influence the adsorption characteristics of the nitrate ion. It has been demonstrated ( I , 8) that the presence of the fluoride ion, also known to be hardly adsorbed at the point of zero charge, has significant influence over the adsorption characteristics of the nitrate ion. Moreover, the fluoride ion is singly charged, whereas the pyrophosphate ion is more highly charged, the average value depending upon the pH of the solution. The presence of multicharged anions, even when not specifically adsorbed, causes the J., us. 4 curve calculated from the Gouy-Chapman theory to be lower for q > 0 than the corresponding curve for an equal concentration of singly charged anions, and a series of curves representing q0 us. 4 and concentration will be quite close together in the potential region where q > 0 (see Figure 10). Any deviation from the assumed adsorption characteristics will therefore cause a significant error in the calculated double layer effects. This difficulty seems to be the cause of the anomalous results for z in the potential region where q > 0. The situation at pH 5.80 is quite different from that at pH 9.18. The mercury pressure dependence of the height of the first wave clearly indicates that this is a kinetic wave, as is verified by the decrease in height of the first wave as the pyrophosphate concentration is increased. However, the dependence of current on concentration of KNO indicates that the reducible species is positively charged. This positive value for z is clearly indicated by the region of negative 6 of Figure 11, from the fact that, at any constant 4 value where q < 0, the current increases as the value of +o increases because of the decreasing supporting electrolyte concentration. The assumptions concerning the exact nature of the adsorption
are essentially inoperative in reaching the conclusion about the sign of z, because the same result was obtained from calculations employing two different sets of electrocapillary data, both with and without the consideration of specific adsorption. There is the possibility, however, that the assumptions concerning the reaction site and the thickness of the reaction layer are in error. While the equilibrium constants previously obtained in alkali free solutions cannot be rigorously applied under the present conditions, it seems evident that a surprisingly high degree of ion pairing exists, causing a positively charged species-such as CuH2KP20?+-to be present. It seems inconsistent to postulate for this case a completely dissociated reactant as would have to be done to explain the positively charged species in the absence of ion pairing. The kinetic parameters obtained from the Gierst-type analysis of the dc polarograms, considering specific adsorption of nitrate, are summarized in Table V. The quantity T K was calculated from the expression (3)
(ZK D)l/2
(24) where D is the diffusion coefficient. For the reaction in Equation 13, the equilibrium constant K should be multiplied by [Py] to obtain the actual equilibrium constant K’ (see Equation 15). u0 =
RECEIVED for review September 8,1968. Accepted January 21, 1969. Research supported in part by PRF grant 1516 A5 to P.E.S. Presented in part, Division of Analytical Chemistry, 153rd National Meeting, ACS, Miami Beach, Fla., April 1967.
Electrochemical Reduction Products of Chromium(VI) in Molten Lithium Chloride-Potassium Chloride Eutectic J. H. Proppl and H. A. Laitinen Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, Ill. 61801 The electrochemical reduction product of K,CrOa dissolved in a LiCI-KCI eutectic containing dissolved MgCl, is characterized as a single, unstoichiometric compound having the empirical formula Li,Mg,CrO, where x 2y = 5.The values of x and yare found to depend upon theconditionsoftheelectro1ysis.X-ray powder diffraction shows the compound to have a face-centered cubic structure with an “a” parameter of 4.182 A. The thermal and pressure stability of the compound are also investigated. The electrochemical reduction of KICrlOi or Cr02Clzin the presence of MgCI, is complicated by chemical attack of the cathode causing a mixture of compounds to be deposited on the electrode surface. Also, oxide ion is released to the bulk melt altering the oxygen to chromium ratio of the Cr(VI) species being transported to the electrode surface.
+
PREVIOUS WORK in this laboratory by Laitinen and Bankert (1) has shown that the electrochemical reduction of KsCrO4 dissolved in a LiC1-KC1 eutectic results in the formation of ‘Present address, Wisconsin State University, Oshkosh, Wis. 54901
(1) H. A. Laitinen and R. D. Bankert, ANAL.CHEM., 39,1790 (1967).
644
ANALYTICAL CHEMISTRY
an insoluble film on the surface of the platinum cathode. The chemical composition of the film was established to be Li6Cr04 using spark emission spectrometry. In the presence of divalent cations such as Mg(I1) or Ca(II), the reduction potential of chromate was shifted from - 1.O V 6s. the Pt/Pt(II) reference to -0.2 V and -0.4 V, respectively, and the composition of the insoluble film was altered. Attempts to establish the composition of these new deposits were not successful because of large and erratic blanks which made the spectrometric data unreliable. In the work presented here, the chemical composition and structure of the reduction product of chromate in the presence of Mg(I1) are investigated. Preliminary results are also given for the electrochemical reduction of Cr(V1) species in the presence of Mg(I1) which contain a lower oxygen-to-chromium ratio, such as K2Cr207and Cr02C12. EXPERIMENTAL
Apparatus. A Sargent Model IV coulometric current source was used for the constant current generation of the platinum reference electrode and for the constant current electrolytic preparation of the electrode deposits. The constant current source used for the coulometric determination of the chromium content of the electrode
deposits and for the chronopoteniometric studies was constructed by use of the Heath Model EUW-19A operational amplifier system and the Heath Model EUA-19-4 operational amplifier stabilizer. The instrument had a current range of approximately 10 PA to 20 mA and was capable of reverse current chronopotentiometry. The details of its construction are given elsewhere (2). A Tektronix 532 oscilloscope served to record chronopotentiograms and to monitor the potential of the working electrode during the preparative electrolysis studies. A Beckman Model D U quartz spectrophotometer with flame attachment employing a Beckman Model 4020 oxyhydrogen atomizer burner was used for the flame photometric determination of lithium in the electrode deposit. The lithium emission was detected using a 1P28 photomultiplier tube mounted in a Heath Built Model EUA 20-20 photohead. The resulting photomultiplier current was measured by a Heath Built Model EUA-20-28X log/linear current module attached to a Heath Built Model EUA-20A servo recorder. X-ray powder patterns of the electrode deposits were made with an 11.47-cm camera using CuKa radiation. The Cu tube was mounted in a Norelco generator. Other apparatus employed in this laboratory for the manipulation of the molten salt has been previously described ( I ) . Electrodes. The Pt/Pt(II) 1M reference electrode which has been characterized previously (3) was used throughout this work. The construction of 0.5-cm2 platinum flag electrodes used in the chronopotentiometric studies was the same as that described by Laitinen and Bankert (1). A spectrometric carbon rod (l/8 inch in diameter) generally served as the counter electrode in all electrochemical investigations in the melt. The cathode used for the preparation of the electrode deposit was constructed from 52-mesh platinum gauze. Each gauze was approximately 1.5 cm on a side and could be conveniently attached and detached from a hook on the supporting electrode. This arrangement permitted the electrode to be weighed before and after electrolysis. Chemicals. All solid chemicals used in this study were reagent grade, vacuum-dried and stored over Mg(ClO& until use. Anhydrous MgClz was prepared by slowly heating the hexahydrate in vacuum to 200 "C over a period of 2 days followed by sweeping of anhydrous HC1 through the salt for 5 hours at 500 "C. Analysis of the MgCl2 prepared in this manner for chloride indicated it to be 99.5 % pure. The LiC1-KC1 eutectic was prepared and purified by Anderson Physics Laboratories, Inc., Champaign, Ill. The method of purification has been previously described ( 4 ) . Solid chemicals were added to the melt with a small glass spoon; chromyl chloride was added in an argon gas stream. Techniques Employed in Preparation and Analysis of Electrode Deposits. The gauze electrodes which served as the cathode in the electrolytic reductions were cleaned in boiling, concentrated HC104, rinsed with distilled water, dried at 130 "C for 24 hours, and stored in a desiccator until use. Because it was found that the electrochemical reduction product of chromate in the presence of Mg(I1) adhered very strongly to the gauze electrode, the electrode could be weighed before and after the electrolysis to determine the weight of the deposit. The large surface area gauze electrodes minimized solvent entrapment problems by keeping the current density low (1-5 mA/cmZ) and the thickness of the deposit relatively thin. After a sufficient amount of deposit had been electrochemically produced on the electrode surface, the gauze was removed from the molten salt, allowed to cool to room temperature, and excess eutectic dissolved with distilled water. (2) J. H. Propp, Ph.D. Thesis, University of Illinois, 1968. (3) H. A. Laitinen and W. S. Ferguson, ANAL.CHEM., 29,4 (1957). (4) H. A. Laitinen, R. S. Tischer, and D. K. Roe, J. Electrochem. SOC.,107, 546 (1960).
The electrode was dried at 110 "C and weighed to the nearest microgram. Dissolution of the electrode deposit directly from the platinum gauze was accomplished by a perchloric acid oxidation procedure described by Smith ( 5 ) . Analytical methods for the determination of Cr, Mg, and Li in the above solution were developed because these three elements appeared to be the only metallic constituents of the deposit as determined by Bankert's spectrometric work ( I ) . The method chosen for the chromium determination was that of Meier, Myers, and Swift (6). In this method Cr(V1) is determined by coulometrically generated Cu(1) using a biamperometric end point. The method was tested on known samples and found to be accurate to approximately 0.1% relative error at the concentrations involved. The magnesium content of the solution was conveniently determined by a simple EDTA titration with Eriochrome Black T as the indicator. The accuracy and precision of this determination was experimentally found to be approximately 0.2%. The lithium content was determined using flame photometry with a relative error of approximately 1.0%. The Li emission was observed at 670.8 mM. RESULTS AND DISCUSSION
K2Cr04-MgC12-LiCl-KCI System. When K2Cr04 is electrochemically reduced at constant current in the presence of MgC12 a green, adherent deposit is formed on the surface of the platinum gauze cathode. The deposit, which is remarkably resistant to acid attack, can be conveniently dissolved by boiling, concentrated HC104. Bankert had observed ( I ) that the deposit was insoluble in hot, concentrated HC104. It was found, however, that dissolution took place if the acid were allowed to boil. According to Smith (5), 70z HC104becomes an effective oxidant for Cr(II1) only after it concentrates to its azeotropic composition (72.5 %). Employing the methods described for the determination of Cr, Mg, and Li in the electrode deposit, several gauze electrodes were electrolytically coated with the reduction product and analyzed. It was of interest to determine if and how the composition of the electrode deposit changed with the conditions of the electrolysis. Three parameters could be conveniently varied-the temperature of the molten salt bath, the current density, and the molar ratio of Mg(I1) to Cr(V1) dissolved in the melt. In all of these studies the current density was kept low enough so that the potential of the working electrode never exceeded -0.4 V cs. the Pt/Pt(II) reference. Table I clearly shows how the composition of the electrode deposit changes with temperature, while holding the current density and the Mg(I1) to Cr(V1) molar ratio constant. The lithium content of the deposit is found to increase substantially when the temperature is increased from 400 to 500 "C. Notice that the Mg(I1) content decreases as the Li(1) content increases. The relationship between the Mg(I1) decrease and the Li(1) increase can be explained if one observes that the oxygen-tochromium atomic ratio in the deposit appears to remain constant at a number approaching 4.0. The electrode deposit could then be described by the empirical formula Li,Mg,Cr04 where x 2y = 5. As seen from the table, typical values of x range between 0.3 and 0.5. It can also be observed in Table I that the total weight percentages of the metallic oxides is nearly equal to 100% in all cases, indicating that the only anion present is oxide. The
+
(5) G. F. Smith, Analyst, 80, 16 (1955). (6) D. J. Meier, R. J. Myers, and E. H. Swift, J. Amer. Chem. SOC., 71,2340 (1949).
VOL. 41, NO. 4, APRIL 1969
645
~~
[K,CrO,] Temperature 400 OC 450 "C 500 OC
=
Table I. Composition of Electrode Deposit as Function of Temperature [MgClZ] = 0.176M 1, = 1 mA/cmz Wt % Li20 Wt % MgO Wt % Cr,03 Total wt Formula 2.29 54.21 43.19 99.7 Lio.z ,Mgz. ~CrO3.99 3.22 53.73 43.78 100.7 Lio. 36Mgz.zsCr03.93 4.13 51.41 43.61 99.2 Lie. 50Mg2.2ZCr03. 97
0.045M
Table 11. Composition of Electrode Deposit as Function of MgClz Concentration Temp.
=
450 OC
Io = 1 mA/cmZ
[K2Cr041
[MgC121
Wt % LizO
Wt % MgO
0.059M 0.051M 0.037M
0.099M 0.185M 0.231M
3.98 3.05 2.63
52.02 52.44 52.54
Total wt %
Wt % Crz03 44.74 44.25 43.89
100.7 99.7 99.1
Formula Li0.4SMg2. 19CrO3.91 3 SMgZ,Z,Cr03.9 0 Lio. ~iMgz.2sCrO3.91
Electricity passed to prepare electrode deposit, peq
Cr(II1) found, peq
Difference, peq
Pt lost as Pt(II), peq
70.93 70.00 70.00
74.12 71.90 71.48
3.19 1.90 1.48
1.80 2.92 1.24
Table 111. Effect of Current Density on Composition of Electrode Deposit [KzCr04]= 0.034M Approximate Io 1 mA/cm2 2 mA/cmz 4 mA/cm2 10 mA/cm2
[MgCl,] Wt % MgO
Wt % Cr203
Total wt %
2.78 2.32 2.20 2.12
52.02 53.23 52.65 52.72
43.01 42.37 42.58 42.67
97.8 97.9 97.4 97.5
Formula Li0.33MgZ.Z8Cr03.9 5 Li0.Z8Mg2.
37cr04.01
33cro3.96 2 SMgZ. 33cro3.95
Li0.26Mg2. LiO.
Electricity passed to prepare electrode deposit, peq
Cr(II1) found, peq
Difference, peq
Pt lost as Pt(II), peq
100.0 100.0 100.2 100.0
106.8 103.2 103.9 105.7
6.8 3.2 3.7 5.7
3.5 3.0 3.8 3.8
100% summation also indicates that the Cr(VI) dissolved in the melt underwent a 3-electron reduction to Cr(II1); otherwise the chromium content of the deposit could not adequately be described as Crz03. The results of changing the molar ratio of Mg(I1) to Cr(V1) dissolved in the melt while holding the current density and temperature constant are shown in Table 11. The Mg(I1) content of the deposit is found to increase as the molar ratio of Mg(I1) to Cr(V1) in the bulk melt is increased. Again it is observed that the oxygen-to-chromium atomic ratio is 4 to 1, indicating that no oxide was lost to the bulk melt during the electrolysis. It is interesting that the number of microequivalents of Cr(II1) found by analysis generally exceeds slightly
Table IV. X-Ray Powder Pattern of Li,MgyCrOc d(A) 2.4024 2.0819 1.4742 1.2585 1.2049 1.0441 0.9584 0.9339 0.8524 0.8038
646
Temperature = 450 "C
= 0.104M
Wt % LizO
I110 20 100 60 20 30 20 10 30 30 5
ANALYTICAL CHEMISTRY
hkl 111 200 220 311 222 400 331 420 422 511,333
4A) 4.166 4.168 4.172 4.178 4.178 4.182 4.182 4.182 4.181 4.182
the number of microequivalents of electricity passed in the preparation of the electrode deposit. The data in Table I1 appear to indicate that the excess Cr(II1) results from a slight chemical oxidation of the pt. Because it was observed that only a very small loss of Pt occurred during the HC104 dissolution, most of the chemical attack of the Pt occurred in the melt. This chemical attack, which was not too serious a problem in the present study, could possibly explain the large and erratic blanks observed by Bankert (1) when he attempted to analyze by spark emission spectrometry microgram quantities of the deposit plated onto platinum rods. It should be pointed out that in all cases the weight percentages of the three oxides were calculated from a corrected tare weight of the Pt gauze-Le., account was taken of the loss in weight of the gauze by the chemical reaction. Generally, this correction, while only 0.2 to 0.3 mg of platinum, was of the order of relative, as seen from Tables I1 and 111. Table I11 shows that there is only a small change in the composition of the electrode deposit upon changing the current density over a 10-fold range while holding the other two variables constant. No absolute values for the current densities employed can be given because gauzes of indeterminate surface area were used; however, a reasonable estimate of the range of current densities used would be of the order of 1 to 10 mA/cm2. The data show a slight decrease in the Li content of the deposit as the current density is increased. This result strongly suggests that Li(1) is not trapped as an impurity,
5z
for if this were the case an increase in current density would tend to increase the Li(1) content in the deposit. X-Ray Powder Diffraction Studies. A typical X-ray powder pattern for the electrode deposit is shown in Table IV. The deposit shows only a single pattern which can be clearly indexed to a face-centered cubic lattice with an “a” parameter equal to 4.182 A. This lattice is the same as that assumed by pure MgO, and the powder patterns for the deposit and pure MgO are nearly identical. The experimentally determined “a” parameter for pure MgO, however, is 4.213 A, so the unit cell for the deposit is slightly smaller. Because the deposit is indicated to be a pure compound and not a mixture, it is not unreasonable to assume that the compound is a solid solution of the three oxides LizO, Cr203,and MgO. Kordes and Petzolt (7) have in fact prepared a series of solid solutions involving LiCrOz and MgO by high temperature sintering of the two oxides. They found that in the range from 0 to 40 mole % LiCrOl, the sintered mass gave an X-ray powder pattern which was face-centered cubic with the “a” parameter decreasing from the pure MgO value of approximately 4.22 A to 4.18 A. Above 40 mole % LiCrOz, the solid solutions exhibited the hexagonal LiCrOz structure. Clearly the compounds prepared by Kordes and Petzolt (7) would always have a Li-to-Cr ratio of 1 ; therefore, the electrode deposit cannot be described exactly by any of their compositions. Still there are many similarities between their solid solutions and the electrode deposit, and it would be of interest to attempt to explain the structure of the electrode deposit in terms of the interpretation offered by Kordes and Petzolt (7). These authors state that the Li(1) and Cr(II1) ions in their compounds randomly substitute for the Mg(I1) ions in a MgO lattice. Because the Mg(I1) ions are octahedrally coordinated, this interpretation would require that the Cr(II1) and Li(1) ions also be octahedrally coordinated. In order to determine experimentally if the Cr(II1) ions in the electrode deposit occupy sites of octahedral symmetry, an ESR spectrum of the powdered deposit was run. A single, broad, symmetrical, Lorentian peak due to Cr(II1) was observed. The “g” value qalculated from this spectrum was 1.979 which is in excellent agreement with a previously reported value of 1.980 (8) for MgO crystals which had been doped with small quantities of Cr(II1). This result may be fortuitous, however, because the concentrations of Cr(II1) in the two studies differed greatly. Nevertheless, the ESR data, the X-ray data, and crystal field stabilization arguments would tend to favor an octahedral coordination about the Cr(II1) ions rather than a tetrahedral one. If the interpretation of Kordes and Petzolt (7) is correct and the Li(1) and Cr(II1) ions do randomly substitute for the Mg(I1) ions, the unstoichiometric nature of the electrode deposit demands that some of the octahedral sites be vacant. Because the total number of lattice sites available which have octahedral symmetry is the same as the number of oxide ions in the lattice, it becomes a simple matter to determine if all the octahedral sites are occupied by the cations. Table V shows that 6 to 10% of the sites are vacant. One way to verify the existence of cation vacancies experimentally would be to compare the density calculated from the X-ray data with the experimentally measured density. The density of the electrode deposit calculated from the X-ray parameters is 4.01 g/cms. By employing a displacement tech(7) V. E. Kordes and J. Petzolt, 2. Anorg. Allg. Chem., 335, 138 (1965). (8) W. Low, Phys. Rev., 105, 801 (1957).
Table V.
Determination of Cation Vacancies in Electrode Deposit
No. of oxygen atoms per molecule
cations per molecule
Difference
No. of vacancies per MgO
4.00 4.00 3.94 3.93 3.97 3.98
3.64 3.63 3.62 3.61 3.71 3.74
0.36 0.37 0.32 0.32 0.26 0.24
0.09 0.09 0.08 0.08 0.07 0.06
No. of
Table VI, Thermogravimetric Data for Li,Mg,CrOr Argon atmosphere Temp. (“C) Wt (g) 25 320 615 700 805 920 1010
0.11825
0.11804 0.11802 0.11784 0.11761 0.11769 0.11774
Air atmosphere Temp. (“C) Wt (9) 25 550 720 920 1000
0.28340 0.28340 0.28340 0.28337 0.28337
nique using water in a 5-ml pycnometer, the density of the deposit was measured to be 3.88 =t0.10 g/cms. The inherent inaccuracies present when trying to measure the density of a powder available only in small quantities makes quantitative interpretation of these data very difficult. Qualitatively, however, it would appear that cation vacancies do exist in the lattice because the measured density is less than that calculated from the X-ray data. Pressure Stability of the Lattice. The presence of cation vacancies might be expected to weaken the lattice causing it to rearrange under extreme pressures. To test this possibility the powdered deposit was subjected to pressures of the order of 80,000 atmospheres and X-ray powder patterns of the pressed deposit were compared with those of the unpressed powder. No change was observed in the lattice type or the lattice dimensions, indicating that either the lattice was not affected by the pressure, or if some transition did occur it was reversible when the pressure was released. Thermal Stability of Li,Mg,CrO 4. Platinum gauze electrodes coated with the deposit were heated in argon and air atmospheres to 1000 “C and their weights were recorded as a function of temperature. The thermogravimetric data shown in Table VI indicate that there was no significant weight loss during the heating process and that the oxygen in the air had no effect. In each case the color of the deposit changed from the original bright green to a brownish green. Chemical analysis of one of the heated samples indicated that it contained 2.34x Li,O, 52.50% MgO, and 42.87% C r z 0 8which corresponds to the original empirical formula, Lio.2sMg2. &r03. 99. Because the analysis corresponded to previous analyses on unheated samples and there was no loss of weight upon heating, it appears that no chemical change occurred. X-ray powder patterns of the electrode deposit shown in Table VII, however, do indicate a change in the structure of the deposit upon heating. At approximately 1000 “C new lines appear in the powder pattern which cannot be ascribed to the original facecentered cubic lattice. Upon further heating to 1400 “Cthese new lines are clearly defined and can be indexed to a diamond cubic lattice having an “a” parameter of 8.318 A as indicated in Table VIII. It is also shown in Table VI1 that the original VOL. 41, NO. 4, APRIL 1969
647
Table VII. X-Ray Powder Pattern of Electrode Deposit as Function of Temperature 25 "C to 800 "C IIIO
2.3981 2.0773 1.4721 1.2561 1.2041 1.0427 0.9576 0.9333 0.8523 0.8035
10 100 80 20 30 20 10 30 30 10
a
1400 "C
1050 "C
d (A)
d (A)
IIIO
4.7995 2.5060 2.4086a 2.3075 2.0892a 1.6053 1.4773a 1.2602a 1.2064a 1.1088 1.0846 1.0456a 0.9582a 0.9352a 0.8535a
40 40 20 5
100 30 100 40
40 1 1 10 3 20 20
Table VIII. Indexing of Diamond Cubic Powder Pattern d(A)
sin28
s
4.7586 2.9284 2.4973 2.0710 1.9068 1.6906 1.5963 1.4669 1.4035 1.2668 1.1984 1.1643 1.1104 1.0819 1.0398 0.9302 0.9128 0.8491
0.0262 0.0691 0.0952 0.1383 0.1632 0.2076 0.2328 0.2756 0.3012 0.3697 0.4130 0.4376 0.4811 0.5068 0.5659 0.6855 0.7121 0.8230
3 8 11 16 19 24 27 32 35 43 48 51 56 59 67 80 83 96
h2/4az x lo3 8.733 8.637 8.655 8.644 8.589 8.650 8.622 8.613 8.606 8.598 8.604 8.580 8.591 8.590 8.446 8.569 8.580 8.573
hkl
111 220 311 400 331 422 511,333 440
531 533 444 711,551 642 731,553 -
-
a
(A)
8.251 8.296 8.288 8.293 8.320 8.291 8.286 8.308 8.312 8.315 8.312 8.324 8.319 8.319 -
Table IX. X-Ray Identification of Heated Electrode Deposit MgCr,O, (ASTM) Electrode deposit
648
4.7586 2.9284 2.4973 2.0993a 2.0710 1.9068 1.6906 1.5963 1.4858a 1.4669 1.4035 1.2668 1.2f~33~ 1.214ga 1.1984
1110 50
15 100 80 80 1 1 50
60 60 15 15 15 15 15
d (A)
1.1643 1.1104 1.0819 1.0533a 1.0398 0.9604a 0.9418a 0.9302 0.9128 0.8596a 0.8491
IIIO
10 5
20 10 10 5
20 20 5
20 20
Face-centered cubic lines
face-centered cubic lines are diminished in intensity and shifted to higher d spacings during the heating process. Upon indexing the new face-centered cubic lines, an "a" parameter of 4.215 8, is calculated which is significantly larger than the
d (A) 4.813 2.945 2.512 2.406 2.083 1.912 1.701 1.603 1.4731 1.4089 1.3176 1.2711 1.2563 1.2028 1.1666 1.1136 1.0850 1.0416
d (A)
Ilia
d (A)
Ilia
65 13 100 13
4.759 2.928 2.497
50 15 100
55
5
3 40 55 13 1 13 9 9 9 3 11 5
ANALYTICAL CHEMISTRY
2.071 1.907 1.691 1.596 1.4669 1.4035
80 1 1 50 60 15
1.2668
15
1.1984 1.1643 1.1104 1.0819 1.0398
15
10
original "a" parameter of 4.182 A. This value now closely approximates the "a" parameter experimentally measured for pure MgO (4.213 A). This observation, coupled with the observation that the diamond cubic lines closely match the ASTM pattern for pure MgCrzO4 as shown in Table IX, appears to indicate that the heating to 1400 "C decomposed the solid solution to a mixture of MgO and MgCrz04. Chemical analysis of the deposit heated to 1400 "C substantiates this interpretation. The deposit was found to contain 53.56% MgO, 45.24% Crz03, and no detectable amount of LizO. Evidently the LizO was either volatilized at this temperature or it reacted with the ceramic crucible used to contain * the deposit. Work by Deren and Haber (9) supports the observation that MgCrzOc is thermally stable at 1400 "C. These workers, while studying the mechanism of formation of MgCra04when mixtures of MgO and C r z 0 3were heated in air, observed that the only solid phases present at temperatures above 600 "C were MgO and MgCrzO4. Deren and Haber also observed that when pure MgCrO4 is heated in air it tends to lose oxygen, which substantiates our observation that no air oxidation occurred during the heating of the electrode deposit. K2Crz0,-MgCIrLiC1-KCl System. It was of interest to investigate the electrochemical reduction of dichromate in the presence of Mg(I1) to see what effect the lower oxygento-chromium ratio would have on the composition of the electrode deposit. Several complications make the results of this investigation difficult to interpret. One serious problem is that in the melt dichromate is a much stronger oxidant than chromate. As a result, substantial chemical oxidation of Pt, C, and Au electrodes is observed. In the case of the t'F oxidation the reduction product was identified as Cr203 by X-ray powder diffraction. This result demands that oxide ion be lost to the bulk melt during the chemical reaction. The oxide ion would immediately be consumed to form C r O F and consequently would alter the oxygen-to-chromium ratio of the Cr(V1) species dissolved in the melt. Table X shows the effect of chemical oxidation on the composition of the electrochemical reduction product of dichromate in the presence of Mg(I1). For each of the three electrodes 100 keq of electricity were passed and the weight
5
20 10
(9) V. J. Deren and J. Haber, Z . Anorg. Allg. Chem., 342, 277, 288
(1966),
~
~~
Table X. Composition of Reduction Product of Dichromate in the Presence of MgCI2 [K2Crz07]= 0.056M Io = 2 mA/cm2
Order of deposition
Cr(II1) due to chemical oxidation, peq 41.7 4.8 2.7
1
2
3
Wt
% Li20
Wt
1.o
2.5 2.2
~ g C I , ]= 0.180M T = 450 "C
% MgO 13.4 32.2 46.3
Wt % Cr203 79.9 61.9 47.7
Total wt % 94.3 96.6 96.2
Formula
Lie.07Mgo.32CrO1.8 5 Lio.2lMg0.98CrOZ. 5 9 Lio.24Mgl.83Cr03.45
Table XI. Composition of Electrochemical Reduction Product of Chromyl Chloride in the Presence of MgClz Electricity passed in electrode preparation, peq
75.00 50.00 Wt % LizO 3.67 3.76
Wt % MgO 29.63 29.21
Cr(II1) found on the electrode, peq 86.60 58.19 Wt % Cr203 66.61 65.18
per cent of each of the three oxides was determined. Note that there was extensive chemical oxidation of the first electrode resulting in a high percentage of Crz03 in the deposit. Subsequent electrodes showed less chemical attack because a portion of the dichromate had been titrated to chromate by oxide ion released in the chemical oxidation. As a result of this chemical oxidation, it has not been possible to ascribe a specific composition to the electrochemical reduction product. Qualitatively it appears, however, that the oxygen-to-chromium ratio in the deposit is lower than that observed for the chromate reduction. CrOzCl~--MgCl~-LiCl-KClSystem. As in the case of the dichromate reduction, chromyl chloride was found to be a strong enough oxidant to oxidize Pt to Pt(I1). The chemical reduction product was again identified as C r z 0 3 by X-ray powder diffraction. Table XI shows the results of chemical analyses of two typical deposits electrochemically prepared on Pt electrodes. The empirical formulas obtained are written to indicate that the chemical reduction product is C r z 0 3and that all other chromium on the electrode surface is due to the electrochemical reduction. The formulas indicate that there
Excess peq 11.6 8.19 Total wt %
Platinum lost as Pt(II), req 19.5 18.2 Formula
is a higher oxygen-to-chromium ratio in the deposit than the expected 2-to-1 ratio. This result would be expected if the chromyl chloride chemically attacked the platinum electrode producing Crz03, Pt(II), and oxide ion. The oxide produced would consequently raise the oxygen-to-chromium ratio of the Cr(VI) species dissolved in the melt, causing the oxygento-chromium ratio in the reduced product to increase. Considerable work remains to be done in an attempt to overcome the problem of chemical oxidation and obtain a pure electrochemical reduction product. ACKNOWLEDGMENT
The authors are indebted to Brian Moores and R. L. Belford for their assistance in obtaining and interpreting the ESR data, and to H. G. Drickamer for his assistance concerning the high pressure studies. RECEIVED for review November 25, 1968. Accepted January 16, 1969. Financial support of this research was provided by the Army Research Office, Contract Number USDA-AROD-G586 and USDA-ARO-D-G968.
VOL. 41, NO. 4, APRIL 1969
649
