CONC.
(NOLES/L.)
Figure 1. Variation of NI4 coupling constant with various metal ions
to the reversible addition of one electron to form the anion radical, is only vary slightly shifted toward more anodic potentials. However, the second peak is shifted markedly t o more anodic
potentials by, say, lithium ion. These results are completely analogous to those recently given by Peover and Davies (3)for the reduction of quinones in DMF and by Holleck and Becher for nitro compounds (2). These effects are important because, in reporting coupling constants of radical ions generated electrolytically, it will now be necessary t o include the supporting electrolyte if this is other than a tetraalkyl salt. The almost complete lack of U N dependence on tetraethylammonium perchlorate concentration (Figure 1) shows that the original choice of tetraalkyl ammonium salts as supporting electrolytes in electrolytic generation of radical anions by Geske and Maki was indeed a judicious one (1). Bowever, as a consequence of the metal ion effects on coupling constants, one is able to study metal ion-radical ion interactions via the EPR method which gives additional information on the physical picture of
the interaction. A detailed report of these studies will be given soon. LITERATURE CITED
(1) Geske, D. H., Maki, A. H., J . Am. Chem. SOC. 8 2 , 2671 (1960). (2) Holleck, L., Becher, D., J. Electroanal. Chem. 4, 321 (1962). (3) Peover, M. E., Davies, J. D., Zbid., 6, 46 (1963). (4) Piette, L. H., Ludwig, P., Adams, R. N., J . Am. Chem. SOC. 84, 4212 (1962).
TOYOKICHI KITAGAWA~ THOMAS LAYLOFF RALPHN. ADAMS Department of Chemistry The University of Kansas Lawrence, Kan. Present address, Department of Chemistry, Osaka City Univereity, Osaka, Japan.
RECEIVED for review January 10, 1964. Accepted January 23, 1964. This work supported by the Air Force through Air Force Office of Scientific Research and by the Atomic Energy Commission through contact AT( 11-1)686.
The Dissolution of Calcium Tungstate with an Acid-Hydrogen Peroxide Mixture SIR: The use of single crystal Ca-
wo4 as a laser host and the associated
problems of doping and charge compensation have created a demand for analytical methods for this material. Although a Na2C03-K2C03 fusion will render CaW04 soluble ( d ) , this procedure is time consuming and not applicable for low-level determinations of alkali metals as in the case with charge-compensated, rare earth-doped crystals. Consequently, a method for the rapid quantitative dissolution of CaW04, involving mild heating with strong acid and hydrogen peroxide, has been developed. This communication reports the details of this procedure, its mechanism, and suggests some possible extensions t o other materials. EXPERIMENTAL
Two milliliters of concentrated acid (HC104 or HN03), 20 ml. of 3001, H202, and 50 ml. of distilled water are normally used for each gram of CaW04. However. the amounts of acid and peroxide ' used are not critical. The rate of dissolution depends critically on solution temperature and surface area of the sample. The optimum solution temperature is SO" to 90" C.-i.e., the highest teniperature which does not cause excessive decomposition of the hydrogen peroxide. Finely divided powder can be dissolved in 2 to 5 minutes while single crystals and sintered material ground with a mortar
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ANALYTICAL CHEMISTRY
pestle require 15 to 30 minutes depending on the degree of fineness achieved. Solutions prepared in this fashion using HClOn are stable for up to 4 months depending on the amount of excess peroxide added. Slow continuous decomposition of the peroxide is evidenced by the slight effervescence of the solutions. Evcntually a yellow tungstic acid precipitate forms. Similar behavior is noted when the solution is boiled. The precipitate can be redissolved by cooling the solution and adding more peroxide. DISCUSSION
For applications such as flame photometry, samples and standards prepared in this manner can be used directly. However, in applications where the solutions must be heated either a t high temperature or for long periods of time, or in cases where the bubbling may be a problem-e.g., x-ray fluorescence spectrometryfurther treatment of the solutions is necessary to stabilize them. A simple method for accomplishing this is to add a slight excess of (ethylenedinitril0)tetraacetic acid (EDTA) t o the solution and then bring the p H of the solution to 12 by addition of base. The majority of the peroxide decomposes spontaneously in basic solution and can be removed entirely with mild heating. The dissolution appears to be a two-
step process. The CaWOa 1s decomposed forming tungstic acid which reacts with the peroxide to form the soluble peroxytungstic acid. No tungstic acid precipitate is formed during the dissolution. Addition of EDTA before making the solution basic complexes the Ca+?which would otherwise precipitate as Ca(OH)?in basic solution. As noted above, particle size and temperature are the most important parameters in the speed of the reaction. Since the primary reaction is the acid decomposition of CaW04, the greatest possible surface area should be presented for the reaction. An optimum temperature exists for the reaction because the CaWO4decomposition is more rapid a t high temperatures while excess heating destroys the hydrogen peroxide. Although stirring is desirable to promote the reaction, Teflon magnetic stirring bars should not be used because they are attacked by the reagent and/or abraded by the CaWO4. Small flakes of Teflon are inevitably found in the solution and repeated use of the same stirrer causes iron from the bar to be released into the solution. Three acids, hydrochloric, nitric, and perchloric, were tested for use in this method. Although the dissolution proceeded slightly faster with HC1 than with the others, effervescence of the resulting solution was more pronounced with this acid. Although no apparent
difference in speed of dissolution was observed between HNO, and HClOd, HC10, solutions are more stable upon heating and with respect t o time. This is in accord with the findings of Dams and Hoste (1). Their work also suggests that, if tungsten is t o be determined by homogeneous precipitation, HNOI should be used in the initial decomposition. If the solution is t o be treated with EDTA and base, any of the acids is acceptable. The meechanism of the reaction suggests possible uti1ii;y not only in this application but also for insoluble or
difficultly soluble titanates, vanadates, chromates, etc., which also form soluble peroxy acids (3). The method can be applied t o barium titanate without modification and is far superior t o the overnight digestion with HCI followed by Na2CO3 fusion of the insoluble residue, which has been used by other workers (4, 6). LITERATURE CITED
(1) Dams, R., Hoste, J., Talanta 8, 664 (1961). ( 2 ) desousa, A., Anal. Chim. Acta 9, 309 (1953).
(3) Moeller, T. M., “Inorganic Chemistry,” p. 512, Wiley, New York, 1952. (4) Morachevskii, Yu. V., Gordeeva, M. N., Prokof’eva, R. V., Zavodsk. Lab. 27, 1200 (1961). ( 5 ) Murphy, T. J., Clabaugh, W. S., Gilchrist, R., J . Research Natl. Bur. Std. M A , 535 (1960).
RONALD H. CURRY J. ROBERT CARTER Sperry Rand Research Center 100 North Road Sudbury, Mass. RECEIVED for review January 6, 1964. Accepted February 5, 1964.
OH Stretching Vibration of Pyromellitic DianhydrideHydroxynaphthalene Complexes SIR: When a cyclic or planar acceptor forms a charge-transfer complex with an aromatic donor, the structure is generally considered to have the planes of the molecules lying; parallel to each other (6). This sandwich-type structure has been proved by x-ray work in a number of cases su3h as chloranilhexamethylbenzene (!), and trinitrobenzene-naphthalene (2-4). An analysis of thc OH stretching vibration of hydroxy-substituted naphthalenes is of value in determining whether or not an acceptor molecule will form sandwich type complexes. Solid charge-transfer complexes were prepared using pyromellitic dianhydride (PMDA) as the acceptor and mono- and
Table 1. O H Stretching Frequencies in Cm.-’of Hydroxy Substituted Naphthalenes and PMDA Complexes
HydroxySubstitution naphthalene 1 2 2,3 2.6 2;7 193 1,4 195 1.6 1;7
3300 3320 3320 3300 3300 3320 3330 3320 3320 3300
PMDA complex 3490 3500 3530 3560 3560 3470 3470 3490 3560 3560
dihydroxynapthalenes as the donors. Infrared spectra are presented for the OH stretching region of these complexes. EXPERIMENTAL
The solid complexes were prepared by mixing boiling acetone solutions of equimolar quantities of PMDA and the donor molecules, and filtering the solid complex that precipitates on cooling. Infrared spectra were obtained on a Perkin-Elmer Model 21 Spectrophotometer, employing standard KBr pelleting procedures. RESULTS AND DISCUSSION
The frequencies of the hydroxynaphthalenes and their PMDA complexes are listed in Table I. The pure crystalline hydroxynaphthalenes have OH strectching frequencies between 3300 and 3330 cm.-l These are all broad, intense bonds indicating there is hydrogen bonding in the pure hydroxynaphthalenes. This is not an unexpected result for solid phendic type compounds of this class (1). The OH stretching frequencies of the hydroxynaphthalene donors in the PMDA4 complexes are sharp bands between 3470 and 3560 cm.+ These sharp bands very closely approximate a free OH group. This result means that the components of the complex cannot come together in the eolid to form hydrogen bonds, nor can the complexed hydroxy-
naphthalenes hydrogen bond with each other to any appreciable extent. A sandwich-type donor-acceptor structure is consistent with the OH stretching vibration bands. With the PMDA lying flat on top of the naphthalene rings, there would be a minimal interaction between the parallel OH and anhydride groups, giving a free OH vibration, Intermolecular hydrogen bonding between units of the complexes must also be relatively unimportant because this interaction would give similar OH bands to the solid hydroxynaphthalenes. LITERATURE CITEP
( 1 ) Bellamy, L. J., “The Infrared Spectra of Complex Molecules,” p. 111, Wiley, New York, N. Y. (2) Briegleb, G., “Elektronen DonatorAcceptor Komplexe,” pp. 175-81, Springer-Verlag, Berlin, 1961. (3) Hertel, E., Bergk, H. W., 2. Physik. Chem. 33, 319 (1936). (4) Powell, H. M., Huse, G., J. Chem. SOC. 1943, 435. (5) Wallwork, S. C., J. Chem. SOC.1961, 494.
Department of Chemistry Rensselser Polytechnic Institut,e Troy, N. Y. RECEIVEDfor review June 12, 1963. Accepted Ja,nuary 10, 1964.. Presented at Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 7. 1963. ~- - ~ H. H. RICHTOT, I
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