tributed between the resin and the external solution, but because of its divalency it displaces two sodium ions. Since magnesium is present in small amounts compared to sodium, the increase in concentration of Na+ in the external solution is negligible and has a negligible effect on the ionic strength. Thus, equilibrium between internal cqncentration of H+ and Naf and external concentration of H+ and Na+ and the rate of movement of Mg2+ down the column are all functions of the pK of the resin. A strongly chelating metal could not be
used for this method since both pK1 and pK2 would be depressed because of chelation (IO). RECEIVED for review February 17, 1971. Accepted May 25, 1971. Taken in part from the thesis submitted by George H. Luttrell to the Graduate School of Humanities and Sciences, Southern Methodist University, Dallas, Texas, in partial fulfillment of the requirements for the degree of Master of Science.
Spectrophotometric Determination of Niobium with 4-(2-Pyridylazo)Resorcinol and Colored Complexes Separated from Oxalic and Tartaric Acid Systems Marija Siroki Laborator!, of AnaiJ,tical Chetnistrj', Fucu1t.v of Science, Institute ,for Inorganic and Annlstical Clier?iistr.v, The Unicersity, Zagreb, Yugosiacia Cirila Djordjevic Ikpartrnent 01 C h e m i v / r ~, College of Wrliiatn and M a r y , Williurnsburg, Vu. 23185 Niobium complex species involved in the spectrophotometric determination of Nb(V) with 4-(2-pyridylazo)resorcinol, (PAR = H,R), in oxalic and tartaric acid media were separated in solid state. Oxooxalato-4(2pyridy1azo)resorcinoI niobate(V) was extracted from aqueous solutions in chloroform by tetraphenyl phosphonium and tetraphenyl arsonium chloride, respectively, and compounds of the formula (CsH5)4X[NbOC204)R],where X = P, As; and R = CI1H7N3O2*-,were obtained. From aqueous tartaric acid solutions, at pH 1-2, a substance corresponding to the formula NbO(C4H40s)(H R). HzO was prepared. The compounds were characterized by analysis, IR and visible spectra, conductivity measurements, and powder photographs. It has been shown that colored species in oxalic and tartaric acid media, absorbing between 560-520 nm, involve mixed ligand spheres, containing coordinated PAR along with an oxalato and tartrato ligand, respectively.
SPECTROPHOTOMETRIC DETERMINATION Of niobium with 4(2-pyridylazo)resorcinol, PAR, and complex species present in different aqueous systems was studied by several authors (1-6). Species containing a NbjPAR ratio of l / l have generally been found to exist in solutions, regardless of the presence of other different reagents capable of forming stable complexes with niobium (such as oxalates, tartrates, fluorides, hydrogen peroxide, etc.). It has been observed, however, that the absorption maximum in the visible spectrum is shifted in Nb-PAR aqueous solutions, as dependent upon the presence of a particular additional reagent. These shifts, as well as the instability of the Nb-PAR complex in solutions (1) T. Belcher, T. V. Ramakrishna, and T. S. West, Tulanta, 10, 1013 (1963). (2) I. P. Alirnarh and Han Si-i, Zh. Altal. Khim., 18, 182 (1963). (3) S. V. Elinson and L. I. Pobedina, ibid., p 189. (4) S. V. Elinson. L. I. Pobedina, and A. T. Rezova, ibid., 20, 676 ( 1965). (5) E. Lassner, Talantu, 10, 1229 (1963). (6) E. Lassner and R. Puschel, Michrochirn. Acta, 1963,950.
that do not contain a large excess of tartrates, oxalates, or similar complexing agents, imply the possibility that additional reagents are taking part in the colored Nb-PAR complex formation. However, solution studies alone cannot show if some of the ions present in large excess are coordinated to the metal or not. Therefore, the nature of the Nb-PAR complexes present in the analytical systems was not known. We have undertaken the experiments to separate the colored complexes and characterize them in the solid state and believe that the work described below offers some evidence on the composition and properties of the complexes involved in this analytical method that is becoming increasingly important for analysis of niobium ( 7 4 ) . EXPERIMENTAL
Reagents and Chemicals. All the chemicals used were Analar grade. Merck monosodium salt of 4-(2-pyridylazo)resorcinol, Fluka tetraphenylphosphonium and arsonium chloride, BDH niobium pentoxide, and chloroform containing 1 % ethanol, were used. Standard Solution of Niobium. Niobium solutions (about 2 x 10-2M) were prepared in 5 % tartaric and oxalic acid, respectively. Nb205(about 0.7 gram) was fused with KHSOa (10 grams) in a platinum crucible. The melt was extracted with hot 10% oxalic acid (50 ml). To remove sulfates, niobium was reprecipitated with ammonia and centrifuged and washed three times with ammonium chloride solution (2 %) and once with distilled water. Freshly precipitated niobium hydroxide was then dissolved in hot 10% oxalic and tartaric acid (100 ml), respectively. The solutions were heated on a water bath, filtered, and diluted to 200 ml. Solutions were standardized by the tannin or cupferron (7) R. 2. Bachrnan and C. V. Banks, ANAL.CHEM., 41, 112R (1969). (8) R. G. Anderson and G . Nickless, Analyst (London), 92, 1093 (1967). (9) A. I. Busev and V. M. Ivanov, Zh. Anal. Khim., 19, 1238 (1964).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
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Compound
N ~ ~ C ~ H I O K X HzR 0) . [(CsHi)(PI[NbO(C*04)Rl [(C~Hj),As][Nbo(Czo,)Rl R CI~H~NIO?.
C 36.8 59.3 56.0
Table I. Analytical Data Calculated, % H N P/As Nb 2.9 8.6 ... 19.0 3.6 3.4
5.6 5.3
4.1 9.4
method. Solutions of lower concentration were obtained by dilution. Preparation of (C,H5),X[NbO(Cz0,)R], X = P, As. R = CllHiN302. To the Nb(V) solution (about 2 X 10-2M) in 5 oxalic acid (25 ml), an equal volume of an equimolar aqueous solution of PAR was added and the pH was adjusted to 5 . 5 . The solution was heated and an equimolar amount of tetraphenylphosphonium (0.24 gram) and arsonium chloride (0.26 gram), respectively, dissolved in a minimum amount of water was added dropwise. The reaction mixture was transferred into a separatory funnel, extracted with an approximately equal volume of chloroform (50 ml), and the layers separated. The chloroform phase wat transferred into a distillation flask and chloroform was distilled off at 40 "C. The volume was reduced to about half of the ini?ial volume. The solution was then transferred to an Erlenmeyer flask and cooled to 0 "C. To the cool solution an approximataly equal volume of petroleum ether was added dropwise. Leaflet-like crystals which formed were filtered OR, washed with petroleum ether, and dried in the vacuum desiccator over calcium chloride. The yield was about 7 0 x and 6 0 x for the phosphonium and arsonium derivative, respectively. Analytical data are given in Table I. Preparation of NbO(C&O&HR) HzO, HR = CllH8N302. To the hot Nb(V) solution (2 X 10-2M) in 5x tartaric acid (25 ml), an equimolar amount of PAR dissolved in hot water ( 5 ml) was added dropwise. After about an hour the precipitate formed coagulated and was centrifuged, washed several times with small amount of water and acetone, and dried in vacuo ( 5 X 10-*M Hg) at 50 "C. Analysis is given in Table I. Analytical Procedures. Niobium. IN COMPOUNDS CONTAINING PHOSPHORUS OR ARSENIC. Substance (about 30 mg) was destroyed in a Kjeldahl flask with concd sulfuric acid (1 ml), to which 4-5 drops of concd H N 0 3 were added. The procedure was repeated three times. The remaining solution was transferred with 5 x oxalic acid (20 rnl) in a beaker, diluted to 50 ml, and precipitated by the tannin method. IN TARTRATO COMPOUND.The substance (about 30 ml) was dissolved in 0.1M NaOH ( 5 ml). The solution was diluted to 50 ml and heated until the red color of the niobium complex disappeared and the solution became yellow because of the free PAR presence. To the hot solution hydrochloric acid (1 :1, 10 ml) was added and the reaction mixture boiled until niobium hydroxide precipitate coagulated. The precipitate was filtered, washed with 2 x ammonium chloride solution, ignited at 900 "C, and weighed as Nb205. Satisfactory results were obtained, too, if the niobium was determined as Nb?05 by igniting the substance in a platinum crucible. Phosphorous was determined by titration with standard lead nitrate solution and PAR as indicator (IO),after igniting the sample by the Schoniger method (11). Arsenic was determined iodometrically (12), after destroying the sample with concd sulfuric and nitric acid. (10) R . Puschel, Michrochim. Acta, 1960, 352. (11) S . W. Schoniger, Mirlirocltim. Acta, 1955, 123; ibid., 1956, 869. (12) M. M. Tuckermann, J. H. Hodecher, B. C. Southworth, and K. D.Fleicher, Anal. Chim. Acra, 21,463 (1959). 1376
12.4 11.7
C
H
36.3 59.2 56.0
3.3 3.9 4.0
Found, % N P/As 8.7 5.8 5.1
... 3.8 9.9
Nb 19.7 12.2 11.2
Carbon, hydrogen, and nitrogen were determined microanalytically. Physical Measurements. Visible spectra of the solutions were recorded on a Perkin-Elmer 137 UV Spectrophotometer. Absorbance measurements were carried out on a Beckman Spectrophotometer Model DU-2. IR spectra of nujol mulls of the compounds were recorded on the Perkin-Elmer Spectrophotometer Model 137 and 221 in the region 4000-650 cm-I. Conductances were measured using a 100-c conductivity bridge and a cell with a cell constant of 0.2 cm-1. Molar conductances in methanol were determined at 25 "C at a concentration of lO-IM with a specific conductance of the solvent not greater than 1.33 X ohm-' cm2. X-Ray powder photographs were obtained in a 0.3-mm capillary with a Phillips 57.54 mm camera, CuK, radiation, and exposure time of 2 hours. Experimental work was done at the Faculty of Science, Zagreb, Yugoslavia. RESULTS
The colored niobium complex formed with PAR in oxalato aqueous solution at pH 5 can be extracted by tetraphenylphosphonium and tetraphenylarsonium chlorides, respectively, in chloroform. This behavior confirms the existence of anionic complexes in aqueous solutions, corresponding by composition to generally used analytical systems. The compounds obtained are only slightly soluble in water, and can be precipitated with onium salts at higher concentration as a fine red-violet solid. However, the substances obtained in this way are impure and cannot be further purified, since they decompose on recrystallization. From chloroform extracts, however, red leaflets of pure complexes, decomposing at 240-245 and 190-195 "C for phosphonium and arsonium derivatives, respectively, are obtained. The analysis agrees with the formula [(C6H5),X][NbO(C2O4)(R)],for X = P, As; R = C11HiN3022-. The compounds are soluble in alcohols, chloroform, and nitrobenzene. The molar conductivity of methanol solutions was found to be 70-75 ohm-l cm2, indicating the presence of a 1:l electrolyte. X-Ray powder photographs of the solids show only diffuse lines. Visible spectra of the oxalato-PAR-Nb(V) derivatives in aqueous and chloroform solutions are given in Figure 1, as compared to the spectra of the reagents alone. The maxima for the Nb(V) species occur at 545 and 560 nm in aqueous and chloroform solutions, respectively, showing a bathochromic shift from the aqueous to the organic phase. PAR, on the other hand, shows a small hypsochromic shift, the maximum occurring at 410 and 400 nm in water and chloroform, respectively. Visible spectra of the solutions made to correspond with analytical systems and spectra of dissolved complexes are identical. It has been observed, however, that the color intensity of Nb(V)-PAR-oxalato solutions depends upon concentration of PAR, oxalates, and onium salts. For a quantitative extraction, at optimal oxalato and onium salt concentration, a molar ratio of
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
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A
A
100
600
nm
Figure 1. Visible spectra
500
600
4w
(1) PAR, PH 6
Aqueous solution, (1) PAR, (2) Nb-PAR-oxalate Chloroform solution, (3) PAR, (4) Nb-PAR-oxalate
System Nb-PAR-tartrates, (2) pH 2; (4) pH 6 NbO(C4H40s)HR.H20 in 0.01M tartaric acid, (3) pH 2; (5) pH 6
Nb:PAR of 1:3 is sufficient, and above this value PAR concentration has no effect. Absorbance of the organic phase has been found to increase with the increase of onium salt concentration and only above the molar ratio of Nb:onium salt of 1 :50 no further effect on the color intensity was detected. The chloroform solutions are stable (more than 24 hours), but the reproducibility of the absorbance in different samples is poor, dependent strongly upon the oxalate concentration in the aqueous phase. Optimal oxalate to 6 X 10-3M. Reproconcentration amounts to 2 X ducibility of the niobium determination is dependent upon the oxa1ate:PAR ratio as well. Best results are achieved at a Nb:PAR:oxalate molar ratio of 1 :IO-30:300-400. On comparing the stability of aqueous and chloroform solutions we have observed that the chloroform solutions are more stable, they obey Lambert-Beer’s law in a much wider range than aqueous solutions, and the sensitivity of niobium determination is higher in chloroform. IR spectra of tetraphenylphosphonium and tetraphenylarsonium salts of oxooxalato-PAR niobate(V) are almost identical. Bands at 1715 and 1680 cm-l along with some other characteristic bands indicate presence of coordinated oxalato groups. A strong band at 885 cm-l obviously contains a contribution from the niobyl stretching group (13), since it is much more intense than the absorption observed normally for oxalates in this region, because of CO and OCO modes (14). Bands at 1570, 1250, and 1211 cm-l could be assigned to coordinated PAR vibrations (25). From acid tartrato aqueous solutions of pH 5-6, which have proved to be most sensitive for niobium determination with PAR, we have not been able to separate complexes in the solid state. They cannot be extracted by organic solvents in the presence of suitable cations and would not precipitate even from concentrated aqueous solutions. However, niobium has been shown to react with PAR in tartaric media even at lower pH ( 4 ) . The complex that forms under such conditions (pH 1-2) is less soluble and can be precipitated from 10-2M aqueous solutions. The substance separated in this way has a red-brown color and corresponds according to the analysis to the formula NbO(C4H40B)(HR). HsO. It decomposes on heating and does not melt, is soluble in (13) V. Katovic and C. Djordjevic, h r g . Chem., 9, 1720 (1970). (14) K. Nakarnoto, “Infrared Spectra of Inorganic and Coordination Co_rnpounds,”Wiley, New York, N. Y.,1963, p 211. (15) M. Siroki and C. Djordjevic, J . Less-Common Metals, 23, 228 (1971).
7w
Figure 2. Visible spectra of aqueous solution
water, and relatively stable in weakly acidic or neutral solutions. In alkaline media the complex is destroyed and PAR ligand released, as shown by spectral evidence. The presence of coordinated PAR and tartrates is confirmed by analysis and other properties of the compound. In weakly alkaline solutions PAR is dissociated off, but niobium hydroxide does not precipitate unless the solution is acidified with hydrochloric acid and boiled. This procedure destroys the tartrato complexes of Nb(V), that are stable in weakly acidic, neutral, and alkaline media. Similar behavior is observed for tartrato Nb(V) solutions alone. The visible spectrum of the isolated substance in water is identical to the spectrum of solutions corresponding to analytical conditions. For comparison, spectra are shown in Figure 2. Aqueous tartrato solutions of this complex at pH 2 show a maximum at 520 nm, shifted to 540 nm in solutions at pH 6. The IR spectrum 01. the oxotartrato-HR-Nb(V) has been helpful for the characterization, confirming the presence of coordinated tartrates, indicated by bands occurring at 1670 and between 1370 and 1200 cm-I. A broad absorption in the region 3500-3000 cm-’ is caused by hydrogen bonded OH stretchings. The spectrum is complex, but strong PAR bands are clearly distinguished at 1595, 1585 (doublet), and 1400 cm-I. However, a strong band between 930-850 cm-l, generally observed for a niobyl group stretching ( I 3 ) , is not present. A broad background absorption in the wide region between 850-700 cm-I may well be due to Nb-0-Nb vibrational modes, implying a polymeric nature of this substance in the solid state, suggested as well by diffuse X-ray powder photographs. DISCUSSION
4-(2-pyridylazo)resorcinol (PAR), is an asymmetric azo compound of the formula
P- k N
N=
p
on
HO
that has been extensively used lately as a spectrophotometric agent for a number of transition metal ions (7-9). However, so far there have been no reports on compounds prepared or separated from the aqueous solutions that contain PAR as a ligand in the solid state (IS).
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Complex species of Wb(V) with PAR in aqueous tartaric
or oxalic acid media cannot be extracted directly by organic solvents and are expected to be anionic in nature. We have thus attempted to extract their salts with large cations that may enable transport into the organic phase. TartratoPAR-Nb(V) species have proved nonextractable, but oxalatoPAR-Nb(V) species can be extracted very well by tetraphenylphosphonium and tetraphenylarsonium chlorides in chloroform or some similar solvent, showing a behavior similar to PAR complexes of vanzdium, cobalt, gallium, copper, and palladium. Study of these metal systems is in progress. The visible spectrum of niobium complexes in chloroform solutions shows a bathochromic shift of 15 nm with regard to the corresponding aqueous solution, the maxima ccciirring a: 560 and 545 nm, respectively. The maximum of PAR itself is shifted in chloroforni for &bout 10 nrn toward lower wavelength. In this way the separation of maxima of the colored complex and reagent i s increased to almost 30 nm. This observation may leRd to a spectrophotometric determination of niobium in the orgzcic phase, that should have advantages over the niobium determination with FAR in aqueous solutions. The optimal pH for Nb-oxalato-PAR co-plex formation was found to be at 5.5, a region where the oxalato species are unstable and hydrolyze in water. Products of such hydrolytic reactions probably interact with PAR in different ways and have a significant influence on the rate of the niobiumPAR colored species formation! and therefore on time needed to reach equilibrium. Under such conditions a number of factors are critical, for example the amount and sequence of reactants used, and that has acrually Seen observed. The fact that the complexes are extracted from aqueous solutions under conditions corresponding to the analytical systems, and that the solutions of complexes prepared show the same optical properties as the solutions used for analytical determination, indicate strmgly that oxooxalato-PARniobate(V) ion characterized in the solid state is the species responsible for the color formation. It has been observed that in more acid Nb(V) oxalato solution, where stable NbO(C20&- anion is present, PAR does not coordinate to the metal. PAR-onium salts of oxotrisoxalato niobate(V) can be prepared from such solutions, very different in properties from the compounds described above (15). The reaction of Nb(V) with PAP in oxalic systems has been studied less than the reaction in tartaric systems, although oxalates are more suitable for a quantitative transfer of Nb(V) into solution. From the reports published it also seems that tartaric systems perform better in the spectrophotometric determination of niobium with PAR. Such behavior is expected on the basis of lower stability of the
1378
tartrato Nb(V) complex species, which are able to react favorably with the azo reagent. Indeed, in oxalato systems, optimal pH for the reaction has been found to be 5.5, where oxalato niobium complexes are less stable and thus more suitable for interaction with an incoming ligand. In solutions with a pH larger than 5 , PAR was found to exist primarily in the form (16-19) of HR-. It is anticipated, too, that PAR acts as a tridentate ligand, with the pyridine and the azo group (the one nearer to the resorcinol ring) (17, 20) nitrogen, and the o-hydroxy group oxygen as donor atoms. In more acid tartrato media PAR may thus interact with Nb(V) by forming a neutral molecule. The substance formed under such conditions, as described in the Experimental Section, analyzed to NbO(C4HaOs)(HR). H 2 0 , containing a PAR and a tartrato ligand. In aqueous solutions, containing an excess of tartrates, the absorption maximum of the complex depends upon pH, and occurs at 520 nm and 540 nm, in solutions with pH 2 and 6, respectively, It is probable that in solutions with pH 2, PAR is coordinated as HR-, forming the neutral complex described, but at higher pH the R2-form of PAR is coordinated and an anionic complex NbO(C4H40e)(R)aK-is present in solution. The shift of spectral maxima in the visible region from 540 to 520 nm may be related to the existence of these different species. The same trend in the shift of absorption maxima is observed for PAR itself, where maxima occur for H2R, HR-, and R2- at 385,415, and 485 nm, respectively. The work described has shown that colored species involved in the spectrophotometric determination of niobium with PAR in oxalic and tartaric acid media contain a PAR ion coordinated to Nb(V) in addition to an oxalato and tartrato ligand, respectively. This structure explains the instability of color in solutions that do not contain a large excess of tartrates or oxalates. It is very likely that the same situation persists in Nb-PAR systems involving other complexing agents, such as fluorides or hydrogen peroxide, implying that mixed ligand spheres are preferred by niobium(V) in complexes with such simple azo molecules. RECEIVED for review April 2,1971. Accepted May 26,1971. (16) M. HniliEkova and L. Sommer, COIL.Czech. Chem. Commun., 26, 2189 (1961). (17) W. J. Geary, G. Nickless, and F. H. Pollard, A m / . Cliim. Acta, 26, 575 (1962); ibid., 27, 71 (1962). (18) A. I. Busev and V. M. Ivanov, Zh. Anal. Chim., 22, 382 (1967). (19) A. Corsini, I. Mai-Ling Yih, Q. Fernando, and H. Freiser, ANAL.CHEM., 34, 1090 (1962). (20) Shun’ichiro Ooi, D. Carter, and Q. Fernando, C/iem. Commun., 1967, 1301.
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