Synthesis and electrochemical properties of heteropolymolybdates

Journal of the Less-Cordon Metals, 36 (1974) 19 - 93 @ Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF HETEROPOLYMOLYBDATES*

G. A. TSICDINOS and C. J. HALLADA Climax Molybdenum

Company of Michigan, Ann Arbor, Michigan (U.S.A.)

SUMMARY

The beta form of 12-molybdosilicic acid was prepared and isolated as a yellow hygroscopic solid from a 1: 1 mixture of water-dioxane. The identity of the beta- isomer was established polarographically and spectrophotometrically. The range of stability of the beta-isomer as a function of hydrogen ion concentration was determined. In addition, techniques were developed for preparing, in pure form, several heteropoly compounds used for polarographic and conductivity measurements. The solvolytic stability, electrolyte strength and oxidation-reduction behaviour of several heteropoly compounds were determined by pH, conductivity, and polarographic measurements. Heteropolymolybdate oxidation-reduction behaviour and stability in solution were examined by direct current and alternating current polarography and cyclic voltammetry in aqueous and water-dioxane solutions. The reduction of the anions proceeds in several steps for which the halfwave potentials were determined. Conductivity and pH measurements on the heteropoly acids H4 [PMollV04e] and Hs [PMo1eVz04eJ at 2.5 “C in aqueous solution, and in mixed oxygen-containing solvents, show that the vanadium compounds exhibit behaviour typical of unsymmetrical strong electrolytes in oxygen-containing solvents and are stable in these media.

INTRODUCTION

The characteri~tion of heteropoly compounds by the physicoch~ical means outlined in this study requires that such compounds be of sufficient purity so that meaningful results are obtained.** Consequently, in the course of this investigation new or improved preparative procedures were developed. As a result, a method for preparing and isolating beta-12-molybdosilicic acid was found and the compound was further characterised. Heteropolymolybdates are very suitable compounds for polarographic studies since several of them are known to undergo reversible oxidation-reduction in both aqueous and mixed solvents. information obtained from such an investigation may lead not only * Presented at the Conference on “The Chemistry and Uses of Moiybdenum”, at the University of Reading, England, September 17 - 21, 1973, sponsored by Climax Molybdenum Co. Ltd., and the Chemical Society (Dalton Division). **The necessity of this is pointed out in the paper.

80

to useful preparative procedures for new heteropoly compounds, but such data may also aid their application to homogeneous catalysis. Prior to this investigation, most of the polarographic work on heteropolymolybdates had been restricted to the reduction of the 12-molybdophosphate and 12-molybdosilicate anions in aqueous solution, but both compounds are very susceptible to hydrolytic degradation even in acid solution. Although Wu [l ] was able to isolate the air-stable two-, four- and six-electron reduction products of the [P2MoIa062] 6- anion, only recently have Pope and co-workers [2,3] used polarographic techniques to study the tungstates and molybdates of the 2: 18 series. Pope showed, by direct current polarography, that the [PZM018062] 6- anion undergoes a three-step reduction of two electrons each in acid solution. In the present work, the oxidation-reduction properties of heteropoly compounds of molybdenum were investigated by applying, for the first time [4] , cyclic voltammetry [5,6] and alternating current polarography [5,7,8]. The compounds studied were (NH4)6[P2Mor8062] in aqueous and water-dioxane solutions; the 12-heteropoly acids Ha[PMor20~a], Hq[SiMor204e], Hq[PMollV04e], and Hs [PMoIOVZ04e] in aqueous and mixed solvents; and heteropoly molybdates with transition metals as central atoms in aqueous solution. In addition to polarography, conductance and pH measurements have been used in the present study to establish the hydrolytic stability, the acid electrolyte type and the strength of the acids in aqueous and mixed solvents. EXPERIMENTAL

Preparation and isolation of beta-12-mo(ybdosilicic acid To 1,4-dioxane (350 ml) was added a solution containing concentrated hydrochloric acid (80 ml) and water (75 ml), followed by the slow addition of NalSi0s.9Hz0 (10 g) dissolved in water (60 ml). NazMo04.2Hz0 (100 g) dissolved in water (160 ml) was slowly added to the above solution. To the deep-yellow solution was added bromine water (10 ml) to prevent the reduction of the heteropoly acid, and the reaction mixture was allowed to stand for 24 h. The ion exchange resin used for preparing the free acid was conditioned as follows. Wet Dowex 5OW-X8 cation exchange resin in the hydrogen form (1100 ml) was put in a column and thoroughly flushed with a 50:50 water-dioxane solution that was 1 M in HCl. The yellow reaction mixture was then passed through the column at a rate of 20 ml/min. The effluent solution was cooled to -20 “C in an acetone-dry ice bath and concentrated hydrochloric acid (850 ml) was added slowly with vigorous stirring of the solution. The temperature was not allowed to exceed -20 “C The yellow precipitate thus produced was quickly filtered, and, whilst wet, was quickly transferred to a vacuum desiccator and thus dried by pumping overnight. The dried bright yellow crystalline acid isolated was stored in the dark since it is easily reduced in the presence of light. The product is also very hygroscopic. (Found: Si, 1.19; MO, 49.0; C, 8.37%; Mo/Si 12.05; C/Si 16.45; Calc. for H4 [SiMor20J, 4C4Hs02.7.5Hz0: Si, 1.21 MO, 49.83; C, 8.32%; Mo/Si, 12; C/Si, 16. The solid contained 0.0016% Na. Preparation of other heteropoly compounds The molybdovanadophosphoric acids used in this work were prepared and analysed according to procedures described previously [9] . The 12-molybdophosphoric acid used

81

was Mallinckrodt, Analytical Reagent. (Found: P, 1.33; MO, 48.91; HaO, 23.50. Calc. for Ha [PMo,,O,e] .30Hz0: P, 1.31; MO, 48.67; HzO, 22.83%). The 12-molybdosilicic acid was prepared by ion exchange from its sodium salt. (Found: Si, 1.35; MO, 55.13. Calc. for & [SiMorzO,e] .l 5Hz0: Si, 1.34; MO, 55.00%). All other compounds were prepared and analysed in this laboratory according to procedures described elsewhere [IO]. Polarographic instruments and procedures A potentiostat was constructed to perform d.c. and a.c. polarography and cyclic voltammetry. This instrument was designed to measure potentials to an accuracy of f 1% over the current range of 0 to 20 mA. The polarographic instrument employed in obtaining the data in this report consisted of the potentiostat just described, a Wavetek Function Generator Model 110, and a Mosely Autograph Model 2FR X-Y recorder. The recorder was employed in the d.c. and a.c. polarography. For the cyclic voltammetry experiments, a Tektronix Type 564 Storage Oscilloscope, equipped with a Polaroid camera, was employed, which permitted the recording of cyclic voltammograms as a function of frequency. The dropping mercury electrode (DME) was used in the experiments involving the heteropolymolybdates with transition metals as central atoms. Since the 12-heteropoly acids of molybdenum with non-transition elements as central atoms are reduced by mercury, a rotating platinum electrode was employed for those d.c. polarograms. A stationary platinum electrode was used for a.c. polarography and cyclic voltammetry experiments. In all experiments, a saturated calomel electrode was used as a reference, along with a platinum counter electrode. An H-cell was used with the rotating platinum electrode. A cup-type electrolysis cell was used with the stationary and dropping mercury electrodes. The solutions were freed of oxygen by flushing with pre-purified nitrogen before electrolysis. In all tests, a blank employing the solvent and the supporting electrolyte was examined polarographically. Conductivity and pH data Conductivities were measured at 25.00 + 0.01 “C, using the weight dilution technique in a flask-type cell containing shiny platinum electrodes. The cell was calibrated with standard potassium chloride solutions and found to have a constant of 0.4093 cm-‘. All measurements were made at 3 kc, using an Industrial Instruments Model RC-18 conductivity bridge. Results from several stock solutions of a given acid and from different preparations of a given acid show that the data were reproducible. Solvent conductivity corrections were made in all cases but this was always less than 1%. Potentiometric measurements of the hydrogen ion activity were made with a Beckman Model 1019 Research pH meter which was calibrated with buffers accurate to f 0.01 pH unit. A glass electrode was used vs. the standard calomel electrode. The 0.100 M NaOH solution used for titrating contained the same solvent as that used in the heteropoly acid solution. The pH vs. pcB+ data were obtained on the above-mentioned acids in water, 20% dioxane, and 20% dioxane containing 3 M NaC104. A standard curve for HCl in the three solvent mixtures was used for comparison. Unsteady values of pH were obtained in 20% dioxane containing 3 M NaC104, indicating adsorption of the electrodes [ 111.

82 TABLE 1 Stabilising solvents for P-H, ISiMo, 20,,) * Stabilising solvents

Non-stabilising solvents

CH,OH CH,CH,OH CHJHOHCH, (CH,), C=O (CHB&CHOHCH,COCH, 1,4-Dioxane *All organic solvents contained 50% water by volume. TABLE 2 Half-wave potentials of p-H,(SiMo,,O,,)* Half-wave potentials Es (V)

24 h after preparation? After ion exchangez After isolation a-form

1

2

0.420 0.410 0.425 0.320

0.305 0.300 0.315 0.190

4

3 0.185 (0.175)s 0.200 -0.055

-0.115 -0.130 -0.110 -0.225

5 -0.260 (-0.280)s -0.250~ -

*In 1 M HCI in 1: 1 water-dioxane. i In situ prepared 12-molybdos~icate anion. Y/I Solution obtained after passage of (a) through ion-exchange. 0 Parentheses indicate weak and poorly defined a.c. peaks. ). The acid contains 7% of the alpha isomer. TABLE 3 Spectroscopic data for H,[SiMo,,O,o] Wavelength h (nm)

Molar extinction coefficient E (mole-’ 1. cm-‘) Beta- form

Alpha-form 450 425 400 375 lO’{H~[SiMo~*O~*)~~

3.9 12.0

1.3 5.9

6.4

1.9 3,s 0.64

6.7

3.6 7.2 0.66

Spectra The absorption spectra of beta-12-molybdosilicic acid in water-dioxane were taken with a Beckman DB automatic recording spectrophotometer employing 1 cm silica cells. RESULTS AND DISCUSSION

(a) Synthesis of ~eta-l2-~o~~dosi~icic acid Strickland [12a] first reported the existence of the beta-isomer of 12-molybdos~icic

83

acid in aqueous solution. We attempted to prepare beta 1Z-molybdos~ici~ acid following a literature method ]12b] but did not obtain a solid upon acidification of sodium molybdate-sodium metasilicate solutions. When such solutions were cooled, we obtained a yellow solid which contained sodium perchlorate when perchloric acid was used as the precipitant. These observations have also been made by others [ 12~1. The lack of additional support is consistent with the fact that the beta-isomer is stable in water solution for periods of up to one hour but it reverts to the more stable and usual alpha-form [ 121. Since then, it has been shown [13, 141 that water-ethanol mixtures stabilise the betaform. Several solvents were examined to ascertain the stabilising effect on the beta-isomer of the acid. With the exception of dioxane, no attempt was made to isolate the free acid from all solvents. The nature of the isomer present was ascertained by ax. polarography. The data obtained are shown in Tables I and 2. Strickland [ 121 pointed out that the molar extinction coefficients for the betaisomer in aqueous solution were roughly twice those for the alpha-isomer. Spectroscopic data (Table 3) of the two isomers of this acid, obtained in 1: 1 dioxane-water solutions, indicate that in this solvent the molar extinction coefficients for the beta-isomer are also roughly twice as large as those for the alpha-isomer. However, Strickland [ 121 and later Langer [I 51 reported that the absorption curves (in aqueous solution) for the two isomers intersect at wavelengths slightly lower than the values obtained here. No intersection was found in the spectra of the two isomers in water-dioxane solutions. The beta-isomer was also shown to be a stronger oxidising agent than the alphaform in water solution ]12]. The half-wave potentials obtained in our work in waterdioxane solution (Table 2) show a similar behaviour of the two isomeric forms in this medium also. (b) Polarographic measurements 18-Molybdodiphosphate anion The hydrolytic stability of the [P~Mo~~O~~] 6- anion enabled Pope and co-workers [2,3] to examine the oxidation-reduction of this anion by conventional (d.c.) polarography. The polarographic behaviour of the same anion obtained in this work is shown in Figures 1 - 3. The cyclic voltammogram shown in Fig. 1 shows the presence of three cathodic and three anodic reversible waves. Values for the cathodic half-wave potentials are tO.47, +0.35 and +0.17.5 V. An irreversible fourth half-wave potential was found at -0.10 V. The anodic potentials occurred at tO.50, t0.38, and tO.22 V. Each of these waves corresponds to the addition of two electrons as ascertained [8J by the a.c. polarogram obtained under the same conditions (Fig. 2). The fourth wave which occurred at -0.10 V is irreversible. While determining this wave, a blue fuzzy precipitate appeared on the platinum electrode possibly resulting from the decomposition of the heteropoly anion. The reduction of the anion in water-dioxane solution is shown in Fig. 3. The values of the half-wave potentials in water and water-dioxane solution show that the heteropoly anion has similar oxidising power in these media. However, addition of more than six electrons to the [PzMo~~O~~] 6- anion in aqueous solution resulted in decomposition of the anion. The cyclic voltammogram and a.c. polarogram (Figs. 1 and 2) could not be reproduced when scanning was carried out beyond -0.1 V which resulted in the addition of more than six electrons to the heteropoly anion cage, although this effect was not observed in water-dioxane solutions.

1

Curve we

3

2: Sweep 3: Sweep

4wI

rata 0~

: % 2

30 75 5o 25

20 IO 0

$0 2

-25

-10

!ial -50

-20

Q

-30

-75 t.0

@S

0.6

@4

0.2

0

300

200

loo

0 +OG3

Am&& potential (V) versus SCE

tO+

HI.4

i-O.3

-l-O,2

+@t

0

-0.1

Applied potential fV) versus SGE

Fig. I. Cyclic voltammogram of 2 X 10m4M (NH,), fP,Mo, 8O62 1 in 1 MH,SO, at 25 “C. Fig. 2. Alternating current polarogram of 8 X 10e9 M(NH,),

[PZMo,8062 j in 1 M H,SO, at 25 “C.

60 60 40 30

20 10 0 Applied potential IV) versus SCE

to+

-to4

+0.2

0

-0.2

Applied potential (V) versus SCE

Fig. 3. Alternating current poiarogram of 2 X 10e4M (NH,),[P,Mo,,O,, dioxane). Fig. 4. Alternating current polarogram of 1 X 10-j M H,[PMo,,O,,] dioxane).

J in 1 M H,SO, (X:1 water--

in 0.5M H,SO, (1 :l water-

Fig. 5. Cyclic voltammogram of 1 X 10-j M H, [ PMo, ,O,,] in 0.5 N H,SO, (I : 1 water-dioxane). Y-axis is: Current (200 pA/div). X-axis is: Applied Potential (0.1 V/div) YS.SCE. Frequency: 10 Hz. Sweep rate: 20 V/s. Fig. 6. Cyclic voltammo~~ of 1 X IO-‘iM H~[SiM~,?O~~] in 0.5 M H,SO, (I:1 water-dioxane). Y-axis is: Current (100 rA/divf. X-axis is: Applied Potential (0.1 Y/d&) 9s. SCE. Frequency: 16 Hz. Sweep rate: 30.7 V/s.

85

Figure 3 shows that in such solutions a well-defined fourth wave occurs at -0.145 V. Beyond this there is an additional wave at -0.25 V which overlaps the hydrogen wave. The stabilising effect of dioxane for highly reduced heteropoly anions may be compared with the same behaviour of this solvent in stabilising the [PMo,,O,,] 3- anion toward hydrolysis. [ 161. Unlike the corresponding heteropoly tungstate [2,3] [P2W,s062] 6; the 18-molybdodiphosphate anion at the acidity studied (in water-dioxane solutions) by cyclic voltammetry did not yield intermediate (one-, three-, and five-electron) reduction products although sweep rates up to 120 V/s were employed. No rationalisation for this difference between heteropolymolybdates and tungstates can be offered at present. 12-Heteropolymolybdates in water-dioxane The polarographic behaviour of the 12-molybdophosphate anion is shown in Figs. 4 and 5. The effect of the heteropoly acid concentration (lo-’ - lo-* M) on the shape of the polarographic wave was ascertained by obtaining polarograms in 0.5 M HzS04. The most well-defined polarograms were obtained in the concentration range of lo4 - low3 M. The shape of the waves was less defined in the more dilute solutions. It was also shown that one-day-old solutions of 12-molybdophosphoric acid in 0.5 M HzS04 gave reproducible polarograms indicating reasonable stability of the acid in that solvent. The reversibility of the oxidation-reduction steps in Ha [PMo,204e] was ascertained from cyclic voltammetry, as shown in Fig. 5, at frequencies from 0.01 to 10 Hz. The general shape of the voltammogram indicates some reversibility of the first three waves at the sweep rates studied. No conclusions can be drawn about the fourth wave since it occurs close to the evolution of hydrogen. Although the above-mentioned data indicate that the 12-molybdophosphate anion is stable in water-dioxane solutions, polarographic data from the literature on aqueous solutions of 12-molybdophosphates should be viewed with caution since these data were obtained without regard to the hydrolytic instability (see pH measurements in this work) and reactivity of these compounds with mercury. For example, the polarographic behaviour of dilute aqueous solution of H3 [PMo r 2040] has been described by Kemula [ 171 employing the dropping mercury electrode. By employing a platinum electrode in water-dioxane media the hydrolysis of the heteropoly anions and spurious reactions have been avoided. 12-molybdosilicic acid, which is more stable hydrolytically than the phosphorus analogue, is reduced in four steps of about equal height, although the first and second steps are not well separated. The first three steps are reversible as shown by the cyclic experiments (Fig. 6). Each step appears to correspond to the addition of two electrons (Fig. 7) consistent with d.c. polarographic and coulometric data obtained in water ethanol solutions for 12-molybdosilicic [ 13, 141 and 12-molybdophosphoric acids [ 131. Behaviour similar to that of the phosphorus and silicon compounds is exhibited by the analogous arsenic anion [AsMoiZOQO] 3-. Although the free acid of the arsenate anion has not been isolated [18] , contrary to previous claims [ 191, the 12-molybdoarsenate anion is stabihsed by mixed solvents [18, 191. The preparation of the free 12-molybdoarsenic acid as described in the literature [19] could not be repeated. It was found in our work that addition of perchloric acid to water-dioxane solutions containing sodium molybdate and arsenate ions resulted in the formation of yellqw solids that contained 1.94 - 2.83% Na, indicating considerable contamination by sodium and perchlorate ions. Thus in view of these results and those

86

120

60

? g ‘;:

80

+0.8

$0.4

to.2

0

- 0.2

Applied potential (V) versus SCE

-0.4

al +0.81 1+0.41 1+0.21 1 01

’I

1

-0.2

’I

11

-0.4

Applied potential (V) W~SUS SCE

Fig. 7. Alternating current polarogram of 1 X 10m3M H,[SiMo,,O,,] dioxane). Fig. 8. Alternating current polarogram of 1 X lo-’ M H, [PMo,,VO,,] (1:l water-dioxane).

in 0.5 M H,SO, (1:l water(fresh solution) in 0.5 M H,SO,

also obtained when such precipitations are carried out from water-ethanol mixtures, it is recommended that the procedure outlined in this paper for preparing beta 12-molybdosilicic acid by ion exchange be adapted to preparing these and other heteropolyacids free of contaminants from mixed solvents by ion exchange. A d.c. polarogram of the arsenate obtained in 0.5 M HzS04 in 1: 1 water-dioxane consists of three well-defined waves of equal height and a fourth wave (ill-defined) at half-wave potentials (in V versus SCE):+0.35,+0.23, -0.01, and -0.17 V. It has the same general shape as that of the [PM0is0~~] 3- anion. However, the more positive value of the half-wave potentials indicates that it is a stronger oxidising agent than the phosphorus analogue. In view of the instability of the 12-molybdoarsenate anion in aqueous solution [ 18, 191 the polarographic data for the “12-molybdoarsenic acid” given by Kemula [20] in water were no doubt obtained on a mixture of [As*Moi 8o62] 6and molybdate species. Although the polarograms of H4 [PM0 llV040] and Hs [PMo 1oVz040] in water solution show that these acids are considerably decomposed, the data in water-dioxane solutions, Fig. 8 for H4 [PMoi IVO4o] and Fig. 9 for Hs [PMo~~V~O~~], show that dioxane stabilises the anion structures toward hydrolysis. Four stepwise reductions, which are fairly reversible, were shown by cyclic experiments. The second and third waves, especially for Hs [PMo,~V~O~~] , overlap considerably. Isolation of reduced forms of these materials may be possible by employing the conditions used in water-dioxane media. Recently, the polarographic behaviour of Hs [PMo~~V~O~~] in aqueous 0.5 M HzS04 and in absolute ethanol (0.5 M H,S04) was reported [21] employing the dropping mercury electrode. Two waves at 0 and at -0.3 V (L’s) were obtained in aqueous solution and a single reduction wave at Ey2 = -0.35 V in absolute alcohol. The waves were reported to be irreversible, the number of electrons corresponding to each reduction being four. In view of the instability of this compound in 0.5 M HzS04 found in this work and the reduction of the 12-heteropolymolybdates by mercury, these data are questionable.

Fig. 9. Alternating current polarogram of I X 10‘” &f N,[PMo,,V,O,,] &SO, (1: 1 water-dioxane).

(fresh solution) in 0.5 M

Fig. 10. Cyclic voltammogam of 3 X lo-* MNa,[TeMo,O,,] in 1 M NaAc (pH = 3.4). Y-axis is: Current (5 pA/div). X-axis is: Applied Potential (0.1 V/div) vs. SCE. Sweep rate (outer cycle): 3.84 V/s (inner cycles lower sweep rates).

Heteropolymolybdates with transition metals as central atoms Few studies have been made of the oxidation-reduction properties of heteropolymolybdate anions containing transition metals as central atoms. No “heteropoly blue” involving reduced species of these anions, is known. Only a brief report has appeared in the literature (221 stating that some of these anions are reduced at the dropping mercury electrode, but no definite conclusions can be drawn from these data. Consequently, the polarographic behaviour of the anions [TeMo60z4J 6; [CrMo6024H6] 3; [AlMo~O*~~~] 3; [CO~MO,~O~~II.+] 6; [FeMo60z4HJ3; and of the anion [IMo6024] ‘- was examined by d.c. and a.c. polarography and cyclic voltammetry. All of these anions showed no reduction waves at positive potentials with respect to the SCE, using the platinum electrode. This fact is consistent with the lower oxidising power of these species compared with the 12-heteropolymolybdates examined in this work. Consequently, all polarographic data on compounds with transition metals as central atoms were taken with the dropping mercury electrode. The 6-molybdotellurate(VI) anion was examined in detail since it presented the most consistent behaviour. A cyclic volt~mogram of [TeMo60z4] ‘- anion in 1 M NaAc (pH = 3.4) is shown in Fig. 10. The reversib~ity of this system was difficult to ascertain since considerable adsorption effects on the electrodes were experienced which could not be overcome with the use of suppressors such as Triton Xl00 and methyl red. Thus “log plots” Of -Ede Ver?8&$ log ~

i

id - i

obtained

from de. polarograms

did not produce a

straight line as expected for a reversible system. The effect of pH on the nature of the polarographic wave was also examined employing sodium acetate - acetic acid buffers. It was found that at pH 2 and 3, considerable adsorption occurred. Substituting a 0.1M Na2S04 (pH = 3.73) solution as the supporting electrolyte did not improve the results, even in the presence of Triton Xl 00. Thus, the system showed the most consistent behaviour in 1 M sodium acetate (pH = 3.4) buffer system. In trying to determine ~oulometri~~ly the number of electrons ~orres~nding to

88

this redox step in the presence of sodium perchlorate as the supporting electrolyte, we found that the diffusion current increased instead of decreasing with time, indicative of a catalytic polarographic current which may be attributed to the presence of perchlorate ions. An estimate of the number of electrons that correspond to the oxidation-reduction step in the [TeMo60a4] 6- was obtained from the Ilkovic equation [S] employing a value of the diffusion coefficient for the anion, D, equal to 7.0 X 10e6 cm2fs [23]. Thus, the redox step in question involves one electron. The cathodic and anodic peaks, as shown in Fig. 10, are separated by - 50 mV, indicating that the oxidation-reduction step in this process is reversible or nearly so. The limited pH range of stability of the 6-molybdotellurate anion and the adsorption problems encountered precluded any further study of this system. The other six heteropolymolybdates studied exhibited different behaviour. For example, the cyclic voltammogram (Fig. 11) of the 6molybdochromate anion is typical of a system [5] which undergoes a rapid electron transfer followed by a slow chemical reaction. That is, the anodic peaks are considerably smaller than the cathodic ones (Fig. 11) suggesting that the reduced species undergo decomposition. The cyclic voltamm~ric experiments for the other 6-heteropolymolybdates of Al(III), Fe(III), I(VI1) and for the anion [Co2Moi&sH4] 6- gave voltammograms similar to that of the chromium complex, suggesting that the reduced species produced are unstable in solutions, and isolation of any heteropoly blues is thus precluded. (c) Conductivity and pH measurements

The conductivity results on the heteropoly acids H4 [PMo,~VO~~] and Hs [PMoi0V204e] at 2.5 “C in aqueous solution and in mixed solvents have been interpreted using the Onsager-Fuoss theory derived for ~mmetri~al electrolytes 124,251. For completely unassociated electrolytes the theory may be written in the form A=/$” -SC’

+EclogctJc

where A is the measured equivalent conductance at concentration c, A” is the equivalent conductance at infinite dilution, c is the concentration in equivalents per litre, S is the Onsager limiting law slope ( a function of A”, temperature, dielectric constant of the solvent, viscosity of the solvent, and valence type of the electrolyte}, E is a constant defined by the same variables as S, and J is a constant defined by the same variables as S and E but including the closest distance of approach of the ions, a’. To find values for A0 and J, values of A’ are calculated. Ai=A+Sc’-Ecfogc=A”iJc. Highly dissociated unsymmetrical electrolytes have been she m to behave in a manner similar to that predicted by the Onsager-Fuoss theory or I an empirical equation of the same form [25]. Reasonable values of the conductivity at infinite dilution and the closest distance of approach have been found when the Onsager-Fuoss theory has been applied to data from unsymmetrical, strong electrolytes, although A’ vs. c plots do begin to curve downward at higher concentrations [26]. It has also been shown that acids obey the conductivity theory despite the different transport mechanism involved for protons [27,28]. Therefore, conductivity data can help establish the stability, the

89

1.5

2.5

2.0 PC,+

3.0

3.5

(calculated)

Fig. 11. Cyclic voltammogram of 1 X 10m3M (NH,), [CrMo,O,,H, ] in 1 M NaClO,. Y-axis is : Current (200 rA/div). X-axis is: Applied potential (0.2 V/div) vs. SCE. Sweep rate (outer cycle): 2 V/s (inner cycles lower sweep rates). Fig. 12. pH data on H,[PMo,,VO,,] and H,[PMo,,V,O,,]. 0 H,[PMo,,VO,,] in aqueous in HNO, solution (Hi: solution; l H, [ PMo, OV,O,,] in aqueous solution; n H,(PMo,,VO,,,] heteropoly anion = 13.7);. H,[PMo,,V,O,,] in HNO, solution (H+: heteropoly anion = 13.7); 0 H,[PMo,,VO,,] in HNO, solution (w: heteropoly anion = 22.4); n H,[PMo,,V,O,,] in HNO, solution (H’: heteropoly anion = 22.4).

1.5

2.0 PC+++

2.5

3.0

3.5

Fig. 13. pHdata on H,[PMo,,VO,,]

1.5

2.0 PC,+

(calculated)

and H,[PMo,,V,O,,]

2.5

3.0

3.5

(calculated)

in 3 MNaClO,. 0 H,[PMo,,VO,,]

;

.H,U’Mo,oVzO,,l. Fig. 14. pH data for HCl in water and 20% dioxane. 0 HCI in water; n HCI in 20% dioxane.

electrolyte type and the mobility of the anion. When association occurs, it is difficult to define the degree of association and the closest distance of approach of unsymmetrical electrolytes from conductivity measurements. The associated species itself is conducting until a neutral species is formed, and this species is probably formed only in very concentrated solutions. However, pH data can help to clarify whether or not ion association is occurring.

90 TABLE 4 Conductivity data at 25 “C for molybdovanadophosphoric

acids A0 (ohm-’ cm1 equiv-’ )

(mp)

Limiting conductivity A0 (ohm-’ cm’ equiv-I)

18.54 60.79 61.3 27.9

8.95 12.92 23.06 48.16

492 330 250 90

142 80

78.54 60.79 42.98 61.3

8.95 12.92 17.8 23.06

453 300 180 215

101 50 15 -

Dielectric constant at 25 “C

Viscosity at 25 “C*

H,P’Mo,,VG,,l Water* 20% dioxane 20% rert-butyl alcohol 60% tert-butyl alcohol

H,lPMo,oV,O,,l Water* 20% dioxane 40% dioxane 20% tert-butyl alcohol

*The viscosities at 25 “C of 20 and 60% t-butyl alcohol solutions were determined with the Ostwald viscometer, using water as a standard (F. Daniels, et al., “Experimental Physical Chemistry” 5th edn., McGraw-Hill, New York, 1956, p. 60).

The conductivity data obtained on I& [PMorlV04e] and H5 [PMoroVz04e] in water [29] and in 20% dioxane and 20% tert-butyl alcohol are summarised in Table 4. Typical phoreograms along with the limiting slopes and A’ vs. c plots for these acids in water and in mixed solvents are illustrated in refs. 29 and 16, respectively. The A’ vs. c plots are linear and A0 and J can be readily found for each acid. However, at high concentrations they begin to curve downward. The a0 values in water for H5 [PMor0VZ04e] (a” = 4.0) is lower than that found for H4 [PMorlV04e] (a” = 3.3). Both are smaller than the 5.5 A radii reported for a0 using X-ray techniques for similar heteropoly anions [ 10, 301. The values h” also vary more than expected for ions that would be assumed to be essentially the same size and which differ only slightly in charge. A small amount of ion association would make a0 appear to be smaller than it should be [ 3 1] . On the other hand, a slight hydrolytic degradation of the [PMor rV04e] 4- anion, similar to that which occurs to a larger extent in [PMor204e] 3- and [PMogV304e] 6- could lead to higher conductivity readings. The pH data in water solution for the two molybdovanadophosphoric acids are shown in Figs. 12 and 13. It should be noted that since a standard curve of pH vs. pcH+ of HN03 in 3 M NaC104 was used to determine a corrected pH from the measured pH of the heteropoly acids in 3 M NaC104, the activity coefficient has essentially been accounted for and the pH values shown for pH (measured) for the heteropoly acids are measured values for pcH+. In aqueous or weak acid solutions without a high ionic strength medium, the pH data appear to indicate increased ion association as the concentration of the acids is increased. However, although the data show some scatter, there is no clear increase in degree of H’ association with increased acid concentration in 3 M perchlorate for either of the acids. Therefore, the data in low ionic strength media are apparently showing activity coefficient effects instead of increased ion association, and

91

both acids appear to be very strong electrolytes which are essentially completely dissociated. The potentiometric titration of 12-molybdosilicic acid, H~[SiM0r204~], with sodium hydroxide shows the acid to be tetrabasic in water, as ascertained by the single point of inflection occurring at the addition of 4 mole of base per mole of acid. The same titration in 3 M NaC104 shows a considerable flattening of the curve, the inflection point becoming indiscernible, indicating appreciable hydrolytic degradation of the heteropoly anion in the high ionic-strength-medium employed. However, the same titration in 20% tert-butyl alcohol solution containing 3 M NaClOd gives a well-defined inflection point at the addition of 4 mole of base per mole of acid. 12-Molybdophosphoric acid, Hs [PMor204,J, has long been known to undergo extensive hydrolytic degradation in water, especially in dilute solutions. This behaviour has accounted for the high basicities reported for this acid in the older literature, especially by the Rosenheim School [32], and this effect has been reported even in fairly modern literature [33]. As already indicated, 12-molybdophosphoric acid when titrated with sodium hydroxide behaves as six-to-seven basic in water or in 20% t-butyl alcohol solution containing 3 M NaC104. However, in 20% t-butyl alcohol or in 20 and 40% dioxane solution, the acid is tribasic, as formulated. It was also found that this acid still behaves as five-to-six basic in 10% dioxane and in 10% t-butyl alcohol; hence, this amount of organic solvent is not sufficient to impart hydrolytic stability to the 12-molybdophosphate anion. Although 12-molybdosilicic acid is stabilised by 20% butyl alcohol in the presence of 3 M NaC104, 12-molybdophosphoric acid (a hydrolytically less stable compound) is not. Figures 14 - 16 represent plots of pH vs. pCH+ for hydrochloric acid, H,[SiMorzO,,] , and H3 [PMo,~O~,], respectively, where pH is the measured value and pCH+ is calculated from the solution concentration. The pH data for hydrochloric acid (Fig. 14) were specifically obtained both in aqueous and in 20% dioxane to show that the effect of liquid junction potentials is very small. The measured values of pH for the same concentration of hydrochloric acid in 20% dioxane are slightly below those in water, as expected for these mixed solvent media [32], since the activity coefficients of hydrochloric acid are nearly identical in water and 20% dioxane.[25]. The data for the heteropoly acids show that within experimental error, the pH is the same for both aqueous and 20% dioxane solutions of HCl, I& [SiMor,O,a] , and Ha [PMor,O4e], i.e., Figs. 14 - 16 are superimposable. However, in water alone, Ha [PMo1s04e] undergoes hydrolytic degradation producing hydrogen ions as shown by the fact that the pH values are below those calculated (shown by solid line) in the more dilute solutions. This effect is more pronounced when such solutions are allowed to stand for one day. It has been shown from conductivity measurements that hydrochloric acid in water-dioxane mixtures behaves like a strong electrolyte, even up to 70% dioxane [35]. Consequently, hydrochloric acid must remain unassociated at the concentrations employed in this work, both in water and dioxane-water. The upward trend of the points at low pc~+, Le., high acid concentration (Fig. 14) must be due to the variation of activity coefficients which are nearly identical in water and 20% dioxane [25], rather than association. Such variation of activity coefficients has been established [36] in solutions less than 0.1 M for the heteropoly compounds (NH4)3 [CrMosOz4Hs] and (NH4)s [AlMo6024HG]. It has already been shown that nitric acid behaves similarly in

Fig. 15. pH data for H,(SiMo, 2040] in water and 20% dioxane. 0 in water; n in 20% dioxane. Fig. 16. pH data for H, [PMo,,O,, ] in water and 20% dioxane. A Water sdution 0 water solution after one day; 0 2-% dioxane solution.

(fresh);

water, but pH us, pCI.j+plots of this acid in 3 M NaC104 are linear due to the high ionic strength medium employed. The similarity in behaviour between H~[S~o~~~~~~ and these known strong acids in both solvents confirms that H4 [SiMor204e~ is stable and a very strong acid in these solutions. On the other hand, Ha [PMo~~O~~], a strong but unstable acid in water, retains its acid strength and is stabilised in 20% dioxane. Such stab&&ion may be imparted by the direct interaction of the heteropoty anion with the organic solvent. Recent infrared evidence points to the direct interaction of heteropoly anions with organic solvents containing electron-donating groups such as ketones (371. In addition, n.m.r. evidence has shown that the protons in heteropoly acids are present as HsO+ ions in the solids and in ether solutions [38]. The acids l$ [PMo,rVO.J and HS [PMo1eV104e] also behave like hydrochloric acid in 20% dioxane and hence are stable, strong acids in this medium. Plots of pH W_p&‘~+for HCf, I& fSiMoraO,ej, HqfPMol ttrO,~J, and HS [PMo~*V~O~*] in 20% dioxane containing 3 M NaC104 were also obtained. However, as already mentioned, adsorption effects on the electrodes make these pH data somewhat unreliable; hence these data are not presented. Souchay has reported [39] that dissociation constants can be determined from petition measurements involving water and organic soIvents. A pK value of 0.75 was reported for the acid dissociation constants of H4 [PMo~~VO~~l,which was measured by the distribution of this acid in water and isoamyl alcohol at varying perchloric acid concentrations. However, Souchay made the assumption that the heteropoly acid is less dissociated in higher percbloric acid concentrations and, hence, more extractable in the organic solvent. This remains only an assumption. It has been shown in this work that the hetero~Iy acids studied are strong electrolytes, even in mixed solvents having a low dielectric constant.

93 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

H. Wu, J. Biol. Chem., 43 (1920) 189. M. T. Pope and E. Papaconstantinou, Inorg. Chem., 6 (1967) 1147. E. Papaconstantinou and M. T. Pope, Inorg. Chem., 6 (1967) 1152. G. A. Tsigdinos, C. J. Hallada and D. K. Means, Abstr. Papers, 156th Nat. Mtg., American Chemical Society, Atlantic City, New Jersey, September 8 - 13, 1968, p. INOR 174. L. Meites, Polarographic Techniques, Interscience, New York, 2nd edn., 1965, pp. 577 - 592. J. B. Headridge, Electrochemical Techniques for Inorganic Chemists. Academic Press, New York, 1969, Ch. 5. D. E. Smith, AC Polarography and Related Techniques: Theory and Practice, from A. J. Bard (cd.), Electroanalytical Chemistry, Vol. 1, Marcel Dekker, New York, 1966, pp. 1 - 155. D. E. Smith, Anal. Chem., 35 (1963) 1811. G. A. Tsigdinos and C. J. HaIIada, Inorg. Chem., 7 (1968) 437. G. A. Tsigdinos, Heteropoly Compounds of Molybdenum and Tungsten, Climax Molybdenum Co. Bull. Cdb-12a (Revised), November, 1969. J. Kucharsky and L. Safarik, Titrations in Non-Aqueous Solvents, Elsevier, Amsterdam, 1965, p. 69. (a) J. D. H. Strickland, J. Am. Chem. Sot., 74 (1952) 862, 868, 872; (b) R. Massart and P. Souchay, Compt. Rend., 257 (1963) 1297, and also reference 13; (c) P. Rabette, Thesis, University Paris VI, 1973, p. 2. R. Massart, Ann. Chim. (Paris) 3 (1968) 507, and references therein. R. Massart and G. Her&, Rev. Chim. MinBrale, 5 (1968) 501. K. Langer, Z. Anal. Chem., 245 (1969) 139. G. A. Tsigdinos and C. J. HaIlada, Inorg. Chem., 9 (1970) 2488. W. Kemula and S. Rosolowski, Roczniki Chem., 37 (1963) 941. G. A. Tsigdinos, Climax Molybdenum Co., unpublished results. P. Souchay and R. Contant, Compt. Rend., 265~ (1967) 723. W. Kemula and S. Rosolowski, Roczniki Chem., 38 (1964) 905. N. A. Polotebnova and L. A. Furtune, Russ. J. Inorg. Chem., 14 (1969) 1124. A. Duca and T. Budiu, Rev. Roumaine Chim., 10 (1965) 193. M. T. Pope and L. C. W. Baker, J. Phys. Chem., 63 (1959) 2083. R. M. Fuoss and F. Accascina, Electrolytic Conductance, Interscience, New York, 1959. H. S. Hamed and B. B. Owen, The Physical Chemistry of Electrolytic Solutions, Reinhold, New York, 3rd. edn., 1958, p. 207. G. Atkinson, M. Yokoi and C. J. HaIlada, J. Am. Chem. Sot., 83 (1961) 1570. F. Franks and D. J. G. Ives, Quart. Rev., 20 (1966) 1. H. 0. Spivey and T. Shedlovsky, J. Phys. Chem., 71 (1967) 2171. C. J. HaIlada, G. A. Tsigdinos and B. S. Hudson, J. Phys. Chem., 72 (1968) 4304. R. Signer and H. Gross, HeIv. Chem. Acta, 17 (1934) 1076. R. L. Kay, J. Am. Chem. Sot., 82 (1960) 2099. A. Rosenheim in R. Abegg (ed.) Handbuch der Anorganischen Chemie, Vol. 4, Part 1, ii, S. Hirzel, Leipzig, 1921, pp. 977 - 1065. E. A. Nikitina and E. V. Buris, Zh. Neorg. Khim., 3 (1958) 2687. 0. Popovych and A. J. DilI, Anal. Chem., 41 (1969) 456. B. B. Owen and G. W. Waters, J. Am. Chem. Sot., 60 (1938) 2371. E. Meyer and R. Huckfeldt, J. Phys. Chem., 74 (1970) 164. M. N. Ptushkina and L. I. Lebedeva, Russ. J. Inorg. Chem., 13 (1968) 1579. T. Wada, Compt. Rend., 259 (1964) 553. P. Souchay, F. Chauveau and P. Courtin, Bull. Sot. Chim. France, (1968) 2384.