Spectrophotometric Determination of Rhenium VlLLlERS W. MELOCHE and RONALD L. MARTIN Department o f Chemistry, University of Wisconsin, Madison 6, W i s .
4 rapid, accurate, and sensitive spectrophotometric method for the determination of rhenium is based on the formation of hexachlororhenate(1V) ion by the chromium(I1) chloride reduction of perrhenate in strong hydrochloric acid solution. The ahsorption maximum of hexachlororhenate at 281.5 m p is used, and the small absorbance of the chromium at this wave length is eliminated by the use of a blank. The stoichiometry of the chromium(I1) chloride-perrhenate reaction is discussed.
A
S P E C T R O P H O T O l I E T R I C method for determination of rhenium, based on the formation of hexachlororhenate(1T’) ion, was proposed by RIeyer and Ru!fs (11). T h e hexachlororhenate was produced by boiling perrhenate with a large excess of hydrazine for an hour in about 1OiY hydrochloric acid. ilbsorbance readings n-ere taken a t the absorption masima for hydrazine and hexachlororhenate, and a two-component anaij..-is n-as applied. An investigation in this laboratory shovc-ed that hesachlororhenate could be produced quantitatively by the chromium(I1) chloride reduction of perrhenate. A spectrophotometric method involving this reaction has two inherent advantages over the RIeyer and Rulfs method. Hexachlororhenate can be produced quantitatively by chromium(I1j chloride reduction in a couple of minutes, thus decreasing the time required for analysis by severalfold, and the two-component analysis can be eliminated. T h e contribution of chromium to the total absorbance a t the hexa-
Table I.
Analytical Data
Rhenium, RZg.
Absorbance Reading at 281.5 m r
Molar Absorbancy Index X 10-1
Average Deviation,
0,0721
0.100 0.098
1291 1266
+1.3 -0 7
0,1444
0.19fi 0.198
1264 1278
-0.9 f0.2
0.2186
0,296 0.300
1274 1291
-0.1 +1.3
0.3247
0,444 0.447
1274 1282
+0.5
0.504
0,589
1267 1278
-0.6 +0.2
0,5412
0,742 0.739
1277 1272
+0.2 -0.2
0,6496
0.887 0.892
1272 I279
-0.2 +0.3
0.7577
1.034 1.038
1271 127G
-0.3 10.1
0,8660
1.184 1.180
1274 1269
-0
0 9712
1.333 1.334
1276 1273
1 ,0824
1.476 1.481
1270 1273
-0.4
1.1906
1.626 1.622
12i2 12c9
-0.2 -0.5
1 ,2089
1.776 1.777
1274 127.5
-0.1 0.0
1 4Oil
1.929 1 932
1277 1279
+0.2 +o 3
0,4330
.1~. 127.5
%
-0.1
-0
1
5
0 0 0 0
0.0
0.31
chlororhenate maximum is small an'.^ :an be simply and precisely cancelled with a blank. This paper describes chromium(11) chloride reduction of perrhenate to hexachlororhenate, and the application of this reaction to the spectrophotometric determination of small amounts of rhenium. EXPERIMENTAL
Reagents. The perrhenate stock solution was prepared from potassium perrhenat,e (University of Tennessee). T h e potassium perrhenate assayed 99.5% by determination with nitron (a). Potassium hexachlororhenat,e(IV) was prepared by reduction of potassium perrhenate n-ith hydroiodic acid in concentrated hydrochloric acid solution ( 6 ) . T h e hexachlororhenate(1V) ion in the preparation assayed 99.1yo by determination as cesium hexachlororhenate (1, 12). Standard chromium( 11) chloride solutions were prepared directly in the storage flask by the procedure described by Lingane and Pecsok (8). T h e concentration of the standard chromium(I1) chloride solution was checked by titration against standard potassium dichromate in the presence of excess ferrous ions (8). Apparatus. Light absorption measurements were made in 0.1-cm. and 1-em. silica cells on the Cary Model 1411 recording spectrophotometer. This instrument was convenient t o use in the spectrophotometric study, because accurate measuremenk could be made over the entire absorbance range. The absorbance measurements a t 281.5 m p were made wit,h a slit width of 0.07 mni. Procedure. About 6 t o 7 ml. of the solution t o be analyzed, containing between 0.07 and 1.4 mg. of rhenium as perrhenate, were placed in a 50-ml. volumetric flask, and 25 ml. of concentrated hydrochloric acid (12.4N) were added. Nitrogen was p%sed through the flask while 1.00 ml. of about 0.0451M chromium(I1) chloride was added. T h e volumetric flask was stoppered and after a m i m t e or two had been allowed for the reduction to be completed, air was bubbled through the solution for 2 to 3 minutes. After dilution t o 50 ml., the absorbance of the solution was read a t 281.5 mp in a 1-cm. silica cell against a blank prepared in the same manner except for the exclusion of the perrhenate. T h e absorbance reading n-as converted to concentration of rhenium by using the Beer’s law relationship and a value for the molar absorbancy coefficient determined previously on the spectrophotometer being used. R E S U L T S AND DISCUSSION
Analytical Data. Varying knon-n amounts of a standard potassium perrhenate stock solution were added t o a series of 50-ml. volumetric flasks by means of a calibrated micrometer syringe buret. T h e perrhenate solutions were treated as described in the procedure. T h e absorbance value for each solution is presented in Table I. T h e solutions analyzed covered the entire concentration and absorbance range applicable to this procedure. The average deviation of the molar absorbancy index values from the average index over this range x a s 0.34%. This indicated that Beer’s law was followed closely over this concentration range, I n order to check the validity of the Beer’s lam relationship a t higher hexachlororhenat,e concentrations, the absorbances of higher rhenium concentrations were measured using 0.1-cm. instead of 1-cm. cells. Beer’s law was found to be rigorously valid for these concentrations also. As a further check on t,he reproducibility of the method, eight replicate determinations were made a t each of two rhenium levels, 0.4330 and 1.0825 mg., according to the procedure described above. The average deviation of the individual absorbance9 from the average absorbance for the two rhenium amounts was 0.20 and 0.187,, respectively, For the smaller rhenium amount, the concentration of the chromium(I1) chloride solution added 1671
*
1672
ANALYTICAL CHEMISTRY
was cut to two thirds of the amount suggested in the procedure so that the error in the cancellation of the chromium contribution from the absorbance would be smaller. For the solutions whose absorbances are tabulated in Table I, the per cent deviation of the molar absorbancy index from the average index also can be thought of as the per cent error in the determination of rhenium in the solution. Rhenium was determined in these solutions with an average deviation of 0.347,. It is necessary to determine the molar absorbancy index on the specific instrument to be used. Values on different spectrophotometers have been found to vary by as much as 10%. Choice of Acidity during Reduction. Figure 1 is a plot of the data obtained when chromium(I1j chloride reduction of perrhenate was performed in solutions of varying hydrochloric acid concentrations. Exactly 1 ml. of 0.045M chromium(I1) chloride was added to each of a series of 50-ml. flasks containing 25 ml. of solution with varying hydrochloric acid concentrations and 1.082 mg. of rhenium as potassium perrhenate. After reduction, air was bubbled through the solutions for 2 t o 3 minutes. Then the contents were diluted t o 50 ml., including in the dilution sufficient hydrochloric acid so that the final acid concentration was 6.2N in each case. The absorbance of each of these solutions was measured against an appropriate blank a t 281.5 mp, the peak absorbance of hexachlororhenate. The per cent of rhenium as hexachlororhenate was calculated by interpolation from absorbances of solutions prepared from pure potassium hexachlororhenate(II-j.
-
100
80
-
60
-
-
11 0
0 0
LX
40-
.\e
20
-
NORMALITY OF HCI Figure 1. Yield of herachlororhenate a8 a function of hydrochloric acid concentration during reduction The plot in Figure 1shows that practically no hexachlororhenate was present in the final solution when the hydrochloric acid concentration was below 4N during reduction. As the concentration of hydrochloric acid a t the time of reduction increased, the concentration of hexachlororhenate increased. When the acid concentration reached about 8N, the yield of hexachlororhenate was 100% and higher acid concentrations caused no change. Any hydrochloric acid concentration between 8- and 11N could be used for the spectrophotometric determination of rhenium. A hydrochloric acid concentration near the middle of the region was selected, so t h a t a slight variation v,-odd not affect the yield of hexachlororhenate. Use of a Blank. One advantage of the chromium(I1) chloride reduction over the hydrazine reduction is that it does not require
a two-component spectrophotometric analysis technique; the absorbance of chromium(II1) ions a t 281.5 mp is small, and can be accurately canceled with an appropriate blank, .411 the chromium in the blank and in the unknowns existed as chromium(II1j chloride after air was bubbled through the solutions. The res ilting chromium(II1) chloride contributed an absorbance of only 0.06 to the total absorbance of the solution after dilution to 50 ml. By adding the same amount of chromium(I1) chloride to the blank and to the unknown, the contribution of the chromium(II1) ions t o the total absorbance was cancelled with good accuracy. A group of ten blanks prepared in this manner all fell within an absorbance range of 0.003 unit The hydrochloric acid content of the solution also contributes a small amount to the total absorbance. However, if the h? drochloric acid added to the sample and to the blank differs by no more than 1 ml., its absorbance nil1 be canceled with an error of no more than 0.001 absorbance unit. The absorbance contribution from the zinc ions picked up from the zinc reductor was negligible. It is important that the chromium be added t o the blank in the same manner as it is added to the unknown. The chromium must not be added to the blank as the unreduced chromiuni(111) ions, but should be added as chromium(I1) ions, just as in the qoliitions containing rhenium. This precaution is necessary because there are several possible species of chromium(II1) ions, n-hich have slightly different absorbancy indices. I n order for the absorbance contribution of the chromium to be canceled accurately, the same chromium(II1) species must be present In the blank and in the unknown. Production of Chromium(I1) Chloride. The apparatus used for storing and dispensing standard chromium(I1) chloride solutions was the same as t h a t described by Lingane and Pecsok ( 8 ) , except that a dry ice carbon dioxide generator ( 4 ) \\-as substituted for the Kipp generator. A IO-ml. buret was used in place of the 50-ml. buret, so that the 1.00 ml. suggested in the procedure could be delivered more accurately. A simpler method, which n orked almost as well, involved pouring chromium(II1) chloride through a buret partly filled with amalgamated granular zinc. Then the amount of chromium(I1) chloride delivered was read directly from the buret. T h e volume of the chromium(I1) chloride delivered could not be reproduced quite as accurately as by the other procedure, because the height of the liquid column drifted slightly, owing to hydrogen evolution. However, if the zinc was well amalgamated and if the aciditj- of the chromium(II1) chloride solution was no greater than O . O l h ' , the results nere satisfactory. The amount of chromium(I1) chloride suggested in the procedure is more than is necessary to reduce easily the maximum amount of rhenium that can be determined by this procedure. The oxygen need not be removed from the original solution. I t is not important that the concentration of the chromium(I1) chloiide solution be accurately known; but the same amount mrist be added to both the blank and the unknown. Reaction Path. I n order t o obtain a quantitative yield of hexachlororhenate, it was necessary t o expose the reduced rhenium species to air. When a large excess of chromium(I1) ions was added to a strong hydrochloric acid solution of perrhenate under an atmosphere of nitrogen, absolutely no peak appeared a t the absorption maximum for hexachlororhenate. However, after air was bubbled through the solution for several minutes, the absorbance a t 281.5 mp showed that quantitative formation of hexachlororhenate had occurred. This strongly indicated t h a t the rhenium had been reduced below the quadrivalent state, and then was air-oxidized to hexachlororhenate(1Y) ion. I n order to ascertain how far the perrhenate had been reduced, 0.I500 mmole of perrhenate was titrated potentiometrically with 0.lOOOX chromium(I1) chloride. The titration was conducted in 9.8'V hydrochloric acid solution, so that it would correspond t o the perrhenate reduction employed in the procedure.
.
1673
V O L U M E 2 8 , NO. 11, N O V E M B E R 1 9 5 6 T h e titration curve gave a large break in poteritial after only 3.0 equivalents of chromium(1Ij had been addei! (Figure 2,.4). This, a t first, seemed to indicate that the perrhenate had been reduced only t’o the quadrivalent state. I n order t o obtain information concerning the course of the Teaction, small samples were withdran-n from the titration flPsk at regular intervals during a titration and replaced under the protection of nitrogen. The absorbances of these samples were measured a t the absorbance peak for hexachlororhenate. Froni the absorbance readings, the per cent of the rhenium present as hexachlororhenate was calculated. T h e amount of hexachlororhenate formed was quantitativt a t the end point of the titration, but fell off rapidly as excesb chromium(I1) chloride was added (Figure 2,B). Evidently, the excess chromium(I1j chloride reduced the hesachlororhenate(1T’) ion t o a lower valence state. This vie\\- was strengthened by the fact that every sample withdrawn after the end point when exposed to air gave an absorbance reading corresponding to quantitat,iveformation of hexachlororhenate. The slope of the line after the end point in Figure 2,B, is suffic4cntly great so that the reduction of hexachlororhenate(1V:’ ion could not involve a change of 2 or more electrons. On thi:, h i s , it seems apparent that the chromium( 11) chloride reduce(. some of the hexachlororhenate(I1-) ion to rhenium(II1) aft?:. the end point of the titration had been passed. That no second break occurs in the titration curve is not p r w f t h a t further reduction does not take place. Even if potential tlift’erences favored a second break in the titration curve, th:s stage would not appear, because a t potentials more negatkre than about -0.2 volt, the platinum indicator electrode approximates the potential of the H + - Hz conple. This is a result of the reaction Cr(I1)
that, although the hexachlororhenate yield was not quantitative a t the lower hydrochloric acid concentrations, the aeration had oxidized the remainder of the reduced rhenium to some form other than hexachlororhenate. Interferences. T h e most important interfering element in the analysis of minerals and residues for rhenium is molybdenum. The authors have verified the use of the chloroform-cupferron extraction, applied by hleyer and Rulfs (11) for the elimination of molybdenum. This procedure can be applied directly, without modification, t o the rhenium determination described in this paper
f H + = l/*H2+ Cr(II1)
u-hich takes place a t the surface of the indicator electrode. This ~vouldobscure the break for the second stage of the rhenium rediiction. The reported evidence concerning the stability of acid solutions of trivalent rhenium is conflicting (3,6, 7 , 9). The findings of 3Iuncahot and Dusing ( 9 ) seem consistent with the results report,ed i n this paper. They reported the rhenium(IT1) in strong hydrochloric acid solution was oxidized by air to hexachlororhenate(Is’ j ion. Their rhenium(II1) was produced by cathodic reduction of pot:tssium hexachlororhenate(1Vj in 2- to 4 5 sulfuric acid with mercury and platinum electrodes. The titration curve of Figure 2 4 , shows that the break using conditions of the xork reported here came a t 3 equivalents, lr-hereas Tribalet. ( I S ) under apparently similar conditions obtained a break after 4 equivalents had been added. Oxidation of Reduced Rhenium Species. One t o 2 minutes of bubbling compressed air through the solution a t a slon- rate was sufficient t o oxidize all t,he reduced rhenium to hexachlororhenate(IIT)ion. Extended aeration times led to no damaging effects, because of t,he extreme stability of hesachlororhenate toward air oxidation ( I O ) . Complete oxidation Tvas also obtained after the Teduced solution was left standing in the unstoppered flask for about a half hour. It was essential that the air oxidation be conducted whilcb the hydrochloric acid concentrat,ion was still high, before dilution t o final volume. Where the hydrochloric acid concentration TS less than 8n’ during aeration, the yield of hexachlororhenate T Y s~ lers than loo%. Aeration of a reduced rhenium solution whil h \\-:is 6 . 2 5 in hydrochloric acid produced an 82% yield of hex.ichlororhenate. When the hydrochloric acid concentration wa * 3.1S, the hexachlororhenate yield dropped to 24y0. \Then the hydrochloric acid concentration of these aerate.1 solutions was increased t o 9.V and the aeration n-as repeatec 1, the yield of hexachlororhenate did not increase. This indicatr.1
0
1
0
1.0 2.0 EQUIVALENTS
Figure 2. A.
B.
I
3.0
4.0
O F CHROMIUM (11)
Effect of exposure to air
Titration of potassium perrhenate with chromium(r1) chloride in 9 . 8 N hydrochloric, acid 55 hexachlororhenate formed a s function of chromium(I1) chloride added
Other interferences to the rhenium determination of JIeyer and Rulfs (11) are almost identical to those for this method. T h e methods described by IIeyer and Rulfs for the elimination of these interferences are also applicable here, LITERATURE CITED
(1) Enk. E.. Ber. 64. 791 (1931). i2j Geillnann, W., Voigt, I{., Z . anorg. allgena. Chem. 193,311 (1930). (3) Geilmann, W., Wrigpe, F. W., Ibid., 214, 248 (1933).
(4) Hershberg, E. B., Wellwood, G. IT., IND.ENG.CHEW,ANAL. ED.9,303 (1937). (5) Hurd, L. C., Reiiiders, V. .1., “Inorganic Synthesis,” vol. 1, p. 178,1IcGraw-Hill, S e w York, 1939. (6) Kolling, 0. IT., Trans. Kansas Acad. Sci. 56, 278 (1953). (7) Latimer, W.lI.,“Oxidation Potentials,” Prentice-Hall, Sew York, 1952. (8) Lingane, J. J., Pecsok, R. L.. .b-.ir,. CHEM.20, 425 (1948). (9) llanchot, W., Dusing, J., Arm. 509, 228 (1934). (10) AIaun. E. I
