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Titrimetric and spectrophotometric methods for the determination of cerium(IV) in aqueous solution
MICROCHEMICAL
JOURNAL
30, 53-57 (1984)
Titrimetric and Spectrophotometric Methods for the Determination of Cerium(lV) in Aqueous Solution S. A. RAHIM, D. AMIN, Deparrment
AND
W. A.
BASHIR
of Chemistry, College of Science, University of Mosul. Mosul, Iruy Received May 25, 1982
INTRODUCTION The titrimetric and spectrophotometric methods available (2, 4-6, 8, 10) for the determination of cerium(IV) are not completely satisfactory from the analytical point of view. In order to develop a reliable and sensitive methods, we have investigated an amplification procedure and a spectrophotometric finish. The first method is based on the reaction of cerium(IV) with iodide (3); the liberated iodine is extracted with chloroform followed by its reduction to iodide, and the latter is determined by the Leipert amplification procedure with either 6- or 36-fold method (1). The spectrophotometric method depends on the reaction of ccrium(IV) with excess of iron(H); the formed iron(W) is reacted with hexacyanoferrate(II) to produce a stable and intense prussian blue color (9) suitable for the determination of the titled ion. EXPERIMENTAL Apparatus Laboratory glassware. All spectrophotometric measurements were done on a Bausch & Lomb Spectronic 20 spectrophotometer using matched l-cm optical glass cells. Reagents Unless otherwise stated, all chemicals used were of analytical-reagent grade. All water is deionized or distilled. Standard cerium(ZV) solution, 1000 pg ml-‘. Dissolve 2.8855 g of ceric sulfate tetrahydrate in 1 N sulfuric acid solution and dilute to 1000 ml with the same acid. Potassium iodide solution, 0.26% (w/v). Dissolve 0.26 g of potassium iodide in water and dilute to 100 ml. This solution contains 2 mg I- ml-‘. Sodium thiosulfate solutions, 0.01 and 0.001 N. Prepare from the pcn53 0026-265X184 $1.50 Copyright C 1984 by Academx Press, Inc All rlghtc of reproductmn m any form reserved.
54
RAHIM,
AMIN,
AND
BASHIR
tahydrated sodium salt and standardize against potassium iodate solutions of similar normality. Sodium sulfite solution, 1% (w/v). Dissolve 1 g of sodium sulfite in water and dilute to 100 ml. Bromine water, saturated solution. Formic acid solution, concentrated. Ammonium iron sulfate solution, 2 x 10m3M. Dissolve 0.0784 g of ammonium iron(I1) sulfate haxahydrate in 1 N sulfuric acid solution and dilute to 100 ml with the same acid. Prepare fresh daily. Potassium hexacyanoferrate(ZZ) solution, 2 x 10m3M. Prepare from the tetrahydrated potassium salt in water. EDTA (disodium salt) solution, 1 x lo- 1M. Titrimetric Procedure In a 50-ml separatory funnel, place a suitable volume (0.3-5 ml) of cerium(IV) solution, 5 ml of 0.26% potassium iodide solution (10 mg of iodide), and add water to make a total volume of 15 ml. Shake and extract the liberated iodine with 3 x 10 ml of chloroform. Collect the extracts in another funnel and shake the iodine solution with 10 ml of water containing 0.5 ml of 1% sodium sulfite solution to reduce iodine to iodide. Allow the phases to separate and discard the chloroform. Withdraw the aqueous phase into a 50-ml Erlenmeyer flask, and rinse the inside of the funnel tip with some water. Add 3 ml of bromine water and shake for 5 min. Destroy the excess of bromine by 5 ml of formic acid. Add about 0.2 g of potassium iodide and titrate the liberated iodine with the thiosulfate solution in the usually way using starch as indicator. Run a blank determination. For the 36-fold amplification, the iodine formed as above, transfer it to a 50-ml separatory funnel, extract with 3 x 15 ml of chloroform, and then reduce to iodide. Oxidize the iodide by bromine water and proceed as described for the 6-fold amplification. For this procedure the amount of cerium(IV) taken should be less than 1 mg. Img Ce = 4.28 ml of 0.01 N and 25.68 ml of 0.01 N thiosulfate solutions for the 6- and 36-fold, respectively. The blank values were 0.34 ml and 1.22 ml of 0.001 N thiosulfate solution in the 6- and 36-fold methods, respectively. Spectrophotometric Procedure Transfer an aliquot of the standard solution containing about 25- 100 kg of cerium(IV) into a series of 25-ml calibrated flasks, add 3.5 ml of 2 x lop3 M of iron(I1) solution, 2 ml of 2 x 10e3M of hexacyanoferrate(I1)
DETERMINATION
OF Ce(IV)
IN AQUEOUS
SOLUTION
55
solution, and 3.5 ml of 1 x 10-r M EDTA solution, dilute to the mark with 1 N sulfuric acid solution. Allow the reaction mixture to stand for 5-7 min and measure the absorbances against the reagent blank at 650 nm using l-cm cells. The color is stable for 60 min. The linear absorbance plot indicates that Beer’s law is followed in the range l-4 ppm with a conditional molar absorptivity (referred to as cerium(IV) of 13.3 x lo3 mol-i cm-’ and a sensitivity index (expressed as the amount of cerium corresponding to an absorbance of 0.001) of 0.0105 Fg cm--‘. RESULTS AND DISCUSSION
Titrimetric Method The present method depends on using an excess of iodide to reduce ceric ion quantitatively to cerous ion in acidic solution. This had been proved experimentally by allowing the acidic solution of cerium(IV) and excess of iodide to react and to liberate an equivalent amount of iodine, then extraction of the resulting iodine into chloroform, followed by its reduction to iodide and then determining the iodide in the aqueous layer. It was found that the reaction is complete within 1 min in the presence of excess iodide; 10 mg of iodide covers the range of cerium(IV) determination by the present method. Spectrophotometric Method In the preliminary investigation, the determination of cerium(IV) was attempted under the experimental conditions described for the determination of chromium(V1) (7). However, the color intensity of the product thus obtained was not measurable, and optimum reaction conditions were therefore sought. Effect of Reagent Concentrations The effect of iron(I1) concentration on the color intensity was first studied. An amount 2.5-4 ml of 2 x 1O-3 M iron(I1) solution gave maximal absorbance, and 3.5 ml of the prescribed iron(I1) solution was used in the subsequent experiments. Next, the optimum amount of 2 x 10P3M hexacyanoferrate(I1) solution was tested. The results of investigation showed that 1.5-2.5 ml of the intended solution gave the most intense absorption, and 2 ml was incorporated in the procedure. Then, the useful concentration of 1 x 10e3 M EDTA solution was sought. The experimental data revealed that 2-3.5 ml resulted in a maximum absorbance, and 3.5 ml was recommended for the method; 5 ml decreased the absorbance by about 10%. The most effective diluents among the tried, was the 1 N sulfuric so-
56
RAHIM, AMIN, AND BASHIR
lution. Water dilution, for example, resulted in an absorbance decrease of about 25%. The use of gelatin, recommended previously (7) here had no practical significance on either sensitivity or stability. Therefore, it was eliminated here. Order of Addition
of Reagents
For maximal absorbance and stability, one has to follow the order given in the procedure. Any other order, than that cited, would result in an absorbance decrease of 40-97%. Rate of Reaction
In spite of the rapid blue color development, the reaction needed to stand for 5-7 min before measurement to attain maximum constant absorbance which remained thereafter for 1 hr. Accuracy
and Precision
of the Methods
Under the prescribed conditions of the methods, the accuracy and precision of the methods were established from the results of five determinations of standard solutions. The results are compiled in Table 1. Suggestion for Interference
Elimination
The possible interfering effect caused by some foreign ions may simply be removed using a preferential ion-exchange resin. Also, the presence of EDTA in the reaction mixture may perhaps play an important role in masking action. TABLE 1 ACCURACYAND
PRECISION OF THE METHODS
Cerium(IV) taken hJ.n)
Relative error m
Relative standard deviation (%)
6-fold
100 1000 5000
-2.0 -0.3 -1.0
3.1 1.0 0.6
36-fold
30 50 100 1000
-3.3 -2.0 -1.0 -0.4
3.5 2.2 1.7 1.1
Spectrophotometric
30 50 100
-6.0 +3.9 -2.9
6.2 1.6 3.1
Method
DETERMINATION
OF Ce(IV) IN AQUEOUS SOLUTION
57
CONCLUSION
Two methods have been developed for the micro and trace determination of cerium(IV). However, many ions commonly present in waters can interfere in the determination. A suitable preliminary ion-exchange cleanup may therefore frequently be required. SUMMARY A titrimetric and spectrophotometric methods for cerium(IV) determination have worked out. The first method relies upon the treatment of cerium(IV) solution with an excessive amount of iodide; the liberated iodine is extracted into chloroform, then reduced to iodide. The latter is iodometrically determined after 6- or 36-fold amplification. The spectrophotometric finish is based upon the reaction of the titled ion with iron( in the presence of hexacyanoferrate(II), to form an intense prussian blue color suitable for the trace determination of cerium(IV) ion.
REFERENCES 1. Geilmann, W., and Barttlingek, H., Determination of traces of iodide in salts. Mikrothem. 30, 217-225 (1942). 2. Idriss, K. A., Issa, I. M., and Temerk, Y. M., Potentiometric and visual titration of
tetravatent cerium with potassium permanganate. Z. Anal. Chem. 278, 364 (1976). 3. Kolthoff, I. M., and Belcher, R., “Volumetric Analysis,” Vol. III, p. 367. Interscience, New York, 1957. 4. Lang, R., and Faude, E., Ferrometric determination of cerium, manganese, chromium and vanadium in the presence of one another. Z. Anal. Chem. 108, 181-189 (1937). 5. Marczenko, Z., Calorimetric determination of cerium by means of formaldoxime. Acta Chim. Acad. Sci. Hung. 26, 347-354 (1961). 6. Penzes, I., and Csanyi, L. .I., Microdetermination of cerium (IV). Acta Phys. Chem. 15, 55-57 (1969). 7. Rahim, S. A., Abdulahed, H., and Milad, N. E., Microdetermination
of chromium: dichromate ion. J. Indian Chem. Sot. 52, 853-854 (1975). 8. Verdizade, A. A., and Gurbanov, A. N., Determination of cerium as its normal orthoperiodate. Ser. Khim. Nuuk. 3, 37-42 (1968); Anal. Absr. 18, 1548(1970). 9. Vogel’s Textbook of macro and semimicro qualitative inorganic analysis. Fifth ed., p. 246, Longman, London, 1979. 10. Willard, H. H., and Young, P., Ceric sulfate as a volumetric oxidizing agent. J. Amer. Chem. Sot. 50, 1368-1385 (1928).
JOURNAL
30, 53-57 (1984)
Titrimetric and Spectrophotometric Methods for the Determination of Cerium(lV) in Aqueous Solution S. A. RAHIM, D. AMIN, Deparrment
AND
W. A.
BASHIR
of Chemistry, College of Science, University of Mosul. Mosul, Iruy Received May 25, 1982
INTRODUCTION The titrimetric and spectrophotometric methods available (2, 4-6, 8, 10) for the determination of cerium(IV) are not completely satisfactory from the analytical point of view. In order to develop a reliable and sensitive methods, we have investigated an amplification procedure and a spectrophotometric finish. The first method is based on the reaction of cerium(IV) with iodide (3); the liberated iodine is extracted with chloroform followed by its reduction to iodide, and the latter is determined by the Leipert amplification procedure with either 6- or 36-fold method (1). The spectrophotometric method depends on the reaction of ccrium(IV) with excess of iron(H); the formed iron(W) is reacted with hexacyanoferrate(II) to produce a stable and intense prussian blue color (9) suitable for the determination of the titled ion. EXPERIMENTAL Apparatus Laboratory glassware. All spectrophotometric measurements were done on a Bausch & Lomb Spectronic 20 spectrophotometer using matched l-cm optical glass cells. Reagents Unless otherwise stated, all chemicals used were of analytical-reagent grade. All water is deionized or distilled. Standard cerium(ZV) solution, 1000 pg ml-‘. Dissolve 2.8855 g of ceric sulfate tetrahydrate in 1 N sulfuric acid solution and dilute to 1000 ml with the same acid. Potassium iodide solution, 0.26% (w/v). Dissolve 0.26 g of potassium iodide in water and dilute to 100 ml. This solution contains 2 mg I- ml-‘. Sodium thiosulfate solutions, 0.01 and 0.001 N. Prepare from the pcn53 0026-265X184 $1.50 Copyright C 1984 by Academx Press, Inc All rlghtc of reproductmn m any form reserved.
54
RAHIM,
AMIN,
AND
BASHIR
tahydrated sodium salt and standardize against potassium iodate solutions of similar normality. Sodium sulfite solution, 1% (w/v). Dissolve 1 g of sodium sulfite in water and dilute to 100 ml. Bromine water, saturated solution. Formic acid solution, concentrated. Ammonium iron sulfate solution, 2 x 10m3M. Dissolve 0.0784 g of ammonium iron(I1) sulfate haxahydrate in 1 N sulfuric acid solution and dilute to 100 ml with the same acid. Prepare fresh daily. Potassium hexacyanoferrate(ZZ) solution, 2 x 10m3M. Prepare from the tetrahydrated potassium salt in water. EDTA (disodium salt) solution, 1 x lo- 1M. Titrimetric Procedure In a 50-ml separatory funnel, place a suitable volume (0.3-5 ml) of cerium(IV) solution, 5 ml of 0.26% potassium iodide solution (10 mg of iodide), and add water to make a total volume of 15 ml. Shake and extract the liberated iodine with 3 x 10 ml of chloroform. Collect the extracts in another funnel and shake the iodine solution with 10 ml of water containing 0.5 ml of 1% sodium sulfite solution to reduce iodine to iodide. Allow the phases to separate and discard the chloroform. Withdraw the aqueous phase into a 50-ml Erlenmeyer flask, and rinse the inside of the funnel tip with some water. Add 3 ml of bromine water and shake for 5 min. Destroy the excess of bromine by 5 ml of formic acid. Add about 0.2 g of potassium iodide and titrate the liberated iodine with the thiosulfate solution in the usually way using starch as indicator. Run a blank determination. For the 36-fold amplification, the iodine formed as above, transfer it to a 50-ml separatory funnel, extract with 3 x 15 ml of chloroform, and then reduce to iodide. Oxidize the iodide by bromine water and proceed as described for the 6-fold amplification. For this procedure the amount of cerium(IV) taken should be less than 1 mg. Img Ce = 4.28 ml of 0.01 N and 25.68 ml of 0.01 N thiosulfate solutions for the 6- and 36-fold, respectively. The blank values were 0.34 ml and 1.22 ml of 0.001 N thiosulfate solution in the 6- and 36-fold methods, respectively. Spectrophotometric Procedure Transfer an aliquot of the standard solution containing about 25- 100 kg of cerium(IV) into a series of 25-ml calibrated flasks, add 3.5 ml of 2 x lop3 M of iron(I1) solution, 2 ml of 2 x 10e3M of hexacyanoferrate(I1)
DETERMINATION
OF Ce(IV)
IN AQUEOUS
SOLUTION
55
solution, and 3.5 ml of 1 x 10-r M EDTA solution, dilute to the mark with 1 N sulfuric acid solution. Allow the reaction mixture to stand for 5-7 min and measure the absorbances against the reagent blank at 650 nm using l-cm cells. The color is stable for 60 min. The linear absorbance plot indicates that Beer’s law is followed in the range l-4 ppm with a conditional molar absorptivity (referred to as cerium(IV) of 13.3 x lo3 mol-i cm-’ and a sensitivity index (expressed as the amount of cerium corresponding to an absorbance of 0.001) of 0.0105 Fg cm--‘. RESULTS AND DISCUSSION
Titrimetric Method The present method depends on using an excess of iodide to reduce ceric ion quantitatively to cerous ion in acidic solution. This had been proved experimentally by allowing the acidic solution of cerium(IV) and excess of iodide to react and to liberate an equivalent amount of iodine, then extraction of the resulting iodine into chloroform, followed by its reduction to iodide and then determining the iodide in the aqueous layer. It was found that the reaction is complete within 1 min in the presence of excess iodide; 10 mg of iodide covers the range of cerium(IV) determination by the present method. Spectrophotometric Method In the preliminary investigation, the determination of cerium(IV) was attempted under the experimental conditions described for the determination of chromium(V1) (7). However, the color intensity of the product thus obtained was not measurable, and optimum reaction conditions were therefore sought. Effect of Reagent Concentrations The effect of iron(I1) concentration on the color intensity was first studied. An amount 2.5-4 ml of 2 x 1O-3 M iron(I1) solution gave maximal absorbance, and 3.5 ml of the prescribed iron(I1) solution was used in the subsequent experiments. Next, the optimum amount of 2 x 10P3M hexacyanoferrate(I1) solution was tested. The results of investigation showed that 1.5-2.5 ml of the intended solution gave the most intense absorption, and 2 ml was incorporated in the procedure. Then, the useful concentration of 1 x 10e3 M EDTA solution was sought. The experimental data revealed that 2-3.5 ml resulted in a maximum absorbance, and 3.5 ml was recommended for the method; 5 ml decreased the absorbance by about 10%. The most effective diluents among the tried, was the 1 N sulfuric so-
56
RAHIM, AMIN, AND BASHIR
lution. Water dilution, for example, resulted in an absorbance decrease of about 25%. The use of gelatin, recommended previously (7) here had no practical significance on either sensitivity or stability. Therefore, it was eliminated here. Order of Addition
of Reagents
For maximal absorbance and stability, one has to follow the order given in the procedure. Any other order, than that cited, would result in an absorbance decrease of 40-97%. Rate of Reaction
In spite of the rapid blue color development, the reaction needed to stand for 5-7 min before measurement to attain maximum constant absorbance which remained thereafter for 1 hr. Accuracy
and Precision
of the Methods
Under the prescribed conditions of the methods, the accuracy and precision of the methods were established from the results of five determinations of standard solutions. The results are compiled in Table 1. Suggestion for Interference
Elimination
The possible interfering effect caused by some foreign ions may simply be removed using a preferential ion-exchange resin. Also, the presence of EDTA in the reaction mixture may perhaps play an important role in masking action. TABLE 1 ACCURACYAND
PRECISION OF THE METHODS
Cerium(IV) taken hJ.n)
Relative error m
Relative standard deviation (%)
6-fold
100 1000 5000
-2.0 -0.3 -1.0
3.1 1.0 0.6
36-fold
30 50 100 1000
-3.3 -2.0 -1.0 -0.4
3.5 2.2 1.7 1.1
Spectrophotometric
30 50 100
-6.0 +3.9 -2.9
6.2 1.6 3.1
Method
DETERMINATION
OF Ce(IV) IN AQUEOUS SOLUTION
57
CONCLUSION
Two methods have been developed for the micro and trace determination of cerium(IV). However, many ions commonly present in waters can interfere in the determination. A suitable preliminary ion-exchange cleanup may therefore frequently be required. SUMMARY A titrimetric and spectrophotometric methods for cerium(IV) determination have worked out. The first method relies upon the treatment of cerium(IV) solution with an excessive amount of iodide; the liberated iodine is extracted into chloroform, then reduced to iodide. The latter is iodometrically determined after 6- or 36-fold amplification. The spectrophotometric finish is based upon the reaction of the titled ion with iron( in the presence of hexacyanoferrate(II), to form an intense prussian blue color suitable for the trace determination of cerium(IV) ion.
REFERENCES 1. Geilmann, W., and Barttlingek, H., Determination of traces of iodide in salts. Mikrothem. 30, 217-225 (1942). 2. Idriss, K. A., Issa, I. M., and Temerk, Y. M., Potentiometric and visual titration of
tetravatent cerium with potassium permanganate. Z. Anal. Chem. 278, 364 (1976). 3. Kolthoff, I. M., and Belcher, R., “Volumetric Analysis,” Vol. III, p. 367. Interscience, New York, 1957. 4. Lang, R., and Faude, E., Ferrometric determination of cerium, manganese, chromium and vanadium in the presence of one another. Z. Anal. Chem. 108, 181-189 (1937). 5. Marczenko, Z., Calorimetric determination of cerium by means of formaldoxime. Acta Chim. Acad. Sci. Hung. 26, 347-354 (1961). 6. Penzes, I., and Csanyi, L. .I., Microdetermination of cerium (IV). Acta Phys. Chem. 15, 55-57 (1969). 7. Rahim, S. A., Abdulahed, H., and Milad, N. E., Microdetermination
of chromium: dichromate ion. J. Indian Chem. Sot. 52, 853-854 (1975). 8. Verdizade, A. A., and Gurbanov, A. N., Determination of cerium as its normal orthoperiodate. Ser. Khim. Nuuk. 3, 37-42 (1968); Anal. Absr. 18, 1548(1970). 9. Vogel’s Textbook of macro and semimicro qualitative inorganic analysis. Fifth ed., p. 246, Longman, London, 1979. 10. Willard, H. H., and Young, P., Ceric sulfate as a volumetric oxidizing agent. J. Amer. Chem. Sot. 50, 1368-1385 (1928).