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Extractive-spectrophotometric determination of molybdenum as an ion-association complex with thiocyanate and adogen
Talanro, Vol. 32, No. 1, pp. 63-65, 1985 Printed in Great Britain
0039-9140/85 $3.013+ 0.00 Pergamon Press Ltd
EXTRACTIVE-SPECTROPHOTOMETRIC DETERMINATION OF MOLYBDENUM AS AN ION-ASSOCIATION COMPLEX WITH THIOCYANATE AND ADOGEN F. SALINAS, J. J. BERZAS NEVAW and M. I. ACEDO VALENZUELA Department of Analytical Chemistry, Faculty of Sciences, University of Extremadura, Badajoz, Spain (Received 12 March 1984. Accepted 30 June 1984) Summary-A selective and sensitive method is described for the determination of trace amounts of molybdenum, based on its reaction with thiocyanate and its extraction (into toluene) as an ion-association complex formed with adogen (methyltrioctylammonium chloride). The molar absorptivity is 2.13 x 104l.mole-‘.cm-l at 1,, 467 nm. The method has been applied to molybdenum determination in steels.
iron(H) solution, and 0.5 ml of stannous chloride solution were added, followed by dilution to 50 ml with demineralized water. The mixture was then shaken with 10 ml of the toluene solution of adogen for 2 min. The toluene layer was collected 30 min later and dried over anhydrous sodium sulphate in a 50-ml beaker, and its absorbance was measured at 467 nm against adogen solution.
The use of thiocyanate for the determination of molybdenum has been widely studied and a number of methods have been reported. The fundamental differences between these methods are the reducing agents and solvents used, and in recent years the use of mixed-ligand complexes or ion-association complexes. Stannous chloride is the reducing agent most commonly used,‘” but ascorbic acid is recommended”‘3 because it gives a high absorbance for the complex and its concentration is not critical. It has also been found that extraction of the molybdenum-thiocyanate complex is complete even in the absence of a reducing agent if the reaction mixture is shaken for at least 30min.“~” However, the presence of a reducing agent is necessary to eliminate interference, such as that of Fe(II1). In this paper, we report a selective, sensitive and reproducible method for determination of molybdenum, based on reduction of Mo(V1) to MO(V) with stannous chloride [in presence of Fe(II)] and then reaction with thiocyanate in hydrochloric acid media, formation of the MO(V)-thiocyanate complex and its subsequent extraction into toluene with adogen.
Determination of MO in steel A 0. l-g sample was dissolved in 5-7 ml of 70% perchloric acid and the solution evaporated to dryness. The residue was heated with 5 ml of concentrated hydrochloric acid and evaporated almost to dryness, then the mass was cooled, dissolved in 15-20 ml of 2M hydrochloric acid, and diluted to 100 ml with demineralized water. When perchloric acid was not satisfactory as the solvent, 10 ml of aqua regia were added and the mixture was heated to fumes of oxides of nitrogen. After the solution was cooled, 3-5 ml of sulphuric acid were added, the solution was evaporated to dryness and the dried mass was cooled and dissolved in 15-20 ml of 2M hydrochloric acid and diluted to 100 ml with demineralized water. An aliquot (containing not more than 5 mg of Fe) was taken in a lOO-ml separatory funnel and molybdenum was then determined as described in the procedure above. When the MO content (in steel) is <0.1x, the standardaddition method is recommended. RESULTS AND DISCUSSION
Spectral characteristics EXPERIMENTAL
The absorption spectrum of the ion-association complex in toluene is shown in Fig. 1. Adogen has negligible absorption at the wavelength of maximum absorption. The absorption peak of the complex (at 467 nm) corresponds to an MO(V)-thiocyanate complex.*
Reagents Molybdenum(VI) solution. Prepared from sodium molybdate dihydrate and standardized gravimetrically. The working solution (MO 11.6 pg/ml) was prepared by appropriate dilution. Tin(U) solution. A 0.2% solution in 2M hydrochloric acid. Adogen solution. A 0.5 g/l. solution in toluene. Zron(ZZ)solution. Freshly prepared by dissolving 1 g of (NH,),Fe(S0,)2.6H,0 in 100 ml of water. Thiocyanate solution, 1M. All solvents and reagents were of analytical grade.
Influence of reducing agents Ascorbic acid and stannous chloride were tried as reducing agents. Ascorbic acid gives a high absorbance for the complex and the amount to be added is not critical when [SCN-] = 0.4M and [HCl] = 2M. Under these conditions the extraction (shaking for 2min) is complete even in the absence of ascorbic acid. Stannous chloride [in the absence of Fe(II)]
Procedure The sample solution, containing 2.5-50 pg of MO, was placed in a lOO-ml separatory funnel, then 6 ml of thiocyanate solution, 9.5 ml of 2M hydrochloric acid, 1 ml of 63
SHORT
64
06
COMMUNlCATlONS
The absorbance was practically constant for aqueous/organic phase-volume ratios from 5: 1 to 1: 1. A shaking time of 2 min was sufficient for complete extraction. For low concentrations of molybdenum (l-2 ppm), the absorbance was constant for at least 6 hr, but for higher concentration (5 ppm) it was constant only for 15-60 min. The order of addition of the reagents affects the absorbance (decreasing it), only when stannous chloride and molybdenum are mixed in the absence of thiocyanate.
-
Spectrophotometric characteristics
I
I 460
I
I
510
560
X(nm)
Fig. 1. Absorption spectrum of molybdenum-thiocyanateadogen complex in toluene. gives a lower absorbance and its concentration must be between 1 x 10m3 and 3.5 x 10w3M when [SCN-] = O.lM and [HCl] = 0.3M. However, if iron(I1) is present, stannous chloride gives a similar absorbance to that obtained with ascorbic acid. When stannous chloride is used, the absorbance can be measured against adogen solution, whereas when ascorbic acid is used, the absorbance must be measured against a reagent blank. Hence the stannous chloride/iron(II) system is preferred. The effect of the tin(I1) concentration is shown in Fig. 2. The presence of iron(I1) increases the extraction efficiency and makes the tin(I1) concentration range less critical. Effect of other variables
The extraction was found to be maximal at [HCl] 2 0.25M and 0.05-Q 15M thiocyanate (Fig. 3); the adogen must have a concentration of at least 0.03%. The presence of iron(I1) increased the extraction of molybdenum. The absorbance was maximal and constant at [Fe(II)] > 5 ppm. A similar effect was observed when copper(I1) was added, and the effect is not inhibited by the presence of EDTA or tartrate.
In the absence of iron( Beer’s law is obeyed in the range 5-50 pg of MO in the aqueous phase, and the molar absorptivity is 1.29 x lo4 l.molee’.cm-‘. The relative error (95% confidence level) for 23 pg of MO was + 1.5% (11 replicates). In presence of iron( Beer’s law is obeyed in the range 2.5-50 pg of MO in the aqueous phase, and the molar absorptivity is 2.13 x lo4 l.mole-‘.cm-‘. The optimum range on the basis of a Ringbom plot is l&40 pg of MO. The relative error (95% confidence level) for 23 pg of MO was f0.6%. Composition of the complex
Different stoichiometries for Mo-thiocyanate complexes are given in the literature. We used the JobI and Yoe and Jones methods” and found the most probable composition of the ion-association complex to be 1:4:3, Mo-SCN-adogen, so the species extracted into toluene is probably MoOz(SCN),(ADG), where ADG is the trioctylmethylammonium ion. This stoichiometry agrees with that reported for the molybdenum-thiocyanate-tetrabutylammonium system.16 Interferences
The effect of other ions on the determination of 23 pg of MO was examined. The tolerance limit was taken as the concentration that did not cause more than f 2% change in the absorbance. The results are summarized in Table 1.
100
z
.-.--.-
-
go-
4, SO
2
4
6
6
10
cSn (II)3 , 10m3M Fig. 2. Effect of Sn(I1) concentration on the extraction of 29 pg of molybdenum: (I) in absence of iron(B); (2) in presence of 700 pg of Fe(U).
1
.
$i
I
I
I
I
I
I
01
02
03
04
05
06
C Reagent I, M Fig. 3. Effect of thiocyanate and hydrochloric acid concentrations on the extraction of 35 pg of MO: (0-O) % E us. [SCN-] (in 0.15M HCI); (0-O) % E us. [HCI] (in 0.1544
SCN -).
SHORT
Table 1. Interference
65
COMMUNICATIONS
of foreign ions in the determination molybdenum
Amount tolerated, ppm
Ion added
>lOOO 150
sulphate, phosphate, EDTA, fluoride, Fe*+ NH:, K+, Na+, Li+, Mgr+, Ca*+ Sr*+, Ba*+ Fe)+ V(V), Cu*+, Bi3+, Pb2+, Cr3+, As(V), Sb(III), Zr(IV), oxalate A13+, Znr+, W(VI), U(VI), Mn*+, Co*+, Hg*+, tartrate Cd’+ Ni*+ Pd2+ ’ Table 2. Determination
BAS 64b
BCS 261/l BCS 432 BCS 219/4 BCS 406/l
100 50 25 5
of molybdenum in steels
% composition certified
MO certified, %
MO found, %
Cr(0.61), S(O.O3),C(2.24) Si(2.00), P(O.l l), Mn(0.64) Ni(2.24) C(O.90), Cr(4.55). V( 1.99) W(7.05)
0.40
Cr(17.4,). Ni(l3.1& C(O.09,) Si(O.SO),Mn(0.83), Nb(0.91) Cu(O.12) Cr(0.21), Ni(0.24), Mn(1.10) Cu(O.16) Cr(O.66), Ni(2.55), C(O.31,) Mn(0.81), Cu(O.088) Cr(2.10). Ni(1.52), C(0.23) Mn(0.61), Si(O.31), Cu(O.28)
0.11
0.41 0.39 0.39 4.96 4.91 4.94 0.10 0.11 0.12 0.05 0.05 0.60 0.62
Sample BAS 33b
of 23 pg of
4.95
0.039 0.58 1.oo
1.00
1.00 I .03
Table 3. Comparison of the proposed method with the existing thiocyanate methods Method s,
Sn*+, Fe*+ Sn*+, Cu*+, Fe’+ Tartaric acid Ascorbic acid Ascorbic acid Ascorbic acid Sn*+, Fe*+ (proposed method)
isoamyl alcohol MIBK ethyl methyl ketone aminopyridines/benzene diary1benzamidines/benzene adogen/toluene
Applications
The validity of the method was tested with six alloy steels. The results are shown in Table 2. The method compares favourably with existing thiocyanate methods for determination of molybdenum. Our method is the most sensitive, as shown in Table 3.
REFERENCES
1. D. D. Perrin, N.Z. J. Sci. Tech., 1946, 27A, 396. and C. E. Johnson, Anal. Chem., 1954, 26, 1284. 3. J. 0. Hibbits. W. F. Davis. M. R. Menke and S. Kallmann, Taianta, 1960, 4, 104. 4. J. 0. Hibbits and R. T. Williams, Anal. Chim. Acta, 2. C. E. Crouthamel
1962, 26, 363.
l.mole-‘.cm
Organic phase
Reducing agent
-’
Applications
Reference
not stated 1.04 x 10’
Soils Steels
1 4
8.0 x IO3 1.5-1.9 x 10’ 1.65-1.85 x 104 2.13 x IO4
Steels Steels, ores Steels, ores Steels
11 12 13 -
5. H. Matsuo and S. Chaki, Bunseki Kagaku, 1967, 16, 551. 6. A. A. Ponomareva, N. A. Agrinskaya and L. G. Anokhina, Novye Melody Khim. Anal. Mater., 1971,88. 7. S. P. Patil and V. M. Shinde, Anal. Chim. Acta, 1973, 67, 473. 8. A. K. Bhadra and S. Banerjee, Talanta, 1973, 20, 342. 9. A. I. Laxarev and V. I. Lazareva, ZavodFk. Lab., 1958,
24, 798. 10. I. Adamiec, Chem. Anal. Warsaw, 1966, 11, 1175. 11. U. Madan and L. R. Kakkar, Talunta, 1982, 29, 623. 12. K. S. Pate1 and R. K. Mishra, ibid., 1982, 29, 791. 13. K. S. Patel, R. M. Verma and R. K. Mishra, Anal. Chem., 1982, 54, 52. 14. P. Job, Ann. Chim. Paris, 1927, 9, 114. 15. J. H. Yoe and A. L. Jones, Ind. Eng. Chem., Anal. Ed., 1944, 16, 111. 16. Yu. G. Eremin, E. F. Kolpikova and T. V. Rodionova, Zh. Analit. Khim., 1976, 31, 732.
0039-9140/85 $3.013+ 0.00 Pergamon Press Ltd
EXTRACTIVE-SPECTROPHOTOMETRIC DETERMINATION OF MOLYBDENUM AS AN ION-ASSOCIATION COMPLEX WITH THIOCYANATE AND ADOGEN F. SALINAS, J. J. BERZAS NEVAW and M. I. ACEDO VALENZUELA Department of Analytical Chemistry, Faculty of Sciences, University of Extremadura, Badajoz, Spain (Received 12 March 1984. Accepted 30 June 1984) Summary-A selective and sensitive method is described for the determination of trace amounts of molybdenum, based on its reaction with thiocyanate and its extraction (into toluene) as an ion-association complex formed with adogen (methyltrioctylammonium chloride). The molar absorptivity is 2.13 x 104l.mole-‘.cm-l at 1,, 467 nm. The method has been applied to molybdenum determination in steels.
iron(H) solution, and 0.5 ml of stannous chloride solution were added, followed by dilution to 50 ml with demineralized water. The mixture was then shaken with 10 ml of the toluene solution of adogen for 2 min. The toluene layer was collected 30 min later and dried over anhydrous sodium sulphate in a 50-ml beaker, and its absorbance was measured at 467 nm against adogen solution.
The use of thiocyanate for the determination of molybdenum has been widely studied and a number of methods have been reported. The fundamental differences between these methods are the reducing agents and solvents used, and in recent years the use of mixed-ligand complexes or ion-association complexes. Stannous chloride is the reducing agent most commonly used,‘” but ascorbic acid is recommended”‘3 because it gives a high absorbance for the complex and its concentration is not critical. It has also been found that extraction of the molybdenum-thiocyanate complex is complete even in the absence of a reducing agent if the reaction mixture is shaken for at least 30min.“~” However, the presence of a reducing agent is necessary to eliminate interference, such as that of Fe(II1). In this paper, we report a selective, sensitive and reproducible method for determination of molybdenum, based on reduction of Mo(V1) to MO(V) with stannous chloride [in presence of Fe(II)] and then reaction with thiocyanate in hydrochloric acid media, formation of the MO(V)-thiocyanate complex and its subsequent extraction into toluene with adogen.
Determination of MO in steel A 0. l-g sample was dissolved in 5-7 ml of 70% perchloric acid and the solution evaporated to dryness. The residue was heated with 5 ml of concentrated hydrochloric acid and evaporated almost to dryness, then the mass was cooled, dissolved in 15-20 ml of 2M hydrochloric acid, and diluted to 100 ml with demineralized water. When perchloric acid was not satisfactory as the solvent, 10 ml of aqua regia were added and the mixture was heated to fumes of oxides of nitrogen. After the solution was cooled, 3-5 ml of sulphuric acid were added, the solution was evaporated to dryness and the dried mass was cooled and dissolved in 15-20 ml of 2M hydrochloric acid and diluted to 100 ml with demineralized water. An aliquot (containing not more than 5 mg of Fe) was taken in a lOO-ml separatory funnel and molybdenum was then determined as described in the procedure above. When the MO content (in steel) is <0.1x, the standardaddition method is recommended. RESULTS AND DISCUSSION
Spectral characteristics EXPERIMENTAL
The absorption spectrum of the ion-association complex in toluene is shown in Fig. 1. Adogen has negligible absorption at the wavelength of maximum absorption. The absorption peak of the complex (at 467 nm) corresponds to an MO(V)-thiocyanate complex.*
Reagents Molybdenum(VI) solution. Prepared from sodium molybdate dihydrate and standardized gravimetrically. The working solution (MO 11.6 pg/ml) was prepared by appropriate dilution. Tin(U) solution. A 0.2% solution in 2M hydrochloric acid. Adogen solution. A 0.5 g/l. solution in toluene. Zron(ZZ)solution. Freshly prepared by dissolving 1 g of (NH,),Fe(S0,)2.6H,0 in 100 ml of water. Thiocyanate solution, 1M. All solvents and reagents were of analytical grade.
Influence of reducing agents Ascorbic acid and stannous chloride were tried as reducing agents. Ascorbic acid gives a high absorbance for the complex and the amount to be added is not critical when [SCN-] = 0.4M and [HCl] = 2M. Under these conditions the extraction (shaking for 2min) is complete even in the absence of ascorbic acid. Stannous chloride [in the absence of Fe(II)]
Procedure The sample solution, containing 2.5-50 pg of MO, was placed in a lOO-ml separatory funnel, then 6 ml of thiocyanate solution, 9.5 ml of 2M hydrochloric acid, 1 ml of 63
SHORT
64
06
COMMUNlCATlONS
The absorbance was practically constant for aqueous/organic phase-volume ratios from 5: 1 to 1: 1. A shaking time of 2 min was sufficient for complete extraction. For low concentrations of molybdenum (l-2 ppm), the absorbance was constant for at least 6 hr, but for higher concentration (5 ppm) it was constant only for 15-60 min. The order of addition of the reagents affects the absorbance (decreasing it), only when stannous chloride and molybdenum are mixed in the absence of thiocyanate.
-
Spectrophotometric characteristics
I
I 460
I
I
510
560
X(nm)
Fig. 1. Absorption spectrum of molybdenum-thiocyanateadogen complex in toluene. gives a lower absorbance and its concentration must be between 1 x 10m3 and 3.5 x 10w3M when [SCN-] = O.lM and [HCl] = 0.3M. However, if iron(I1) is present, stannous chloride gives a similar absorbance to that obtained with ascorbic acid. When stannous chloride is used, the absorbance can be measured against adogen solution, whereas when ascorbic acid is used, the absorbance must be measured against a reagent blank. Hence the stannous chloride/iron(II) system is preferred. The effect of the tin(I1) concentration is shown in Fig. 2. The presence of iron(I1) increases the extraction efficiency and makes the tin(I1) concentration range less critical. Effect of other variables
The extraction was found to be maximal at [HCl] 2 0.25M and 0.05-Q 15M thiocyanate (Fig. 3); the adogen must have a concentration of at least 0.03%. The presence of iron(I1) increased the extraction of molybdenum. The absorbance was maximal and constant at [Fe(II)] > 5 ppm. A similar effect was observed when copper(I1) was added, and the effect is not inhibited by the presence of EDTA or tartrate.
In the absence of iron( Beer’s law is obeyed in the range 5-50 pg of MO in the aqueous phase, and the molar absorptivity is 1.29 x lo4 l.molee’.cm-‘. The relative error (95% confidence level) for 23 pg of MO was + 1.5% (11 replicates). In presence of iron( Beer’s law is obeyed in the range 2.5-50 pg of MO in the aqueous phase, and the molar absorptivity is 2.13 x lo4 l.mole-‘.cm-‘. The optimum range on the basis of a Ringbom plot is l&40 pg of MO. The relative error (95% confidence level) for 23 pg of MO was f0.6%. Composition of the complex
Different stoichiometries for Mo-thiocyanate complexes are given in the literature. We used the JobI and Yoe and Jones methods” and found the most probable composition of the ion-association complex to be 1:4:3, Mo-SCN-adogen, so the species extracted into toluene is probably MoOz(SCN),(ADG), where ADG is the trioctylmethylammonium ion. This stoichiometry agrees with that reported for the molybdenum-thiocyanate-tetrabutylammonium system.16 Interferences
The effect of other ions on the determination of 23 pg of MO was examined. The tolerance limit was taken as the concentration that did not cause more than f 2% change in the absorbance. The results are summarized in Table 1.
100
z
.-.--.-
-
go-
4, SO
2
4
6
6
10
cSn (II)3 , 10m3M Fig. 2. Effect of Sn(I1) concentration on the extraction of 29 pg of molybdenum: (I) in absence of iron(B); (2) in presence of 700 pg of Fe(U).
1
.
$i
I
I
I
I
I
I
01
02
03
04
05
06
C Reagent I, M Fig. 3. Effect of thiocyanate and hydrochloric acid concentrations on the extraction of 35 pg of MO: (0-O) % E us. [SCN-] (in 0.15M HCI); (0-O) % E us. [HCI] (in 0.1544
SCN -).
SHORT
Table 1. Interference
65
COMMUNICATIONS
of foreign ions in the determination molybdenum
Amount tolerated, ppm
Ion added
>lOOO 150
sulphate, phosphate, EDTA, fluoride, Fe*+ NH:, K+, Na+, Li+, Mgr+, Ca*+ Sr*+, Ba*+ Fe)+ V(V), Cu*+, Bi3+, Pb2+, Cr3+, As(V), Sb(III), Zr(IV), oxalate A13+, Znr+, W(VI), U(VI), Mn*+, Co*+, Hg*+, tartrate Cd’+ Ni*+ Pd2+ ’ Table 2. Determination
BAS 64b
BCS 261/l BCS 432 BCS 219/4 BCS 406/l
100 50 25 5
of molybdenum in steels
% composition certified
MO certified, %
MO found, %
Cr(0.61), S(O.O3),C(2.24) Si(2.00), P(O.l l), Mn(0.64) Ni(2.24) C(O.90), Cr(4.55). V( 1.99) W(7.05)
0.40
Cr(17.4,). Ni(l3.1& C(O.09,) Si(O.SO),Mn(0.83), Nb(0.91) Cu(O.12) Cr(0.21), Ni(0.24), Mn(1.10) Cu(O.16) Cr(O.66), Ni(2.55), C(O.31,) Mn(0.81), Cu(O.088) Cr(2.10). Ni(1.52), C(0.23) Mn(0.61), Si(O.31), Cu(O.28)
0.11
0.41 0.39 0.39 4.96 4.91 4.94 0.10 0.11 0.12 0.05 0.05 0.60 0.62
Sample BAS 33b
of 23 pg of
4.95
0.039 0.58 1.oo
1.00
1.00 I .03
Table 3. Comparison of the proposed method with the existing thiocyanate methods Method s,
Sn*+, Fe*+ Sn*+, Cu*+, Fe’+ Tartaric acid Ascorbic acid Ascorbic acid Ascorbic acid Sn*+, Fe*+ (proposed method)
isoamyl alcohol MIBK ethyl methyl ketone aminopyridines/benzene diary1benzamidines/benzene adogen/toluene
Applications
The validity of the method was tested with six alloy steels. The results are shown in Table 2. The method compares favourably with existing thiocyanate methods for determination of molybdenum. Our method is the most sensitive, as shown in Table 3.
REFERENCES
1. D. D. Perrin, N.Z. J. Sci. Tech., 1946, 27A, 396. and C. E. Johnson, Anal. Chem., 1954, 26, 1284. 3. J. 0. Hibbits. W. F. Davis. M. R. Menke and S. Kallmann, Taianta, 1960, 4, 104. 4. J. 0. Hibbits and R. T. Williams, Anal. Chim. Acta, 2. C. E. Crouthamel
1962, 26, 363.
l.mole-‘.cm
Organic phase
Reducing agent
-’
Applications
Reference
not stated 1.04 x 10’
Soils Steels
1 4
8.0 x IO3 1.5-1.9 x 10’ 1.65-1.85 x 104 2.13 x IO4
Steels Steels, ores Steels, ores Steels
11 12 13 -
5. H. Matsuo and S. Chaki, Bunseki Kagaku, 1967, 16, 551. 6. A. A. Ponomareva, N. A. Agrinskaya and L. G. Anokhina, Novye Melody Khim. Anal. Mater., 1971,88. 7. S. P. Patil and V. M. Shinde, Anal. Chim. Acta, 1973, 67, 473. 8. A. K. Bhadra and S. Banerjee, Talanta, 1973, 20, 342. 9. A. I. Laxarev and V. I. Lazareva, ZavodFk. Lab., 1958,
24, 798. 10. I. Adamiec, Chem. Anal. Warsaw, 1966, 11, 1175. 11. U. Madan and L. R. Kakkar, Talunta, 1982, 29, 623. 12. K. S. Pate1 and R. K. Mishra, ibid., 1982, 29, 791. 13. K. S. Patel, R. M. Verma and R. K. Mishra, Anal. Chem., 1982, 54, 52. 14. P. Job, Ann. Chim. Paris, 1927, 9, 114. 15. J. H. Yoe and A. L. Jones, Ind. Eng. Chem., Anal. Ed., 1944, 16, 111. 16. Yu. G. Eremin, E. F. Kolpikova and T. V. Rodionova, Zh. Analit. Khim., 1976, 31, 732.