- Email: [email protected]
Extraction-spectrophotometric determination of niobium with dibenzo-18-crown-6 and thiocyanate
0039-9140/82/090761-05$03.00/0 Pergamon Press Ltd
Talanta, Vol. 29, pp. 761 to 765. 1982 Printed in Great Britain
EXTRACTION-SPECTROPHOTOMETRIC DETERMINATION OF NIOBIUM WITH DIBENZO-1%CROWN-6 AND THIOCYANATE D. BLANCO GOMIS, S. ARRIBAS JIMENO and Departamento
(Received
de Quimica
4 November
A. SANZ-MEDEL
Analitica, Facultad de Ciencias, Oviedo, Espatia
1981. Revised 27 January
Universidad
1982. Accepted
de Oviedo,
21 March
1982)
Summary-A method is described for the direct spectrophotometric determination of micro-amounts of niobium by extraction into a benzene solution of dibenzo-l&crown-6 (L) from 3M hydrochloric acid containing potassium thiocyanate. The molar absorptivity of the extracted complex is 3.85 f 0.03 x lo4 1. mole-’ .cm- ’ (relative standard deviation 0.8%). Co-ordinatively unsaturated complexes of the type [NbO(SCN),]zL and NbO(SCN)3L are extracted, along with ion-pairs, especially when small amounts of L are used for extraction. The ion-pair complex [NbOCI,(SCN),][(LK),] seems to be the main species formed in the organic phase. _
Pedersen’ was the first to report the potential selectivity of crown ethers, L, as hgands complexing most strongly those metal cations having ionic radii which best match the radius of the cavity formed by the polyether ring. The positively charged complex is usually not coloured or fluorescent, although a colorimetric’ or fluorimetric3 determination of the cation is possible by solvent extraction of the ion-association species formed by the cationic complex and a coloured or fluorescent organic anion. We have previously reported the impressive sensitivity and selectivity that can be achieved by using this principle in the fluorimetric determination of potassium with 18-crown-6 and eosin.4 In continuance of our studies on the analytical application of crown ethers a different approach has been investigated: the use of cationic alkali metal-crown ether complexes for extraction of anionic species containing the cation to be extracted or determined. In this type of extraction, ion-association species L of the type [LK+][MX;] can be expected in the organic phase.5,6 We are also engaged in the search for improved analytical methods for determination of niobium7-9 and as this element forms anionic thiocyanate complexes” its extraction into an organic phase might be accomplished by addition of a cationic crown ether complex, and the extraction followed spectrophotometrically. The spectrophotometric determination of niobium with thiocyanate, with and without extraction, has become very popular although the reagent concentrations are critical.” Affsprung” reported that this determination was improved by use of tetraphenylarsonium counter-ions for extraction of the anionic thiocyanatoniobate complex into a chloroformacetone mixture. In the present work the potassium complex of 761
dibenzo-18-crown-6 is used to extract the thiocyanatoniobate complexes (from hydrochloric acid medium) into benzene (in chloroform the extracted species precipitate, although partially halogenated hydrocarbons such as 1,2-dichloroethane can be used for the extraction). Investigation of the optimal conditions and the nature of the extracted species has allowed us to establish a new extraction-spectrophotometric determination of niobium and has shown the involved mechanism of this extraction process. EXPERIMENTAL Reugents
All reagents
used were of analytical-reagent grade. solution (10 pg/ml). Stock solution (200 pg/ml), prepared as described previously,’ diluted with 2% tartaric acid solution. Potussium thiocyanute solution, 3M. Prepared daily to avoid thiocyanic acid polymerization. Dibenzo-/X-crown-(, solution in benzene, 5 x 10e3M. Nb(V)
standard
Turturic Ascorbic Generul
acid solution, ucid solution.
3M. 10%. Freshly
prepared.
procedure
Pipette a portion of the sample containing up to
20 pg of niobium into a lOO-ml separating funnel, add 5 ml of 6M hydrochloric acid and 3 ml of 3M potassium thiocyanate, and dilute to 10 ml with redistilled water. Add 10 ml of the crown-ether solution and extract the niobium by mechanical shaking (5 min) to achieve a rapid distribution of the extraction reagent between the two phases. Allow the phases to separate, filter the benzene solution through a dry filter paper and measure the absorbance at 398 nm against benzene.
RESULTS AND DISCUSSION Spectral
characteristics
qf the complex
The absorption spectra of the extracted species and the reagent blank are given in Fig. 1. The absorbance of the blank is very small at the wavelength of the
D. BLANCO-GOMIS rt a/
162
0.4
r
Intetference
1
A
Wavelength,
Fig. 1. Absorption benzene (dashed
nm
spectrum of the niobium complex in line corresponds to the reagent blank).
absorbance
maximum of the complex, so benzene can be used as reference. The calibration graph is linear over the range l-20 pg of niobium, and the molar absorptivity at 398 nm is 3.85 x lo4 l.mole- ’ .cm-‘. The relative standard deviation (10 replicates, 10 pg of niobium) is 0.8%.
Efect
@reagent
0500 C B
concentratiom I?
The optimization studies were done with a fixed amount of 10 pg of niobium and a single extraction step. Figure 2 shows that the optimum concentrations are 2-4M hydrochloric acid and not less than 0.3M thiocyanate and 0.004M crown ether. Extraction
studies
The effect of various metals (those most frequently associated with niobium in steels and niobium ores) on the determination is shown in Table 1. Table 2 gives the influence of some common anions that mask niobium. As can be seen from Table 1, some elements interfere even at low levels (W, V, Ti, Sn and Cu) but their interference may be reduced by addition of ascorbic acid, tartaric acid or both as shown in Table 1. Mo(VI) interference can be eliminated by a preextraction step, introduced into the general procedure, consisting of addition of potassium fluoride to the aqueous phase to prevent extraction of Nb(V) while the Mo(VI) is extracted with the crown ether. After this step, boric acid is added to the aqueous phase to demask Nb(V) and allow it to be extracted with a further portion of crown-ether solution. In general, the extraction is fairly selective, and the method could be especially advantageous for niobium ores containing rare earths. As shown in Table 2, fluoride is the most efficient masking agent for Nb(V), but the effect of up to 14.7
: 0.300 k 9
conditions
The extraction is maximal after 4 min shaking time, and the yellow colour produced remains constant for at least 1 hr. The efficiency of the extraction was determined by stripping the extracted niobium and determining it; a single extraction was found to be 99.4 * 0.4’4 complete.
Table
I. Effect of foreign
Foreign
ion
Th(IV), Zr(IV). U(Vl), La(III), Ce(III), Cr(III), Mn(II), Ni(I1) Co(I1) Fe(II1) Ta(V) Cu(I1) Sn(IV) W(VI) V(V) Ti(IV) Mo(V1)
Fig. 2. Extraction efficiency as a function of reagent concentration: (A) molar concentration of HCI in the aqueous phase; (B) molar concentration (x 103) of dibenzo-lgcrown-6 in the organic phase; (C) molar concentration (x 10) of KSCN in the aqueous phase.
ions on the determination Tolerance Without masking agent
1000 150 50 100 IO Interfere Interfere Interfere Interfere Interfere
of IO pg of Nb(V) with DBC and KSCN
limit,* pg With masking agent
2000 350 50 20 150 50 5 > 50t
Masking
Ascorbic
agent
acid. IO g/l.
Tartaric acid, 150 g/l. + ascorbic acid, IO g/l. Tartaric F-.
*Level causing an error not exceeding 27: in absorbance for IO peg of Nb. tMasking and demasking procedures are given in the text.
acid, 150 g/l.
1.5 g/l.
Determination Table
of niobium
2. Effect of some common masking anions on Nb(V) determination Tolerance
limit, mg 1500 80 80 2.5 1 14.7
Tartrate Phosphate EDTA Oxalate F- alone F- ( + boric acid, 8.5 g/l.)
*Level causing an error not exceeding the absorbance for 10 pg of Nb.
2”/:, in
mg of fluoride in 10 ml of aqueous phase can be eliminated by adding 85 mg of boric acid for demasking.
Nature qf thr extracted extraction
Equilibrium-shift method applied to the system. Nb = 1.08 x lo-‘M. (A) Relationship SCN = 0.9M; (B) relationship Nb/SCN, Nb/L, of mixture, A, = absorbance L= 5 x 10-SM. A,,, = saturation absorbance with excess of reagent present. Fig.
4.
Nb-L-SCN
species and mechanism of the
The stoichiometry of the potassium dibenzo-18crown-6 complex has been clearly established13-’ 5 as I : 1 (LK+). As the anionic complexes of niobium and thiocyanate are of the type” NbOX,(SCN); or NbO(SCN):-, the expected Nb/L ratio should be 1: 1 or 1:2 for neutralization of the charge. Experimental results from investigation of the Nb/SCN ratio by the Asmus and equilibrium-shift methods invariably gave a value of 1:3 (Figs. 3 and 4). The experimental Nb/L ratio obtained by the same methods unexpectedly gave a value of 0.5 by the Asmus method. The equilibrium-shift method gave 0.45 at low L concentration but 0.77 at higher L concentration (Figs. 3 and 4). It seems unlikely that the formation and extraction of any NbO(SCN),-LKf ion-pair could correspond to the Nb/L ratio observed. Thus it appears that neuand NbCISL tral adducts of the type 16s1’ Nb,Cl,,L might be formed. On this basis, the equilibrium between an aqueous solution containing niobium and thiocyanate, and an organic solution containing a cyclic polyether (L) could be simply expressed as
Fig. 5. Determination of molar ratios in the ternary system Nb-L-SCN. (A) L = 5 x lo-‘M, SCN = 0.9M; (B) Nb = 2.15 x 10-4M, SCN = 1.5M; (C) Nb = 1.08 x lo-~M. L = 5 X lO_‘M.
CNbnWWJ-,I,,,,
mNb + nSCN + pL,,,, G?
16
X Fig. 6. Continuous-variations
Total concentration of Nb + L = 1.8 x 10-4M; wavelength
Nb
of Nb-L systems. (A) Nb + L = 2.4 x 10-4M; (B) (C) Nb + L = 1.2 x 10-4M; study
= 398 nm, l.OO-cm cells.
(I/V)” Fig. 3. The Asmus method applied to the Nb-L-SCN system. (A) Relationship Nb/SCN, Nb = 1.08 x 10-5M, relationship L = 5 X 10~3M; Nb/L, (B) A = absorbance, Nb = 1.08 x 10m5M, SCN = 0.9M. V = volume
(ml).
where the subscript org refers to the benzene phase, and charges are omitted for simplicity. The extraction constant is
K, = C~~,~S~~~,~pl,,,,/C~~lmC~~~l”C~lf,,, (1)
164
D. BLANCO-GOMIS et d.
and assuming that only one species given range of reagent concentrations,
CW,WWJJ,,,, = AId where E = molar length. Substituting log A = mlog[Nb]
(2)
absorptivity and I = cell path(2) into (1) and simplifying gives + nlog[SCN] + plog[L],,,.,
DISCUSSION
The results obtained can be explained in terms of an extraction process dependent on the concentration of L. If this concentration is low, the most probable species formed is a neutral adduct [NbO(SCN),],L analogous to (NbCl,),L (recently demonstrated in the reaction of NbCl, with L” or with cyclic polythiaethers).” When excess of L is used, the species NbO(SCN),L would also be formed (cf. formation of NbCl,L”). Simultaneous formation of both com-
C N
H Nb Cl
K
3.
Analysis of the isolated complex Experimental, %
Theoretical,* %
44.8 3.7 4.2 7.x 6.6 7.1
45.34 3.69 4.25 8.16 6.22 6.86
*For NbOCI,(SCN):-(LK+),.
Sample Stainless steel BCS 261/l Pyrochlore OKA-
of niobium
Niobium present, % 0.91 0.36
Niobium found, % 0.91, 0.34,
0.914 0.90, 0.909 0.356 0.34, 0.352
+ logK,el
It is possible to determine m, n or p by maintaining all concentrations constant except that of the appropriate component. The values found in this way were m = 1.02 and n = 2.8, but for p two values were obtained, 0.54 at low L concentration and 0.94 at high L concentration (Fig. 5). Moreover Job plots ([Nb] and [L] varied, their sum being constant) showed three absorption maxima, located at Xyh = 0.33, 0.5 and 0.66 (corresponding to 1:2, 1 :l and 2:l stoichiometries) (Fig. 6). To get more information about the nature of the extracted complexes, attempts were made to isolate them from the benzene extract. The procedure already described was applied to 0.5 mg of niobium in 10 ml of aqueous phase, and a yellow-orange crystalline compound was obtained. This was filtered off, rinsed with pure benzene to eliminate the excess of LK+SCNand LK+Cll present, and finally dried at 100”. Elemental analysis gave the results in Table 3, which agree with the values calculated for a complex of formula [NbOCl,(SCN)J(LK+),. This formulation is supported by comparative conductivity measurements on 10m4M acetonitrile solutions of this species, L and LK+Cl-. The observed specific conductance of the solution of the complex is about 50 times that of the L solution and only slightly lower than that of the LK+ ‘Cl solution.
Table
Table 4. Determination
is formed in a we may write:
pounds explains the slope of 0.77 obtained for the Nb/L ratio by the equilibrium-shift method (Fig. 4). Under the conditions of the procedure, during the shaking L is mostly changed into the cationic LK+ complex, which could be responsible for the formation of the ion-pair NbOCl(SCN); LK+. With a large excess of L, as in the analytical procedure, the species formed in the organic phase could be ion-association complexes having a higher content of L, such as NbOCl,(SCN):-(LK+),, as demonstrated by the analysis of species isolated, supported by the Nb:L ratio of 1:2 found in the continuous-variations experiments. The proton NMR data reportedi6.” for such adducts suggest a weak bond between the Nb(V) and the available oxygen co-ordination sites of the polyether. The low dielectric constant of the organic solvent, its solvating properties and especially the large excess of L (as LK+) would promote further stabilizing ion-association reactions of such adducts in the organic phase. Conclusion The mechanism of extraction anionic metal complexes with cationic crown-ethers becomes quite involved when the metal has great affinity for the oxygen atoms of the polyether. In such cases the species extracted may be neutral adducts of the unsaturated co-ordination type’* which are extracted along with the expected ion-pair LK+MX;.5.6 For the system studied here, it seems that further reaction can take place in the organic phase between the excess of L and the ion-association species containing the cation to be determined, so that species such as NbO(SCN)3C12(LK), are the final species responsible for the absorption properties of the organic extract. To assess the reliability and utility of the method, a stainless steel (B.C.S. 261/l) and a niobium pyrochlore ore from OKA (Canada) were analysed. The steel was dissolved by the method proposed by Sanz-Medel et ~1.‘~ and the ore sample by a modification of Faye’s method.’ The results agreed very well with the expected values (Table 4).
REFERENCES
1. C. J. Pedersen, J. Am. C/tern. Sot., 1967. 89, 7017. 2. H. Sumiyoshi, K. Nakahara and K. Ueno. T&mu. 1977, 24, 763.
Determination 3. K. Kenyu, S. Katsuhiko and I. Nobuhiko, Bunseki Kagaku, 1978, 27, 291. 4. A. Sanz-Medel, D. Blanc0 Gomis and J. R. Garcia. Talanta, 1981, 28, 425. H. Noguchi and M. Naga5. M. Yoshio, M. Ugamura, matsu, Anal. Lett., 1978, 1I, 281. 6. Idem. ibid.. 1980. 13. 1431. 7. A. Sanz-Medel and C. Camara, Anal. Chem.. 1980, 52. 1035. 8. M. M. Bonilla and A. Sanz-Medel, An. Quim.. 1978, 74, 595. 9. A. Sanz-Medel and M. E. Diaz, Analyst, in the press. Anal. Chem., 1968, 40, 10. C. Djordjevic and B. Tamhina, 1512. 11 P. F. Sattler and I. E. Schreinlechner, ibid., 1977, 49, 80. 12. H. E. Affsprung and J. L. Robinson, Anul. Chim. Acta, 1967, 37, 81.
of niobium
765
Synthetic Multidentate Macrocyclic 13. R. M. Izatt, Compounds, p, 216. Academic Press, New York, 1978. 14. R. M. Izatt and J. J. Christensen. Proyress in Macrocyclic Chemistry. Vol. 1, p. 120. Wiley, New York, 1979. and A. Yu. Nazarenko, Zh. Neorgan. 15. I. V. Pyatnitskii K&m., 1980, 25, 1064. 16. R. E. DeSimone and T. hl. Tighe, J. Inorg. Nucl. Chem., 1976, 38, 1623. and M. Tsunoda, Inorg. Chim. 17. L. G. Hubert-Pfalzgraf Actu, 1980, 38, 43. in Trace Char18. T. S. West, Chemical Spectrophotometry acterization-Chemicrrl and Physical, W. W. Meinke and B. F. Scribner (eds.), pp. 215-301. NBS Monograph 100, NBS, Washington, 1967. An Quim., 19. M. Bonilla, C. Camara and A. Sanz-Medel, 1979. 75, 565.
Talanta, Vol. 29, pp. 761 to 765. 1982 Printed in Great Britain
EXTRACTION-SPECTROPHOTOMETRIC DETERMINATION OF NIOBIUM WITH DIBENZO-1%CROWN-6 AND THIOCYANATE D. BLANCO GOMIS, S. ARRIBAS JIMENO and Departamento
(Received
de Quimica
4 November
A. SANZ-MEDEL
Analitica, Facultad de Ciencias, Oviedo, Espatia
1981. Revised 27 January
Universidad
1982. Accepted
de Oviedo,
21 March
1982)
Summary-A method is described for the direct spectrophotometric determination of micro-amounts of niobium by extraction into a benzene solution of dibenzo-l&crown-6 (L) from 3M hydrochloric acid containing potassium thiocyanate. The molar absorptivity of the extracted complex is 3.85 f 0.03 x lo4 1. mole-’ .cm- ’ (relative standard deviation 0.8%). Co-ordinatively unsaturated complexes of the type [NbO(SCN),]zL and NbO(SCN)3L are extracted, along with ion-pairs, especially when small amounts of L are used for extraction. The ion-pair complex [NbOCI,(SCN),][(LK),] seems to be the main species formed in the organic phase. _
Pedersen’ was the first to report the potential selectivity of crown ethers, L, as hgands complexing most strongly those metal cations having ionic radii which best match the radius of the cavity formed by the polyether ring. The positively charged complex is usually not coloured or fluorescent, although a colorimetric’ or fluorimetric3 determination of the cation is possible by solvent extraction of the ion-association species formed by the cationic complex and a coloured or fluorescent organic anion. We have previously reported the impressive sensitivity and selectivity that can be achieved by using this principle in the fluorimetric determination of potassium with 18-crown-6 and eosin.4 In continuance of our studies on the analytical application of crown ethers a different approach has been investigated: the use of cationic alkali metal-crown ether complexes for extraction of anionic species containing the cation to be extracted or determined. In this type of extraction, ion-association species L of the type [LK+][MX;] can be expected in the organic phase.5,6 We are also engaged in the search for improved analytical methods for determination of niobium7-9 and as this element forms anionic thiocyanate complexes” its extraction into an organic phase might be accomplished by addition of a cationic crown ether complex, and the extraction followed spectrophotometrically. The spectrophotometric determination of niobium with thiocyanate, with and without extraction, has become very popular although the reagent concentrations are critical.” Affsprung” reported that this determination was improved by use of tetraphenylarsonium counter-ions for extraction of the anionic thiocyanatoniobate complex into a chloroformacetone mixture. In the present work the potassium complex of 761
dibenzo-18-crown-6 is used to extract the thiocyanatoniobate complexes (from hydrochloric acid medium) into benzene (in chloroform the extracted species precipitate, although partially halogenated hydrocarbons such as 1,2-dichloroethane can be used for the extraction). Investigation of the optimal conditions and the nature of the extracted species has allowed us to establish a new extraction-spectrophotometric determination of niobium and has shown the involved mechanism of this extraction process. EXPERIMENTAL Reugents
All reagents
used were of analytical-reagent grade. solution (10 pg/ml). Stock solution (200 pg/ml), prepared as described previously,’ diluted with 2% tartaric acid solution. Potussium thiocyanute solution, 3M. Prepared daily to avoid thiocyanic acid polymerization. Dibenzo-/X-crown-(, solution in benzene, 5 x 10e3M. Nb(V)
standard
Turturic Ascorbic Generul
acid solution, ucid solution.
3M. 10%. Freshly
prepared.
procedure
Pipette a portion of the sample containing up to
20 pg of niobium into a lOO-ml separating funnel, add 5 ml of 6M hydrochloric acid and 3 ml of 3M potassium thiocyanate, and dilute to 10 ml with redistilled water. Add 10 ml of the crown-ether solution and extract the niobium by mechanical shaking (5 min) to achieve a rapid distribution of the extraction reagent between the two phases. Allow the phases to separate, filter the benzene solution through a dry filter paper and measure the absorbance at 398 nm against benzene.
RESULTS AND DISCUSSION Spectral
characteristics
qf the complex
The absorption spectra of the extracted species and the reagent blank are given in Fig. 1. The absorbance of the blank is very small at the wavelength of the
D. BLANCO-GOMIS rt a/
162
0.4
r
Intetference
1
A
Wavelength,
Fig. 1. Absorption benzene (dashed
nm
spectrum of the niobium complex in line corresponds to the reagent blank).
absorbance
maximum of the complex, so benzene can be used as reference. The calibration graph is linear over the range l-20 pg of niobium, and the molar absorptivity at 398 nm is 3.85 x lo4 l.mole- ’ .cm-‘. The relative standard deviation (10 replicates, 10 pg of niobium) is 0.8%.
Efect
@reagent
0500 C B
concentratiom I?
The optimization studies were done with a fixed amount of 10 pg of niobium and a single extraction step. Figure 2 shows that the optimum concentrations are 2-4M hydrochloric acid and not less than 0.3M thiocyanate and 0.004M crown ether. Extraction
studies
The effect of various metals (those most frequently associated with niobium in steels and niobium ores) on the determination is shown in Table 1. Table 2 gives the influence of some common anions that mask niobium. As can be seen from Table 1, some elements interfere even at low levels (W, V, Ti, Sn and Cu) but their interference may be reduced by addition of ascorbic acid, tartaric acid or both as shown in Table 1. Mo(VI) interference can be eliminated by a preextraction step, introduced into the general procedure, consisting of addition of potassium fluoride to the aqueous phase to prevent extraction of Nb(V) while the Mo(VI) is extracted with the crown ether. After this step, boric acid is added to the aqueous phase to demask Nb(V) and allow it to be extracted with a further portion of crown-ether solution. In general, the extraction is fairly selective, and the method could be especially advantageous for niobium ores containing rare earths. As shown in Table 2, fluoride is the most efficient masking agent for Nb(V), but the effect of up to 14.7
: 0.300 k 9
conditions
The extraction is maximal after 4 min shaking time, and the yellow colour produced remains constant for at least 1 hr. The efficiency of the extraction was determined by stripping the extracted niobium and determining it; a single extraction was found to be 99.4 * 0.4’4 complete.
Table
I. Effect of foreign
Foreign
ion
Th(IV), Zr(IV). U(Vl), La(III), Ce(III), Cr(III), Mn(II), Ni(I1) Co(I1) Fe(II1) Ta(V) Cu(I1) Sn(IV) W(VI) V(V) Ti(IV) Mo(V1)
Fig. 2. Extraction efficiency as a function of reagent concentration: (A) molar concentration of HCI in the aqueous phase; (B) molar concentration (x 103) of dibenzo-lgcrown-6 in the organic phase; (C) molar concentration (x 10) of KSCN in the aqueous phase.
ions on the determination Tolerance Without masking agent
1000 150 50 100 IO Interfere Interfere Interfere Interfere Interfere
of IO pg of Nb(V) with DBC and KSCN
limit,* pg With masking agent
2000 350 50 20 150 50 5 > 50t
Masking
Ascorbic
agent
acid. IO g/l.
Tartaric acid, 150 g/l. + ascorbic acid, IO g/l. Tartaric F-.
*Level causing an error not exceeding 27: in absorbance for IO peg of Nb. tMasking and demasking procedures are given in the text.
acid, 150 g/l.
1.5 g/l.
Determination Table
of niobium
2. Effect of some common masking anions on Nb(V) determination Tolerance
limit, mg 1500 80 80 2.5 1 14.7
Tartrate Phosphate EDTA Oxalate F- alone F- ( + boric acid, 8.5 g/l.)
*Level causing an error not exceeding the absorbance for 10 pg of Nb.
2”/:, in
mg of fluoride in 10 ml of aqueous phase can be eliminated by adding 85 mg of boric acid for demasking.
Nature qf thr extracted extraction
Equilibrium-shift method applied to the system. Nb = 1.08 x lo-‘M. (A) Relationship SCN = 0.9M; (B) relationship Nb/SCN, Nb/L, of mixture, A, = absorbance L= 5 x 10-SM. A,,, = saturation absorbance with excess of reagent present. Fig.
4.
Nb-L-SCN
species and mechanism of the
The stoichiometry of the potassium dibenzo-18crown-6 complex has been clearly established13-’ 5 as I : 1 (LK+). As the anionic complexes of niobium and thiocyanate are of the type” NbOX,(SCN); or NbO(SCN):-, the expected Nb/L ratio should be 1: 1 or 1:2 for neutralization of the charge. Experimental results from investigation of the Nb/SCN ratio by the Asmus and equilibrium-shift methods invariably gave a value of 1:3 (Figs. 3 and 4). The experimental Nb/L ratio obtained by the same methods unexpectedly gave a value of 0.5 by the Asmus method. The equilibrium-shift method gave 0.45 at low L concentration but 0.77 at higher L concentration (Figs. 3 and 4). It seems unlikely that the formation and extraction of any NbO(SCN),-LKf ion-pair could correspond to the Nb/L ratio observed. Thus it appears that neuand NbCISL tral adducts of the type 16s1’ Nb,Cl,,L might be formed. On this basis, the equilibrium between an aqueous solution containing niobium and thiocyanate, and an organic solution containing a cyclic polyether (L) could be simply expressed as
Fig. 5. Determination of molar ratios in the ternary system Nb-L-SCN. (A) L = 5 x lo-‘M, SCN = 0.9M; (B) Nb = 2.15 x 10-4M, SCN = 1.5M; (C) Nb = 1.08 x lo-~M. L = 5 X lO_‘M.
CNbnWWJ-,I,,,,
mNb + nSCN + pL,,,, G?
16
X Fig. 6. Continuous-variations
Total concentration of Nb + L = 1.8 x 10-4M; wavelength
Nb
of Nb-L systems. (A) Nb + L = 2.4 x 10-4M; (B) (C) Nb + L = 1.2 x 10-4M; study
= 398 nm, l.OO-cm cells.
(I/V)” Fig. 3. The Asmus method applied to the Nb-L-SCN system. (A) Relationship Nb/SCN, Nb = 1.08 x 10-5M, relationship L = 5 X 10~3M; Nb/L, (B) A = absorbance, Nb = 1.08 x 10m5M, SCN = 0.9M. V = volume
(ml).
where the subscript org refers to the benzene phase, and charges are omitted for simplicity. The extraction constant is
K, = C~~,~S~~~,~pl,,,,/C~~lmC~~~l”C~lf,,, (1)
164
D. BLANCO-GOMIS et d.
and assuming that only one species given range of reagent concentrations,
CW,WWJJ,,,, = AId where E = molar length. Substituting log A = mlog[Nb]
(2)
absorptivity and I = cell path(2) into (1) and simplifying gives + nlog[SCN] + plog[L],,,.,
DISCUSSION
The results obtained can be explained in terms of an extraction process dependent on the concentration of L. If this concentration is low, the most probable species formed is a neutral adduct [NbO(SCN),],L analogous to (NbCl,),L (recently demonstrated in the reaction of NbCl, with L” or with cyclic polythiaethers).” When excess of L is used, the species NbO(SCN),L would also be formed (cf. formation of NbCl,L”). Simultaneous formation of both com-
C N
H Nb Cl
K
3.
Analysis of the isolated complex Experimental, %
Theoretical,* %
44.8 3.7 4.2 7.x 6.6 7.1
45.34 3.69 4.25 8.16 6.22 6.86
*For NbOCI,(SCN):-(LK+),.
Sample Stainless steel BCS 261/l Pyrochlore OKA-
of niobium
Niobium present, % 0.91 0.36
Niobium found, % 0.91, 0.34,
0.914 0.90, 0.909 0.356 0.34, 0.352
+ logK,el
It is possible to determine m, n or p by maintaining all concentrations constant except that of the appropriate component. The values found in this way were m = 1.02 and n = 2.8, but for p two values were obtained, 0.54 at low L concentration and 0.94 at high L concentration (Fig. 5). Moreover Job plots ([Nb] and [L] varied, their sum being constant) showed three absorption maxima, located at Xyh = 0.33, 0.5 and 0.66 (corresponding to 1:2, 1 :l and 2:l stoichiometries) (Fig. 6). To get more information about the nature of the extracted complexes, attempts were made to isolate them from the benzene extract. The procedure already described was applied to 0.5 mg of niobium in 10 ml of aqueous phase, and a yellow-orange crystalline compound was obtained. This was filtered off, rinsed with pure benzene to eliminate the excess of LK+SCNand LK+Cll present, and finally dried at 100”. Elemental analysis gave the results in Table 3, which agree with the values calculated for a complex of formula [NbOCl,(SCN)J(LK+),. This formulation is supported by comparative conductivity measurements on 10m4M acetonitrile solutions of this species, L and LK+Cl-. The observed specific conductance of the solution of the complex is about 50 times that of the L solution and only slightly lower than that of the LK+ ‘Cl solution.
Table
Table 4. Determination
is formed in a we may write:
pounds explains the slope of 0.77 obtained for the Nb/L ratio by the equilibrium-shift method (Fig. 4). Under the conditions of the procedure, during the shaking L is mostly changed into the cationic LK+ complex, which could be responsible for the formation of the ion-pair NbOCl(SCN); LK+. With a large excess of L, as in the analytical procedure, the species formed in the organic phase could be ion-association complexes having a higher content of L, such as NbOCl,(SCN):-(LK+),, as demonstrated by the analysis of species isolated, supported by the Nb:L ratio of 1:2 found in the continuous-variations experiments. The proton NMR data reportedi6.” for such adducts suggest a weak bond between the Nb(V) and the available oxygen co-ordination sites of the polyether. The low dielectric constant of the organic solvent, its solvating properties and especially the large excess of L (as LK+) would promote further stabilizing ion-association reactions of such adducts in the organic phase. Conclusion The mechanism of extraction anionic metal complexes with cationic crown-ethers becomes quite involved when the metal has great affinity for the oxygen atoms of the polyether. In such cases the species extracted may be neutral adducts of the unsaturated co-ordination type’* which are extracted along with the expected ion-pair LK+MX;.5.6 For the system studied here, it seems that further reaction can take place in the organic phase between the excess of L and the ion-association species containing the cation to be determined, so that species such as NbO(SCN)3C12(LK), are the final species responsible for the absorption properties of the organic extract. To assess the reliability and utility of the method, a stainless steel (B.C.S. 261/l) and a niobium pyrochlore ore from OKA (Canada) were analysed. The steel was dissolved by the method proposed by Sanz-Medel et ~1.‘~ and the ore sample by a modification of Faye’s method.’ The results agreed very well with the expected values (Table 4).
REFERENCES
1. C. J. Pedersen, J. Am. C/tern. Sot., 1967. 89, 7017. 2. H. Sumiyoshi, K. Nakahara and K. Ueno. T&mu. 1977, 24, 763.
Determination 3. K. Kenyu, S. Katsuhiko and I. Nobuhiko, Bunseki Kagaku, 1978, 27, 291. 4. A. Sanz-Medel, D. Blanc0 Gomis and J. R. Garcia. Talanta, 1981, 28, 425. H. Noguchi and M. Naga5. M. Yoshio, M. Ugamura, matsu, Anal. Lett., 1978, 1I, 281. 6. Idem. ibid.. 1980. 13. 1431. 7. A. Sanz-Medel and C. Camara, Anal. Chem.. 1980, 52. 1035. 8. M. M. Bonilla and A. Sanz-Medel, An. Quim.. 1978, 74, 595. 9. A. Sanz-Medel and M. E. Diaz, Analyst, in the press. Anal. Chem., 1968, 40, 10. C. Djordjevic and B. Tamhina, 1512. 11 P. F. Sattler and I. E. Schreinlechner, ibid., 1977, 49, 80. 12. H. E. Affsprung and J. L. Robinson, Anul. Chim. Acta, 1967, 37, 81.
of niobium
765
Synthetic Multidentate Macrocyclic 13. R. M. Izatt, Compounds, p, 216. Academic Press, New York, 1978. 14. R. M. Izatt and J. J. Christensen. Proyress in Macrocyclic Chemistry. Vol. 1, p. 120. Wiley, New York, 1979. and A. Yu. Nazarenko, Zh. Neorgan. 15. I. V. Pyatnitskii K&m., 1980, 25, 1064. 16. R. E. DeSimone and T. hl. Tighe, J. Inorg. Nucl. Chem., 1976, 38, 1623. and M. Tsunoda, Inorg. Chim. 17. L. G. Hubert-Pfalzgraf Actu, 1980, 38, 43. in Trace Char18. T. S. West, Chemical Spectrophotometry acterization-Chemicrrl and Physical, W. W. Meinke and B. F. Scribner (eds.), pp. 215-301. NBS Monograph 100, NBS, Washington, 1967. An Quim., 19. M. Bonilla, C. Camara and A. Sanz-Medel, 1979. 75, 565.