RU2241983C1 - Method of determining rhenium and rhenium in presence of molybdenum by inversion voltammetry technique - Google Patents

Abstract

FIELD: analytical methods.

SUBSTANCE: determination is accomplished against 1 M HNO₃ with addition of 0.1 g KNO₃ by inversion voltammetry technique wherein rhenium is concentrated on graphite electrode, after which precipitate is oxidized.

EFFECT: increased accuracy of determination by more than 2-3 orders of magnitude and reduced analysis time.

2 tbl, 2 ex

Description

The invention relates to analytical chemistry, and in particular to methods for determining metal ions, and can be used in hydrometallurgy, in various geological explorations for prospecting and exploration in the case of ore analysis, as well as in petrochemistry for determining the concentration of ions in solutions, ores and ore concentrates rhenium by inversion voltammetry (IVA).

The polarographic behavior of rhenium was studied in acid solutions (HCl, HClO ₄ , H ₂ SO ₄ , H ₃ PO ₄ , HNO ₃ , acetic acid), in neutral solutions of potassium and sodium chlorides, as well as alkaline solutions [L.V. Borisova, A.N. Ermakov. Analytical chemistry of rhenium, - M: Publishing house "Science", 1974, 315 S.].

The behavior of rhenium in hydrochloric and perchloric acid solutions. The maximum wave height of the reduction of rhenium is observed in 2-4.3 N Hcl [Lingune JJJAm.Chem.Soc., 64, 1001, 2182 (1942); 65, 866 (1943)]. The half-wave potential of E _1/2 for rhenium (VII) in 2 M Hcl is -0.45 V, and in 4.2 M Hcl -0.31 V. The disadvantage of this process is that NO ions interfere with the determination of rhenium $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4}}$ , PO $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4}}$ , which distort the background line, as well as molybdenum and tungsten, which are accompanying elements of rhenium in ores and ore concentrates.

The behavior of rhenium in sulfuric and phosphoric acid solutions. Geyer showed [Geuer RZ anorg. allgem. Chem., 263, 47 (1950)] that the number of recovery waves and their nature varies with the concentration of H ₂ SO ₄ . The potential of the half-wave E _1/2 for rhenium (VII) in 3.5 M H ₂ SO ₄ is from 0.2 to -0.45 V. In this case, the anions Cl ^- , NO ^- , PO interfere with the determination of rhenium $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4}}$ as well as Mo, Fe and Ti.

The behavior of rhenium in neutral, alkaline and buffer solutions. The use of polarography in neutral and alkaline solutions is most appropriate for determining small concentrations of rhenium [Rubinskaya T.Ya., Mairanovsky SG Electrochemistry, 7, 1403 (1971)]. Catalytic waves suitable for analytical purposes respond from -1.2 to -1.6 V. In this process, the ions Cl ^- , NO interfere $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4}}$ , PO $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4}}$ and Mo, Cu, Zn, Cd, Pb, Bi, Se, Co and W.

The behavior of rhenium in nitric acid solutions. The oscillographic method made it possible to observe the cathode wave of rhenium reduction. In 1M HNO _3, the half-wave potential for rhenium is -0.6 V. The detection limits were 10 ^-3 -10 ^-4 %. Interfering elements include Mo, Cu, Zn and W, and the anions Cl ^- and others.

Known polarographic methods for the determination of rhenium, which are divided into 2 groups:

a) methods for determining large quantities (from 10 ^-3 % and above), based on the measurement of perrenate recovery waves;

b) methods for determination of trace amounts of rhenium in the range of 10 ^-3 -10 ^-7 %, using catalytic currents and the concentration of rhenium on the electrode in the form of a sparingly soluble oxide film.

The rhenium content in alloys of 2 · 10 ^-1 % or more is usually determined by a wave with E _1/2 = (- 0.3) - (-0.4) V against a background of 2.5 M H ₂ SO ₄ . Smaller amounts of rhenium (up to 10 ^-3 %) in the products of copper-molybdenum production are determined by the catalytic wave of hydrogen with E _1/2 = -1.2 V against the background of a phosphate buffer solution (pH 7-8). The disadvantage of this method is that molybdenum is preliminarily separated in the form of sparingly soluble calcium molybdate by sintering the sample with CaO at 600-700 ° C for 2 hours. A detailed analysis is given in [Kryukova TA, Sinyakova SI, Aref'eva T. IN. Polarographic analysis. M., Goskhimizdat, 1959, p. 351].

Molybdenum concentrates are also recommended to decompose conc. HNO ₃ followed by distillation of Re ₂ O ₇ from a sulfate solution [Duca A., Stanescu D., Puscasu M. Studii sicercetari chim. Acad. RPRFil. Cluj, 6, 123 (1955); 13.197 (1962)]. The determination of rhenium is carried out against the background of a solution of NaCl + Na ₂ SO ₃ (pH 11.3-11.5) at a molar ratio of Mo: Re≤200: 1 and E _1/2 = -0.45 V. The opening minimum is 1-2 mcg Re / ml. The determination is also carried out after sample preparation, during which molybdenum, tungsten and other related elements are separated from rhenium.

The sensitive method is the inverse voltammetric method for determining rhenium against the background of 4M Н ₃ РО ₄ using an oscillographic polarograph and a mercury stationary microelectrode [Demkin AM, Sinyakova SI Sat "Determination of microimpurities." M., DNTP, 1968, p. 31] (prototype). The determination of rhenium is carried out according to the following procedure. An aliquot of 5-10 ml (4M H ₃ PO ₄ ) containing ~ 0.01-0.1 μg Re is placed in an electrolytic cell. Nitrogen is blown through the solution for 3 minutes, the anodic polarization mode and the potential of the mercury microelectrode equal to 0.95 V are set on an oscillographic polarograph. The wave is recorded at this potential. A necessary condition for the process is that rhenium in the samples is separated from molybdenum and tungsten. This condition is one of the disadvantages of this method.

The main objective of our solution is to determine rhenium and rhenium in the presence of molybdenum on an impregnated graphite electrode by the IVA method.

The task is achieved by the fact that rhenium is electrochemically concentrated on various types of graphite electrodes (glassy carbon (SU), mercury-film (RP) or impregnated graphite (IG)) with subsequent registration of anode voltammograms. New in the method is that the accumulation of rhenium in a stirred solution is carried out for 120-150 s with electrolysis potentials E _e = (- 0.7) - (- 1.0) V in the background: 1 M HNO ₃ with the addition of 0, 1 g of KNO ₃ with subsequent registration of the anode peaks in the cumulative mode of shooting voltammograms at a sweep speed of 30-50 mV / s. The concentration of rhenium and rhenium in the presence of molybdenum is determined by the height of the anode peak in the potential range from 0.150 to 0.250 V relative to the saturated silver chloride electrode (n.h.e.).

In the prototype, the quantitative determination of rhenium is based on the reduction reaction and is possible only after carefully separating it from molybdenum, which is often a concomitant of rhenium in ores and ore concentrates.

In the proposed method, the ability of rhenium to oxidize on various types of graphite electrodes was first established. As an indicator, IG, SU, and RP were used (a mercury-drop electrode was used in the prototype). The use of such electrodes is due to the high chemical and electrochemical stability of graphite, a wide range of working potentials, as well as the simplicity of mechanical surface renewal and safety requirements. In the RP electrode, mercury consumption is significantly reduced, as mercury is toxic.

The maximum value of the recorded current is observed at the IG electrode. The SU and RP electrodes are also suitable for use, however, due to the large residual current, they turned out to be less convenient in operation than the IG electrode. The IG electrode was first used to determine rhenium and rhenium in the presence of molybdenum.

The prototype describes the use as a background of 4 M phosphoric acid solution. Under these conditions, it is possible to determine only large quantities of rhenium (~ 10 ^-2 %). When determining the micro amounts of rhenium under these conditions, it is necessary to isolate it from complex compositions, since most of the accompanying elements are interfering (for example, molybdenum, tungsten and copper). The backgrounds proposed in the claimed invention, 1 M H ₂ O ₂ , or 0.5 M HNO ₃ , or 1 M HCl, or 0.5 M HOOH, or 1 M HNO ₃ with the addition of 0.1 g KNO ₃ allow rhenium to be determined with good reproducibility. The use of 0.5 M HNO ₃ and 1 M HCl backgrounds is difficult because a sufficiently large residual current was observed in the determination of rhenium, and when using 1 M H ₂ O ₂ and 0.5 M HCOOH backgrounds, it is impossible to determine rhenium in the presence of molybdenum, since against these backgrounds, the molybdenum analytical signal drowns the rhenium analytical signal. The highest sensitivity coefficient was observed against a background of 1 M HNO ₃ with the addition of 0.1 g of KNO ₃ . The use of this background also solves the problem of the joint determination of rhenium and molybdenum. The detection limit is 10 ^-6 -10 ^-2 % (in the prototype 10 ^-3 -10 ^-2 %).

The results of using different electrodes on different backgrounds are given in table. 1. From table 1 it is seen that the IG electrode has the highest sensitivity coefficients. The same picture is observed if the determination is carried out against a background of 1 M HNO ₃ with the addition of 0.1 g of KNO ₃ , in addition, against this background, rhenium can be quantified in the presence of molybdenum. Thus, we proposed the determination of rhenium and rhenium in the presence of molybdenum on the IG electrode and on the background of 1 M HNO ₃ with the addition of 0.1 g of KNO ₃ .

Another distinctive feature is the established conditions of electrochemical accumulation: electrolysis potential E _e = (- 0.7) - (- 1.0) V. Experimental data showed the dependence of the rhenium oxidation current on E _e (Table 2). The value of the anode current increased and reached a maximum value in the potential region (-0.7) - (- 1.0) V. Using pre-electrolysis at potential values of -0.8 ± 0.1 V allows to record voltammograms with a clearly defined maximum. This improves the accuracy and resolution of the method.

The pre-electrolysis time (τ _e ) is selected depending on the concentration of the analyte. The maximum value of the oxidation current is achieved at τ _e equal to 120-150 s. When τ _e <120, the sensitivity of determination decreases and the error of determination increases, and when τ _e > 150, the expressivity decreases.

Important for determining rhenium is the choice of potential sweep speed. The optimal speed is 30-50 mV / s. An increase in speed of more than 50 mV / s increases the sensitivity, but the residual current increases and the resolution of the method decreases. Using a speed of 30 mV / s significantly reduces the anode current and decreases the sensitivity of the determination of rhenium.

Thus, the established conditions made it possible for the first time to quantify rhenium and rhenium in the presence of molybdenum based on the electrooxidation reaction. To increase the detection sensitivity, preliminary concentration of rhenium on the surface of graphite electrodes was used. The proposed voltammetric method allowed to significantly improve the metrological characteristics of rhenium analysis, to increase the detection sensitivity (1 · 10 ^-6 mg / kg), which is 3-4 orders of magnitude lower compared to the prototype, the analysis time does not exceed 120-150 s versus 30 minutes. The definition does not interfere with molybdenum, tungsten and copper, which are accompanying elements.

As a prototype, the method for determining rhenium from the current-voltage curves of electroreduction Re (7+) → Re (4+) was chosen. We proposed the determination of rhenium by the IVA method, in which electroconcentration of rhenium is carried out on a graphite electrode with subsequent oxidation of the precipitate. The analysis was performed on a STA-1 instrument (Tomsk, Russia). The method we have chosen allows us to significantly expand the range of determined concentrations from 10 ^–3 –10 ^–2 to 10 ^–6 –10 ^–2 %), and also allows us to simplify the hardware design of the process. In addition, this method allows the determination of rhenium in the presence of molybdenum, which solves the issues of sample preparation for analysis.

Case Studies

Example No. 1. Measurements were carried out on artificial mixtures. 10 ml of background electrolyte is placed in a quartz glass. Without stopping stirring, electrolysis of the solution is carried out under the condition: E _e = -0.800V, τ _e = 120 s. The volt-ampere electrooxidation curve is taken at a sweep speed of 40 mV / s, starting from potential E _beg = 0.000 V. The absence of peaks indicates the purity of the background. Then add 0.05 ml of a standard sample (CO) of rhenium and conduct electrochemical concentration of the precipitate under similar conditions. The peak for the indicated concentration of the substance is recorded in the potential range from 0.150 to 0.250 V (relative to H.s.). A 0.05 ml addition of rhenium CO was made and the analytical signal was recorded again. The rhenium peak current difference was used to calculate the rhenium concentration in the solution. The determination of rhenium in the presence of molybdenum is carried out similarly, only molybdenum is added to the background electrolyte (from 0.05 to 5 ml of molybdenum CO). It was found that with a 100-fold excess of molybdenum (compared with rhenium) rhenium can be quantified. CO of rhenium was obtained by diluting 0.05 g of pure rhenium powder in 50 ml of a solution containing 25 ml of concentrated HNO ₃ and distilled water. Molybdenum CO was prepared in the same manner. The background electrolyte was obtained by diluting concentrated nitric acid (10 ml conc. HNO ₃ in a 100 ml volumetric flask) and adding 0.1 g KNO ₃ . The measurement error is about 1-5%.

Example No. 2. 0.1 g of rhenium-containing catalyst powder was dissolved in 15% HNO ₃ in a 100 ml flask for 15-20 minutes with stirring. 10 ml of the background electrolyte was placed in a quartz glass, an aliquot of 1-1.5 ml of the resulting catalyst solution was added. The current-voltage curve of the rhenium electrooxidation was taken. An addition of rhenium CO 1 ml was made and the analytical signal was recorded again. The rhenium peak current difference was used to calculate the rhenium concentration in the solution. The peak of rhenium current was recorded at a potential of E = 0.200 V. The concentration of rhenium was ~ 65%. CO rhenium and background electrolyte is prepared in the same way as in example No. 1.

Thus, for the first time, the ability to quantitatively analyze rhenium by the peaks of its oxidation at the IG electrode was established (in the prototype, rhenium is quantified by reduction waves at a mercury droplet electrode). The sensitivity of determination significantly increased (by more than 2-3 orders of magnitude). The conditions used in the prototype do not allow the analysis of rhenium in the presence of molybdenum.

The proposed method is simple, eliminates the use of toxic mercury, does not require large labor costs, a large number of reagents, and can be used in any chemical laboratory with computerized analyzers such as CTA, TA or polarograph. The proposed method can be used to determine rhenium and rhenium in the presence of molybdenum in ores and ore concentrates.

Method for determination of rhenium and rhenium in the presence of molybdenum by inversion voltammetry

Claims (1)

A method for determining rhenium and rhenium in the presence of molybdenum by inversion voltammetry, which consists in transferring rhenium from a sample to a solution and performing a voltammetric determination, characterized in that rhenium is accumulated on an impregnated graphite electrode in a stirred solution for 120-150 s at potentials electrolysis (-0.7 ÷ -1.0) V in a relatively saturated silver chloride electrode against a background of 1 M HNO ₃ with the addition of 0.1 g KNO ₃ followed by registration of the anode peaks in the accumulation mode amperograms at a potential sweep speed of 30-50 mV / s and concentration are determined by the height of the peak in the potential range 0.150 - 0.250 V by the method of additives of certified mixtures.