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High-temperature Heat Contents of Sodium Metasilicate and Sodium Disilicate' RS B. F. NAYLOR' The therrnodynainic properties of silicates have, in recent years, received only nominal attention in spite of the importance of these substances as constituents of glasses and inetallurgical slags. Earlier publications of the Pacific Experiment Station have dealt with low-temperature specific heats and entropies of several silicates.3 The present paper reports high-temperature heat conleiit nieasurernents of sodium inetasilicate and sodiuin disilicate. Previous siniilar data arc lacking for both these substances. Method and Materials The heat conteilt measurements were made by the "drop" method in ;in apparatus previously tIescrlbed.* The calorimeter was calibrated with clectrical energy, measured in international joules, sild the results were converted to the conventional thermochemical calorie by the relation,5 1 cal. = 4.1833 int. joules (NBS). The samples were enclosed in platinumrhodium alloy capsules, the heat contents of which were measured separately. Each capsule was se:ilcd with platinum after filling; however, on reaching the liquid range of either substance, small pili holes developed in the capsule walls. A close check was maintained and weigllt corrections were made for any loss of sample, which for any single measurenient did not exceed 0. The sodium metasilicate and disilicate were furnished by the Philadelphia Quartz Co. The mctasi1ic:tte was part of the material used previously by Kelley3 for low-temperature specific heat measurements. This saniple was reheated to 1100" and then slowly cooled to assure ccmllization. A4nalysisshowed it to be :it
liquid state, and the results for crystals, glass, and liquid have been listed separately. The sample weights were corrected t o vacuum, usiiig the densities sodium metasilicate, 2.61 ; and sotiiuin disilicxte 2.50, g./cc. The molecular weights accord with the 1941 International Atomic Weights. Xo attempt was made to correct for the impurities present, since they were sniall and not definitely known. Sotliuiri inetasilicate is represented by curve A i i i Fig. 1. The melting point, 13G1°K., is indicated by ;1. dotted line and the heat of fusion was cLilculatcd to be 12,4i0 calories per mole. Slight premelting was indicated about 100' below the melting point. 'I'he nie:tsureinents of crystalline sodium disilie made before the liquid and glass were studied, care being taken not to nielt the sample; the results are plotted as curve H in Fig. 1. Some premelting occurred on approaching the melting point, 1147°K. No heat of fusion is obtainable because of glass formation when the liquid cooled in the calorirneter. The upper portion of curve C represents the 1ie:tt coiitciits obtained for liquid disilicate, while the lower portion shows the results of nieasurements made on the glass. The values for the glass from 877'K. to the melting point :ire not considered reliable, as several erratic results in this temperature range, listed in Table I but not plotted, showed that while heating in the furnace, the glass was tr:msforming spontaneously to crystals. I t could, hmvever, be re-formed by heating the sample above 1147'K. and dropping into the calorimeter. This was done before and tiurin;: the nieasurenietits of the glass. TAHLEI
The sodium disilicate was a specially decolorized sample which was heated at 700' before using; its purity w;is 995.0.
Results 'I'he results of the experimental measurements are presented in Table I and Fig. 1. The column lnbcled I', OK., lists the temperature of the saniple before dropping into the calorimeter and IIT TI2$+ 16, the heat liberated per gram molecular weight in cooling to 29S.lfi"K. The heat coiltent values that appear to involve premelting have been designated " t'p) ." Sodiuin disilicate fori1i.s :i glass upon cooling quickly froiu the \ I ' I'ublished b y perm%ssionof tlir Director. U t i r e , ~ n of h l r n c - , S o t copyriyhtcd I>,epartmerit of t h e I i i t e r i o r i . l i e m i , t , I'icifie E x p e r i m e n t Station, Uure.i
( I ! J & i , , 6 5 , .3;i!t (1943). ,l) J C Su:itIiard, i D i d . , 63, 3112 ( 1 ! J 4 1 ) ) E. 1' ',h::kt ari,l r 1) X o i i i i i . A n ; .I (1944:
l'iiisfr,
12, I--7
H E A T C O N T E N T S ABOVE
Sa?SiOs,~nol.wt. = 122 051 I , 'K 360 5
406.0 804.3 750 804 ii 891.y ion7 0 1124 0 1261 l i p ) 130.1 (1,) 13.54 1.11 I 1.jti7 1010 171;
ml' -
Hles.ia 1,890
Sa2SisOj,mol. w t . T , 'IC.
6,17*
376.8 503.2
570.3
31.040 35,760 :>8,2?f) 51,840 54,580 58.640
fizw.ia IIT
=
T,'K.
Crystal
15,0fi" 17,060 20,400 ~Z.(IOO
298.16 "K.
641.1 f,73.S 719 3 751 4
835.2 874 6 8'37 1 039 j 074 7
182.114 HT H 1 9 8 . 1 6
Gl2,bS
3,390 9,2011 12.410 16,020 17,800 20,1;!) 21 970 "(;,890
445.5 677.4 800.6 577.3 '330 2
6.570 15,250 25,450 30,710
34.0-iU ~ ~ 6 . ::G 0 2ou 101s.i aY,i~ii 1120 7 .17.4;,0
28,91U
;S[),IS,I 3'7 e ! )
I, i q u id
~
2,
i67tI 17.14
S0,57U
84.00O
hlarch, 1945
Heat content values read from smooth curves a t 100' intervals and corresponding entropy increments are given in Table.11. TABLE I1 HEATCONTENTS AND ENTROPIES ABOVE 298.16'K. Na2SiOs, cryst. and liquid NazSilOs, rrystnl HT - H ~ s . l a , ST - SWIR, HT - H z m . ~ , ST - S W I ~ .
T , OK.
400 500 600 700 800 900 1000 1100 1147 1200 1300 1301 1361 1400 1500 1600 1700 1800
cal./mole
467
HEATCONTENTS OF SODIUM METASILICATE A N D DISILICATE
cal./deg./mole
3,080 6,300 9,650 13,190 16,910 20,730 24,700 25,770
8.83 16.04 22.14 27.60 32.56 37.08 41.24 45.11
32,940 37,210 39,870(s) 52,310(1) 54,010 5S,290 62,570 66,850 71,130
48.74 52.16 54.16 63.32 64.53 07.48 70.24 72.84 75.28
cal./mole
cal./dey./molt:
4,410 9,040 13,980 19,160 24,670 30,400 36,320 42,430 45,360
12.70 23.02 32.02 40.00 47.35 54.09 (30.33 66.15 08.76
Using the method previously described by Shomate6 and the specific heats7 a t 298.16'K. (NazSiOs,cp295.16 = 26.72 and Na2Si206,cfi298.16 = 07.41 calories per degree), heat content equations were derived for the crystalline state of both silicates. A linear equation based upon heat contents of 54,010 calories a t llOO°Ki. and 82,5'70 calories a t 1(500°K. satisfactorily fitted the experimental data for liquid sodium metasilicate, the mean deviation being O.lyl,. *Issuming that liquid sodium disilicate cooled to the same final state in each measurement, a value was obtained for its specific heat from the slope of the best straight line through the experimental heat content points. The heat content equations, followed by the appropriate temperature range and mean percentage deviation of the equation from the experi:nental data, are given below. The heat contents influenced by preincltiiig were riot used in computing the niem deviatinn.
Xa2Si205(s): HT NazSi03(1): HT
H ~ W M= 42.80T
-
5910 (1361'1800°K.; O.lC/o) H~8.16 = 44.38T 0 00843P (l,067,000/T) - 17,560 (298"-1147"K.; 196)
+
+
The specific heat equations are NazSi03(sj: Z, NaZSiOs(1): C, NazSiz05(s):c, Sa2Siy05(l):C,
= 31.14 = 42.80 = 44.38
= 62.35
+ 0,009607' - (fi47,000/7? + (JI)t68fir - (l,OCi7,000/l
1J
80,000
0
E 6U,OOO h
c. I+r
.-CI
8.40,000 5 (0 0
2 I il
=: 20.000
800
HUO
10ou
14uu
1800
T , OK. Fig. 1.-Crystalline and liquid NazSiOs, A ; crystalline NaZSizO5, B ; glassy and liquid NazSiZOs, C.
Summary High-temperature heat contents above 295.16'K. of sodium metasilicate and sodium disilicate were determined from room temperature to about 1773'K. The heat of fusion of the metasilicate also was obtained, but glass formation precluded a heat of fusion determinatiop for the disilicate. The results have been summarized by algebraic equations and a table giving heat content and entropy increments above 29S.1Ci°K. a t 100' intervals. BERKELEY. CALIF.
RECEIVEDDECEMBER 15, 1944
