827
Anal. Chem. 1986, 58,827-829
6:;; 6
3 LH; + Fez' + (LH2)3 FeZt+ 3 H t lLHZ)3Fe2+: H3C
Fez+
CH3
Z L H ~t
NH-CS-NHz
F d + d(LHJ2Fe++
=A\]/
(LHJzFe' : H3C
2H'
LITERATURE CITED
Fe3'
=y'OH
CH3
Applications. In Tables 111-V are the results obtained in the determination of iron in wines, foods, and minerals. These results are compared with those obtained by atomic absorption spectrophotometry or with 2,2'-bipyridine. These results are concordant and the proposed method is more rapid and easier. Registry No. Fe, 7439-89-6; LH2, 99688-02-5; (LH&Fe2+, 99688-00-3; (LH)2Fe+,99688-01-4.
N=C;'
"2
Flgure 3. Probable structure of the Fe(I1)and Fe(II1) complexes wlth DCDT.
absorptivity was 8.9 X lo3 L-mol-l-cm-'. The detection limit (10) was 0.05 pg-mL-l of Fe(I1) and the determination limit (11) was 0.1 pg-mL-l of Fe(I1). The optimum working range, as evaluated by Ringbom's method was 0.7-5.0 MgmL-' of iron. The relative error (11determinations, 3.0 ppm of iron(II), 95% confidence level) is f0.5%. Effects of Foreign Ions. In the determination of 3.0 pg-mL-l of iron, foreign ions can be tolerated (less than a 2.5% change in absorbance) a t the levels given in Table 11. The great selectivity of the method should be emphasized because only Co(I1) interferes seriously.
Slngh, R. B.; Grag, B. S.; Slngh, R. P. Talanta 1978, 25, 647. Can0 Pavbn, J. M.; PBrez Bendlto, D.; Valclrcel, M. Oulm. Anal. 1982, I , 118. Can0 Pavbn, J. M. Mlcrochem. J . 1981, 26, 155. Haas, P. J . Chem. SOC. 1907, 91, 1437. Wittemberger, W. "Chemlsche Laboratoriumstechnik",4th ed.;Sprlnger: Vienna, 1959, p 101. Stenstrom, W.; Goldsmlth, N. J . Phys. Chem. 1926, 3 0 , 1683. Salinas. F.: JImBnez SBnchez, J. C.; Lemus Gallego, J. M. Talanfa, in press. Romin Ceba, M.; JimBnez Sanchez, J. C.; Galeano Dlaz, T. Afinldad 1981, 375,439. Gonzilez Garcia, D. V.; Arrebola Ramhez, A.; R o m h Ceba, M. Talanfa 1979, 26, 215. IUPAC Nomenclature, Symbols, Units and Their Usage in Spectrochemical Analysis Pure Appl. Chem. 1978, 105, 45 pp. ACS Committee on Environmental Improvement Subcommittes on Environmental Analytlcal Chemistry Anal. Chem. 1980, 52, 2242.
RECEIVED for review July 29, 1985. Accepted October 23, 1985.
Extraction-Spectrophotometric Determination of Tungsten as a Mixed Thiocyanate-Propericiazine Complex Ankapura T. Gowda*
Department of Chemistry, A . V.K. College for Women, Hassan-573201, Karnataka, India Kanchugarakoppal S . Rangappa
Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OW0
A new extractive spectrophotometric method for the determination of mlcrogram amounts of tungsten, based on the extraction of yellow tungsten(V)-thlocyanate-properlclazlne ion assoclatlon complex Into chloroform from 4.5-8.0 M hydrochloric acid medium Is descrlbed. Beer's law Is valld over the concentration range 0.6-14.8 ppm of tungsten. The complex, whlch is stable over a week, has an absorption maximum at 410 nm, and Its molar absorptivity is 1.82 X IO4 L mol-' cm-'. The effects of acid concentratlon, time, temperature, concentration of reagent, and the Interferencesfrom various Ions are investigated. The proposed method Is employed for the determination of tungsten In tungsten steel.
Several spectrophotometric methods have been reported for the determination of tungsten (1-11). Most of them are unsatisfactory for one reason or another. One of the most often used is the tungsten thiocyanate method (1-8) applied to either the aqueous solution or the organic extract. In these methods the interference of various metals, the stability of the complex, and the reproducibility have been the main problems. It was found during our investigation that the stability and the sensitivity of the tungsten-thiocyanate complex could be enhanced by adding 10-[3-(4-hydroxy-
piperidino)propyl]phenothiazine-2-carbonitrile (CZ1Hz30N3S) or propericiazine (PPC), which forms an ion association complex that can be extracted into an organic solvent. The structure of the PPC molecule is as follows:
I The proposed method offers the advantages of rapidity, reproducibility, sensitivity, and selectivity without the need for heating the solution. The application of the method in the determination of tungsten content in tungsten steels has also been studied.
EXPERIMENTAL SECTION Apparatus. A Beckman spectrophotometer (Model DB) with matched 1-cm silica cells was used for absorbance measurements. Reagents. Tungsten(VI) Solution. A stock solution of tungsten(V1) was prepared from sodium tungstate (AnalaR) in double distilled water and standardized by the 8-hydroxyquinoline method (12).The stock solution was further diluted as required. PPC Solution. A 2 X lo-' M solution of PPC (Rhone Poulenc, Paris) in chloroform (AnalaR) was prepared and stored in an amber bottle and kept in a refrigerator for further use.
0003-2700/86/0358-0827$01.50/0 .. . 0 1986 American Chemical Soclety
828
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL
1986
Table I. Precision and Accuracy in the Determination of Tungsten(V1) amt. of tungsten, PPm
taken
found*"
2.00
2.023 4.970 8.042 10.964
5.00 8.00 11.00
re1 error, '70
std. dev, ppm
f1.12 -0.60 +0.525 -0.327
0.004 0.0051 0.0110 0.0072
Each result in the average of five separate determinations. Ammonium Thiocyanate. A 10% solution of ammonium thiocyanate (AnalaR) was prepared in double distilled water. T W O Chloride Solution. A 10% tin(I1) chloride solution was prepared from SnC12.2Hz0(AnalaR) in 2 M hydrochloric acid. All other reagents, of analytical grade, were used without further purification. Procedure. An aliquot of the stock solution containing 6-148 bg of tungsten(V0, 12 mL of 10 M hydrochloric acid; 1 mL of 10% tin(I1) chloride, and 3 mL of 10% ammonium thiocyanate was taken in a 100-mL seb.arating funnel and diluted to 20 mL with double distilled water. The solution was left at room temp. (25 f 2 "C) for 15 min. Five milliliters of 2 X M PPC in chloroform was added, and the mixture was equilibrated for 2 min. The chloroform layer was transferred t o a 25-mL beaker containing about 0.5 g of anhydrous sodium sulfate. The extract was transferred to a 10-mL volumetric flask and diluted t o the mark with chloroform. The absorbance was measured at 410 nm against a reagent blank prepared in the same manner. The concentration of tungsten in the sample solution was determined from the calibration graph. RESULTS AND DISCUSSION Tungsten(V) formed by the reduction of tungsten(V1) with tin(I1) chloride combines with ammonium thiocyanate to form a yellow anionic tungsten(V)-thiocyanate complex in 4.5-8.0 M hydrochloric acid solution. This complex reacts with the radical cation of PPC, formed by the oxidation of PPC with tungsten(VI), and forms an ion association complex (13) in the same range of acid. The ion association complex can be extracted into chloroform while the binary tungsten-thiocyanate complex cannot be extracted. Absorption Spectra. The absorption spectra of the tungsten(V)-thiocyanate complex, tungsten(V)-thiocyanatePPC ion association complex, and the reagent blank are shown in Figure 1. Tungsten(V)-thiocyanate complex in 6 M hydrochloric acid has an absorption maximum a t 395 nm, whereas the chloroform extract of tungsten(V)-thiocyanatePPC ion association complex has an absorption maximum at 410 nm, thus showing a bathochromic shift of 15 nm. All subsequent measurements were made at 410 nm.
Effect of Various Experimental Variables. The effect of acid concentration on the formation and extraction of the ternary complex into an organic phase was investigated with hydrochloric, sulfuric, phosphoric, and acetic acids. Maximum and constant absorbance readings were obtained in 4.5-8.0 M hydrochloric or sulfuric acid. But the sensitivity is less in phosphoric acid, and the complex is not formed in acetic acid medium. The interference from various foreign ions is more in sulfuric acid. Hence, 6 M hydrochloric acid medium was selected for further studies as the reaction is more sensitive and selective. The effect of reagent concentration was investigated by varying the concentration of ammonium thiocyanate and PPC. Both 1-5 mL of 10% ammonium thiocyanate and 2-10 mL of 2 X lo-' M PPC were required for maximum color development. Hence, optimum concentrations of 3 mL of 10% ammonium thiocyanate and 5 mL of 2 X M PPC in chloroform were selected for the determination of tungsten. The intensity of the color depends on the reducing agent. Tin(I1) chloride, thiourea, thioglycolic acid, hydrazine sulfate, ascorbic acid, and acetone were examined as reducing agents. Tin(I1) chloride was found to be the most suitable reducing agent as it increased the sensitivity of the reaction and gave reproducible values. A minimum concentration of 0.2 mL of 10% tin(I1) chloride was required for the reduction of 6 ppm of tungsten(V1). Addition of an excess of tin(I1) chloride had no effect on the absorbance readings. Many organic solvents such as chloroform, benzene, amyl alcohol, chlorobenzene, and n-butyl alcohol have been tried for the extraction of the ion association complex from the aqueous phase. Among these, chloroform was found to be the most suitable solvent for the extraction of the ion association complex as it increased the sensitivity and stability of the reaction. The absorbance readings of the chloroform extract were stable for over a week in the temperature range 8-42 "C. Calibration, Range, and Sensitivity. The chloroform extract of the ion association complex obeyed Beer's law in the range of 0.6-14.8 ppm tungsten. The optimum concentration range according to Ringbom's method was 2.0-14.4 ppm. The sensitivity of the reaction as defined by Sandell (14) is 12.8 ng cm-2. The molar absorptivity calculated from Beer's law data is 1.82 X lo4 L mol-' cm-' a t 410 nm. Precision and Accuracy. The precision and accuracy of the method were studied by analyzing solutions containing known amounts of tungsten(V1) in the absence of other possibly interfering metals. The results are presented in Table I. Effect of Diverse Ions. In order to assess the usefulness of the proposed method, the effect of diverse ions that often accompany tungsten was studied. The following amounts
-
Table 11. Determination of Tungsten in Tungsten Steels tungsten content of solution, ppm proposed steel sample T12SW
14c05crV4
T75W18Co1~Cr4V&h
certified composition, '70
C, 1.5; Si, 0.2; Mn, 0.3; Cr, 5.0; V, 5.0; Co, 5.0; W, 12.5
C, 0.76; Si, 0.2; Mn, 0.3; Cr, 4.3; Mo, 0.9; V, 1.6; Co, 9.5; W, 18.0
TmW&o&r,VMom T72W18Cr4V1
C, 0.82; Si, 0.2; Mn, 0.3; Cr, 4.3; Mo, 0.9; V, 16; Co, 4.8; W, 18.0 C, 0.75; Si, 0.2; Mn, 0.3; Cr, 4.3; V, 1.1; W, 18.0
Average of five determinations.
certified value 2.5 4.5 6.0 2.0 3.8 6.5 3.8 6.4 8.2 4.0 7.5 9.0
methodn thiocyanate method" 2.51 4.46 5.91 2.00 3.77 6.54 3.78 6.37 8.24 4.02 7.50 8.94
2.53 4.56 6.08 1.98 3.75 6.43 3.88 6.49 7.10 3.89 7.52 9.12
re1 error, '70
std dev
+0.40 -0.88 -1.50 0.00 -0.80 +0.63 -0.53 -0.47 +0.49
0.0141 0.0007 0.0072
0.01 0.0028 0.0022
+0.50
0.0109
0.00 -0.66
0.007 0.0092
0.001 0.002
0.007
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4,APRIL 1986
A
O“1
01
c
t
400
503
m
WAVELENGTH nm
Figure 1. Absorption spectra of (A) tungsten(V)-thiocyanate-PPC Ion association complex in 6 M hydrochloric acld (tungsten(V)= 5 ppm), (B) tungsten(V)-thiocyanate complex in 6 M hydrochloric acid (tungsten(\/) = 5 ppm), and (C) reagent blank in 6 M hydrochloric acid. (ppm) of foreign ions were found to give an error of less than 2% in the determination of 6 ppm tungsten(VI): Fe(III), 2200, Cr(III), 2000; Ru(III), 800; Al(III), 1800; Bi(III), 2000; Ir(III), 200; Rh(III), 220; Au(III), 120; Ti(IV), 80; Pt(IV), 60; Th(IV), 1200; Zr(IV), 1000; V(V), 100; Ta(V), 50; Nb(V), 40; U(VI), 1300; Mo(VI), 30; Mo(V), 10; Ni(II), 200; Cu(II), 350; Co(II), 400; Zn(II), 1400; Mn(II), 1500; Pb(II), 1400; Ca(II), 1250; Mg(II), 1500; Ba(II), 1400; Ag(I), 60; fluoride, 5000; bromide, 2600, iodide, 1400; acetate, 5000; oxalate, 2500; tartrate, 2000; citrate, 2500; sulfate, 10 000; and phosphate, 10 000. The tolerance limit of titanium(IV), niobium(V), vanadium(V), and tantalum(V) could be raised to 600-fold and molybdenum(V) to 15-fold by complexing with fluoride (0.2-1 mL of 1 M sodium fluoride) and retaining in the aqueous phase. Stoichiometry of the Complex. The stoichiometry of the ion association complex was studied by Job’s method and the equilibrium shift method. These methods indicated that the molar ratio of tungsten to propericiazine was 1:2 and to thiocyanate was 1:5. Thus the complex extracted was (PPCH+)2[Wo(SCN)52-] where PPCH+ represents protonated propericiazine. This formula was confirmed by chemical analysis of the precipitate obtained by shaking a 2 X M chloroform solution of propericiazine with an aqueous solution of the thiocyanato-tungsten complex at a molar ratio of 2:l. The organic phase was separated and the solvent evaporated.
829
The precipitate was dried in a vacuum desiccator and analyzed. Anal. Calcd for (PPCHt)2[Wo(SCN)52-]:C, 25.54; H, 2.05; N, 9.16; W, 15.05. Found: C, 25.2; H, 2.0; N, 9.25; W, 14.8. Application. Determination of Tungsten in Tungsten Steels. About 0.5 g of tungsten steel was accurately weighed into a 250-mL beaker and treated with 25-mL of concentrated hydrochloric acid and 0.5 mL of concentrated nitric acid. The mixture was boiled to remove the oxides of nitrogen and diluted to 6 M hydrochloric acid. Iron(II1) was then extracted with three 5-mL portions of diethyl ether (12). The aqueous layer was transferred to a 100-mLvolumetric flask and diluted to the mark with double distilled water. A suitable aliquot of this solution was taken in a separating funnel, and the tungsten content was determined following the recommended procedure. The results are presented in Table 11. The results are also compared with the standard thiocyanate method. The proposed method is selective, reliable, and reproducible over the thiocyanate method.
ACKNOWLEDGMENT We thank Messrs. Visvesvaraya, Iron and Steel, Ltd., Bhadravathi, India, and Rhone-Poulenc, Paris, for supplying alloy steels and pure PPC, respectively. Registry No. PPC, 2622-26-6; W, 7440-33-7;tungsten steel, 11114-36-6;thiocyanate, 302-04-5. LITERATURE CITED (1) Fogg, A. G.; Marriot, D. R.; Burns, D. T. Analyst (London) 1970, 95, 848-854. (2) British Iron and Steel Research Association Methods of Analysis Committee J. Iron Steel Inst. London 1952, 172,413-428. (3) Norwitz, G.; Codell, M. Anal. Chim. Acta 1954, 7 1 , 359-365. (4) Wood, D. F.; Clark, R. T. Analyst (London) 1958, 8 3 , 325-332. (5) Hobart, E. W.; Hurley, E. P. Anal. Chim. Acta 1062, 27, 144-152. (6) Gottschalk, G. 2.Anal. Chem. 1962, 787, 164. (7) Sultanova, 2. Kh.; Chuchalin, L. K.;Jofa, B. 2.;Zolotov, Yu. A. Zh. Anal. Khim. 1973, 28,413-421. (8) Topping, J. J. Talanta 1078, 25, 61-65. (9) Yathirajan, V.; Sudershan, D. Talanta 1075, 22, 760-762. (IO) Donaldson, E. M. Talanta 1075, 22, 837-841. (11) Cogger, N. Anal. Chim. Acta 1976, 8 4 , 143-148. (12) Vogel, A. I. “A Text Book of Quantltative Inorganlc Analysis”; The ELBS and Longmans: London, 1968; pp 566, 897. (13) Ramappa, P. G.; Sanke Gowda, H.;Manjappa, S. Curr. Sci. 1979, 4 8 , 10 16- 10 17. (14) Sandell, E. B. “Colorimetric Determination of Trace Metals”, 3rd ed.; Interscience: New York, 1959.
RECEIVED for review February 1, 1985. Resubmitted August 26, 1985. Accepted August 26, 1985. A.T.G. thanks the University Grants Commission, New Delhi, and the University of Mysore, Mysore, for the award of a Teacher Fellowship.
