Analysis of rare earth sulfides, selenides and tellurides

Talanta,1966.Vol. 13,pp. 265to 269. Pergamon PressLtd. Printedin NorthernIreland

ANALYSIS

OF RARE EARTH SULFIDES, AND TELLURIDES OWEN

H.

KRIEGE

and

MARY

SELENIDES

LOUISE THEODORE

Research and Development Center, Westinghouse Electric Corporation Pittsburgh, PeMsylvania 15235, U.S.A. (Received 23 June 1965. Accepted 16 November 1965) Summary-The rare earth content of sulfides, selenides and tellurides is determined by titration with EDTA using Xylenol Orange as indicator. Sulfur is measured by titration of sulfate with lead nitrate solution after removal of rare earths on a cation exchanger. Selenium metal is precipitated from hydrochloric-sulfurous acid solution. Metallic tellurium is deposited from strongly acidic solutions containing sulfurous acid and hydrazine sulfate. An evaluation of the accuracy and precision of these methods is made. INTRODUCTION THE

sulfides, selenides and tellurides of the rare earths have recently become of increasing interest. While many authors have described the preparation and physical properties of these materials, only occasionally is the composition reported. In some cases one element is determined (usually the rare earth) and the other element is calculated by difference. More often, high purity materials are mixed together in known mole ratios and allowed to react at high temperature with the assumption that the desired stoichiometric reaction occurs. The errors inherent in such techniques are obvious. Because no thorough investigation has previously been reported in this area, a study was undertaken in this laboratory to develop procedures of measured reliability for the quantitative chemical analysis of rare earth sulfides, selenides and tellurides. All samples studied in this program were shown by spectrographic examination to contain only small amounts of the other rare earths; consequently, no attempt was made chemically to differentiate between mixtures of rare earths. Because this effort was in support of a program to study the variation of physical properties with mole ratio, samples of widely varying composition were analyzed. Samples to be analyzed for rare earth content were dissolved with hydrochloric acid (most of the non-metal being distilled as hydrogen sulfide, selenide and telluride), adjusted to a pH of 5.5 and titrated with EDTA using Xylenol Orange as indicat0r.l Sulfur was determined in rare earth sulfides by solution of the samples with nitric acid under a bromine atmosphere, removal of rare earths on a cation-exchange resin and titration of sulfate with lead nitrate using dithizone as indicator.* Selenium was precipitated from hydrochloric acid solution with sulfur dioxide after selenide samples had been dissolved in nitric acid. 3 Tellurium was determined in rare earth tellurides by precipitation as the element from strongly acidic solution containing sulfur dioxide and hydrazine sulfate .* Telluride samples were dissolved with nitric acid using the special technique discussed in a later section of this paper. 265

266

OWEN H. KRIEGEand MARY LOUISETHEODORE

APPLICABILITY

The chemical and physical similarities of the rare earths are well known; however, there are significant variations in the solubility and chemical reactivity of rare earth sulfides, selenides and tellurides. For example, some samples of cerium telluride are pyrophoric and react violently with dilute nitric acid, while other samples of cerium telluride having a different composition dissolve only slowly in concentrated nitric acid. The methods detailed in this paper should not be used indiscriminately until they have been verified for the particular type of sample analyzed; however, it is expected that after only slight modifications (usually in solution techniques) these procedures may be used to analyze all rare earth sulfides, selenides and tellurides. Samples actually studied include the sulfides of lanthanum, cerium, praseodymium, neodymium, samarium, terbium and hohnium; cerium selenide; and the tellurides of lanthanum, cerium and praseodymium. EXPERIMENTAL Reagents All reagents used were of reagent grade whenever available. 0.1 y0 Bromphenol Blue solution 0.1% Dithizone solution in acetone 0.03M EDTA solution. Prepared from the disodium salt of ethylenediaminetetra-acetic 0.02M Lead nitrate solution 0.1 y0 Xylenol Orange solution

acid.

Apparatus In order to ensure quantitative separation of rare earth from sulfate ions, it is necessary that the dimensions of the ion-exchange columns be as specified. The resin bed of 20-50 mesh Dowex 50 X8 cation exchanger has a depth of 6 inches and is in a column with an inside diameter of 0.75 inches. For a detailed account of the techniques of ion exchange see the book by Samuelson. Determination of rare earths in rare earth sulfides Place a 200- to 500-mg sample (which has been weighed in an inert atmosphere if it reacts rapidly with air) in a 400-ml beaker and cover with a watch glass. Transfer the beaker to a fume hood and slowly add 10 ml of 1 f 1 hydrochloric acid. After the vigorous reaction ceases, rinse and remove the watch glass and replace with a Speedyvap cover. Evaporate the sample to dryness on a hot plate, but do not bake. Dissolve the residue with 5 ml of 1 + 1 hydrochloric acid, dilute to 150 ml with water, add 5 g of hydroxylamine hydrochloride and adjust the pH to 5.5 f 0.5 with aqueous ammonia. Add 5 drops of Xylenol Orange indicator and titrate with EDTA solution until the color of the solution changes from red to yellow. Correct for impurities in the sample which react with EDTA at this pH. Determination of sulfur in rare earth sulfides Weigh a 300- to 400-mg sample (in an inert atmosphere if the sample reacts readily with air), place in a dry 400-ml beaker and cover with a watch glass. Transfer the beaker to a fume hood and add 10 ml of 2 + 1 carbon tetrachloride-bromine mixture and age for 15 min. Add 10 ml of concentrated nitric acid and warm gradually on a hot plate. Add 50 mg of sodium nitrate (to prevent loss of sulfuric acid), replace the watch glass with a Speedyvap cover and evaporate until crystals start to form. Take up in 10 ml of concentrated hydrochloric acid and evaporate until crystals just start to form. Immediately dilute to 35 ml with water and warm to effect complete solution of the sample. If a precipitate persists at this point, alternate evaporations with nitric acid and hydrochloric acid are required to dissolve the sample completely. Evaporate most of the acid in order that the solution formed by adding 75 ml of water will not have a pH < 1. Without delay, add to the ion-exchange column and elute with water at the rate of 75 ml in the tirst 10 min, then at a flow rate of 20 ml/min until a total volume of 250 ml has been collected in a volumetric flask. Place a 50-ml aliquot in a 400-ml beaker, add 200 mg of sodium nitrate to prevent loss of sulfuric acid and evaporate to dryness. Add 10 ml of nitric acid and evaporate to dryness again. Take up with 15 ml of water, add 2 drops of Bromphenol Blue indicator and adjust the solution to a light yellow color with 0*02M nitric acid or aqueous ammonia. Add 1 ml of 20% acetic acid, 50 ml of acetone and 1 ml of freshly prepared

Rare earth sulfides dithizone indicator. Titrate with lead nitrate solution (standardised permanent bright red end-point.

267 against sodium sulfate) to a

Determination of rare earths in rare earth selenides The procedure described above for the determination of rare earths in stides may be used for any insoluble selenium metal present in the sample after selenides with the following modification: treatment with hydrochloric acid is removed on Whatman #41 filter paper before the sample is diluted to 150 ml. Determination of selenium in rare earth selenides Weigh a 300- to 400-mg sample (in an inert atmosphere if the sample reacts readily with air), place in a dry beaker and cover with a watch glass. Add 10 ml of concentrated nitric acid. After the vigorous reaction ceases, rinse and remove the watch glass and replace with a Speedyvap cover. Evaporate just to dryness on a hot plate, cool to room temperature and dissolve, stirring if necessary, in 100 ml of concentrated hydrochloric acid. Add, with stirring, 50 ml of concentrated hydrochloric acid which has been saturated with sulfur dioxide. Allow the solution to age for at least 3 hr, stirring occasionally to coagulate the precipitate. Remove the precipitate on a tared, medium-porosity filter crucible, wash with concentrated hydrochloric acid, then water and tinally methanol. Dry for 1 hr at 105” and weigh as selenium. Determination of rare earths in rare earth tellurides The procedure described above for the determination of rare earths in selenides may be used to determine rare earths in tellurides with the following modification: to prevent the formation of insoluble tellurites, 90% of the stoichiometric amount of EDTA is added before adjustment of the pH to 5.5. Determination of tellurium in rare earth tellurides Because of the great variability in the reaction rates of rare earth tellurides with nitric acid, individual solution techniques must be established for the particular type of sample analyzed. Factors affecting the reaction rate include the percentage of rare earth in the sample, the particular rare earth present, the particle size and previous heat treatment. The following procedure is applicable to most samples. Weigh a 300- to 400-mg sample in an inert atmosphere and place in a 2-liter beaker. Invert a 400-ml beaker over the sample and place a cover glass over the 2-liter beaker. Slowly add 20 ml of 1 + 1 nitric acid until vigorous reaction ceases and the sample is in solution. (Caution: Some telluride samples are pyrophoric.) Rinse all glassware with 1 + 1 nitric acid and transfer the sample to a 400-ml beaker. Evaporate just to crystals and take up with 50 ml of 3M hydrochloric acid. Heat to boiling and add 15 ml of water saturated with sulfur dioxide, then add 10 ml of 15 % hydrazine sulfate and 25 ml of water saturated with sulfur dioxide. Continue boiling for about 5 min until the precipitate coagulates. Allow the precipitate to settle for about 10min. then remove on a tared medium-porosity filter crucible, washing thoroughly with 300 ml of hot water and 4Oml ofmethanol. It is important that the precipitate not be permitted to compact on the filter crucible until the methanol wash is completed. After the tellurium is sucked dry, heat the crucible for 1 hr at 105” and weigh the precipitate as tellurium metal. RESULTS

AND

DISCUSSION

Sample weighing Because of high reactivity of some rare earth sulfides, selenides and tellurides, these samples were weighed by difference in a dry argon atmosphere. Samples were placed in bottles (with tight-fitting caps) in a glove box, transferred to a balance and weighed. After the sample was removed for analysis, the container was returned to the glove box, recharged with argon, removed from the box and reweighed. Dissolution of samples In order to prevent loss of hydrogen sulfide by hydrolysis, all samples analyzed for sulfur were dissolved under a bromine atmosphere. The solution of rare earth tellurides required special precautions because of the great variability in their solution

OWEN H. KRIEGE and MARY LOUISETHEODORE

268

Table I-ANALYSISOFRAREEARTH

SIJLFIDES,~ELENIDESAND TELLIJRIDES

Sample

R.E., %

S, %

Se, %

Cerium sulfide Lanthanum sulfide Praseodymium sulfide Terbium sulfide Neodymium sulfide Hohnium sulfide Samarium sulfide Cerium selenide Cerium telluride Lanthanum telluride Praseodymium telluride

80.78 74.38 75.61 75.33 75.20 77.88 78.50 47.34 52.28 52.85 51.21

18.92 24.05 24.36 22.64 23.96 22.65 18.43 -

52.67 -

Te, %

4G4 47.14 47.69

Other metals, %

Total, %

0.18 N.D. N.D. N.D. N.D. N.D. 1.33 0.22 N.D. N.D. N.D.

99.88 98.43 99.97 97.97 99.16 100.53 98.26 100*23 100.02 99.99 98.90

N.D. = not detected, insuthcient sample.

rates. A small portion of sample was treated with varying concentrations of nitric acid until a desired rate of solution was determined. The double-beaker technique was used to ensure quantitative recovery of volatile tellurium compounds. All beakers were rinsed with dilute nitric acid to remove traces of tellurium deposited on the surfaces. Formation of insoluble double sulfates

In the analysis of sulfides of neodymium, samarium, terbium and holmium, it should be noted that there is a tendency to form insoluble double sulfates6 Care Table II-PRECISION STUDYOF CERIUMSULFIDE, SELENIDE AND TELLURIDE Sample

Ce, %

S, %

Cerium sulfide

80.82 80.81 80.65 80.85 80.80 80.94 80.78 80.61 0.11

19.08 18.91 18.89 18.77 18.89 18.86 18.92 19.03 0.10

Standard deviation Cerium selenide

Standard deviation Cerium telluride

Standard deviation

47.32 47.35 47.39 47.31 47.38 47.40 47.34 41.27 0.04 52.32 52.25 52.34 5222 52.23 52.27 52.29 0.05

Se, %

Te, %

52.62 52.67 52.72 52.68 52.65

004 47.32 48.21 48.22 47.22

Rare earth sulfides

269

should be taken that the samples are placed on the ion-exchange column as soon after solution as possible. If a precipitate forms, it can usually be redissolved by alternate evaporations with nitric or hydrochloric acids. In the case of samarium sulfide, the sample must be added to the column in 0.1M nitric acid to eliminate insoluble salts. Determination of sulfate by precipitation with barium

The sulfur content of rare earth sulfides may also be determined by precipitating the sulfate with barium, provided that corrections are made for the solubility of the precipitate and coprecipitation of barium chloride. It is necessary to remove rare earths on a cation-exchange column before precipitation because barium sulfate formed in the presence of rare earths is badly contaminated by rare earth sulfates. Accuracy No standards are available by which the accuracy of these techniques could be evaluated other than by summation of the constituents in the sample. Results for the analysis of rare earth sulfides, selenides and tellurides are given in Table I. These are representative of the many samples of widely varying mole ratios analyzed by these procedures. Precision

Only limited amounts of most samples were available for analysis; consequently it was not possible to make a comprehensive precision study for all samples. However, representative data were collected from the sulfides, selenides and tellurides of cerium. Results are shown in Table II. Zusammenfassung-Der Gehalt von Sulfiden, Seleniden und Telluriden an seltenen Erden wird durch EDTA-Titration mit Xylenolorange als Indikator bestimmt. Die Schwefehnenge wird gemessen durch Titration von Sulfat mit Bleinitratliisung nach Entfemune Y der seltenen Erden an einem Kationenaustauscherr Selen wird als Element aus Salzs%_ueschwefliger S&e gefiillt. Metallisches Tellur wird aus stark sauren Losungen mit schweftiger Slure und Hydrazinsulfat abgeschieden. Genauigkeit und Richtigkeit dieser Methoden werden ermittelt. R&mm&La teneur des terres rares en sulfures, s6leniures et tellurures est determinee par dosage a l’aide d’EDTA en presence de Xylenol orange. Le soufre est dose a l’etat de sulfate par le nitrate de plomb apres elimination des terres rares sur &changeur cationique. Le selenium metal est pr&cipite de la solution acide chlorhydriquesulfureuse. Le tellure metal est isole par precipitation dune solution fortement acide contenant de l’acide sulfureux et du sulfate d’hydrazine. Une etude de l’exactitude et de la precision de ces methodes est effectuee REFERENCES 1 J. Kinnunen and B. Wennerstrand, Chemist-Analyst, 1957,46,92. 2 E. E. Archer, Analyst, 1957,82,208. 3 I. M. Kolthoff and P. J. Elving, Treatise on Analytical Chemistry, Part II, Vol. 7, p. 158. Interscience, New York, 1961. 4 Idem, ibid., p. 174. 6 0. Samuelson, Ion Exchangers in Analytical Chemistry, 2nd Ed. Wiley, New York, 1962. BI. M. Kolthoff and P. J. Elving, Treatise on Analytical Chemistry, Part II, Vol. 8, p. 11. Interscience, New York, 1963.