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Extractive spectrophotometric determination of molybdenum(V) in molybdenum steels
MICROCHEMICAL
32, 281-285 (1985)
JOURNAL
Extractive Spectrophotometric Determination Molybdenum(V) in Molybdenum Steels K. N. THIMMAIAH’ Depurtment
of Chemisrry. AND
Department
AND W. D. LLOYD
CJni~vrsiry of’ 7i,xtrs. El Puso, Texus 79968
G. T. CHANDRAPPA
of Post-Graduate Studies und Research in Chemistry, Mannsn Gangorri, Mysore-570006, Indiu Received
of
February
University
of Mysore,
22, 1985; accepted July 22, 1985
An extraction spectrophotometric method has been developed for the determination of traces of molybdenum present in molybdenum steels which is based on the extraction of the orange-red molybdenum-thiocyanate-acetonethiosemicarbazone complex into chloroform from hydrochloric acid medium. The complex has an absorption maximum at 472 nm with a molar absorptivity of 1.9 x IO4 liters mol-’ cm-‘. Beer’s law is valid over the concentration range 0.1-9.5 ppm of molybdenum with an optimum concentration range of 0.4-9 ppm. The equilibrium shift method indicates 1:4:2 composition for molybdenumthiocyanate-acetonethiosemicarbazone complex. The effect of acidity, reagent concentra% 1985 Academic tions, temperature, and interferences from Various ions are reported. Press, Inc.
INTRODUCTION
Most extensively used thiocyanate-tin(I1) chloride methods for the spectrophotometric determination of molybdenum have several drawbacks such as low sensitivity and lack of analytical reproducibility (2-6). In the present communication the authors have found that the sensitivity and stability of molybdenumthiocyanate complex could be increased by adding acetonethiosemicarbazone (AT) to form a new ternary complex which could be extracted into an organic solvent. The proposed method offers the advantages of sensitivity, selectivity, and stability. MATERIALS
AND METHODS
Solutions
AT was prepared from thiosemicarbazide and acetone and purified by the recommended method. A 2.5% solution of AT in acetone-water (3:2) was prepared. Molybdenum solution. A stock solution of molybdenum(W) was prepared from AR grade ammonium molybdate in doubly distilled water containing a few drops of ammonia and standardized gravimetrically using 8-hydroxyquinoline (7). Thiocyanate and ascorbic acid solutions. Stock solutions (10%) of ammonium ’ To whom correspondence
should be addressed. 281 0026-265X/85 $1.50 Copyright 0 1985 by Academic Press. Inc. All rightc of reproduction in any form reserved.
282
THIMMAIAH,
LLOYD,
AND
CHANDRAPPA
thiocyanate and ascorbic acid were prepared in doubly distilled water. All other reagents were analytical grade and were used without further purification. Apparatus
A Varian Cary Model 219 spectrophotometer for absorbance measurements.
with matched l-cm cells was used
Procedures Recommended procedure for the determination of molybdenum. An aliquot of the stock solution containing I-95 p.g of molybdenum, 6 ml of 10 M hydrochloric acid, 2 ml of 10% ascorbic acid, and 6 ml of 10% ammonium thiocyanate was transferred into a 50-ml separatory funnel and diluted to 25 ml with doubly distilled water. After 15 min at room temperature (27”(J), 8 ml of 2.5% AT solution was added, the solution was shaken well and extracted twice with 4-ml portions of chloroform. The chloroform extracts were dried with anhydrous sodium sulfate, diluted to 10 ml with chloroform, and the absorbance measured at 472 nm against a corresponding reagent blank prepared under similar conditions. The molybdenum content of the sample solution was determined from a standard calibration graph. Recommended procedure for the determination of molybdenum in molybdenum steels. An exact amount of molybdenum steel (approx 0.5 g) was transferred into
a 250-ml beaker and treated with I5 ml of 5 M sulfuric acid, 2 ml of phosphoric acid (sp. gr. = 1.75), and 1 ml of coned nitric acid. The solution was gently boiled to expel oxides of nitrogen, cooled, and then diluted to 50 ml with doubly distilled water. A small amount of 0.1 N potassium permanganate solution was added dropwise until the solution turned pale pink. After 5 min, 0.01 N oxalic acid was added dropwise with stirring until the pale pink of the solution disappeared and the solution was diluted to 100 ml. An aliquot of this solution was used for the determination of the molybdenum content following the recommended procedure. The results are given in Table 1. RESULTS
AND DISCUSSION
Molybdenum(V) formed by the reduction of molybdenum(W) with ascorbic acid combines with ammonium thiocyanate to form a red molybdenum(V)-thiocyanate complex in 0.8-3.2 M hydrochloric acid solution. On adding AT solution an orange-red ternary complex is formed in the same acid concentration. The ternary complex is extracted into chloroform while the binary molybdenum(V)thiocyanate complex is not. A double extraction is necessary to remove molybdenum completely from the aqueous phase. Effect
of Acid Concentration
on the Formation
of the Complex
The effect of concentration of acid on the formation and the extraction of the ternary complex into chloroform was investigated. The complex was not formed in acetic or perchloric acid medium. It was formed only in hydrochloric, sulfuric, phosphoric, or nitric acid medium but the absorbance reading of the chloroform extract from 0.5 to 5 M hydrochloric acid is most sensitive. The hydrochloric acid medium therefore was selected for further investigations.
SPECTROPHOTOMETRY Determination
Sample No. 1. Eo.300
TABLE 1 of Molybdenum in Molybdenum
Steels Molybdenum
General composition (%)
Certified
Found”
Mn-0.41, Ni-0.16, Cr-1.25, V-0.96, Cu-0.11, Ti-0.06, B-0.0046, C-2.0, Si-0.35, s-o.01 I
0.940
0.945
Mn-1.11, Ni-0.11, G-1.13, V-0.16, P-0.018, Si-0.59, C-l.01
0.035
0.036
0.81
0.82
0.12
0.13
Sample Sample No. 4. 20 Cr, MO. 95 V-85, Ti. B.
283
OF MO(V)
HSL,ASP, Durgapur H. No. 7-2034, 1SCDV,
Mn-0.88, Cr-1.32, V-0.19, Ni-0.37, Si-0.10, C-0.14, P-0.027, Al-0.032, S-O.010
HSL, ASP, Durgapur H. No.6-4828, 58CrV,
Mn-0.86, Cr-1.12, V-0.09, Si-0.29, C-0.62, Ni-0.14, Al-0.017, P-0.033, S-O.01 1
LIAverage of five determinations.
Role of Ascorbic Acid Ascorbic acid is a more suitable reducing agent for molybdenum(V1) than tin(H) chloride as it enhances the sensitivity of the reaction, gives reproducible values, and masks many interfering ions. A minimum concentration of 1 ml of 10% ascorbic acid is sufficient for the reduction of 2 ppm of molybdenum(V1) to molybdenum(V). More ascorbic acid than this has no effect on the absorbance readings. Effect of Ammonium
Thiocyanate
Concentration
The absorbance of the complex increases with increasing concentration of ammonium thiocyanate and AT. For maximum color development, 2.5-6 ml of 10% ammonium thiocyanate and 5-9 ml 2.5% AT were required. The optimum amounts of 6 ml of ammonium thiocyanate and 8 ml of AT were used for further studies. Selection
of Extracting
Solvent
Solvents such as cyclohexanone, benzene, carbon tetrachloride, toluene, isopropyl ether, butyl alcohol, and tetraline do not extract the complex while chloroform, n-butylacetate, amyl alcohol, and o-dichlorobenzene extract the complex. The molar absorptivity values for the complex in the solvents are: chloroform, 1.9 x 104; n-butyl acetate, 1.53 x 104; amyl alcohol, 1.4 x 103, and o-dichlorobenzene, 4.65 x lo2 liters mallr cm- ‘. Therefore, chloroform was selected for further studies. Absorption Spectra The binary complex in 1.5 it4 hydrochloric
acid has an absorption
maximum
at
284
THIMMAIAH,
LLOYD,
AND CHANDRAPPA
TABLE 2 Interference of Diverse Ions on the Determination Ion added Cu(I1) Co(H) Ni(I1) Mn(I1) Zn(I1) Fe(II1) Cr(II1) wm Ba(I1) Pb(I1) Cd(H) Bi(II1) Ti(IV) AI(II1) Pd(I1) Rh(II1) Pt(IV)
Tolerance limit (rvm) 400 102 2000 4000 4000 5000b 2000 500 8000 5000 650 loo 300 12 12.5 20
of Molybdenum” Tolerance limit (wm)
Ion added Ir(II1) Os(VII1) Rh(II1) VW) NW)
300 80 12 15 20 15 2000 8000 150
TaW
Chloride Bromide Iodide Citrate EDTA Phosphate Sulfate Nitrate Thiosulfate
0 Concentration of molybdenum(V1) = 3 ppm. b In the presence of 5 ml of 10% ascorbic acid.
440-450 nm. The chloroform extract of the ternary complex has an absorption maximum at 465-480 nm showing a bathochromic shift of 25 nm. The reagent blank shows little absorbance at these wavelengths. All subsequent studies were made at 472 nm. Calibration Graph and Sensitivity The complex obeys Beer’s law over the range 0.1-9.5 ppm of molybdenum. The optimum range is 0.4-9 ppm. The molar absorptivity is 1.9 x lo4 liters mol-’ cm-‘. The standard deviation calculated from 10 determinations on a solution containing 2 ppm of molybdenum(V) is 0.009 and the relative error is less than 2%. The absorbance readings of the chloroform extract of the ternary complex were stable for about 2 weeks in the temperature range 6-50°C. There was no appreciable change in the absorbance of the complex if the order of addition of reagents was varied. Composition
of Molybdenum(
V)- Thiocyanate-AT
Complex
The equilibrium shift method (3) indicates the formation of a 1:4:2 complex between molybdenum, thiocyanate, and AT. The apparent stability constant of the complex as evaluated by Bjerrum’s method (I) has the value log p = 7.0 at 27°C and Gibb’s free energy, - AG, of the system is 11 .O kcal mol-I. Effect of Diverse Ions The effect of diverse ions which often accompany
molybdenum
was studied
SPECTROPHOTOMETRY
OF MO(V)
285
with 3 ppm of molybdenum. An error of 2% in the absorbance readings was considered tolerable. The results presented in Table 2 show that molybdenum can be selectively determined in the presence of large excess of many cations and anions. Determination of Molybdenum
in Molybdenum Steels
The results in Table 1 show that the values obtained by the proposed method compare favorably with the certified values of molybdenum. ACKNOWLEDGMENTS The authors wish to express their sincere thanks to Ms. Lynette Williams for her assistance in preparing the manuscript. One of the authors, G. T. Chandrappa, gratefully acknowledges the University Grants Commission, New Delhi, for awarding a Junior Research Fellowship.
REFERENCES 1. Bjerrum, J.. “Metal Amine Formation in Aqueous Solutions,” p. 298. Hasse, 1941. 2. Khosla, M. M. L.. and Rao, S. P., Selective extraction of Molybdenum(V)-thiocyanate with NBenzylaniline. And. Chim. Acfrr 57, 323-329 (1971). 3. Puzanowska-Tarasiewicz, Grudniewska, A., and Tarasiewicz. M., An examination of chlorpromazine hydrochloride as indicator and spectrophotometric reagent for the determination of molybdenum (V). And. Chim. Acta 94, 435-441 (1977). 4. Sandell, E. B., “Calorimetric Determination of Traces of Metals.” Interscience, New York, 1959. 5. Savariar, C. P., Arunachalam, M. K., and Hariharan, T. R., Spectrophotometric determination of molybdenum (VI) with 2-mercaptobenzo-v-thiopyrone and ammonium thiocyanate. Anal. Chim. Acra 69, 305-31 I (1974). 6. Verdizade, N. A., and Melikov, S. R., Extraction-photometric determination of molybdenum in steels. Azerb. Khim. Z/I. 3, 130-131 (1975). 7. Vogel, A. I., “Quantitative Inorganic Analysis,” p. 506. The Elbs and Longmans, London, 1968.
32, 281-285 (1985)
JOURNAL
Extractive Spectrophotometric Determination Molybdenum(V) in Molybdenum Steels K. N. THIMMAIAH’ Depurtment
of Chemisrry. AND
Department
AND W. D. LLOYD
CJni~vrsiry of’ 7i,xtrs. El Puso, Texus 79968
G. T. CHANDRAPPA
of Post-Graduate Studies und Research in Chemistry, Mannsn Gangorri, Mysore-570006, Indiu Received
of
February
University
of Mysore,
22, 1985; accepted July 22, 1985
An extraction spectrophotometric method has been developed for the determination of traces of molybdenum present in molybdenum steels which is based on the extraction of the orange-red molybdenum-thiocyanate-acetonethiosemicarbazone complex into chloroform from hydrochloric acid medium. The complex has an absorption maximum at 472 nm with a molar absorptivity of 1.9 x IO4 liters mol-’ cm-‘. Beer’s law is valid over the concentration range 0.1-9.5 ppm of molybdenum with an optimum concentration range of 0.4-9 ppm. The equilibrium shift method indicates 1:4:2 composition for molybdenumthiocyanate-acetonethiosemicarbazone complex. The effect of acidity, reagent concentra% 1985 Academic tions, temperature, and interferences from Various ions are reported. Press, Inc.
INTRODUCTION
Most extensively used thiocyanate-tin(I1) chloride methods for the spectrophotometric determination of molybdenum have several drawbacks such as low sensitivity and lack of analytical reproducibility (2-6). In the present communication the authors have found that the sensitivity and stability of molybdenumthiocyanate complex could be increased by adding acetonethiosemicarbazone (AT) to form a new ternary complex which could be extracted into an organic solvent. The proposed method offers the advantages of sensitivity, selectivity, and stability. MATERIALS
AND METHODS
Solutions
AT was prepared from thiosemicarbazide and acetone and purified by the recommended method. A 2.5% solution of AT in acetone-water (3:2) was prepared. Molybdenum solution. A stock solution of molybdenum(W) was prepared from AR grade ammonium molybdate in doubly distilled water containing a few drops of ammonia and standardized gravimetrically using 8-hydroxyquinoline (7). Thiocyanate and ascorbic acid solutions. Stock solutions (10%) of ammonium ’ To whom correspondence
should be addressed. 281 0026-265X/85 $1.50 Copyright 0 1985 by Academic Press. Inc. All rightc of reproduction in any form reserved.
282
THIMMAIAH,
LLOYD,
AND
CHANDRAPPA
thiocyanate and ascorbic acid were prepared in doubly distilled water. All other reagents were analytical grade and were used without further purification. Apparatus
A Varian Cary Model 219 spectrophotometer for absorbance measurements.
with matched l-cm cells was used
Procedures Recommended procedure for the determination of molybdenum. An aliquot of the stock solution containing I-95 p.g of molybdenum, 6 ml of 10 M hydrochloric acid, 2 ml of 10% ascorbic acid, and 6 ml of 10% ammonium thiocyanate was transferred into a 50-ml separatory funnel and diluted to 25 ml with doubly distilled water. After 15 min at room temperature (27”(J), 8 ml of 2.5% AT solution was added, the solution was shaken well and extracted twice with 4-ml portions of chloroform. The chloroform extracts were dried with anhydrous sodium sulfate, diluted to 10 ml with chloroform, and the absorbance measured at 472 nm against a corresponding reagent blank prepared under similar conditions. The molybdenum content of the sample solution was determined from a standard calibration graph. Recommended procedure for the determination of molybdenum in molybdenum steels. An exact amount of molybdenum steel (approx 0.5 g) was transferred into
a 250-ml beaker and treated with I5 ml of 5 M sulfuric acid, 2 ml of phosphoric acid (sp. gr. = 1.75), and 1 ml of coned nitric acid. The solution was gently boiled to expel oxides of nitrogen, cooled, and then diluted to 50 ml with doubly distilled water. A small amount of 0.1 N potassium permanganate solution was added dropwise until the solution turned pale pink. After 5 min, 0.01 N oxalic acid was added dropwise with stirring until the pale pink of the solution disappeared and the solution was diluted to 100 ml. An aliquot of this solution was used for the determination of the molybdenum content following the recommended procedure. The results are given in Table 1. RESULTS
AND DISCUSSION
Molybdenum(V) formed by the reduction of molybdenum(W) with ascorbic acid combines with ammonium thiocyanate to form a red molybdenum(V)-thiocyanate complex in 0.8-3.2 M hydrochloric acid solution. On adding AT solution an orange-red ternary complex is formed in the same acid concentration. The ternary complex is extracted into chloroform while the binary molybdenum(V)thiocyanate complex is not. A double extraction is necessary to remove molybdenum completely from the aqueous phase. Effect
of Acid Concentration
on the Formation
of the Complex
The effect of concentration of acid on the formation and the extraction of the ternary complex into chloroform was investigated. The complex was not formed in acetic or perchloric acid medium. It was formed only in hydrochloric, sulfuric, phosphoric, or nitric acid medium but the absorbance reading of the chloroform extract from 0.5 to 5 M hydrochloric acid is most sensitive. The hydrochloric acid medium therefore was selected for further investigations.
SPECTROPHOTOMETRY Determination
Sample No. 1. Eo.300
TABLE 1 of Molybdenum in Molybdenum
Steels Molybdenum
General composition (%)
Certified
Found”
Mn-0.41, Ni-0.16, Cr-1.25, V-0.96, Cu-0.11, Ti-0.06, B-0.0046, C-2.0, Si-0.35, s-o.01 I
0.940
0.945
Mn-1.11, Ni-0.11, G-1.13, V-0.16, P-0.018, Si-0.59, C-l.01
0.035
0.036
0.81
0.82
0.12
0.13
Sample Sample No. 4. 20 Cr, MO. 95 V-85, Ti. B.
283
OF MO(V)
HSL,ASP, Durgapur H. No. 7-2034, 1SCDV,
Mn-0.88, Cr-1.32, V-0.19, Ni-0.37, Si-0.10, C-0.14, P-0.027, Al-0.032, S-O.010
HSL, ASP, Durgapur H. No.6-4828, 58CrV,
Mn-0.86, Cr-1.12, V-0.09, Si-0.29, C-0.62, Ni-0.14, Al-0.017, P-0.033, S-O.01 1
LIAverage of five determinations.
Role of Ascorbic Acid Ascorbic acid is a more suitable reducing agent for molybdenum(V1) than tin(H) chloride as it enhances the sensitivity of the reaction, gives reproducible values, and masks many interfering ions. A minimum concentration of 1 ml of 10% ascorbic acid is sufficient for the reduction of 2 ppm of molybdenum(V1) to molybdenum(V). More ascorbic acid than this has no effect on the absorbance readings. Effect of Ammonium
Thiocyanate
Concentration
The absorbance of the complex increases with increasing concentration of ammonium thiocyanate and AT. For maximum color development, 2.5-6 ml of 10% ammonium thiocyanate and 5-9 ml 2.5% AT were required. The optimum amounts of 6 ml of ammonium thiocyanate and 8 ml of AT were used for further studies. Selection
of Extracting
Solvent
Solvents such as cyclohexanone, benzene, carbon tetrachloride, toluene, isopropyl ether, butyl alcohol, and tetraline do not extract the complex while chloroform, n-butylacetate, amyl alcohol, and o-dichlorobenzene extract the complex. The molar absorptivity values for the complex in the solvents are: chloroform, 1.9 x 104; n-butyl acetate, 1.53 x 104; amyl alcohol, 1.4 x 103, and o-dichlorobenzene, 4.65 x lo2 liters mallr cm- ‘. Therefore, chloroform was selected for further studies. Absorption Spectra The binary complex in 1.5 it4 hydrochloric
acid has an absorption
maximum
at
284
THIMMAIAH,
LLOYD,
AND CHANDRAPPA
TABLE 2 Interference of Diverse Ions on the Determination Ion added Cu(I1) Co(H) Ni(I1) Mn(I1) Zn(I1) Fe(II1) Cr(II1) wm Ba(I1) Pb(I1) Cd(H) Bi(II1) Ti(IV) AI(II1) Pd(I1) Rh(II1) Pt(IV)
Tolerance limit (rvm) 400 102 2000 4000 4000 5000b 2000 500 8000 5000 650 loo 300 12 12.5 20
of Molybdenum” Tolerance limit (wm)
Ion added Ir(II1) Os(VII1) Rh(II1) VW) NW)
300 80 12 15 20 15 2000 8000 150
TaW
Chloride Bromide Iodide Citrate EDTA Phosphate Sulfate Nitrate Thiosulfate
0 Concentration of molybdenum(V1) = 3 ppm. b In the presence of 5 ml of 10% ascorbic acid.
440-450 nm. The chloroform extract of the ternary complex has an absorption maximum at 465-480 nm showing a bathochromic shift of 25 nm. The reagent blank shows little absorbance at these wavelengths. All subsequent studies were made at 472 nm. Calibration Graph and Sensitivity The complex obeys Beer’s law over the range 0.1-9.5 ppm of molybdenum. The optimum range is 0.4-9 ppm. The molar absorptivity is 1.9 x lo4 liters mol-’ cm-‘. The standard deviation calculated from 10 determinations on a solution containing 2 ppm of molybdenum(V) is 0.009 and the relative error is less than 2%. The absorbance readings of the chloroform extract of the ternary complex were stable for about 2 weeks in the temperature range 6-50°C. There was no appreciable change in the absorbance of the complex if the order of addition of reagents was varied. Composition
of Molybdenum(
V)- Thiocyanate-AT
Complex
The equilibrium shift method (3) indicates the formation of a 1:4:2 complex between molybdenum, thiocyanate, and AT. The apparent stability constant of the complex as evaluated by Bjerrum’s method (I) has the value log p = 7.0 at 27°C and Gibb’s free energy, - AG, of the system is 11 .O kcal mol-I. Effect of Diverse Ions The effect of diverse ions which often accompany
molybdenum
was studied
SPECTROPHOTOMETRY
OF MO(V)
285
with 3 ppm of molybdenum. An error of 2% in the absorbance readings was considered tolerable. The results presented in Table 2 show that molybdenum can be selectively determined in the presence of large excess of many cations and anions. Determination of Molybdenum
in Molybdenum Steels
The results in Table 1 show that the values obtained by the proposed method compare favorably with the certified values of molybdenum. ACKNOWLEDGMENTS The authors wish to express their sincere thanks to Ms. Lynette Williams for her assistance in preparing the manuscript. One of the authors, G. T. Chandrappa, gratefully acknowledges the University Grants Commission, New Delhi, for awarding a Junior Research Fellowship.
REFERENCES 1. Bjerrum, J.. “Metal Amine Formation in Aqueous Solutions,” p. 298. Hasse, 1941. 2. Khosla, M. M. L.. and Rao, S. P., Selective extraction of Molybdenum(V)-thiocyanate with NBenzylaniline. And. Chim. Acfrr 57, 323-329 (1971). 3. Puzanowska-Tarasiewicz, Grudniewska, A., and Tarasiewicz. M., An examination of chlorpromazine hydrochloride as indicator and spectrophotometric reagent for the determination of molybdenum (V). And. Chim. Acta 94, 435-441 (1977). 4. Sandell, E. B., “Calorimetric Determination of Traces of Metals.” Interscience, New York, 1959. 5. Savariar, C. P., Arunachalam, M. K., and Hariharan, T. R., Spectrophotometric determination of molybdenum (VI) with 2-mercaptobenzo-v-thiopyrone and ammonium thiocyanate. Anal. Chim. Acra 69, 305-31 I (1974). 6. Verdizade, N. A., and Melikov, S. R., Extraction-photometric determination of molybdenum in steels. Azerb. Khim. Z/I. 3, 130-131 (1975). 7. Vogel, A. I., “Quantitative Inorganic Analysis,” p. 506. The Elbs and Longmans, London, 1968.