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Extraction and spectrophotometric determination of niobium by tetraphenylarsonium and phosphonium chloride from a hydrochloric acid solution
MICROCHEMICAL
JOURNAL
30, 178-185 (1984)
Extraction and Spectrophotometric Niobium by Tetraphenylarsonium Chloride from a Hydrochloric B. TAMHINA Laboratory
Determination of and Phosphonium Acid Solution
AND A. GOJMERAC IvSIC
of Analytical
Chemistry, I;aculty of Science, University Strossmayerov trg 14, 41000 Zagreb, Yugmlnviu
qf Zagreb,
Received July 20, 1982
INTRODUCTION In a hydrochloric acid solution niobium(V) forms chloro complexes whose composition depends on hydrogen and chloride ion concentration. They can be in the molecular, anionic, or cationic form. Kanzelmeyer and Freund (3, 4) reported that only three soluble chloride containing species of niobium exist to any appreciable extent in the systems studied: [Nb(OH),ClJ at both high chloride and high hydrogen ion concentration, [NbOHCl,] + at extremely high hydrogen ion concentration and low chloride ion concentration, and Nb(OH),Cl, at hydrogen ion and chloride ion concentrations both in the range of about 3 mol dm 3. The lowering of hydrogen ion concentration below about 2 mol dme3 at higher chloride ion concentration causes a colloid formation probably of the type Nb(OH),Cl,. Lapenko and Gibalo (6) studied the solvent extraction of chloride, oxalate, tartrate, and citrate complexes of niobium(V) and some other elements with tetraphenylarsonium chloride solution in dichloroethane. These authors found that niobium(V) can be extracted only at high hydrochloric acid concentration (X.5-9.5 M HCl) when niobium is present in the anionic form (NbOCl,-). Nabivanec (7) reported that in HCl < 5 M the cationic form of niobium dominated, in HCl > 5 M the anionic form was predominant and in 8 M HCl all of niobium was in the anionic form. The same author also reported that for the formation of the niobium anionic chloride complexes it was necessary to increase H+ and Cl- concentrations simultaneously, whereas for the formation of the anionic sulfate complexes it was sufficient to increase the concentration of SOJ2- at constant H+ concentration. In the present paper on extractionspectrophotometric method for the determination of niobium is described. It is based on the extraction of niobium into chloroform as an ion-associated complex, formed between the niobium(V) chloride anion 178 0026-265X/84 $1.50 Copyright 0 1984 by Academic Press. Inc. All rights of reproduction m any form reserved.
EXTRACTION
OF NIOBIUM
179
and tetraphenylarsonium (TPA) or tetraphenylphosphonium (TPP) cation from a hydrochloric solution with high chloride ion concentration in a wide an relatively low region of hydrogen ion concentration. The composition of the extracted niobium species-[(C,H,),As] [NbOCl,] and [(C,H,),P] [NbOCl,]-was determined spectrophotometrically, radiometrically, and by characterization of the crystalline compounds isolated. MATERIALS
AND METHODS
Reagents A standard solution of niobium (about 2 x 10-z M> was prepared in 10 M hydrochloric acid. Nb,O, was fused with KHSO, in a platinum crucible. The melt was extracted with a hot 10% oxalic acid solution. The niobium was precipitated with ammonia, centrifuged, and washed three times with a 2% ammonium chloride solution and once with distilled water. The freshly precipitated niobium hydroxide was then dissolved in 10 M HCl. The solution was filtered and standardized by precipitating niobium with tannin (5). The solutions of lower concentrations were prepared by diluting the standard solution with 10 M HCl. 95Nb was obtained from the Radiochemical Centre, Amersham, in the form of oxalato complex in a 0.5% solution of oxalic acid. 95Nb in the chloride form was prepared by dissolving freshly precipitated 95Nb hydroxyde in 10 M HCI. Tetraphenylarsonium and tetraphenylphosphonium chloride (analytical grade, Fluka) were dissolved in reagent-grade chloroform. All chemicals used were of analytical purity. Apparatus Absorption spectra and absorbance measurements of solutions were made on a Perkin-Elmer Hitachi-200 spectrophotometer. Infrared spectra of the isolated complexes were recorded on a Perkin-Elmer spectrophotometer Model 167. Radioactivity measurements (95Nb) were performed with a well-type gamma scintillation counter (NaI/Tl) from Ecco Electronic. For extraction a Griffin flask shaker with a time switch was used. Determination of the Distribution Ratio The distribution of niobium was determined at ambient temperature (about 22°C) by shaking an equal volume (5 ml) of the organic and aqueous phase of a given composition in a stoppered flask in a mechanical shaker for 20 min. The time necessary to attain equilibrium was experimentally established as 5 min. The phases were separated, and an aliquot
180
TAMHINA
AND
IVsI6
(1 ml) of each phase was taken for radiochemical analysis. Distribution ratios were calculated from counts/100 set of both phases. Spectrophotometric
Determination
of Niobium
in Chloroform
A solution containing lo- 100 pg niobium(V) in 10 M HCl (1 ml), 3 ml of 10 M HCl, and 6 ml of 10 M LiCl was placed in an Erlenmeyer flask. After the addition of 10 ml of 6 x lop3 M TPA or TPP in chloroform the solution was shaken for 20 min with a mechanical shaker. After the phases were separated, the absorbance of the organic phase was measured at 282 nm (TPA) and 285 nm (TPP) against a reagent blank. Preparation
and Analysis
of Solid Complexes
Solid complexes of niobium were prepared by shaking chloroform solutions of TPA and TPP (6 x lop3 M) with the aqueous solution of niobium in which HCl and LiCl concentrations were 4 and 6 M, respectively. Niobium was in excess to extractant. (Molar niobium to TPA (TPP) ratio was 1.5.) White crystals were precipitated immediately on the bounds of the phases. The crystals were dried in vacua over calcium chloride and analyzed. The analytical data were consistent with the formula [(C,H,),fl [NbOCl,], where X = As, P. C,,H,,OCl,AsNb (Calcd: C, 45.46; H, 3.18; 0, 2.52; Cl, 22.37; As, 11.81; Nb, 14.65%. Found: Cl, 22.71; As, 11.85; Nb, 14.55%). C,,H,,OCl,PNb (Calcd: C, 48.84; H, 3.42; 0, 2.71; Cl, 24.03; P, 5.25; Nb, 15.74%. Found: Cl, 24.25; P, 4.91; Nb, 15.67%). Carbon and hydrogen were analyzed by the standard microanalytical procedure. The results obtained were a little lower than the theoretical values because of the experimental error probably due to the presence of phosphorus or arsenic. Arsenic and chloride were determined by a final EDTA titration after the substance had been destroyed by the flask combustion method (9, 1). Phosphorus was determined by the molybdate method (8) after decomposition of the substance with concentrated sulfuric and nitric acids. Niobium was determined by the tannin method (5) after decomposition of the substance with concentrated sulfuric and nitric acids. RESULTS AND DISCUSSION Optimum Conditions for Extraction
Niobium can be extracted as a niobium(V) chloride complex from aqueous hydrochloric solutions containing an excess of chloride ions by TPA and TPP in chloroform. The solution of the complex in chloroform has a maximum absorbance at 282 nm (TPA) and 285 nm (TPP). Radiometrical and spectrophotometrical studies (Fig. 1) show that niobium(V)
EXTRACTION
181
OF NIOBIUM
0.6 1
1,
/
2
I,
L cot-c Cl-
I
1,
1
6
6
10
(mot dmb3)
b)
hi-L-
350
300 Wavelength
(nm 1
FIG. 1. (a) The dependence of percentage extraction of niobium(V) on the concentration of chloride ions. Nb(V) 5 x 10e5M, H+ 4 M, TPA 6 x lo-) M. (b) The dependence of the absorption spectra of niobium complex on chloride ion concentration. Nb(V) 5 x 10m5M, H+ 4 M, TPA 6 x 10e3M. Concentration of chloride ions (1) 4 M; (2) 5 M; (3) 6 M; (4) 7 M; (5) 7.5 M; (6) 8 M; (7) 9 M; (8) IO M.
is extracted practically completely (about 95%) from a solution containing more than 9 mol dmM3 chloride and that niobium is not extracted by chloroform solutions of TPA or TPP at chloride concentrations less than 6 mol dmp3. The effect of the hydrogen ion concentration at the constant and optimum concentration of chloride ions on the extraction of niobium was also studied (Fig. 2). The optimum hydrogen ion concentration for the extraction of niobium with TPA is 2-5 M H’ and with TPP l-5 M H+. By increasing hydrogen ion concentration niobium extraction very slowly decreases. If chloride ions are not present at a concentration higher than 9 mol dmp3 a complete extraction of niobium cannot be achieved regardless of the hydrogen ion concentration (Fig. 2b, curve 2). The maximum extraction and constant absorbance of the organic phase was achieved with a 60-fold excess of TPA or TPP as shown in Fig. 3. The extracted niobium can be quantitatively stripped with a 10% (w/v) solution of oxalic acid. Moreover, it was established that none of the investigated anions (citrate, acetate, tartrate, phosphate, cyanide, perchlorate, fluoride, oxalate, nitrate, and sulfate) forms with niobium a complex extractable by TPA or TPP in chloroform.
182
TAMHINA
cont.
AND IV$GIC
H* (mol dmh3)
cont.
H* lmol dm-3)
FIG. 2. (a) The dependence of percentage of extraction of niobium on hydrogen ion concentration. Nb(V) 5 x 1O-5M, Cl- 10 M, R 6 x 10m3M, (0) R = TPA, (0) R = TPP. (b) The dependence of absorbance on hydrogen ion concentration. Nb(V) 5 x 10m5M, TPA 6 x 1O-3M, curve l-(O) Cl- 10 M, curve 2-(O) Cl- 8 M.
Spectrophotometric Determination of Niobium in Organic Phase At optimum conditions for extraction: 10 M chloride concentration and 4 M hydrogen ion concentration (6 M LiCl and 4 M HCl) in the aqueous phase and 6 x lop3 M extractant concentration in the organic phase niobium can be determined spectrophotometrically in the organic phase. The calibration graph is linear all over the range suitable for spectrophotometric measurements, corresponding to 1- 10 kg/ml of niobium in the measured solution. The effective molar abosrptivity of the chloroform extract at 282 (TPA) or 285 nm (TPP) is (1.33 k 0.03) x lo4 dm3 mol- 1
-2
L
6
6
[TPA] ~10~ lmol dme3)
log
CO~C.
[TPA]
FIG. 3. (a) The dependence of percentage of extraction of niobium (curve 1) and absorbance (curve 2) on TPA concentration. Nb(V) 5 x 1O-5M, Cl- 10 M, H+ 4 M. (b) The dependence of the distribution ratio of niobium(V) on the TPA concentration. Nb(V) 5 x 1O-5M, Cl- 10 M, (0) H+ 8 M, (cl) H+ 10 M.
EXTRACTION
183
OF NIOBIUM
TABLE 1 THE EFFECTOF FOREIGNIONSON THE DETERMINATIONOF NIOBILJMBY THE RECOMMENDED PROCEDURE Foreign ions Acetate, sulfate, citrate, cyanide. tartrate, ammonium, Na, K, Mg, Ca, Cd, Ni, Ba, Sr. Zn, Cu, Pb, Cr(III), Mn(I1)
Concentration tolerated (Molar ratio)
IOOO-fold
Phosphate, nitrate, Hg(II), Zr(IV), Ti(IV), Th(lV)
IOO-fold
Perchlorate, fluoride, oxalate, Sn(lI), W(V1). U(VI), Mo(V1). Eu(III), Pd(I1) Cu(II), V(V), Fe(iI1)”
I O-fold
” Ascorbic acid (0.15 M) present.
cm-‘. The sensitivity expressed by Sandell’s index (for A = 0,010) is 0.074 pg/cm2. The extracted complexes are stable for at least 24 hr. The reproducibility of the results expressed as a relative standard deviation is l-5% depending on the niobium concentration. The determination of niobium by the proposed extraction spectrophotometric procedure is feasible in the presence of many foreign ions (Table 1). Composition of Extracted Complexes The ratio of niobium to TPA or TPP was determined spectrophotometrically and radiometrically. The results obtained by Job’s method show that the molar ratio of niobium to onium ion is I:1 (Fig. 4). The slopes of the curves, obtained by the radiometric studies for the dependence of the log D on the log concentration tetraphenyl onium ion also indicate the equimolar ratio of the niobium to onium ion (Fig. 3b). Since the extraction mechanism can be different for high and low niobium concentrations, we determined the dependence of the niobium extraction on its initial concentration (Fig. 5). By plotting the molar ratio of TPA (TPP) and niobium in the organic phase against the initial niobium concentration in the aqueous phase, we found that one molecule of the extractant is bonded to one niobium atom. These results and the fact that the maximum absorbance of the chloroform extract does not shift with the changing niobium concentration, and does not depend on the concentration ratio of reactants implies that only one niobium species is extracted. These investigations show that the anionic niobium(V)-chloride complex, bearing one negative charge with one onium ion forms an ion-associated complex which is transferred into the organic phase. It was not possible to determine the niobium to chloride ratio but chemical analysis
184
TAMHINA
0.2
0.4
0.6 0.6
AND IVSIC
0.2
1.0
0.6 0.8
0.4
1.0
[Nbl
[Nbl [Nb]. [TPA]
[Nb]
l
[TPP]
FIG. 4. Determination of complex composition by Job’s method. H+ 4 M, Cl- 10 M, (0) INb] + [TPA] = 5 x 1O-4 M, (0) [Nb] + [TPA] = 3 x 1O-4 M, (El) [Nb] + [TPP] = 5 x 10-4M, (W) [Nb] + [TPP] = 3 x 10-4M.
and strong bands at 930 and 946 cm-‘, generally observed for niobyl group stretching (2), in ir spectra of isolated complexes show that the extracted complexes are [(C,H,),As] [NbOCl,] and [(C,H,),P] [NbOCl,]. The absorption spectra of the isolated complexes dissolved again in chloroform were identical with the spectra of the original chloroform extract, showing that no decomposition occurred during isolation.
/ 5
===@I----~ I-I/ [Nb] ap x ‘d:
•~-.-~-20
(mol dmb3j l5
FIG. 5. Determination of TPA (TPP): Nb ratio as a function of the initial niobium (V) concentration in the aqueous phase. R 4 x 10m4M, H+ 4 M, Cl- 10 M, the Nb(V) concentration is varied. (0) R = TPA, (0) R = TPP.
EXTRACTION
OF NIOBIUM
185
Therefore, the mechanism of niobium extraction can be presented as [(C,H,),XI to, +
WbOCL,l(aq)*
KGJ3,)4XI
[N’3OC141,0,
where X = As or P. SUMMARY The extraction of niobium(V) in the form of a chloro complex has been studied. Radiametrical and spectrophotometrical studies show that niobium(V) is extracted practically completely from a solution containing more than 9 mol dmm3chloride in the range of 2-5 M hydrogen ion concentration by chloroform solutions of tetraphenylarsonium (TPA) and tetraphenylphosphonium (TPP) chloride and that niobium is not extracted at chloride concentrations less than 6 mol dme3. The mechanism of extraction is based on the formation of the ion-associated compounds that form between the onium cation and the oxotetrachloroniobate(V) anion. The extracted complexes in chloroform have a maximum absorbance at 282 nm (TPA) and 285 nm (TPP); they obey Beer’s law in the range of l-10 +g Nb ml-t, and are stable for at least 24 hr. The molar absorptivity of the method is 1.33 x lo4 dm3 mol-t cm-i. The composition of the extracted species [(C,H,),Xl [NbOCl,] where X = As or P was determined spectrophotometrically, radiometrically, and by characterization of the crystalline compounds isolated.
REFERENCES 1. Flaschka, H., Mikrotitrationen
2. 3. 4. 5. 6.
7. 8. 9.
mit Athylendiamintetraessigsaure. V. Die indirekte Bestimmung von Silber und Halogenen. Mikrochem. 40, 21-26 (1953). Haritonov, Ju. Ja., and Buslaev, Ju. A., Infrakrasnie spektri poglosenija oksoftoridov nekatorih metalov. Izv. Akad. Nauk SSSR, Ser. Khim. 5, 808-814 (1964). Kanzelmeyer, J. H., and Freund, H., Ultraviolet spectrophotometric determination of niobium in hydrochloric acid. Anal. Chem. 25, 1807-1809 (1953). Kanzelmeyer, J. H., Ryan, J., and Freund, H., The Nature of Niobium(V) in Hydrochloric Acid Solution. J. Amer. Chem. Sot. 78, 3020-3023 (1956). Kolthoff, I. M., and Elving, P. J., “Treatise on Analytical Chemistry,” Part 2, Vol. 6, p. 284. Interscience, New York, 1964. Lapenko, L. A., and Gibalo, I. M., Solvent Extraction of niobium, tantalum and some other elements with tetraphenylarsonium chloride. Zh. Anal. Khim. 31, 481-484 (1976). Nabivanec, B. I., Sastojanie niobija(V) v rastvomah asotnoi, soljnoi i semoi kislot. Zh. Neorg. Kern. 9, 1079-1084 (1964). Pregl, F., and Grant, J., “Quantitative Organic Micro Analysis,” p. 141. Churchill, London, 1951. Stefanac, Z., Chelometrische Arsenbestimmung und ihre Anwendung auf die MikroElementaranalyse. Mikrochim. Acta 1115-l 120 (1962).
JOURNAL
30, 178-185 (1984)
Extraction and Spectrophotometric Niobium by Tetraphenylarsonium Chloride from a Hydrochloric B. TAMHINA Laboratory
Determination of and Phosphonium Acid Solution
AND A. GOJMERAC IvSIC
of Analytical
Chemistry, I;aculty of Science, University Strossmayerov trg 14, 41000 Zagreb, Yugmlnviu
qf Zagreb,
Received July 20, 1982
INTRODUCTION In a hydrochloric acid solution niobium(V) forms chloro complexes whose composition depends on hydrogen and chloride ion concentration. They can be in the molecular, anionic, or cationic form. Kanzelmeyer and Freund (3, 4) reported that only three soluble chloride containing species of niobium exist to any appreciable extent in the systems studied: [Nb(OH),ClJ at both high chloride and high hydrogen ion concentration, [NbOHCl,] + at extremely high hydrogen ion concentration and low chloride ion concentration, and Nb(OH),Cl, at hydrogen ion and chloride ion concentrations both in the range of about 3 mol dm 3. The lowering of hydrogen ion concentration below about 2 mol dme3 at higher chloride ion concentration causes a colloid formation probably of the type Nb(OH),Cl,. Lapenko and Gibalo (6) studied the solvent extraction of chloride, oxalate, tartrate, and citrate complexes of niobium(V) and some other elements with tetraphenylarsonium chloride solution in dichloroethane. These authors found that niobium(V) can be extracted only at high hydrochloric acid concentration (X.5-9.5 M HCl) when niobium is present in the anionic form (NbOCl,-). Nabivanec (7) reported that in HCl < 5 M the cationic form of niobium dominated, in HCl > 5 M the anionic form was predominant and in 8 M HCl all of niobium was in the anionic form. The same author also reported that for the formation of the niobium anionic chloride complexes it was necessary to increase H+ and Cl- concentrations simultaneously, whereas for the formation of the anionic sulfate complexes it was sufficient to increase the concentration of SOJ2- at constant H+ concentration. In the present paper on extractionspectrophotometric method for the determination of niobium is described. It is based on the extraction of niobium into chloroform as an ion-associated complex, formed between the niobium(V) chloride anion 178 0026-265X/84 $1.50 Copyright 0 1984 by Academic Press. Inc. All rights of reproduction m any form reserved.
EXTRACTION
OF NIOBIUM
179
and tetraphenylarsonium (TPA) or tetraphenylphosphonium (TPP) cation from a hydrochloric solution with high chloride ion concentration in a wide an relatively low region of hydrogen ion concentration. The composition of the extracted niobium species-[(C,H,),As] [NbOCl,] and [(C,H,),P] [NbOCl,]-was determined spectrophotometrically, radiometrically, and by characterization of the crystalline compounds isolated. MATERIALS
AND METHODS
Reagents A standard solution of niobium (about 2 x 10-z M> was prepared in 10 M hydrochloric acid. Nb,O, was fused with KHSO, in a platinum crucible. The melt was extracted with a hot 10% oxalic acid solution. The niobium was precipitated with ammonia, centrifuged, and washed three times with a 2% ammonium chloride solution and once with distilled water. The freshly precipitated niobium hydroxide was then dissolved in 10 M HCl. The solution was filtered and standardized by precipitating niobium with tannin (5). The solutions of lower concentrations were prepared by diluting the standard solution with 10 M HCl. 95Nb was obtained from the Radiochemical Centre, Amersham, in the form of oxalato complex in a 0.5% solution of oxalic acid. 95Nb in the chloride form was prepared by dissolving freshly precipitated 95Nb hydroxyde in 10 M HCI. Tetraphenylarsonium and tetraphenylphosphonium chloride (analytical grade, Fluka) were dissolved in reagent-grade chloroform. All chemicals used were of analytical purity. Apparatus Absorption spectra and absorbance measurements of solutions were made on a Perkin-Elmer Hitachi-200 spectrophotometer. Infrared spectra of the isolated complexes were recorded on a Perkin-Elmer spectrophotometer Model 167. Radioactivity measurements (95Nb) were performed with a well-type gamma scintillation counter (NaI/Tl) from Ecco Electronic. For extraction a Griffin flask shaker with a time switch was used. Determination of the Distribution Ratio The distribution of niobium was determined at ambient temperature (about 22°C) by shaking an equal volume (5 ml) of the organic and aqueous phase of a given composition in a stoppered flask in a mechanical shaker for 20 min. The time necessary to attain equilibrium was experimentally established as 5 min. The phases were separated, and an aliquot
180
TAMHINA
AND
IVsI6
(1 ml) of each phase was taken for radiochemical analysis. Distribution ratios were calculated from counts/100 set of both phases. Spectrophotometric
Determination
of Niobium
in Chloroform
A solution containing lo- 100 pg niobium(V) in 10 M HCl (1 ml), 3 ml of 10 M HCl, and 6 ml of 10 M LiCl was placed in an Erlenmeyer flask. After the addition of 10 ml of 6 x lop3 M TPA or TPP in chloroform the solution was shaken for 20 min with a mechanical shaker. After the phases were separated, the absorbance of the organic phase was measured at 282 nm (TPA) and 285 nm (TPP) against a reagent blank. Preparation
and Analysis
of Solid Complexes
Solid complexes of niobium were prepared by shaking chloroform solutions of TPA and TPP (6 x lop3 M) with the aqueous solution of niobium in which HCl and LiCl concentrations were 4 and 6 M, respectively. Niobium was in excess to extractant. (Molar niobium to TPA (TPP) ratio was 1.5.) White crystals were precipitated immediately on the bounds of the phases. The crystals were dried in vacua over calcium chloride and analyzed. The analytical data were consistent with the formula [(C,H,),fl [NbOCl,], where X = As, P. C,,H,,OCl,AsNb (Calcd: C, 45.46; H, 3.18; 0, 2.52; Cl, 22.37; As, 11.81; Nb, 14.65%. Found: Cl, 22.71; As, 11.85; Nb, 14.55%). C,,H,,OCl,PNb (Calcd: C, 48.84; H, 3.42; 0, 2.71; Cl, 24.03; P, 5.25; Nb, 15.74%. Found: Cl, 24.25; P, 4.91; Nb, 15.67%). Carbon and hydrogen were analyzed by the standard microanalytical procedure. The results obtained were a little lower than the theoretical values because of the experimental error probably due to the presence of phosphorus or arsenic. Arsenic and chloride were determined by a final EDTA titration after the substance had been destroyed by the flask combustion method (9, 1). Phosphorus was determined by the molybdate method (8) after decomposition of the substance with concentrated sulfuric and nitric acids. Niobium was determined by the tannin method (5) after decomposition of the substance with concentrated sulfuric and nitric acids. RESULTS AND DISCUSSION Optimum Conditions for Extraction
Niobium can be extracted as a niobium(V) chloride complex from aqueous hydrochloric solutions containing an excess of chloride ions by TPA and TPP in chloroform. The solution of the complex in chloroform has a maximum absorbance at 282 nm (TPA) and 285 nm (TPP). Radiometrical and spectrophotometrical studies (Fig. 1) show that niobium(V)
EXTRACTION
181
OF NIOBIUM
0.6 1
1,
/
2
I,
L cot-c Cl-
I
1,
1
6
6
10
(mot dmb3)
b)
hi-L-
350
300 Wavelength
(nm 1
FIG. 1. (a) The dependence of percentage extraction of niobium(V) on the concentration of chloride ions. Nb(V) 5 x 10e5M, H+ 4 M, TPA 6 x lo-) M. (b) The dependence of the absorption spectra of niobium complex on chloride ion concentration. Nb(V) 5 x 10m5M, H+ 4 M, TPA 6 x 10e3M. Concentration of chloride ions (1) 4 M; (2) 5 M; (3) 6 M; (4) 7 M; (5) 7.5 M; (6) 8 M; (7) 9 M; (8) IO M.
is extracted practically completely (about 95%) from a solution containing more than 9 mol dmM3 chloride and that niobium is not extracted by chloroform solutions of TPA or TPP at chloride concentrations less than 6 mol dmp3. The effect of the hydrogen ion concentration at the constant and optimum concentration of chloride ions on the extraction of niobium was also studied (Fig. 2). The optimum hydrogen ion concentration for the extraction of niobium with TPA is 2-5 M H’ and with TPP l-5 M H+. By increasing hydrogen ion concentration niobium extraction very slowly decreases. If chloride ions are not present at a concentration higher than 9 mol dmp3 a complete extraction of niobium cannot be achieved regardless of the hydrogen ion concentration (Fig. 2b, curve 2). The maximum extraction and constant absorbance of the organic phase was achieved with a 60-fold excess of TPA or TPP as shown in Fig. 3. The extracted niobium can be quantitatively stripped with a 10% (w/v) solution of oxalic acid. Moreover, it was established that none of the investigated anions (citrate, acetate, tartrate, phosphate, cyanide, perchlorate, fluoride, oxalate, nitrate, and sulfate) forms with niobium a complex extractable by TPA or TPP in chloroform.
182
TAMHINA
cont.
AND IV$GIC
H* (mol dmh3)
cont.
H* lmol dm-3)
FIG. 2. (a) The dependence of percentage of extraction of niobium on hydrogen ion concentration. Nb(V) 5 x 1O-5M, Cl- 10 M, R 6 x 10m3M, (0) R = TPA, (0) R = TPP. (b) The dependence of absorbance on hydrogen ion concentration. Nb(V) 5 x 10m5M, TPA 6 x 1O-3M, curve l-(O) Cl- 10 M, curve 2-(O) Cl- 8 M.
Spectrophotometric Determination of Niobium in Organic Phase At optimum conditions for extraction: 10 M chloride concentration and 4 M hydrogen ion concentration (6 M LiCl and 4 M HCl) in the aqueous phase and 6 x lop3 M extractant concentration in the organic phase niobium can be determined spectrophotometrically in the organic phase. The calibration graph is linear all over the range suitable for spectrophotometric measurements, corresponding to 1- 10 kg/ml of niobium in the measured solution. The effective molar abosrptivity of the chloroform extract at 282 (TPA) or 285 nm (TPP) is (1.33 k 0.03) x lo4 dm3 mol- 1
-2
L
6
6
[TPA] ~10~ lmol dme3)
log
CO~C.
[TPA]
FIG. 3. (a) The dependence of percentage of extraction of niobium (curve 1) and absorbance (curve 2) on TPA concentration. Nb(V) 5 x 1O-5M, Cl- 10 M, H+ 4 M. (b) The dependence of the distribution ratio of niobium(V) on the TPA concentration. Nb(V) 5 x 1O-5M, Cl- 10 M, (0) H+ 8 M, (cl) H+ 10 M.
EXTRACTION
183
OF NIOBIUM
TABLE 1 THE EFFECTOF FOREIGNIONSON THE DETERMINATIONOF NIOBILJMBY THE RECOMMENDED PROCEDURE Foreign ions Acetate, sulfate, citrate, cyanide. tartrate, ammonium, Na, K, Mg, Ca, Cd, Ni, Ba, Sr. Zn, Cu, Pb, Cr(III), Mn(I1)
Concentration tolerated (Molar ratio)
IOOO-fold
Phosphate, nitrate, Hg(II), Zr(IV), Ti(IV), Th(lV)
IOO-fold
Perchlorate, fluoride, oxalate, Sn(lI), W(V1). U(VI), Mo(V1). Eu(III), Pd(I1) Cu(II), V(V), Fe(iI1)”
I O-fold
” Ascorbic acid (0.15 M) present.
cm-‘. The sensitivity expressed by Sandell’s index (for A = 0,010) is 0.074 pg/cm2. The extracted complexes are stable for at least 24 hr. The reproducibility of the results expressed as a relative standard deviation is l-5% depending on the niobium concentration. The determination of niobium by the proposed extraction spectrophotometric procedure is feasible in the presence of many foreign ions (Table 1). Composition of Extracted Complexes The ratio of niobium to TPA or TPP was determined spectrophotometrically and radiometrically. The results obtained by Job’s method show that the molar ratio of niobium to onium ion is I:1 (Fig. 4). The slopes of the curves, obtained by the radiometric studies for the dependence of the log D on the log concentration tetraphenyl onium ion also indicate the equimolar ratio of the niobium to onium ion (Fig. 3b). Since the extraction mechanism can be different for high and low niobium concentrations, we determined the dependence of the niobium extraction on its initial concentration (Fig. 5). By plotting the molar ratio of TPA (TPP) and niobium in the organic phase against the initial niobium concentration in the aqueous phase, we found that one molecule of the extractant is bonded to one niobium atom. These results and the fact that the maximum absorbance of the chloroform extract does not shift with the changing niobium concentration, and does not depend on the concentration ratio of reactants implies that only one niobium species is extracted. These investigations show that the anionic niobium(V)-chloride complex, bearing one negative charge with one onium ion forms an ion-associated complex which is transferred into the organic phase. It was not possible to determine the niobium to chloride ratio but chemical analysis
184
TAMHINA
0.2
0.4
0.6 0.6
AND IVSIC
0.2
1.0
0.6 0.8
0.4
1.0
[Nbl
[Nbl [Nb]. [TPA]
[Nb]
l
[TPP]
FIG. 4. Determination of complex composition by Job’s method. H+ 4 M, Cl- 10 M, (0) INb] + [TPA] = 5 x 1O-4 M, (0) [Nb] + [TPA] = 3 x 1O-4 M, (El) [Nb] + [TPP] = 5 x 10-4M, (W) [Nb] + [TPP] = 3 x 10-4M.
and strong bands at 930 and 946 cm-‘, generally observed for niobyl group stretching (2), in ir spectra of isolated complexes show that the extracted complexes are [(C,H,),As] [NbOCl,] and [(C,H,),P] [NbOCl,]. The absorption spectra of the isolated complexes dissolved again in chloroform were identical with the spectra of the original chloroform extract, showing that no decomposition occurred during isolation.
/ 5
===@I----~ I-I/ [Nb] ap x ‘d:
•~-.-~-20
(mol dmb3j l5
FIG. 5. Determination of TPA (TPP): Nb ratio as a function of the initial niobium (V) concentration in the aqueous phase. R 4 x 10m4M, H+ 4 M, Cl- 10 M, the Nb(V) concentration is varied. (0) R = TPA, (0) R = TPP.
EXTRACTION
OF NIOBIUM
185
Therefore, the mechanism of niobium extraction can be presented as [(C,H,),XI to, +
WbOCL,l(aq)*
KGJ3,)4XI
[N’3OC141,0,
where X = As or P. SUMMARY The extraction of niobium(V) in the form of a chloro complex has been studied. Radiametrical and spectrophotometrical studies show that niobium(V) is extracted practically completely from a solution containing more than 9 mol dmm3chloride in the range of 2-5 M hydrogen ion concentration by chloroform solutions of tetraphenylarsonium (TPA) and tetraphenylphosphonium (TPP) chloride and that niobium is not extracted at chloride concentrations less than 6 mol dme3. The mechanism of extraction is based on the formation of the ion-associated compounds that form between the onium cation and the oxotetrachloroniobate(V) anion. The extracted complexes in chloroform have a maximum absorbance at 282 nm (TPA) and 285 nm (TPP); they obey Beer’s law in the range of l-10 +g Nb ml-t, and are stable for at least 24 hr. The molar absorptivity of the method is 1.33 x lo4 dm3 mol-t cm-i. The composition of the extracted species [(C,H,),Xl [NbOCl,] where X = As or P was determined spectrophotometrically, radiometrically, and by characterization of the crystalline compounds isolated.
REFERENCES 1. Flaschka, H., Mikrotitrationen
2. 3. 4. 5. 6.
7. 8. 9.
mit Athylendiamintetraessigsaure. V. Die indirekte Bestimmung von Silber und Halogenen. Mikrochem. 40, 21-26 (1953). Haritonov, Ju. Ja., and Buslaev, Ju. A., Infrakrasnie spektri poglosenija oksoftoridov nekatorih metalov. Izv. Akad. Nauk SSSR, Ser. Khim. 5, 808-814 (1964). Kanzelmeyer, J. H., and Freund, H., Ultraviolet spectrophotometric determination of niobium in hydrochloric acid. Anal. Chem. 25, 1807-1809 (1953). Kanzelmeyer, J. H., Ryan, J., and Freund, H., The Nature of Niobium(V) in Hydrochloric Acid Solution. J. Amer. Chem. Sot. 78, 3020-3023 (1956). Kolthoff, I. M., and Elving, P. J., “Treatise on Analytical Chemistry,” Part 2, Vol. 6, p. 284. Interscience, New York, 1964. Lapenko, L. A., and Gibalo, I. M., Solvent Extraction of niobium, tantalum and some other elements with tetraphenylarsonium chloride. Zh. Anal. Khim. 31, 481-484 (1976). Nabivanec, B. I., Sastojanie niobija(V) v rastvomah asotnoi, soljnoi i semoi kislot. Zh. Neorg. Kern. 9, 1079-1084 (1964). Pregl, F., and Grant, J., “Quantitative Organic Micro Analysis,” p. 141. Churchill, London, 1951. Stefanac, Z., Chelometrische Arsenbestimmung und ihre Anwendung auf die MikroElementaranalyse. Mikrochim. Acta 1115-l 120 (1962).