Anodic Dissolution and Analysis of Ruthenium Alloyed with Molybdenum and Tungsten 1. M. COTTON
and A. A.
WOOLF
Research laboratory, Associated Electrical Industries, Aldermaston, Englond
b Ruthenium-tungsten and rutheniummolybdenum alloys can b e dissolved uniformly by anodic oxidation in aqueous alkali hydroxides. Potassium persulfate is a constituent of the electrolyte for those alloys which tend to passivate. The ruthenium, present in solution as ruthenate and perruthenate, is completely converted to the latter b y oxidation in sulfuric acid-sodium bismuthote and distillation of the resulting ruthenium tetroxide into sodium bicarbonate solution. The perruthenate concentration is measured spectrophotometrically a t 31 5 mp where the molar absorptivity is 2600. Extension of the method to other alloys is indicated.
T
H E SOLVTION STEP in the analysis of certain ruthenium alloys is difficult because of their chemical inertness (3). Ruthenium itself is usually dissolved in a mixture of fused salts containing alkali and oxidizing components. This process is slow with consolidated metal, and leaching the solidified melt with water often results in a suspension of ruthenium dioxide rather than a clear solution. Because some of the molybdenum and tungsten alloys were even more intractable, an alternative mode of solution was sought. Anodic oxidation in aqueous alkali hydroxide solutions was the method finally adopted. Stable solutions were produced in which the ratio of ruthenate to perruthenate depended on the alkalinity of the electrolyte and the alloy composition. With ruthenium, a limiting ratio, together with a maximum current efficiency, was obtained with electrolytes stronger than about 3.5NJ and the absorbance of these solutions at any wavelength afforded a direct measure of the total ruthenium in solution. The same limiting ratio cannot be obtained with alloys under the same conditions, and varying the conditions of electrolysis for each alloy is a n impractical procedure for routine analysis. Woodhead and Fletcher (8) have recently proposed that ruthenium be estimated spectrophotometrically in mixtures of ruthenate and perruthenate at the isosbestic wavelength 415 mp, using a value of 1047 for the molar
absorptivity. Such a measurement would seem a n ideal finish for the analysis of ruthenium in our alloys after anodic dissolution, because the absorptivity of tungstates or molybdates is negligible at this wavelength. I n practice we obtained greater accuracy by converting all the ruthenium in solution t o perruthenate. Larsen and Ross ('7) effected this conversion via oxidation with sodium bismuthate in sulfuric acid and distillation of' the ruthenium tetroxide formed into sodium hydroxide t o which sodium hypochlorite had been added as a perruthenate stabilizer. We obtained perruthenate solutions of adequate stability by collecting the tetroxide in sodium bicarbonate solutions (0.5 to 1M) and were then able to utilize the most sensitive perruthenate absorption peak a t 315 mp u-ithou;t interference from the broad hypochlorite maximum around 296 mp. In the molybdenum-ruthenium equilibrium diagram ( I ) a considerable range of compositions, on either side of a narrou- sigma phase region, is twophase. Differential attack on the separate phases could occur. R e have checked that selective dissolution does not occur under the electrolysis conditions described later, an important consideration for alloys which are difficult to comminute. Tungstenruthenium alloys (6) are solid solutions not subject t o differential solution with respect t o their ruthenium content, but still subject to intergranular attack which could lead t o detachment of alloy fragments if the wrong conditions were chosen. The conditions for uniform attach are similar t o those required for electropolishing. The solution rate should be governed by transport across a boundary layer of ele trolyte rather than across a boundary film on the solid. The former type of control was indicated by the increased rate of solution caused by stirring (2). The latter type of control became predominant after a short electrolysis with alloys rich in molybdenum and some alloys may even passivate. Passivation was avoided, or its onset sufficiently delayed, by adding potassium persulfate to the electrolyte. The passivating oxide film which formed
on alloys could be removed by a short electrolysis in an alkaline mannitol solution. EXPERIMENTAL
Solutions. Sodium hydroxide (4N) solution containing 5y0 w./v. potassium persulfate (ammonium persulfate cannot be substituted for the potassium salt); 4N sodium hydroxide solution containing 2% y./v. mannitol; 6 N sulfuric acid; 0.5M sodium bicarbonate. Metals and Alloy Preparation. Ruthenium, tungsten, and molybdenum powders were consolidated by arc melting on a water-cooled copper hearth in a purified argon atmosphere. Alloy beads (1-2 grams) were prepared from small premelted beads of t h e componenw and homogenized by frequent turning and remelting. The weight change on alloying never exceeded 0.1%. The ruthenium analyzed on the A.E.I. Model MS7 solid state mass spectrometer was about 99.9 atomic % ' purity (Fe, Ca, Si, 0, N main impurities; other platinum metals, below 0.002 atomic yo; K, Sa, Zn, Cu, Si, Co traces). Apparatus. -4 Unicani Model SP500 spectrophotometer with 1-cm. silica cells was used. The electrolysis setup consisted of a 100-ml. polyethvlene beaker, platinum foil electrodes (2 x 2 x 0.01 cm.) welded to platinum wires (0.05 cm. in diameter) which were twisted onto carbon rods (6 mm. in diameter). The electrolyte level was kept below the carbon leads. The anode foil was positioned horizontally, the cathode foil vertically. .4lso used n-ere a d.c. supply variable up to 12 Volt?, 24 watts and a distillation apparatus similar t o that described previously ( 7 ) nith a sintered glass bubbler on the condenser outlet to ensure complete abqorption of the distillate. Dissolution of Ruthenium and Alloys. i i n arc-melted bead of ruthenium (about 1.5 grams) was placed on the horizontal anode, covered with sodium hydroxide solution, and a current of 0.45 ampere passed for 15 minutes. The amount dissolved was found by weighing the bead after electrolysis. The same bead was used in a series of electrolyses with alkali concentrations between 0.1 and .% 16' The absorbance of each solution after electrolysis mas measured a t 315 mp to calculate the R u + ~ / / R u +ratio. The molar absorptivity of perruthenate VOL. 34, NO. 1 1 , OCTOBER 1962
1385
electrolysis (less than a minute) in the alkali-mannitol solution to remove any passivating film, before repeating the electrolysis in the usual electrolyte. RESULTS AND DISCUSSION
Figure 1 . Variation of solution rate of ruthenium in unstirred solutions with concentration of sodium hydroxide solutions
a t this wavelength was established by taking aliquots from some of the solutions and converting them completely to perruthenate by the distillation procedure described below. The dissolution rates of alloys in different electrolytes and the Ru+e/Ru+7 ratios were determined similarly. Recommended General Procedure A for R u - M o and Ru-W Alloys.
weighed bead of the alloy on the horizontal anode is covered with the alkali-persulfate electrolyte (about 30 ml.) and 5 to 50 mg. of ruthenium in the alloy dissolved electrolytically a t current densities of 0.5 to 1.5 amperes per in 10 to 20 minutes. The polarity is then reversed for a few seconds to dissolve any cathodically deposited ruthenium. The alloy bead is removed and reweighed. The ruthenium solution is diluted to 100 ml. with water, a 10-ml. aliquot placed in the distillation flask, and 50 ml. of 6 N sulfuric acid added to the cooled flask. Sodium bismuthate (0.5 gram) in a small tube is held above the liquid level until the distillation apparatus is assembled and is then brought in contact with the liquid by tilting the apparatus. Ruthenium tetroxide is distilled in an air stream into an ice-cold solution of 0.5M sodium bicarbonate during 5 minutes. The bicarbonate solution is diluted to 100 ml. with further bicarbonate and the absorbance measured in a 1-em. cell a t 315 mp after 15 minutes. A solution containing 39.2 p.p.m. of ruthenium has unit absorbance. The alloy bead can be given a short
Table I.
Dissolution of Ruthenium. The solution rate for ruthenium as a function of alkalinity is shown in Figure 1. The absolute value of the solution rate is overestimated because the specimen is assumed strictly spherical -Le., having a minimum surface to volume ratio-to calculate surface areas from weights. The current efficiency a t different alkalinities is plotted in Figure 2 and the R u + ~ / R u + 'ratio in Figure 3. The limiting ratio 7.00 f 0.05 together with a maximum efficiency is reached when the normality of the sodium hydroxide is greater than 3.5. Perruthenate Solutions in Sodium Bicarbonate. The mean molar ab-
sorptivities of perruthenate solutions in sodium bicarbonate ( M / 2 t o M ) were 2600 10 a t 315 mp, 1204 f 10 at 385 mp, and 131 2 a t 465 mp. When the spectrophotometer was checked with potassium chromate solutions (6), the molar absorptivity found was 4825 a t the 373-mp max-
*
*
%TO
w '
...
0.817
1.047
...
1,723
0.385 0.364 0.337 0.814
...
0.389
1.113 0.620 0.825
... ... ...
1386
Variation of R d 6 / R u + '
Ruthenium, %
Weights of metal in alloy, grams
Ru
ANALYTICAL CHEMISTRY
imum. The absorbance of the perruthenate solutions conformed with Beer's law when they were diluted. The solutions were stable for days, provided the glass containers were free
NORMALITY
Figure 3.
Determination of Ruthenium in Standard Alloys with Molybdenum and Tungsten
0.351
Figure 2. Variation of current efficiency with alkalinity
Electrolyte 1 . 3 1 N NaOH 1.84N NaOH 2.88N NaOH 3.61N NaOH 1.86N NaOH 2.79N NaOH 4.50N NaOH 1.92N NaOH 4 N NaOH 5% K&Oe 4 N NaOH 5% KpSnOa 4N KaOH 5% KzS208
++ +
calcu-
lated
Found
30.0
30.0 30.2 30.1 29.7 37.8 37.7 37.6 50.0 24.7 34.9 49.3 49.8
37.8 49.8 24.7 35.2 49.7
ratio with alkalinity
from organic matter. Absorbances measured after 20 hours were unchanged. The variation of the absorbance with temperature was negligible in the range 10' to 30" C. Application to Alloys. The recommended general procedure was applied to three standard ruthenium-molybdenum alloys which were selected to be in the different phase fields of the equilibrium diagram (1). The standard ruthenium-tungsten alloys dissolved easily in weaker electrolytes so that additions of potassium persulfate were unnecessary. The results are collected in Table I. A single analysis can be completed in 30 minutes.
This niethod of analyzing ruthenium in alloys has been mainly applied to tungsten alloys containing 40 t o 80% of ruthenium. It should be capable of extension to other alloys whose components are soluble in alkali solutions, because the electrolyte or electrolysis conditions can be varied to suit the particular alloy and the distillation procedure separates ruthenium from most other elements. I t is even unnecessary for the electrolyte to be alkaline. For example, ruthenium metal can be dissolved anodically in dilute sulfuric acid, al-
though a t a slower rate. However, the simplicity of the method is then lost, a closed electrolysis cell being needed to prevent loss of ruthenium as the tetroxide (4). ACKNOWLEDGMENT
We are indebted to A. Velschou for the data shown in Figure 3 and J. A. James for the mass spectrographic analysis.
(2) Bircumshaw, B. L., Riddiford, A. C., Quart. Rev. (London) 6, 157 (1952). (3) Geach, G. A., Knapton, A. G., Woolf, A. A., “Plansee Proceedings 1961,” Springer-Verlag, Vienna, (in press). (4) Guebeley, M. A., Haissinsky, M., J . Chim. Phys. 51, 290, (1952). (5).Hansen, M., Anderky: K., “Constitution of Binary Alloys, p. 1160, Mc-
Graw-Hill. New York. 1958. (6) Haupt, G. W., J. Opt. SOC.Am. 42, 441 (1952). (7) Larsen, R. F., Ross, E., ANAL.CHEM. 31, 176 (1959). (8) Woodhead, J. L., Fletcher, J. M., J . Chem. SOC.1961, 5039.
LITERATURE CITED
(1) Anderson, E., Hume-Rothery, W., J . Less-Common MPtals 2, 443 (1960).
RECEIVEDJanuary 4, 1962. Accepted June 22, 1962.
Anion Exchange Separation of Thorium Using Nitric Acid JAMES 5. FRITZ and BARBARA B. GARRALDA Institute for Atomic Research and Deparfment of Chemistry, lowa State Universify, Ames, lowa
b Thorium is quantitatively retained b y an anion exchange column from aqueous 6M nitric acid solution. Most other metal ions are completely eluted from the column with 75 to 100 ml. of 6M nitric acid, Following the separation, thorium is stripped from the column with 0.5M nitric acid and titrated with EDTA.
S
AUTHORS have separated thorium using a n anion exchange column and nitric acid. Some of the most comprehensive work is by Korkisch and Tera (7, 9 ) . These authors separated thorium on an anion exchange column in the nitrate form from a solution consisting of 90% methanol and 10% of aqueous 5 X nitric acid. They separated thorium from a long list of other elements but reported that barium, lead, bismuth, lanthanum, and rare earths are also taken up by the column. Danon (2, 3) reported distribution coefficients for thorium and certain rare earths using an anion exchange resin a t various concentrations of nitric acid. Using a column, he was able to separate quantitatively thorium from lanthanum, neodymium, samarium, europium, and yttrium using 5J1 to 8111 nitric acid. Carswell (1) successfully separated thorium and uranium using an anion exchange resin and aqueous 4.M nitric acid as the eluting agent. This separation was carried out a t 77” C. Ichikawa (5) reported distribution coefficients of various elements between nitric acid and anion exchange resin. Distribution eoeffiEVERAL
cients reported by Nelson and Kraus (8) showed that thorium, bismuth, and lead are taken up by anion exchange resins from strong aqueous solutions of nitric acid. This paper studies in a more comprehensive manner the analytical separation and determination of thorium using nitric acid eluent with anion exchange columns. Although the method of Korkisch and Tera is excellent for the separation of thorium from some elements, the method does not work well for mixtures containing macro amounts of thorium and rare earths. The distribution coefficients of rare earths (6) (Dy = 25.4; Gd = 31.0) are such that on a macro scale these elements break through early but are difficult to elute completely. Using aqueous 6 M nitric acid, we obtained excellent separation of thorium from aluminum, rare earths, iron, zirconium, and other elements. The nonaqueous nitric acid separations required larger volumes of eluent for quantitative elution, and a slower flow rate had to be employed. Furthermore, 6-11 aqueous nitric acid solutions caused less deterioration of the resin than methanol-water solutions that were more dilute in nitric acid. EXPERIMENTAL
Ion Exchange Columns. Conventional glass columns of 12 mm. i.d. are used. T h e column is filled with a slurry of resin, in nitric acid of the concentration to be used as the eluent, to a bed height of 16 cm. T h e eluent is added to the column dropwise
from a cylindrical separatory funnel fitted into the column through a onehole stopper a t a flow rate of about 2 ml. per minute. Ion Exchange Resin. Dowex I X 8, 100- to 200-mesh, anion resin in the nitric form is used. The analyzed reagent grade resin is in the chloride form and must be converted to the nitrate form before use. This is done by placing the resin in a large column and backwashing to remove fine particles. The resin is then washed with 5 M nitric acid until the effluent gives a negative chloride test with silver nitrate. The excess nitric acid is rinsed off with distilled water, the water is removed by suction filtration, and the resin is air-dried. Metal Salts. Solutions (O.05M) of the salts are made using reagent grade nitrate salts of the metals and nitric acid of the concentration to be used a? the eluent. PROCEDURE
Distribution Coefficients. Air-dried anion exchange resin is weighed into a 125-ml. glass-stoppered Erlenmeyer flask. The desired amount of metal ion in nitric acid solution is pipetted into the flask, and nitric acid of the desired concentration is added to bring the total solution volume to 50 ml. The flask is stoppered and shaken for 24 hours. An aliquot is pipetted from the flask and the metal ion content is determined with (ethylenedinitri1o)tetraacetic acid (EDTA) titration. The water content of the airdried resin is determined by oven drying a sample of known weight. The distribution coefficient is then determined on a dry resin basis. The relationship used is: VOL 34, NO. 1 1 , OCTOBER 1962
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