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ONR Contract No. Nonr 591-(10) Technical Report No. 21

R.P.I, Project No. 441.35 April, 19640

Migh Temerature Heat Content and P~sion Propekties for

Binary Carbonate Nixttzres. Li,,KCk)3 and NaKIC

by

G.J. Janz and Jeanne L. Perano

Department of ChemistryRensselaer Polytechnic Institute

Troy, New Yfrk

To be ptblished in Transactions of the Faraday Society, 1964

D D C

IINO Oi f~

Hig Temperature Heat Content and Fusion Properties f.or

• Binary Carbonate Mixtures: Li, K/CO3 and Na, K/CO3

by

Geotge J. Jana and Jeanne L. Purano

Departmnt of Chemistry, Rensselaer Polytechnic Institute, tOY, N.Y.

ABSTRACT

The enthalpy changes between temperatures ranging from 150" to

900"C have been measured by drop calorimetry for the equimolar Na, K/CO3

minimum melting mixture (a. pt. 710"C) and the equimolar Li, K/CO1 mix-

ture (m. pt. 505'C). Heats of fusion (kcal mol-A) thus determined are:

Na, 'K/C03 , 4.7; Li, K/C03, 8.7, respectively. Entropies of fusion and f;

specific heats ara likewise reported. The heat of mixing (kcal -mol-)

and the excess entropy of mixing (cal deg- mok-A) at 8980C are, respec-

tively, *1.8 and +0.2 for the NA, K/CO3 mixture, and -0.1 and -1.5 for

the Li, K/CO, mixture. the thermodynamic relations are discussed.

Introduc tion

The high temperature heat content and fusion properties for

Li2CO3 , Na2CO 3 , K2CO and the ternary eutectic mixture have been reported'

from this laboratory. Thermal data for binary mixtures of the alkali

metal carbonates appear nonexistent. This coamenication reports the

results for the temperature range from 150" to 900"C for two binary mix-

tures, the equimolar miniwum melting mixture (m. pt. 7100C) in theNa3 CO3-K2 C03 system, and the equitoolar mixture (m. pt. SO5'C), in the

Li 3(X)-K 2C0 3 system. Heats and entropies of mixing, calculated from

these data, are also reported and discussed in the light of current

viewpoints for inorganic ionic melts.

fterimen tal

T LL3CO3 , Na CO3 and K3C03 (analytical reagent grade

The i2CO1 NZC03 2C0

-, 2

chemicals) were dried to constant weight under a C03 atmosphere at 6000C

and stored over P.O., in desiccators until used.

The high-temperature phase-transition calorimeter and its op-

eration were the asm as used previously and have been described else-

where in detail; 2. 2 the calorimetric assembly is capable of measuring

entropy changes greater than 1 17/T cal degi mok-A. The carbonates were

contained in thin "ampoules" of Au-Pd alloy (80 wt. % Au) and these am-

poules were hermetically sealed in stouter Pt crucibles. Copper, her!-

mtically sealed in platinum, served for calibration of the calorimetricassembly. As noted previously, the water equivalent of the calorimeter

showed a slight linear dependence on the initial temperature of the

capsule; this source of error was minimized by constructing calibration

and sample capsules as similar in heat content as possible.

25* Correction: The enthalpy corrections, for the small toot-

perature diffekrencet between th2 final calorimeter temperature and 25-C,

were in all-cases lae than 1% of the total enthalpy change.

Results

Heat content changes are shown in Figure 1 for the tempera-

ture range investigated. The results for the coefficients of molar en-

thalpy and heat capacity functions, and the solid-state transition and

fusion parameters are in Tables I and 2, respectively,. The standard

deviation from the least squares analysis of the data for the two sys-

tems (Fig. 1) was 0.1 kcal.

Lia2 O3-KCO3: Reipman' has reported solid-state transitions

at ii22*C in KC03 and at '0*C in LiaCO3,, while Jaffrey3 has noted two

transitions in Li2C 3 at 4100C and 465*C, respectively. These tran-

sitions were not detectable in the mixture by least-squares analyses of

the present work (Fig. 1). The observed results for the fusion tran-

sition (Fig. 1, A) are in Table 2.

NaC0 3 -K2CO3: In Na2CO3 . two transitions have been reported

by Reisman4 (355* and 485*C, respectively) and by Jaffrey3 (360* and 480*C,

respectively, while four polymorphic forms were reported by Khlapora:6

a, rowm temperature to 340*-350*C; ~,3J40*-350 to i47O*-JISSC; y,

t

4700-4850 to 5650-620"C; and 8, above 5600 to 710C (melting point).

'In K2 CO3 , Reisman4 noted one transition at 422"C. A least-squares an-

alysis of the data for the mixture (Fig. 1) indicated the results couldbe expressed, within the limits of accuracy l %), by the distinctly

separate equations for the solid-state temperature ranges: 1600-3550C,

355'-422-C, 422"-485-C, and 485-710"C (Table 1). The bracketed values

for the solid-state transitions at 355" and 422'C (Table 2 and Fig. 1,

E, B) indicate that these are indirect values, calculated from the en-

thalpy equations. The 485*C transition (Table 2 and Fig. 1, C) was

directly observed. The paramters for the fusion transition (Fig. 1, B)

are in Table 2.

Discussion

The temperature dependence of the heats of fusion and entropy r i;of fusion we given by:

I AT T ACp dT (1)

TM

and AST = ASTM . " dT (2)

At constant temperature, the expressions for a mixture ae:1% = XA AHA +XB 6HB + aH (3)

B I mix

and aSM = XA ASA + xB AHB + ASmix + asmix (4)

where A and B are the pure components of the binary mixture, the sub-

-acript H refers to the mixture, the other symbols have the conventional

significance, and AS" is simply 1.363 e.u. for a binary mixture.

The difference in heat capacity [Cp(solid) - C (malt)] in

* equation (1) and equation (2) is given by:

AC =-a' * b'T (5)P

The coefficients a' and b' are given in Table 3. The C equations ofPthe solid mixtures were calculated from the data of the pure components

and the appropriate summatior, i CP; for the liquid mixtures, the

equations were those experimentally observed. From equations (1), (2),,

(3), and (4), the heats and excess entropies of mixing for the two binary

mixtures were calculated at the melting points of the highest mlting

cosponent. K2 CO3 (898"C). The results are in Table 4, together with

the values for the ternary eutectic for comparison.

Examination of the values for the heats and excess entropies

of mixing relative to the known liquidus-solidus phase equilibria4 ' 7 is

of interest. Where a solid solution from two ionic salts is formed

with cations of the same charge and a common anion, the value of AH mix

is always positive* ' when a chemical canpound is formed, the value of

AHmi x is large and always negative;' for similar binary systems where

neither solid solutions nor commounds are formed, the heats of mixing

are always negative and not s8 -large. s It is apparent (Table 4) that

for the system of known solid solutions," Na, K/C03, ARmix is positive

as would be predicted from the preceding generalizations; for the other

binary systems, AHmi x is negative and small. The phase diagram shows7

that compound, (LiK)CO, melts congruently at a rather flat maxim

(5O0C) and is closely flanked by two eutectic mixtures of 42.7 and

62.0 o), % Li3CO3 , both melting in this temperature tange (i.e., 498

and 488"C, respectively). The very small heat of mixing observed for

the equimolar Li, K/CO3 mixture undoubtedly reflects these features of

the phase equilibrium diagram. Relative to the values foz the excess

entropies of mixing (Table 4), randomization in melts from the binary

solid-solution system (Na, K/CO 3) appears practically ideal. For the

Li, K/CO3 binary and the Li, Na, K/CO3 ternary mixtures the randomize-

tion appears markedly less than would be predicted for ideal mixing in

the liquid state.

Acknowledgments: This work was made possible, in large part,

by support received from the U.S. Office of Naval Research, Division of

Chemistry, Washington, D.C. Active participation of F. J. Kelly in the

early phases of this study is acknowledged with pleasure.

5

References

1. G. J. Jans, E. Neuenschwander, and F. J. Kelly; Trans. Faraday

Soc., 59, 841 (1963). . r

2. G. J. Jan:, F. J. Kelly, and J. L. Porano; J. Chem. Erg. Data, 9,

eeee (1964).

3. J. Goodkin, C. Solomons, and G. J. Jans; Rev. Sci. Instr., 29, 105

(1958).

4. A. Reisman; J. Am. Chem. Soc., 80, 3558 (1958).5. J. Jaffrey and P. Martin; J. Phys. Radium, 14, 553 (1953).

6. A. N. Khlapora; Dokl. Akad. Nauk SSSR, 116, 979 (1957).

7. G. J. Jans and H. R. Lorenz; J. Chem. Eng. Data, 6, 321 (1961).

8. 0. J. Kleppa and L. S. Hersh; J. Chem. Phys., 34, 351 (1961).

9. I. G. Murguloscu and S. S. Sternberg; Acad. Rep. Populae Rasine,

Studii Cercetari Chim., 9, No. 1, 39-54(1961).

I)

Table 1.

Coefficients of Molar Enthalpy and Heat Capacity Function

Composition Phase (*K) (cal o t-A)

Li2CO3-K2CO3 S 457-778 33.82 - -11.24

(50-50 ool %) L 778-1200 4.65 - -10.96

(m. 505'C)

Na2 C03 -K2 CO3 S 440-628 34.24 - -10.98

(50-50 ool %) S 628-695 43.50 - -16.71

(W. 7100C) S 695-758 65.05 - -31.64

S 758-983 47.63 - -17.82

L 983-1197 44.26 - -9.84

Table 2.

heats of Transition and Heats and Entropies of Fusion

Car bonate Mixture AHtr Alf ASf

Composition Tamp. (OC) (cal wol) (kcal mot-&) (e.u.)

Li2 QO3 -K2 0 3 505 - 8.17 . 0.1 11.2 -. 0.1

(50-50 aMol %)

Naa 3 -KaCO3 355 *(88) -

(50-50 mci %) 422 &(55) -

4 e5 606 -

710 - 4.7 0.1 4.7 . 0.1

*The brackets indicate that these values were too small to

gain directly from the graphs and were accordingly calculated

from the enthalpy data.

Table 3

Coef ficients of~ the Heat Capacity Difference Fujnction

Cp (solid) -Cp (liquid) a ' + b'T]

system -&I b' X 103

Li223 1)27.03 29.20

Na2 C03 ~ 25.85 24.6oK2C0 3 C 17.76 15.42

L1'20 34C2C0, 33.1 34.L4

N&2C03-K3CO 3 31.3 31.2

Table 4.

Heats and Entropies of Mixing

Tap. A 6 8'Ae

(c) (kcal mol-&) (e.u.)

Li3M3 -K 2 OXJ3 (50-50 mol S; m. pt. SOS'C)

898 -0.1 .t 0.1 -1.5 . 0.1

Na2 CO3-K 2 CO3 (50-50 tol ,; . pt. 701"C)

898 +1.8 .t 0.1 +0.2 .t 0. 1

Li2CO3 (43.5), NaaCO3 (31.5), K2C03 (25.0); (m. pt. 397C)

898 -0.8 .t 0.2 A-2.8 -t 0.2

*The value of ASe for the ternary is given in reference I as +1.3

and appears in error due to the addition rather than subtraction of

the ideal entropy of mixing term.

J0

Fig. 1. Heat content changes as a function of

tempErature for binary carbonate mixtures.

Legend:. C' Li 2IC03 (50%)' K2C0 3 (50%);

N4aC0 3 (56%), K2 C03 (44d%);IA: 505*C; OHft 8700 cal; B: 710*C;allf, 4673 cal; C: 1&85*C, AliH 606 cal;

trD: 422*C, 6Wj 55 cal; 9: 355*C,

AHtre 88 cal.

Is

42

5.

54 0/

to 00o4M 00 w m

T (*C

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