Spectrophotometric determination of rhenium with dithio-oxamide in strongly alkaline medium

0039-9140/88 $3.00+ 0.00 Copyright0 1988Pergamon Journals Ltd

Talonta,Vol. 35, No. I, PP. 62-64, 1988 Printedin Great Bntam. All rights reserved

SPECTROPHOTOMETRIC DETERMINATION OF RHENIUM WITH DITHIO-OXAMIDE IN STRONGLY ALKALINE MEDIUM 0. D. BOZHKOVand N. JORDANOV Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, BG-1040 Sofia, Bulgaria L. V. BORIKWA V. I. Vemadskii Institute of Geochemistry and Analytical Chemistry, USSR Academy of Sciences, Moscow, USSR (Received

10 October 1986. Revised 6 August 1987. Accepted 21 August 1987)

Sammary-The interaction between rhenium(VI1) and dithio-oxamide in strongly alkaline medium in presence of tin(I1) chloride as reductant has been studied. A purple complex is obtained, with J_,,._ 526 nm and L,, = 4.0 x lo3 l.mole-‘.cm-‘. The reaction has been applied to the determination of rhenium in tungsten-rhenium alloy after its anodic electrochemical dissolution in alkaline medium. A IOOO-foldexcess of molybdenum or tungsten does not interfere. A modification of the proposed method can be used as a spot-test for rapid control of rhenium content in industrial solutions.

It is often necessary to analyse strongly alkaline rhenium solutions. A spectrophotometric method is known for determination of rhenium in slightly alkaline or ammoniacal media, based on measuring the perrhenate absorbance at 230 nm.‘s2 Two other methods are also available for determinaton of rhenium in strongly alkaline medium. The first is based on the interaction between perrhenate and thiourea in the presence of tin(I1) chloride as reducing agent3 and the second (6300“In- 1.5 x lo3 l.mole-‘.cm-‘) on the interaction between perrhenate and hydroxylamine4 (tjW nm= 7.9 x lo3 l.mole-L.cm-i). Both procedures involve absorbance measurement in the ultraviolet region, which results in several interferences. The direct perrhenate and thiourea methods suffer from poor sensitivity and the hydroxylamine method suffers from systematic errors when large amounts of molybdenum and tungsten are present. The interaction between rhenium(IV) and dithiooxamide (DTO) has so far been studied only in acidic medium.j It was shown that a mixture of rhenium(IV) sulphide and elemental sulphur is obtained. The interaction between H,(ReOCl,) and DTO has also been studied in acidic medium.6 A compound with composition ReOCl,(DTO), was isolated. The ReDTO bond was reported to be unidentate and formed with a sulphur atom of the DTO. The present paper deals with a new possibility for rhenium determination in a strongly alkaline medium, by interaction of rhenium with DTO in the presence of a reducing agent.

Reagents Standardrhenium solution. Dissolve 0.1553 g of potassium perrhenate in 100 ml of distilled water; 1 ml contains 1000 pg of Re. Dithio-oxamide, 0.04M solution in ?M sodium hydroxide. Dissolve 0.240 g of the reagent in 50 ml of 7M sodium hydroxide. Use only fresh solutions. Tin(D) chloride, 0.24M solution in 7M sodium hydroxide.

Dissolve 5.42 g of the dihydrate in a very small volume (2-3 ml) of distilled water. Add 7M sodium hydroxide ( < 95 ml) with continual stirring until a clear solution is obtained, and then make up to 100 ml with more of the alkali. Use only fresh solutions. Hydrazine hydrochloride solution. Dissolve 7.55 g of the hydrochloride in 100 ml of 7M sodium hydroxide. Use only fresh solution. Standard molybdenum and tungsten solutions. Prepared by dissolving sodium molybdate dihydrate and sodium tungstate dihydrate in 7M sodium hydroxide. Procedures Calibration

graph. Pipette 0.1, 0.2, 0.3, 0.4 and 0.5-ml portions of the standard rhenium solution (1000 pg/ml) into dry lo-ml graduated cylinders fitted with ground-glass stoppers. Add 1.2 ml of DTO solution and 8 ml of stannous chloride solution, make up to volume with 7M sodium hydroxide, mix, and let stand for 45 min. Measure the absorbance at 526 nm against a reagent blank. Plot the calibration graph. Qualitative

EXPERIMENTAL Apparatus

A Beckman trophotometer

and semiquantitative

spot-test

for

rhenium.

Prepare a colour scale by pipetting 0, 5, 15, 25 and 40 ~1 of 1000~pg/ml rhenium solution into wells in a Teflon spot-test plate, and to each add 90 ~1 of DTO solution, 130 ~1 of stannous chloride solution, and make up to 260 ~1 with 7M sodium hydroxide and stir with a thin glass rod. To 20 ~1 of the unknown solution add the quantities of the reagents prescribed above. Compare the resulting colour with the reference scale. The limit of detection is 5 pg/ml. Up to IOOO-fold ratio of MO or W does not interfere with the colour reaction. Procedure for determination in the presence of IOOO-fold ratio of Cu. Add DTO solution to the sample solution until

DK-2A double-beam scanning specwas used with l-cm fused-silica cells.

formation of a brownish-black 62

precipitate ceases. Centri-

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COMMUNICATIONS

fuge, and decant the supematant solution through a medium fast filter paper. Take an appropriate volume of this solution, add the stannous chloride solution, stir and let stand for 45 min. Measure the absorbance. Analysis of w--Re ciIoy. Weigh 0.2 g of the alloy (chips) into a platinum spoon (to be used as anode). Place the spoon and a platinum coil (as cathode) in 20 ml of 5M sodium hydroxide. Connect to a stabilized rectifier delivering 5-6 V and OS-O.9 A. About 40 mg of alloy will dissolve within 20 min. Find the amount of sample dissolved, by removing, rinsing, drying and weighing the anode and residual sample after the electrolysis. Before switching off the current, reverse the polarity of the electrodes until the brown deposit of metallic rhenium on the cathode has dissolved (as little as 20 pg of Re will darken the cathode). Make up the solution to known volume and analyse an aliquot for rhenium as described above. Spectrophotometric &termination of rhenium in jnishing product (a mixture of metallic particles of Mo- W-Re based altoy, fzb~usiues and ~u~rieat~g oil). Treat the sample preliminarily with chloroform to eliminate the organic components and dry it to constant weight. Weigh 1 g of the treated sample and mix with 1 g of sodium hydroxide and 1 g of sodium nitrate. Stir the mixture well and place it in an iron crucible. Heat it to 600” in a mufIle furnace and keep it at this temperature for 1 hr. Cool the melt and treat it with l&-IS ml of distilled water. Filter through a “rapid” filter. Transfer the clear filtrate to a 25ml standard flask and make up to volume with distilled water. Analyse an aliquot as above. Spectrophotometric determination of rhenium in a sublimation product (mixlure of tungsten and rhenium oxides). Weight 1 g of the sample-into-a Teflon beaker and add 10 ml of 5M sodium hydroxide. Place the beaker in an autoclave and digest the-sample in oxygen at a pressure of 5-6 atm, at 110°C for 1 hr. Transfer the resulting clear solution into a 2%ml standard flask with 5M sodium hydroxide and make up to volume. Take an aliquot and analyse it as above. lII?SULTs

AND DISCUSSION

When perrhenate solution is mixed with DTO solution in I-12M sodium hydroxide medium, no colour appears but on addition of excess of reducing agent-tin(I1) chloride or hydrazine hydrochloridea purple colour is formed. The absorption spectra of 5?6nm

23

A

“U

63 -

0.6

-

0.5

-

A

0.3 0.1 IO

11

NaOH A at

absorption 720, 526 and 328 run. The solution DTO and tin(H) chloride has its absorp tion maximum at 526 mu, and the pure DTO solution absorbs at 315 nm. The of the rhenium-DTO maxima-at

conditions

Figure 2 shows that complex formation takes place in 1M sodium hydroxide medium but only slowly and incompletely. alkalinity the The reaction is still rather slow, however, maximal being attained in 40 min, then remaining constant for at least another hour or so. A large excess of reductant (at least 500: 1 molar ratio to rhenium) is needed for complete reduction of and to provide a medium to prevent possible oxidation of DTO and the complex by air. A complex with identical spectral istics is obtained with hydrazine hydrochloride reductant, which shows that tin(I1) chloride acts only as a reductant and does not form part of the complex. The effect of DTO concentration

720nm

Xlnm)

Fig

Absorption __

spectra,

t = 45 mitt,

I = 1 cm,

DTO-tin(H) chloride solution, C, 1.61 x IO-‘&f, Cnrc 4.8 x 10-3M, C,, 0.19M, C,,ou 7M; of DTO solution in 7M NaOH recorded against distilled water, Cnro 4.8 x lO-3M; of DTO + tin(I1) chloride solution in 7M NaOH recorded against distilled water, Cnrc 4.8 x 10-3M, C,, 0.192M.

CDTOl/CRrl Fig. 3. A at 526 nm plotted us. C&o; Ca, 1.07 x 10m4M, C, O.l92M, C,, 7M; spectra recorded against a blank solution.

64

SHORT

CO~MJN~CATIONS

Table 1. Determination of rhenium in various samples Present method Amount dissolved, mg

Sample W-Re alloy Mo-W-Re finishing product W-Re subhmation product

Tbiourea method

Number of replicates

Mean, %

Std. devn., %

Mean, %

Std. devn., %

6 5 5

7.30 0.603 0.517

0.50 0.007 0.007

7.18 0.587 -

0.50 0.028 -

40 1000 1000

takes place, precipitation of hydrolysis products being observed, since the hydroxide ions compete with the DTO reaction. At higher DTO: Re ratios the degree of complex formation increases and at 20: 1 molar ratio constant absorbance is attained for a fixed amount of rhenium. Further increase in the ratio causes no shift in the absorption maximum at 526 nm, indicating formation of a single complex species in the system. For the determination a 30-fold molar ratio of DTO to rhenium is used,

We therefore suggest that the composition of the complex is [Re(OH),(DTO)J-. The molar absorptivity at 526 nm is (4.0 f 0.3) x lo3 l.mole-‘. cm-‘. Beer’s law is obeyed over the rhenium concentration range 5-50 ,ug/ml. The standard deviation of the absorbance is 0.016 for 20 and 40 fig/ml Re (9 replicates), and the relative standard deviations 3.6 and 1.7% respectively.

Composition of the complex

MO and W do not form complexes with DTO under the conditions described. It is found that a 1~0-fold ratio of MO and W does not interfere with the determination.

When excess of tin(I1) chloride is added to an alkaline perrhenate solution a brownish black precipitate (most probably of ReOZ) is formed. In the presence of excess of DTO no precipitate is formed, but a purple colour appears (complex formation). The same absorption spectrum is obtained with tin(H) sulphate as reductant. Addition of excess of alkaline DTO solution to potassium hexachlororhenate(IV) solution produces a purple colour, which has an absorption maximum at 526 nm but is unstable, fading within 30 min. The addition of an excess of reductant-tin(H) chloride or hydrazine-stabilizes the colour and the absorbance at 526 nm is stable for about an hour after complete formation of the complex. The reductant will reduce higher oxidation states of rhenium and prevents both complex and ligand from oxidation by atmospheric oxygen. These facts suggest that the rhenium in the complex is in oxidation state (IV). Ion-exchange and electrophoresis show that the complex is negatively charged. All attempts to isolate the solid complex from alkaline medium have failed. The complex is hydrophilic and is not extracted by a wide variety of organic extractants. The Re:DTO ratio in the complex is 1:2, as determined according to the procedure of Staric and Barbanel.’ The stability constant of the complex, determined by the mole-ratio and Babko’ methods is fl = (1.2 f 0.15) x 106. It is very likely that in strongly alkaline medium the ionized thiol form of DTO is present and the Re-DTO bond should be bidentate.

--S--C=NH -s--L -NH The thiol form of DTO

Interferences

Applications

Three industrial samples were analysed by the proposed method. Comparison analyses of the alkaline solution were performed as follows. Rhenium was separated from the matrix by extraction with acetone from alkaline medium. The extract was evap orated to dryness. The dry residue was dissolved in distilled water and an aliquot of the resulting solution was analysed by the thiourea method.8 The results are shown in Table 1.

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372. 2. L. V. Borisova, Yu. B. Gerlit and A. N. Ermakov, PerspektrOy razvitiya i melody izulecheniya redkikh i rasseyannikh elementov iz rud i mestorozhdenii Armenii i Soyuza, Izd. Akad. Nauk Arm. SSR, Erevan, 1962. 3. L. V. Borisova, A. B. Ismagulova and E. I. Ponomareva, Zh. Kompleksnoe Zspol’zovanie Mineral’nogo sirya (Alma Ata), 1984, No. 12, 22. 4. L. V. Borisova, A. N. Ermakov and A. B. Ismagulova, Anufyst, 1982, 107, 495. 5. V. M. Taravan and A. G. Gaibakvan. Arm. Khim. Zh., 1966, 19, 662. 6. V. Yatirajam and M. L. Kantan, Polyhedron, 1983, 2, 1199. 7. M. I. Bulatov and I. P. Kalinkin, Frakticheskoe IYJkov~~o po fotometr~cheskim i s~ktrokh~icheskim metodam analiza, pp. 222, 227. Izd. Khimiya, Leningrad, 1976. 8. L. V. Borisova and A. N. Ermakov, Analiticheskaya khimiya reniya, p. 101. Izd. Nauka, MOSCOW, 1974.