Anclytica Chimica Acta, 90 (1977) 283-287 OElsevier Scietific Publishing Company, Amsterdam - Printed in The Netherlands

Short Communication

DETERMINATION OF ALKALI METAL IN TETRAGONAL AND HEXAGONAL TUNGSTEN BRONZES

ALTAF HUSGAIUN and LARS KIHLBORG

Deparlment of tr:organic Chemistc. Arrhenius Laboratory. Uniccrsifg of Stochholm. S-l OJO5 Stockholm (Sweden)

(Received 25th September 1976)

The tungsten bronzes have the general formula hI,\YOI (where M represents a relatively electropositive metal and 0 < x < 1) and can be divided into the perovskite temgsten bronzes, PTB, tetragonal tungsten bronzes, TTB, and hesagonal tungsten bronzes, HTB. A fourth group, intergrowth tungsten bronzes, ITF, has recently been discovered [ 11. The PTB phases are formed with Li and Xa, TTB with Na and K, and HTB and ITB with K, Rb and Cs, in a!1 cases within certain limits of x. During a systematic study of the K, Rb and Cs systems the alkali content was determined conveniently by atomic absorption spectrometry. The phase analysis will be published elsewhere.

The analysis of tungsten bronzes has offered certain problems in the past. These compounds are chemically inert and cannot be dissolved without first being aidized. Spitzin and Kaschatnoff [ 2J analyzed sodium tungsten bronzes afte.. oxidation in oxygen at 550°C; blagneli [3] found this method convenient for bronzes rich in alkali but slow for sodium TTB and described another method in which the bronzes were oxidized in a melt of ammonium perosydisulphate. Van Dupn [4] analyzed for sodium gravimetricaliy after oxidizing the bronze with mercury(I1) oxide and heating the product in a stream of chlorine and sulfur chloride. Raby and Banks [ 51 decomposed the samples with bromine trifluoride and separated all three constituents in a specially designed vacuum system. Wechter and Voigt [6] determined K, Rb and Ba in tuslgsten bronzes by neutron or high-energy photon activation analysis. These methods [ 4-61 are rather complicated and laborious. In contrast, atomic absorption spectrometry is rapid and sensitive with a small risk of interference, so that the most convenient way of oxidation and dissolution of the samples can be used.

Preparation The tungsten bronzes were synthesized from mixtures of the appropriate

amounts of oungsten trioside (prepared by dehydration of tungstic acid, Merck, puris.), tungsten dioxide (prepared from WO, by reduction with H,/HZO mivturej and alkali metal tungstate. Rbl\VO.: and Cs2WOJ were

23-I

made from WO, and Rb2COJ (BDH, reagent grade) or Cs2CO~ (Merck, reagent grade) by heating the appropriate amounts in a platmum crucible at 950°C for 5-6 h. K2W04 was commercially available (BDH, reagent grade). The intimate mixtures were heated in sealed, evacuated silica tubes at 700-950°C for various periods of time. The phases formed were identified from s-ray powder patterns (Guinier-HZgg focusing camera).

The bronzes were purified from silica or unreacted materiai by successive treatment with boiling aqueous solutions of HF (40%) and 12 PVI HCl and NaOH (5%), with thorough washing with water after each step, and finally with ethanol.

Procedure About 15 mg of finely powdered tungsten bronze was accurately weighed

in a thoroughly cleaned, clear silica tube (length 7 cm, diam 4 mm) closed at one end. The tube was placed in a muffle furnace at 550-600°C for about 10 h. A known volume (l-2 ml) of 1 AI NaOH was added and the tungstate was dissolved by heating for about 1 h on a water bath. The clear solutions obtained were diluted to concentrations within the optimum range (Table 1).

A Varian Techtron AA-5 atomic absorption spectrophotometer was used with an air-propane flame and the instrumental parameters given in Table 1. The photomultiplier used initially, for most potassium and rubidium analyses, was type R-213 (Hamamatsu TV Co. Ltd.). For cesium a detector with higher response in the long wavelength range (type R-466) was used.

Standard solutions made by dissolving known amounts of tungstate in 1 AI NaOH (Merck, p.a.), were stored in polyethylene bottles after dilution. -4s a check they were standardized against solutions of the corresponding me+A chloride. The observed result was always in very good agreement with ;he calculated composition.

Possible sources of error The compcsition of the bronze may change during the purification

treatment. Remeika et al. [7] found that the superconducting transition temperature is considerably higher for K- and Rb-HTB treated with hot acids than for the unetched powder. They explained this as the result of

TABLE 1

Instrumental parameters used in the analyses

Element Wavelength Spectral band Lamp current Optimum working

(nm) Pass (mW range

(nm) (mg 1-l 1

K 766.5 1.0 5 0.6-1.5 Rb 780.0 0.2 15 2.0-5.0 CS 852.1 0.4 20 15.0-35.0

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reduction in the alkali concentration and mentioned that this had been confirmed by chemical and x-ray fluorescence analysis, but gave no data. To check this, one sample of each of K-HTB and K-TTB was analyzed before and after purification. The unpurified sampies were examined very carefully by’microscope to ensure that they did not contain estraneous material. The results (Table 2) show no significant difference in the alkali content. Electron microprobe analysis of purified and unpurified Rb-ITB previously gave the same result [I] _ Our acid-alkali treatment therefore does not alter the composition significantly.

A part of the sample may volatilize during the oxidation. This was checked by careful weighing of some samples during oxidation or prolonged heat treatment. No weight loss occurred below 850°C; the temperature required for complete oxidation is at least 200°C lower.

The sodium from the hydroxide solution may influence the absorbance. Blanks with NaOH solutions of different concentrations were run with all the different lamps to investigate this possibility. Only with potassium was there an increase in absorbance with increasing NaOH concentration. This could be traced to the presence of potassium as an impurity in the sodium hydroside used: with the highest grade NaOH (Merck, p-a., K < 2.0 - 10~%), no interference could be detected. Measurements lvere also made with a fixed concentration of the alkali to be determined and different sodium hydroxide concentrations. The results showed that the absorbance recorded is independent of the hydroxide concentration within the range 0.002-0.1 M. For K and Rb, the absorbance with no hydroxide was slightly lower than with a concentration of 0.002 hi or higher. All analyses were made with a concentratiun above that value.

TABLE a

Alkali metal content of a number of tungsten bronzes, Mx\VO, ---_ -

Bronzes Gr0Si XIcan x nb sb Bronzes Gross hlenn x n b Sb

c-ntent-l iOl_ldb content“ foundb x _T

K-HTB 0.20 0.197 -I 0.010 Rb-HTB 0.17 0.192 3 0.006 0.30 0.283 5 0.013 0.30 0.287 2 0.011 0.32 0.317 -1 0.010 0.32 0.302 1 - 0.32c 0.308 1 - 0.36 0.321 5 0.015 0.35 0.341 3 0.011 0.40 0.336 2 0.024

K-TTB 0.45 0.421 3 0.009 0.60 0.313 1 -

0.4 SC 0.428 3 0.007 Cs-HTB 0.13 0.148 5 0.003 0.50 0.435 2 0.006 0.30 0.276 4 0.012 0.60 0.464 3 0.010 0.40 0.319 1 - OS0 0.481 2 0.013 0.60 0.344 3 0.010

aGross content of alkali metal in the preparation. %ean value with standard deviation (s) and number of determinations (n). =&me preparation as preceding but unpurified.

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The tungsten present in the solution may interfere. The tungsten concentration was varied in K and Cs samples with fived alkali metal content and had no measurable effect on the absorbance in the range O-40 mg \V l-‘.

Results Calibration graphs prepared from measurements on solutions of tungstate

in 0.003 M NaOH are shown in Fi g. 1. The ccncentration ranges normally used are given in Table 1.

The results obtained for representative samples of different bronzes are given in Table 2. Since the samples were prepared in a closed system their compositions should generally be those of the starting mistures. O;lly if the products contained phases which were wholly or partly removed in the purification process could the alkali metal content be expected to deviate significantly from the initial value.

Table 2 shows that the differences between the initial values and the results are in most cases within the standard deviation. The discrepancy for bronzes mith low alkali conterlt may be esplained by the loss of unreacted lY0, removed during the purification. The ma..imum alkali contents found in Rb- and CsHTB are x = 0.336 and 0.344, respectively. The results fit well with the theoretical upper limit x = l/3 for HTB [S]. Rubidium and ca?sium do not form a TTB phase; the excess of alkali in samples with a higher initial value probably formed tungstate which was removed during purification_

In K-TTB the masimum potassium content obsewed was x = 0.451 although the theoretical limit for the TTB structure is x = 0.60 [ 91. hIag&li found x-values as high as 0.57 but only in samples prepared differently from ours, namely by reduction of polytungstates in a stream of hydrogen.

The above method has also been used for the analysis of ITB phases and potassium polytungstates. The oxidation of ITB bronzes requires a slightly

K Rb

CS

Fig. 1. Typicd cdihration curves.

higher temperature (650°C) to go to completion within a reasonable time, and the dissolution in hydroside is often slower in these two cases. Apart from this there are no differences.

There should be no difficulty in applying this method to Li- and Na- tungsten bronzes (in the latter case with NaOH replaced by KOH) but these bronzes were not included in the present investigation.

This study forms a part of a research program supported by the Swedish Naturai Science Research Council.

REFERENCES

1 A. Hussain and L. Kihlhorg, Acta Crgsbllogr.. Sect. A. 32 (1976) 3.51. 2 V. Spitzin and L. Kaschatnoff. 2. Anal. Chwn., 75 (19%) 410. 3 A. MagnOli, t\rk. licmi, 1 (1919) 2i3. -I D. Van Duyn. Rec. Tram. Chim. Pays-Bas. Gl (I 942) (iB9. 5 B. _A. Hnhy and C. V. Banks, 2\nal. Chum.. 3r3 (196-1) 1101;. 6 AI. A. Wechter and :\. F. VoiKt, Anal. Chrm., 3s (196(i) lGH1 _ ‘7 J. P. Remeika. T. ii. Gcbullc, B. T. hl~lthias, ..\. S. Cooper, (;. \\‘. 111111 and E. Xl. Kelly.

Phys. Lett: _A. 2-I (19Gi) 565. 8 A. Xlagn~li, Acta Chem. Stand., i (1953) :< 15. 9 2\. >IagnGli, Ark. Kemi, 1 (1949) 213.