Hot-zone reaction calorimetry the enthalpies of formation of copper oxides

J. Chem. Thermodynamics 1!%9,1,31-43

Hot-zone

reaction

The enthalpies LISARDO NU-Z, Chemistry Department, Manchester 13, U.K.

calorimetry

of formation

of copper

oxides

G. PILCHER, and H. A. SKINNER University of Manchester

(Received 12 August 1968) A reaction calorimeter, in which a hydrogen + oxygen flame is used to heat the reaction zone, is described. Measurements of the enthalpies of reduction by Ha of CuO(granular, and fine powder), and of CuaO(powder) are reported, from which the enthalpies of formation at 25 “C of the copper oxides are derived. Measurements of the enthalpies of oxidation (by reaction with 0,) of CuzO and of granules of metallic Cu are also given. The enthalpy of formation of cupric oxide is found to depend on its mode of preparation, state of subdivision, and previous heat treatment: annealed, granular CuO is more stable by about 1 kcal/mol than the finely powdered form.

1. rntroduction Values in the literature for the standard enthalpies of formation of cupric and cuprous oxides vary widely, ranging from - 33 .0 (Wbhler and Jochum( to - 38.5 kcal/mol (von Wartenburg and Werth@‘) for AH,” (CuO, c), and from -35.9 (Chiche and Dodt’3’) to -43.0 kcal/mol (Wbhler and Jochum”‘) for AH; (Cu,O, c). To some extent the spread of the reported AH: values may reflect genuine differences in enthalpy of different samples of the same oxide: Fricke, Gwinner, and Feichtnert4’ have shown that cupric oxide samples prepared from a solution of a cupric salt become coarser grained on heating, and that heat treatment at 600 “C can result in energy changes of as much as 1.45 kcal/mol in a given CuO sample. The range in the reported AH,” values for the copper oxides appears too large, however, to be accounted for solely on these terms. Mah and co-workers”’ have recently reported measurements of the energy of combustion of fine copper turnings in oxygen (15 to 40 atm), using a bombcalorimeter. The copper sample burned completely in most experiments, producing a dark brown fused material. The amount of oxygen combining with the copper, as measured by the mass increase on combustion, indicated an oxide composition about half way between that for cuprous and cupric oxides (from 38 to 47 per cent of CuO). This small range of composition prevented direct measurement of the values for pure CuO and Cu,O and, instead, the literature value for the dissociation reaction: 2CuO(c) = C&O(C) +$0,(g); AH”(298 K) = (33.63kO.05) kcal mol-’

(1)

32

L. NUi;lEZ,

G. PILCHER,

AND

H. A. SKINNER

was used to correct the measured energies of combustion of copper to those for idealized oxidations giving entirely CuO or Cu,O as product. The results, AH;(Cu,O,

c) = -(40.83 +0.3) kcal/mol,

and AH;(CuO,

c) = -(37.23&O-15)

kcal/mol,

lie well within the range of the literature values, and are consistent with Gibbs energies of formation of both oxides recently reported by Bidwell.@’ There remain, however, questions of definition of the states of the copper oxides to which these AHf” values refer. This paper describes measurements of the enthalpies of reduction by gaseous hydrogen of samples of copper oxides, prepared in ways that are well defined. Reduction of the oxides by hydrogen takes place readily at elevated temperatures, and the present studies were made using a “hot-zone” reaction calorimeter of novel design.

2. Experimental THE “HOT-ZONE” CALORIMETER The calorimeter, of the isothermal-jacket type, is adapted from the conventional “flame calorimeter”, described in detail elsewhere. (7*8) Built into the combustion chamber, and positioned above the tip of the silica jet supporting the H, + O2 flame, was a silica reaction vessel (encased in a platinum shield) containing the reactants (figure 1). The Hz + 0, flame, playing on the reaction vessel, heated the reaction zone within the combustion chamber, thus enabling the reaction under investigation to take place at a satisfactory rate. In reduction experiments, the reaction vessel was charged with copper oxide, weighed, and sealed (piceined cone-joint) inside the combustion chamber. The combustion unit was placed in position in the calorimeter can inside the thermostat jacket, and the connexions made (ball and socket cone-joints) to the gas inlet lines and to the exit absorption tubes. The H,+Oz flame was spark-ignited (calorimeter temperature 23.3 “C), and the flame maintained for about 12 min, sufficiently to raise the calorimeter temperature by about 3 “C!, and bring it close to the temperature of the thermostat jacket (26.30 “C). Almost immediately after ignition a controlled flow of pure dry Hz gas was directed over the oxide sample in the flame-heated reaction vessel, and continued throughout the reaction period. Time-temperature measurements (platinum resistance thermometer, Mueller bridge) were made during the fore-period (20 min), reactionperiod, and after-period (30 min). Water vapour carried from the burner and heat exchange spiral by the excess gas stream during the reaction period was collected in absorption tubes (Mg(CIO,), + P205) and weighed. Liquid water remaining in the burner and exchange spiral was subsequently removed by passing dry air through the system for about 24 h, and collecting the vapour in absorption tubes. Similar measurements were made

HOT-ZONE REACTION CALORIMETRY

33

FIGURE 1. The calorimeter. A, silica jet; B, reaction vessel (RV); C, high voltage spark lead; D, heat exchange spiral, combustion chamber; E, heat exchangespiral (RV); a, inlet Oa; b, inlet Hz; c, inlet to reaction vessel; d, exit from reaction vessel; e, exit from combustion chamber.

34

L. NUREZ,

G. PILCHER,

AND

H. A. SKINNER

to determine the total mass of water formed in the reaction vessel: the liquid water remained mainly in the heat exchange spiral attached to the exit from the reaction vessel, and was removed by passing dry argon as carrier gas. CALIBRATION

OF THE CALORIMETER

The calorimeter was calibrated in the standard way by burning hydrogen in oxygen, and collecting the total water formed. For this reaction, i.e.

H,(g) + f o,(s) = fW(l)s

(2)

the recommended value(‘) -AH(Q/cal

g-’ = 3791.88-(f?/“C-25)0.424,

where AH(d) is the enthalpy of formation of unit mass of water at Celsius temperature 8, was accepted. The energy equivalent of the calorimeter was calculated from

where Es = energy equivalent of the standard calorimeter system; E, = heat capacity equivalent of one half of the water remaining in the burner after the combustion. The factor of one half allows for the formation of the products of combustion during the main reaction. This is considered equivalent to introducing the total products formed and remaining in the calorimeter at the mid-point temperature, and then heating them to the final temperature; qHzO = heat produced by the calibration reaction, referred to the mid-temperature 8” of the experiment. is the total mass of water formed); where mHzO h-I20 = -AH@ “) x mHzO, qg = heat introduced into the calorimeter by the gases entering at room temperature, calculated from the amount of gas added, the temperature difference (P-0,,,), and the molar heat capacities of the gases involved;(*) qv = heat introduced by the evaporation of water from the combustion vessel. This was obtained from the mass of vapour escaping from the vessel during the experiment, plus the mass remaining in the vapour state in the combustion chamber and heat exchange spiral. The internal volume of the burner and exchange spiral was 270 cm3; qi = ignition energy (spark energy); AR, = corrected temperature rise expressed as the resistance change of the thermometer. The calibration results on the original calorimeter are summarized in table 1. Following repairs and alterations to the burner, the E, value changed and was measured at (35 220.18 + 4.16) cal Q- 1: this value applies to experiments presented in tables 3, 5, and 6. Throughout this paper cal = 4.184 J.

mmolg --__.-3.181 37 3.202 68 3.225 99 3.202 57 3.187 47 3.181 87

HOT-ZONE

REACTION

TABLE

1. Calibration

qH,O/=l

12063.76 12 144.56 12 232.99 12 144.15 12 086.89 12 065.65

of calorimeter? E&al C!- 1

-9,/d

--%lcal

-- 9.14 8.04 12.99 9.69 8.20 11.84

35

CALORIMETRY

A&P

31.91 15.63 0.340 26.65 15.79 0.343 24.51 16.13 0.345 28.92 15.77 0.343 23.51 15.93 0.341 29.76 15.65 0.340 mean value standard deviation of mean

922 926 737 37 I 818 890

E./Cal R- 1 __ 35 258.48 35 268.28 35 268.50 35 247.99 35 260.71 35 265.71 35 261.61 *3.19

i q, = 6.29 cal; 0” = 24.7O”C. REDUCTION

OF CUPRIC

OXIDE

Cupric oxide was B.D.H. Ltd. standard microanalytical reagent, carbon free, coarse grained, and only slightly soluble in dilute sulphuric acid. For reduction experiments, the reaction vessel was charged with 10 g of oxide, and heated at dull red heat to constant mass in a stream of dry oxygen. The vessel was then swept out with dry argon, reweighed, and fitted in position in the calorimeter. At the end of a reduction experiment, the vessel was again swept out with argon, removed from the calorimeter, and reweighed to determine the mass loss from the sample. Reduction of the sample was not total, and the product contained unchanged black grains of CuO admixed with red specks of reduced copper. The experimental results are summarized in table 2. The symbols Es, 8, AR,, qg, q,, and qi have the same meanings as given earlier. Additional symbols used have the following meanings. m,,,(l)

= mass of liquid water formed and remaining in the burner (B), and in the reaction vessel (RV); mHZo(v) = mass of water escaping as vapour from the burner (B), and from the reaction vessel (RV); E; = heat capacity equivalent of one half of the water remaining in the burner and in the reaction vessel after the combustion, plus the heat capacity equivalent of the unchanged CuO plus one half the heat capacity equivalent of the Cu formed and CuO reacted ; qtotal = (4 +JZW, qHzO hune

= =

-

AJW”>

qHl0

+Yi

; X @‘h20(1,

+qvP+

B) RV)

+ %zob’,

+4&B+

B))

;

RV)*

(Note that qy(RV) includes the contribution from water vapour remaining in the reaction vessel and its heat exchange spiral, volume = 140 cm3); 4 re d UC 1’lo”

-An,,,

=

qtota

I -

4flam

;

= amount of CuO reduced; AHreduction = AH(F) of the reaction: CuO(c) + H,(g) = Cu(c)+H,O(I).

lB * RV

gB . RV

gB . RV

2B * RV B 3* RV B 4* RV

B I’ RV

Experiment

2.643 0.359 2.892 0.584 2.898 0.438 3.000 0.458 2.840 0.420 3.092 0.426 3.044 0.393

mH20(l)/g

45 83 81 31 08 44 01 10 11 66 14 04 19 28

0.047 0.136 0.024 0.102 0.028 0.112 0.027 0.111 0.030 0.111 0.025 0.111 0.031 0.117

ma20Wlg

31.44 81.48 18.19 61.99 20.32 67.76 19.72 67.02 21.70 67.14 18.95 66.71 22.10 70.65

Experiment

33 37 62 98 28 87 24 60 64 80 92 06 32 82

--q&l

0.357

11 385.45 10 790.17 II 732.34 11 561.26

11 478.46 10 885.88 11 825.96 11 662.22

d/i-l;

0,027 52 0.027 0.038 23 0.038 0.030 65 0.030 0.031 56 0.031 0.029 53 0.029 0.029 74 0.029 0.028 26 0.028 mean values standard deviations of mean

54 29 65 62 59 81 37

- Anc,O/mol from from mass loss Hz0 formed

0.330

11 001.62

11 096.76

656

858

24.7

24.8

24.6

24.7

24.7

24.7

24.5

e”I”C

$zO.ll

10.10 q, = 6.29 cal

-29.77 -29.42 -29.86 -29.43 -29.96 -29.35 -29.22 -29.57

-29.79 -29.47 -29.86 -29.48 - 30.02 -29.42 -29.33 -29.62

29.25

30.80

28.70

30.67

31.03

31.41

28.51

E,/calfi-l

AH,,auet~onlkcal mol - 1 from from mass loss Hz0 formed

0.351 089

222

872

0.348 971

0.337

0.342

10 974.01

11 062.89

0.308 995

A&D

of CuOt

10 085.00

qrlsme/cal

of reduction

10 203.44

mzO/cal

2. Enthalpy

t Es = (35 261.62f3.19)

819.88 1 126.49 915.16 930.54 886.41 874.89 828.98

6.27 6.08 8.18 7.35 7.32 6.57 6.95 6.15 7.17 6.53 7.91 6.88 7.91 7.13

-q,l=l

TABLE

12 390.24

12 607.23

11 676.58

12 315.99

11 916.78

12 100.50

10 904.88

qtotsl/cal

828.98

874.89

886.41

930.54

915.16

1 126.49

819.88

qreduction/cal

,N

5 2’

r

3B ’ RV

ZB * RV

B I* RV

Experiment

0.040

0.049

0.053

3.067 36

3.128 16

mmowg

3.186 24

mH20(l)/g

1. 2. 3.

Experiment

26

86

44

34.36

32.38

26.88

---%/Cal

AdRV)Ig

cal/R;

476.80 0.195 430.74 0.179 432.45 0.180 mean value standard deviation

%x&al

14.65

37.74

6.29

q&al

3. Oxidation

t E. = (35 220.18+4.18)

12 682.74 12 247.85 12 467.18

9,,dcal

8.40 0.64 8.08 0.67 8.40 1.10

-9&l

TABLE

0.012 0.011 0.011

8” = 24.73

of mean

84 03 42

“C.

24 19 28

38.95 38.49 38.34 38.59 50.18

-AH/kcal

12 034.73

11 817.11

12 205.94

9fmle/Cal

Ann,,dmol

12 063.94

11 820.50

12 235.57

qHzO/cal

of Cu metalt

549

882

mol-1

0.353 765

0.347

0.359

AR&

21.24

20.47

21.20

E&d

R -1

12467.18

12 247.85

12 682.74

4todcal

X

5 2 E

F s

8

2

E

s 8 3

38

L. NUfiEZ,

G. PILCHER,

AND

H. A. SKINNER

Accepting the mean of the values determined by mass loss and mass of water formed, AHreduction = - (29.60 kO.15) kcal/mol at 24.7 “C: the correction to 25 “C is virtually negligible. Using AH: (H,O, 1) = - (68.3250.01) kcal/mol, the measured enthalpy of reduction requires that AH: (CuO, c) = - (38.72 f 0.30) kcal/mol, assuming that the copper grains formed on reduction approximate as far as the enthalpy is concerned to the standard state of copper metal. The uncertainty quoted is twice the standard deviation of the mean. OXIDATION OF COPPER METAL Measurements of the enthalpy of formation of cupric oxide by the direct method, i.e. of the reaction WC> +-to,(g) = ca4, (4) were made on samples of Cu+CuO mixture formed in the reaction vessel by reducing CuO with hydrogen. The extent of oxidation was determined by the mass increase of the charged reaction vessel. Results are summarized in table 3, and are based on the assumption that the oxidation led exclusively to black CuO, and that CuZO was not formed as a by-product. The reaction heat in these experiments amounted to only 4 per cent of the total heat measured (burner plus reaction heat), so that this approach to the enthalpy of formation of CuO is less satisfactory calorimetrically than that via the enthalpy of reduction by H, gas. The measured enthalpy of reaction (4), AH” = -(38.59 kO.18) kcal/mol at 24.7 “C is unchanged on correction to 25 “C, and gives a direct measure of the enthalpy of formation, AH,” (CuO, c) = -(38.59+0.35) kcal/mol, in good agreement with the value obtained from the enthalpy of reduction. In table 3 E’i is the heat capacity equivalent of the water remaining in the burner, plus the heat capacity equivalent of unchanged CuO in the reaction vessel, plus one half the heat capacity equivalent of CuO formed and Cu reacted. REDUCTION OF CUPROUS OXIDE Cuprous oxide was B.D.H. Ltd. standard sample, red coloured, and finely powdered. Samples used in calorimetric experiments were dried beforehand by strong heating in uucuo to constant mass. The reaction vessel was charged with about 5 g Cu,O, and the reduction carried out in the same way as described for the similar experiments on cupric oxide. The mass loss method of estimation of the extent of reaction was, however, less reliable in the case of cuprous oxide, due to the fineness of the powder. Particles of the powder carried away by the excess gases flowing through the reaction vessel made the measured mass loss of the reaction vessel too high. The “spurious” mass loss could be made small by reducing the flow-rate of Hz gas through the reaction vessel, but this in turn reduced the amount of reduction taking place. The earlier studies did not pay sufficient attention to the preparation of thoroughly dry samples of red cuprous oxide, resulting in artificially low values for the heat of reduction. Table 4 summarizes results using carefully dried oxide, and based on the mass of water formed.

gB * RV

gB . RV

qB * RV

gB * RV

2B * RV

B I. RV

Experiment

3.055 0.496 3.054 0.498 3.146 0.218 3.007 0.454 3.041 0.423 3.041 0.410

mH&)/g

07 94 15 76 56 10 98 51 20 74 99 55

0.019 0.048 0.022 0.049 0.027 0.028 0.065 0.055 0.063 0.050 0.083 0.065

m20Wlg

7

11 631.44 11 995.76 11 573.17 11 694.91 11 752.23

- An(CuaO)/mol

11 683.72 12 036.53 11 653.84 11 771.20 11 851.52

Amaolg

cal/Q; Cal/R;

q, = q, =

6.29 cal (experiments 10.49 cal (experiments

0.353

0.352

0.350

0.350

0.353

0.352

ARclfi

26.98 27.09 26.44 27.20 27.38 26.49 26.93 ztO.16 ho.32

-AH’/kcal

343

291

253

244

005

169

Q - 1

“C.

mole1

21.54

21.65

22.02

19.17

24.15

23.49

Elkal

1 to 3); 4 to 6); 0” = 24.75

0.030 26 0.030 44 0.013 66 0.028 33 0.026 3 1 0.026 43

11 610.01

9fhdcal

oxide?

11 660.06

9HzO/Cd

of cuprous

816.31 0.545 16 824.61 0.548 32 361.12 0.246 18 770.52 0.510 39 720.47 0.474 00 700.19 0.476 16 mean value standard deviation of mean uncertainty interval

qlBdUCtlO&al

Experiment 1. 2. 3 4 5. 6.

6.96 3.93 6.98 3.77 7.34 1.49 10.33 4.41 10.58 4.38 11.83 5.26

-d-l

4. Reduction

15.40 30.05 16.99 30.83 19.95 18.25 40.92 34.48 40.60 31.22 52.51 40.18

deal

E, = 35 261.62f3.19 Es = 35 220.1814.18

84 22 57 56 63 08 29 88 02 26 41 61

-

TABLE

120.47 700.19

12 452.42

770.52

361.12

824.61

816.31

9reduotionlcal

12 415.38

12 343.69

12 356.88

12 456.05

12 426.32

4toral!cal

‘* B RV 2B’ RV 3* B RV

:: 3.

Experiment

o.oG -

3.103 75 -

51

0.053 56 0.044 75

3.078 19 3.122 20

WRWg

oxidet

ml/Cl; qi =

11 927.51

11 979.83

11 843.12

10.49 cal;

8” = 24.77

20 00 32

-An(C&O)/mol

11 960.95

12 009.07

11 875.60

of cuprous

677.35 638.15 0.303 93 0.323 19 0.019 0.020 791.53 0.358 86 0.023 mean value standard deviation of mean

7.63 0.80 9.29 1.04 9.96 1.21

5. Oxidation

qo.i&al

t Es = (35 220.18f4.18)

12 617.98 520.47 12 719.04

qtotpl/cal

3G6 -

2tiO

34.54

TABLE

“C.

33.53 33.59 33.94 33.68 zto.13

-AH/kcal

0.360

0.358

mold1

922

076

0.355 293

20.22

18.08

19.67

12 719.04

12 617.98

12 520.47

-

2.828 30 0.737 45

3B’ RV

0.097 41 0.047 52

0.054 0.037 03 09 0.086 15 0.095 23

mazOW/g

-- -... .-....------.---.-.----.-- -----

2.857 13 0.622 74 2.834 17 0.530 35

mazO(l)lg

1’ B RV 2B* RV

Experiment

.- .._-

. .._.. .-

Es = (35 220.1814.18)

1 393.85value mean standard deviation

3.

j-

1 166.52 932.22

(fine

11 093.36

11 039.12 11 073.04

qH20/cal

of CuO

kcal/mol;

..---.----

qi = 10.49 cal.

31.97 31.82 Ito.

31.87 31.66

- AH/kcal

10 997.01

10 982.31 10 961.72

qrlama/cal

powder)t

0.043 57 of the mean

0.036 60 0.029 44

- An(CuO)/mol

-

I. 2.

25.47

24.10 25.38

tPJT

4reducrion/cal

11.32 5.23

7.73 3.11 8.90 4.07

-q&al

~~_______

6 Reduction

Experiment ---...~.

60.67 29.62

34.82 21.64 53.56 55.28

-a&l

TABLE

mol - l

0.351

0.344 0.337

AJW

581

761 512

rR - 1

23.08

18.34 17.96

E&d

12 390.86

12 148.83 11 893.30

4tataW

1 393.85

1 166.52 932.22

qreduotianlcal

P

3 *

E B

9

5!

FE 2

k 2

42 OXIDATION

L. NUfiEZ,

OF CUPROUS

G. PILCHER,

AND

H. A. SKINNER

OXIDE

A few direct measurements of the enthalpy of oxidation of cuprous oxide were made, and are summarized in table 5. In this reaction, Cu,O(c) +&O,(g) = 2CuO(c), (5) the oxide formed wasafinely divided powder, different fromthe hard granular oxide used inthe reduction experiments summarized in table2.The measured enthalpyof reaction, AH”(298 K) = - 33.68 kcal/mol, is in close agreement with the value (- 33.63 + 0.05) kcal/mol given by Mah et al., (V which was based on a re-evaluation of literature data on the dissociation pressure of cupric oxide. In conjunction with the value found for AH,” (Cu20, c), the measured enthalpy of reaction (5) leads to AH: (CuO, fine powder) = - (37.54kO.2) kcal/mol. REDUCTION OF CUPRIC OXIDE (FINE POWDER)

Measurements of the enthalpy of reduction by hydrogen of finely powdered black cupric oxide are summarized in table 6. The powdered oxide was prepared from Cu,O powder by heating to constant mass in a stream of oxygen. Reduction of the oxide in the calorimeter was not total, and the final product contained copper powder admixed with unchanged black CuO. The measured enthalpy per unit amount of water formed (AH” = 31.8 kcal/mol) is decidedly higher than found for reduction of granular cupric oxide (AH” = -29.6 kcal/mol), and corresponds to AH,“(CuO, fine powder) = -(36.50+0.2) kcal/mol. We cannot exclude the possibility that the reduction product also contained cuprous oxide; in this event, the derived value for AH; (CuO, fine powder) would be too small in magnitude. 3. Discussion The measurements of the enthalpy of reduction of granular CuO, and of the enthalpy of oxidation of granules of copper to reform the oxide are in accord, and lead to a mean value, AH,“(CuO, granular) = - (38.65 + 0.3) kcal/mol. Earlier calorimetric measurements by von Wartenburg and Werth (‘) of the enthalpy of reduction of CuO gave AH” = -(29.9+0.5) kcal/mol (AH,0 = -38.4 kcal/mol), in close agreement with the present result. On the other hand, our measurements of the enthalpy of reduction of finely powdered CuO gave AH” = - (3 1.8 k 0.2) kcal/mol, corresponding to AH,“(CuO, fine powder) = -(36.50&0.2) kcal/mol, but this value, as already stated, may be too small in magnitude. Preliminary measurements of the enthalpy of reduction of red cuprous oxide were in fair agreement with the calorimetric measurements (AH” = -25.3 kcal/mol) of Wiihler and Jochum, but later measurements, using very thoroughly dried oxide, indicated a higher value, AH’ = -(26.93 +0.32) kcal/mol, corresponding to AH,“(Cu,O, powder) = - (41.39 t0.32) kcal/mol. Combination of this value with the measured enthalpy of oxidation of cuprous to cupric oxide (AH” = - (33.68 kO.26) kcal/mol) yields AH,“(CuO, fine powder) = -(37.54+0.2) kcal/mol.

HOT-ZONE REACTION CALORIMETRY

43

The differences of from 1 to 2 kcal/mol in the measured AHf” of granular and finely divided forms of CuO are consistent with earlier findingqc4’ and such fluctuations may be characteristic of non-stoichiometric metallic oxides of this type. Evidently the enthalpies of these solids are influenced by their previous history, heat treatment, state of subdivision, and imprecise stoichiometry.

This research has been sponsored in part by the Air Force Office of Scientific Research through the European Office of Aerospace Research (OAR), United States Air Force, under contract AF 61(052)-928. REFERENCES I. 2. 3. 4. 5.

WGhler, L.; Jochum, N. 2. phys. Chem. 1933, 167A, 169. Wartenburg, H.; Werth, H. 2. Elektrochem. 1932, 38, 401. Chiche, P.; Dodd, M. Compt. rend. 1951,232, 618. Fricke, R.; Gwinner, E.; Feichtner, C. Ber. dt. Chem. Ges. 1938, 71, 1744. Mah, A. D.; Pankratz, L. B.; Weller, W. W.; King, E. G. Bureau of Mines Report No. 7026,1967, September. 6. Bidwell, L. R. J. Electrochem. Sot. 1967, 114, 30. 7. Rossini, F. D. In Experimental Thermochemistry, Vol. 1. F. D. Rossini; editor. Interscience: New York. 1956. Chap. 4. 8. Pilcher, G. ; Skinner, H. A.; Pell, A. S.; Pope, A. E. Trans. Faraday Sot. 1963, 59, 316.