ml aliquot of the 1604
acidic wash solution. Measure the absorbance at 318 nm against a procedure blank. Construct a calibration graph of absorbance a t 318 nm c s . cerium concentration. III. Procedure for Indirect Determination of Cerium b), A A S . Follow procedure I1 above. Instead of measuring the absorbance in the organic phase by solution absorptiometry, nebulize the washed organic phases into the nitrous oxide-acetylene flame. Measure the molybdenum atomic absorption at 313.2 nm in each case using pure isobutyl acetate to set zero absorbance. Construct a calibration graph of absorbance a t 313.2 nm cs. cerium concentration. The instrumental conditions employed should be optimized for the determination of molybdenum in an isobutyl acetate solvent. With our experimental assembly there are: slitwidth 100 p m (0.33 nm spectral half band pass^: wavelength. 313.2 nm; lamp current, 10 m 4 : nitrous oxide flow rate. 6.5 (min-1); acetylene flow rate adjusted to give a slightly luminous flame whilst nebulizing isobutyl acetate, (-3.21 min 1). The burner height was adjusted so that radiation from the hollow cathode source passed through the red reducing interconal zone of the flame ca. 4 to 8 m m above the burner head.
RESULTS AND DISCUSSION Formation and Composition of iMCPA. The wavelength of maximum absorbance for MCPA in aqueous solution is similar to t h a t for molybdophosphoric acid and occurs a t 318 nm. In Figure 1, curve 1 shows the absorption spectrum of MPA formed by mixing sodium molybdate and phosphate solutions in 0.1M “ 0 3 . Curve 2 is the absorption spectrum obtained for SICPA by mixing molybdate, cerium(IV1. and phosphate solutions. Curve 3 is the spectrum obtained for a n aqueous solution of ammonium ceric nitrate in 0.4M “ 0 3 . Using a solution of 10-2M with respect to molybdate and 1.2 X lO-3M in phosphate, the effect of variation of the molarity of the nitric acid concentration on the absorbance produced for MCPA at 318 nm was investigated. As shown in Figure 2A, maximum absorbance was produced for 300 p g of cerium in a medium 0.4 to 0.6M in nitric acid. A concentration of 0.4MH N 0 3 was chosen for use in all subsequent.work. The effect on absorbance a t 318 nm of the variation of the phosphate concentration of the solution containing 144 r g of cerium and which was 4.16 X lO-3M in molybdate and 0.4M in nitric acid is shown in Figure 2B. The effect of variation of the molybdate concentration using a solution 4 x lO-5M with respect to phosphate and 0.4M in nitric acid on the absorbance produced for 81 p g of cerium is shown in Figure 2C. When a n overall nitric acid concentration of 0.4M was used, the molybdate and phosphate concentrations employed were 4.16 x 10-3 and 4 X lO-SM, respectively. The order of addition of the phosphate and molybdate has no influence on the absorbance produced for the complex a t 318 nm when the solution is allowed to stand for ten minutes before measurement: this confirms the finding of previous workers ( 7 ) . The combining ratio of cerium to phosphate in MCPA was determined by plotting mole ratios cs. absorbance as the phosphate, molybdate, and acid concentrations were maintained constant and the cerium concentration was varied. As shown in Figure 3 a cerium:phosphate combining ratio of 2 : l was obtained. Because of the necessary presence of a large excess of molybdate to ensure complete formation of the heteropoly acid, no direct spectrophotometric data could be obtained on the phosphate to molybdate combining ratio in MCPA. Direct Ultraviolet Spectrophotometric Determination of Cerium. Conformity to Beer’s law was obtained in the range 1.6 to 10 pg ml-1 of cerium when absorbance measurements were made a t 318 nm in aqueous medium with the recommended conditions. The molar absorptivity for MCPA in aqueous medium at 320 nm was found to be ’7,300 1. (mole-cm)-l.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
A solution of the heteropoly complex formed according to the recommended general procedure reaches its maximum absorbance within 10 minutes and no change in absorbance a t 318 nm occurs for a t least a 60-minute period. An estimate of the precision of the direct molecular absorptiometric method a t 318 nm was obtained from the absorbances obtained for 10 samples each containing 200 pg of cerium. These samples gave a mean absorbance of 0.329. The standard deviation was 0.0045 which corresponds to a coefficient of variation of 1.36%. A study was made to determine the effect of 100-fold weight excess of' various ions on the determination of 200 p g of cerium by the recommended procedure. An ion was considered not to interfere when it produced an error in absorbance of less than twice the coefficient of variation ( i . e , , 3%). The ions Ag, Al, Ca, Co, Cu, K, Na, NH4+, U(VI), C104-, ?1:03-. and so42- do not interfere. Large positive interference was experienced from Fe(III), Cr(VI), Pb(II), and Si032-. Both Fe(II1) and Cr(V1) absorb a t the wavelength of measurement; in the presence of Pb(IIj, a suspension of lead molybdophosphate is formed. Silicate combines with the excess molybdate to produce molybdosilicic acid, which also absorbs a t the wavelength employed. Slight negative interference was caused by V(V) .and C1- but large negative interference occurs with As(III), Mn(II), Ti(IV), W(V1). and F - . Arsenic(II1) probably reduces Ce(IV1 to Ce(II1) and the Ti(1V) was added as oxalate which may also have reduced the cerium. Fluoride may interfere through complex formation with the cerium. Indirect Ultraviolet Spectrophotometric Determination of Cerium. Cerium may be determined indirectly uia the formation of its MCPA complex. In the method developed, the MCPA complex is formed in aqueous medium, the excess MPA is selectively extracted with chloroform: butanol, the MCPA is decomposed by heating the solution, and the MPA formed from the phosphate present in the original ILlCPA complex is selectively extracted into isobutyl acetate for absorbance measurement a t 320 nm. In this method. the optimum concentration range in the initial aqueous solution is 0.8 pg ml-1 to 9.6 pg ml-1 of cerium(1V j : this corresponds to the absorbance values between 0.05 and 0.58. The effective molar absorptivity obtained a t 320 nm in this method was 8,800 1. (mole, cmj-I. It is not possible t o form MCPA in aqueous solution in the presence of excess phosphate and molybdate without the simultaneous formation of MPA. This must be removed before the extraction of the phosphate in the MCPA complex as MPA. Preliminary extractions with a 1:3 v 'v mixture of n-butano1:chloroform serve to extract the excess >lPA and leave the MCPA in the aqueous phase. The non-extraction of cerium during this step was demonstrated by use of cerium-141 radioisotope counting and by the experiments performed to determine the ratio of cerium to final measured molybdenum (see below). The removal of' 51PA b y three 25-ml aliquots of the extractant mixture has been demonstrated previously ( 1 4 ) . Spectrophotometric absorbance measurement of the solution after the extraction demonstrated that the MCPA was unaffected. MCPA is not extracted or reproducibly decomposed by oxygen-containing solvents as are other ternary heteropoly complexes ( 1 1 ) . To determine the cerium in the MCPA complex either the molybdenum or phosphorus associated with it may be measured. Measurement of the molybdenum associated with the cerium in the complex would involve separation of the associated molybdenum from the excess molybdate in the aqueous solution: this is difficult
I*
1:2 I:I 2:l Molar Ratio Ce:P
4:l
Figure 3. Determination of cerium to phosphate mole ratio in molybdocerophosphoric acid complex
to achieve. The phosphate ion associated with each two cerium atoms in MCPA, however, may be determined uia its extraction and measurement as MPA. The conditions for the formation and extraction of MPA have been reported elsewhere (14, 15). The decomposition of MCPA before formation of MPA may be achieved either by reduction of the Ce(1V) to Ce(III), by the addition of alkali, or by heating the solution. Most reducing agents, however, will also tend to reduce MPA to molybdenum blue. MCPA is decomposed above 40 "C (9) and heating to ca. 55 "C for five minutes ensures its complete decomposition. The MPA may then be formed using the conditions previously specified; this requires the use of additional molybdate and acid. Although both Wadelin and Mellon (15) and Kirkbright et al. (14) used hydrochloric acid medium for formation of MPA, the former authors found no difference in its formation characteristics when nitric acid was employed. A single aliquot of isobutyl acetate effects selective and quantitative extraction of the MPA formed ( 1 4 ) . Any excess molybdate reagent mechanically transferred or extracted into the organic phase may be removed using a single rapid wash with dilute hydrochloric acid. An estimate of the precision of the recommended method was obtained from the results obtained for ten samples each containing 200 p g ml-1 of cerium. These samples gave a mean absorbance value of 0.508 a t 318 nm. The standard deviation was 0.018 which corresponds to a relative standard deviation of 3.6%. A study was made to determine the effect of the presence of 100-fold weight excess of various ions on the determination of 200 pg of cerium by the recommended procedure. An ion was considered not to interfere when it produced an error in absorbance less than twice the relative standard deviation ( i , e . , 7%). The ions Ag, Al, Ca, Co, Cr(VI), Cu, Fe(III), K, La, Na, "4, U(VI), Y, C104-, NOS-, SO42- and SiO32- do not interfere. Those ions which suppress the formation of MCPA interfere and include As(III), Mo(II), W(VI), V(V), Ti(IV), F-, and C1-. Indirect AAS Determination of Cerium. The procedure developed for the indirect determination of cerium by molecular absorptiometry in isobutyl acetate is directly applicable to its determination by AAS. When the isobutyl acetate extract was nebulized into the nitrous oxideacetylene flame for measurement of the molybdenum absorbance a t 313.2 nm, the calibration graph obtained was (14) G. F. Kirkbright. A. M. Smith, and T. S. West, Analyst (London), 92, 411 (1967). (15) C. Wadelin and M . G. Mellon, Anal. Chem., 25, 1668 (1953).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
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linear over the range 40 to 400 pg Ce-ie., 1.6 to 16 pg ml-1 of cerium in the initial aqueous solution. The absorbance values which correspond to these concentrations in the aqueous solution are 0.07 and 0.73. A sensitivity for cerium by this indirect method of 0.093 pg ml-1 (for 1% absorption) was obtained. The procedure blank was reproducible and gave rise to an absorbance of 0.04 a t 313.2 nm us. an isobutyl acetate solvent blank. The virtually quantitative extraction of phosphate into the isobutyl acetate medium was demonstrated by taking several different known weights of cerium through the procedure. The MPA in the final isobutyl acetate phases was decomposed and back-extracted by shaking with 10 ml of 4M ammonia solution. The solutions were then diluted to 25 ml with distilled water. The concentration of molybdenum in the ammoniacal phase was determined by AAS using an AAS calibration graph prepared from aqueous molybdate solutions which were also prepared to contain 4M ammonia. The results of these experiments gave a ratio of cerium to final measured molybdenum of 1:5:9 f 0.2. The Mo:Ce ratio of 6:1, the well-known 12:l Mo:P ratio in MPA (14) and the 2 : l ratio in MCPA (11) confirms that MCPA is quantitatively formed in the initial aqueous phase, that MCPA is not extracted by the nbutano1:chloroform solvent used to remove excess MPA, and the complete extraction as MPA of the phosphorus associated with the cerium after decomposition of MCPA into the single isobutyl acetate aliquot used for the final extraction. An estimate of the precision of the recommmended method was obtained from the results obtained for ten samples each containing 200 pg ml-1 of cerium. These samples gave a mean absorbance value of 0.388. The stan-
dard deviation was 0.018 which corresponds to a relative standard deviation of 4.7%. A similar interference pattern to that in the indirect ultraviolet spectrophotometric method %’as found for the indirect AAS method because the selectivity results from the high selectivity of the solvent extraction step with isobutyl acetate.
CONCLUSION Two solution spectrophotometric methods are reported for the determination of cerium via the formation of MCPA. The AAS method reported is approximately 900 times more sensitive than the direct method for cerium in a nitrous oxide-acetylene flame (1%absorption is given by ca. 83 pg ml-1 of cerium by the direct method, whereas by this method 1%absorption results with an initial aqueous cerium solution of 0.093 p g m1k1). The enhancement is achieved not only because the AAS sensitivity is greater for molybdenum than cerium but also due to the fact that six molybdenum atoms are associated with each cerium atom; solvent extraction of the measured molybdenum also increases the nebulization efficiency as well as the molybdenum concentration in the final solution. The method is less sensitive than the solution spectrophotometric methods based on MCPA but is more rapid than the indirect ultraviolet molecular absorptiometric method. Received for review October 11, 1972. Accepted January 15, 1973. We are grateful to Alcan Research and Development Ltd. for financial support of this work and t o the Science Research Council for the grant of a CAPS studentship to one of us (H.N.J.).
Direct Determination of Lead Airborne Particulates by Nonflame Atomic Absorption J. P. Matoukek and K. G. Brodie Varian Techtron Pty. Ltd., North Springvale, Victoria, 3 171, Australia
The direct determination of airborne lead particulates was performed by atomic absorption using nonflame atomization. Particulate matter was collected on a disk of Millipore filter (pore size 0.22 p m ) which had been previously inserted in a modified graphite sampling cup. An air sample volume of 200 ml was sufficient. Prior to the analysis an excess of phosphoric acid was added to the sample in order to produce a single lead absorption peak. The absolute sensitivity achieved (for 1% absorption) was 1.7 X l o - ’ ’ gram of lead which represents 0.1 p g Pb/m3 for a 200-ml air sample. It was shown that aqueous lead solutions added to the standard graphite cup could be used for standardization. The re1 std dev calculated for aqueous solutions (equivalent to a level of 1 p g Pb/m3 in air) was 4.2%. Results determined by this method using aqueous standards correlated well with those by a conventional method. 1606
Atmospheric contamination by lead from automobile exhausts presents a potential hazard to human health. According to Schroeder and Nason ( I ) , the use of lead additives to gasoline has resulted in human body burdens of 120-480 mg of lead. A number of methods for analysis of atmospheric lead have been suggested-a summary of such methods has been given by Loftin et al. ( 2 ) . Such conventional methods require sampling of large volumes of air to obtain an adequate analytical signal. The volume requirement can be drastically reduced by using nonflame atomic absorption. The inherent sensitivity of nonflame methods makes them eminently suitable for such trace metal analysis. Collection of lead particulates on a filter from air vol(1) H. A. Schroederand A. P. Nason, Clin. Chem.. 17,461 (1971). (2) H. P. Loftin. C. M. Christian, and J. W. Robinson, Spectrosc. Lett.. 3, 161 (1970).
A N A L Y T I C A L CHEMISTRY, V O L . 45, NO. 9, A U G U S T 1973
