Spectrophotometric Catalytic Determination of Small Amounts of Rhenium in Mineralized Rocks and Molybdenite F.
0.SIMON'
The American University, Washington, D. C.
F. S. GRlMALDl U. S. Geological Survey,
Washingfon, D. C.
b
Rhenium is determined b y spectrophotometry of the tellurium sol formed b y the reduction of tellurate b y stannous chloride under the catalytic influence of rhenium. A detailed investigation of the conditions for high sensitivity and stability a t lowest concentration levels of rhenium is presented as well as the behavior of 2 6 ions. The method is applied to the determination of some tenths of 1 p.p.m. or more of rhenium in a 1-mg. aliquot of mineralized rocks, mixtures of molybdenite and rocks, and molybdenite concentrates. The practical quantity limit of detection is 2 X lo-'' gram of rhenium. Samples are decomposed with a mixture of C a O , CaCIz, and M g O . O n leaching, most constituents of the sample are precipitated either as calcium salts or hydroxides, except for rhenium and a small amount of molybdenum which pass into the filtrate. Residual molybdenum is removed b y extraction with 8-quinolinol in chloroform. Better than 95% recoveries are obtained with two fusions with flux.
P
proposed a very sensitive colorimetric method for the determination of small amounts of rhenium, based on his discovery that rhenium catalyzes the reduction of sodium tellurate to tellurium metal by stannous chloride (1-4). Although Poluektov indicated that as little as 0.001 pg. of rhenium can be determined, most of the data he presents, including the conditions for carrying out the reaction, were based on experiments involving relatively large amounts of rhenium in the tenths of a microgram range. Also, the determination of 0.001 pg. of rhenium requires an inconvenient 20-hour wait for color development. Because of the importance of the reaction, a more detailed investigation a t the low end of the rhenium concentration range seemed warranted. At the Present address, U. S. Geological Survey, Washington, D. C. OLUEKTOV
rhenium per ml. were stable for a t least 6 months. GUMARABIC,O.47yG(w./v.) in water. The addition of 0.1 ml. of hydrochloric acid to 200 ml. of solution prevents mold formation. TARTARIC - ~ C I D ,50% (w./v.) in water. Dissolve 50 grams of anhydrous tartaric acid in sufficient water to make 100 ml. of solution. MIXED REAGENT. Mix 2.5 ml. of tartaric acid solution, 5.2 ml. of 1 to 1 hydrochloric acid, 1.0 ml. of stannous chloride, and 4.0 ml. of gum arabic solution and dilute to 25-ml. volume with water. Prepare the solution fresh at the time of use. ~-QUINOLIXOL SOLUTION. Dissolve 25 grams of 8-quinolinol in 58 ml. of glacial acetic acid and dilute to 1 liter. EXPERIMENTAL SODIUM ACETATE SOLUTION, 1-v. Apparatus. A Beckman DU specDissolve 82 grams of sodium acetate in trophotometer with 1-em. Corex cells water and dilute to 1 liter. Procedure. Mix in a 30-ml. porcewas used. Reagents. SODIUM TELLURATE, lain crucible 0.5 gram of finely pow0,7570 (w./v.) in water. The sodium dered sample with 1 gram of CaC12. tellurate was prepared from 99.99% 2 H z 0 , 2 grams of CaO, and 0.75 tellurium metal (obtained from the gram of MgO. Starting with a cold American Smelting and Refining Co.) furnace, heat to 900" C. and maintain according to the procedure of Poluektov this temperature for 11/* hours. Cool and leach the sinter with 100 ml. of (3). High purity is important, in that a number of elements such as Se and Cu water for 15 minutes a t boiling temdisturb the reaction. perature, stirring intermittently. Convert 3 grams of metal to tellurium Filter on a tight paper such as Whatman 42, collecting the filtrate in a 200dioxide by heating with 30 ml. of 3 to 1 nitric acid. Decant and wash with ml. volumetric flask, and wash with 5 cold water. Dissolve 2 grams of dry ml. of water. Dry the paper in an oven TeOz in 35 ml. of 2501, (w./w.) KaOH and transfer the residue to another solution in a glass beaker. Add 5 ml. porcelain crucible. -4dd 1 gram of of 30% hydrogen peroxide, and heat for CaClz.2Hz0 and mix. Heat and leach 1 to 2 hours to oxidize the tellurium as before, collecting filtrate in the same and destroy excess peroxide. Decant flask. Reject residue. Adjust filtrate the solution from the precipitated to 200 ml. by addition of water (some sodium tellurate and wash by decantawater is lost during the boiling), tion repeatedly with cold water to reTransfer a 20-ml. aliquot to a 60-ml. move NaOH. Dry in air. separatory funnel. Add 1 to 2 drops STANNOUS CHLORIDE SOLUTION.Disof phenolphthalein indicator (O.lyGin solve 375 grams of stannous chloride alcohol) and titrate the solution caredihydrate in 100 ml. of hydrochloric fully with 1 to 1 HCl until decolorized. acid. Adjust volume to 250 ml. with Adjust to a very faint pink with 1% water. NaOH solution. STANDARD RHENIUM SOLUTIONS. PreAdd by pipet 2 ml. of 8-quinolinol pare from pure potassium perrhenate a solution and 2 ml. of sodium acetate stock solution containing 1 mg. of solution and mix. Extract with three rhenium per ml. in 10% (v./v.) hydro10-ml. portions of chloroform. Reject chloric acid. Prepare less concentrated chloroform layers. Transfer the water standard solutions down to 0.001 pg. layer to a 50-ml. beaker and heat on a per ml. by dilution with water. Solusteam bath to remove chloroform. tions containing 0.01 pg. or more of Cool, transfer to a 50-ml. volumetric same time an increase in sensitivity was sought. The conditions finally arrived a t in the method proposed in this report are considerably different from those of Poluektov. The time for the determination of 0.001 ug. of rhenium has been reduced by more than a factor of 5, and about 0.0002 ug. of rhenium is the smallest practical amount that can be conveniently determined. The method is applied to the determination of some tenths of a part per million or more of rhenium in a milligram aliquot of mineralized silicate rock, mixtures of molybdenite and silicates, and molybdenite concentrates.
VOL 34, NO. 1 1 , OCTOBER 1962
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1361
flask, dilute to volume, and mix. For a sample containing 1 p.p.m. or less, take 1-ml, aliquots for color development. For samples containing greater amounts of rhenium, make further dilutions so that the aliquot taken for color development contains between 0.003 and 0.03 pg. of rhenium. Transfer a 1- or 2-ml. aliquot of the final diluted solution to a 30-ml. beaker. At the same time prepare a blank using 2 ml. of water and two standards containing 0.005 and 0.03 pg. of rhenium. Adjust all volumes to 2 ml. by addition of the required amount of water by PWJ. Add by pipet 2 ml. of mixed reagent to each and mix by swirling. Add rapidly by pipet 1 ml. of tellurate solution and mix. Allow the solutions to stand 1 to 5 hours, depending on the rhenium content. As a guide, 1 to 2 hours are sufficient for 0.01 to 0.03 pg. of rhenium and 2 to 5 hours for 0.001 to 0.01 pg. of rhenium. Determine absorbances of standards and samples at 400 mp in 1-cm. Corex cells. Obtain the rhenium content of the sample from a working curve relating absorbance of the standards to their rhenium content.
tests increased with time. Some tar-
4 08
0 6-
04-
-
-A
= 56
04
Figure 1.
RFSULTS AND DISCUSSION
Table I.
Re concn., pg./5 ml. Blank
Effect of Acidity
0.5
0.0003 0.0005 0.001
1362
Hours 2
4 1 2 4 4 1
H. 0.03
2
in identical concentrations as in the rhenium samples. Where one variable is sinlged out for discussion, other variables were maintained a t eoncentrations according to operating conditions. Unless otherwise indicated, no provision was made for controlling the temperature. The effects of hydrochloric and tartaric acid concentrations are shown in Figures 1 and 2, respectively. Absorption curves are given in Figure 3. The tartaric acid curves do not indicate a linear relation between absorbance and time for a given rhenium concentration, because the temperature during the
(Based on 4 replicas) Coefficient of variation, yo after time in hours 1 1.5 2 4 6 5.2 5.2 5.2 5.2 14.3 50 17 13.5 12 12
0.0025 0.005
0.02 0.03
A. 0.001 6. 0.001 C. 0.005 D. 0 . 0 0 5 E. Blanks against water F. 0 , 0 0 5 G. 0.03
Coefficient of Variation of Absorbance for a Given Rhenium Concentration
0.0001
0.01
IO
38
Normality of H C I
Re, fig.
Operating Conditions. Factors considered most important were high sensitivity, low reagent blank, and the attainment of a direct linear relation between the absorbance of the tellurium sols and the concentration of rhenium over a large range of rhenium concentration and time. A direct linear relation between absorbance and time for a given rhenium concentration was also desirable. The conditions meeting these requirements and finally adopted were: volume, 5 ml.; HC1 concentration, 0.58N; tartaric acid, 0.1 gram; stannous chloride dihydrate, 0.12 gram; sodium tellurate, 7.5 mg.; gum arabic, 1.5 mg., and wavelength, 400 mp. In all experiments, three levels0.001, 0.005, and 0.03 pg. of rheniumwere taken. Absorbances were measured after definite intervals of time against blanks containing all reagents
4
I
02
5.6 3.3
2.5
ANALYTICAL
2.0
1.2
1.1
0.6
1.7
CHEMISTRY
0.8
5.8 2.7
1.8
5.2 1.1
1.4
9.0
5.5 1.2
3.8
8
5.0 11
8.8 6.0
taric acid is desirable, because the sensitivity of the reaction is drastically reA 0.1-gram duced in its absence. amount of tartaric acid was selected to allow for some complexing of cations and yet sufficient distance from the regions of sharp drop in the absorbance curves. The wavelength of 400 mu was selected over the shorter and more sensitive wavelengths on the expectation that there would be less interference from various ions. Up to a t least 4 hours for 0.001 and 0.005 pg of rhenium and 2 hours for 0.03 pg. of rhenium, absorbances at all levels of rhenium concentration are independent of stannous chloride dihydrate concentration in the range from 0.06 to 0.23 gram per 5 ml. and independent of gum arabic concentration from 0.5 to 2.5 mg. per 5 ml. Absorbances for 0.03 pg. are too high to read after 2 hours. For a given time up to 2.5 hours (maximum tested) and a given rhenium concentration, absorbances increase linearly with increase in sodium tellurate concentration up to 11.3 mg. per 5 ml., the maximum tested. Although less sensitive, the choice of 7.5 mg. of sodium tellurate was based on somewhat greater stability of the sols. At constant temperatures of 21' and 26" C. absorbance increases linearly with time up to at least 4 hours for 0.001 and 0.005 ug. of rhenium and up to a t least 2 hours for 0.03 pg. These data also allow calculation of a temperature coefficient of reaction expressed as a ratio of absorbances a t 26' and 21' C. This ratio calculated for various times at the three concentration levels of rhenium averages 1.52 with an average deviation of 0.06. On this basis it was concluded that temperature need not be controlled in applications of the method. Reproducibility of Absorbances. Absorbances of rhenium concentrations varying from 0.0 to 0.03 pg. per 5 ml. were determined in quadruplicate under operating conditions. Calculated coefficients of variation are listed in Table I for various periods of time up to 8 hours. The absorbances of the blanks were read against water and those of rhenium against blanks. About 0.0002 pg. of rhenium seems to be a practical lower limit of detection. Linearity of Relation between Absorbance and Rhenium Concentration. Ratios of absorbance to rhenium concentration under operating conditions were calculated for various ranges of rhenium concentration. A constant ratio within a given set a t a given time would indicate a direct linear relation between absorbance and concentration. Should this be obtained reasonably well, only a few standards would be required to set wc..king curves, thus eliminating the
P+\
r
\
+\
1
I
1
I
+\
o.E12sd 300
350
450
400
500
Wovelength
Figure
2.
Effect of
Absorption Curves Hours
1.75 4.75 1.75
A.
Tartaric
0.001 6. 0.001 c. 0.005 0 . 0.005 E. 0.03
Acid
Re, f i g .
Hours
0.001 6. 0.001 6. 0.005 D 0.001 E. 0.005 F 0.03 G. 0.005 H 0.03
1
A.
necessity for close matching of sample to standard concentration. Coefficients of variation for this ratio are given in Table 11. Although somewhat higher than the reproducibility of absorbances, the coefficientsare small enough to allow the use of one working curve for the range of 0.00025 to 0.01 fig. of rhenium in 5 nil. of solution. For best results, however, a sample should be matched closely to a standard. Behavior of Various Elements or Ions. Aluminum, Co, C r ( I I I ) , K, Mg, NH4+, Ni, Pb, PO4+, SO,-*, and
Zn do not interfere in 1-mg. amounts calculated as elemental oxides, maximum tested. Two milligrams of CaO and 5 mg. of MnO are without effect, more giving increasing negative errors. The final 5 ml. of solution for color development can also contain a mixture of sodium acetate and acetic acid, each a t 0.032N concentration. Elements that interfere are listed in Table 111. The maximum amount tolerable for an error of less than 10% is based on tests with 0.002 pg. of Re. More can be tolerated when rhenium is
4.75
1.75
present in larger amounts. Except for molybdenum and titanium, where effects are additive, the interferences of other elements are variable and not linear. They depend on both rhenium and element concentration, thus precluding use of an addition technique. About 150 pg, of MOO, and 2500 pg. of TiOz are equivalent to 1 pg. of rhenium in their catalytic effect.
Table 111. Maximum Permissible Amount of Various Oxides for Error of Less Than 10% a t 0.002-pg. Level of Rhenium
Tolerable amount,
Oxide
pg.
MOO8
0.03 0.06
Pt02 Table II.
Coefficient of Variation of Ratio Absorbance to Re Concentration
(Assuming a straight-line relationship through origin) Coefficients of variation, yoafter time in hours Range, pg. Re/5 ml. 0.5 1.0 1.5 2.0 4.0 5.0 6.0 0.0001-0.0005 33 20 9.5 9.0 0.0005-0.0025 12 0.0025-0.01 12 7.1 7.4 3.7 0.01-0.03
0.00025-0.01
4.0
6.3
6.3
7.9
CUO SeOz
8.0 12
600
mp
Re, pg.
T a r t a r i c Acid g/5rnl.
Figure
3.
550
TiOz V2Oa Fez03 Ceros
LaZO? WOP HorOs AszOa
0.1 0.7 0.8 3
10
20 35 40
50 200
Nature of error Pos. Neg. below 0 . 5 Pos. above 1 Neg. Pos.
Pos. Neg.
Pos.
Neg. Neg. Pos. Neg. Neg.
VOL. 34, NO. 1 1 , OCTOBER 1962
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Table IV. Results with One Fusion and Leach on Standard Diabase W-1 Containing Added Rhenium
Re added,
Re found,
0.5 0.5 5 5 50 50 500 500
0.46 0.51 4.9 4.9 45.2 45.5 475 463 (1st fusion)
rg.
PP.
l7
/a
Recovery 92 102
42 (2nd fusion)
Table V.
98
98
90.4 91
92.6 8.4
On leaching the melt, most constituents are rendered insoluble either as hydro.;ides or calcium salts, except for rhenium and a small amount of molybdenum. A rather complete separation of the elements is obtained, as shown by the fact that no precipitate is formed in the filtrate from rocks on adjusting the pH to 7.5 with ammonium hydroxide. About 680 and 440 fig. of hIoOa, respectively, were found in the filtrate when 0.5 and 0.3 gram of M o S 2 were tested; 50 ug. and less than 5 fig. of MoOs mere found when 0.5 gram of W-I was taken to which 1 mg. and 100 ug. of 1\1003,respwtiwly, had been added.
Test of Procedure on Mixtures of W-1 and Molybdenite Containing 70 p.p.m. of Re
Mix taken, gram R-1 0.2 0.4 0.025 0.475 hIoSs 0.3 0.1
Re in mix, pg.
21 7.0 1.75
1st fusion 17.6 6.0 1.6
Re recovered, pg. 2nd fusion 3.1
0.9 0.25
Total 20.i 6.9 1.85
more than 90% complete nith a single fusion and leach, as shown in Table IT. Experiments with molybdenite concmtrate containing 7 0 p.p.m. showed thnt of the rheniuni 57.4 p.p.m. or about is made soluble by a single fusion and leach. On drying the residue, and fusing again after adding 1 gram of CaCI2.2H20, 11.2 p.p.m, more of Re or about 90% of the remainder was recovered. X third fusion and leach reco\ ered 1 .-I p.p,m. more. Experiments rvhere the same molybdenite m s fused only once but four separate leaching5 with water were made indicated that rhenium cannot b~ leached readily in this manner, 57.2, 4.1, I, and 0.3 p.p.m. being obtained, respectively, in each successive leach. The procedure was tested also on mixtures of M7-l with the same molyhdenite (Table V). Nore than 95% recovery of the rhenium is obtained in two fusions. The percentage recover) of rhenium in the first fusion is greatpi-. the smaller the amount of molybdenite taken. LITERATURE CITED
Applications of
The procedure has been applied t o mixtures of a standard diabase W-1 with added rhenium, mixtures of TV-1 and molybdenite, and a molybdenite concentrate. Samples were decomposed with a mixture of CaO, CaC12, and MgO. The MgO acts as a combustion aid, CaCh decomposes silicates, and CaO fixes molybdenum as insoluble Cahfo04. Procedure.
Molybdenum and other elements finding their may into the filtrate are separated by extraction of their 8-quinolinates in chloroform from buffered acetate solution as proposed by Poluektov (4). This step is probably unnecessary for samples containing less than a few hundredths of a per cent molybdenum trioside. The recoveries of rhenium added as a solution of KRe04 to 0.5 gram of TV-1 are
(1) Poluektov, N. S., J . A p p l . C h m .
USSR 9, 2312 (1936). (2) Ibid., 11, 534 (1938). (3) Ibid., 14, 695 (1941). (4) Poluektov. N. S..Kononenko. L. I . ' Zavodskaya Lab. 25; Part 5, 548'( 1959).
RECEIVEDfor review June 7 , 1962. Accepted July 25, 1962. Submitted by F. 0. Simon in partial fulfillment of requirements for master of science degree in chemistry a t the American Vniversity, Washington, D. C.
Comparative Study on Determination of Oxygen in Vanadium Metal VELMER A. FASSEL, WAYNE E. DALLMANN, RODNEY SKOGERBOE,' and VIRGINIA M. HORRIGAN? Institute for Atomic Research and Department o f Chemistry, Iowa State University, Ames, Iowa
b A comparative study on the determination of oxygen in vanadium b y the vacuum fusion, inert gas fusion, and d.c. carbon-arc extraction, emission spectrometric, techniques has demonstrated that concordant analytical results can b e obtained b y the three methods. Synthetic standards were employed to validate the absolute accuracy. The experimental conditions for each technique are tabulated.
V
is included among the many metals whose physical and mechanical properties are markedly influenced by the presence of trace quantities of interstitial oxygen (5, 6 ) . ANADIUM
1364
ANALYTICAL CHEMISTRY
To appraise these effects and to provide analytical control during production operations, accurate and sensitive analytical methods for determining the interstitial oxygen content in vanadium metal are required. The classical vacuum fusion (29) and the more recently developed inert-gas fusion (21, 23) techniques have found widest application to the determination of oxygen in metals. Both techniques involve high temperature fusion of the metal specimen in a graphite crucible, under environmental conditions such that the carbon reduction reaction [hl
+ RIxOy] + c
4
bTC
leads to the quantitative evolution of carbon monoxide. To aid in the quantitative extraction of carbon monoxide, the partial pressure of the carbon monoxide above the fused sample is reduced to negligible proportions either by vacuum pumping or by a stream of inert carrier gas. When either of these furnace fusion techniques is applied to a metal system, quantitative accuracy of the determination depends on affirmative answers to Present address, Department of Chemistry, M ontana State University,Bozemon t, Mont.
+M +yco
(1)
2 Present address, Anaconda American Brass Go., Waterbury, Conn.
