B arium Carbonate

M . L . N I C H O L S A N D R . I I . L A F F E R T Y , J R . 1, C o r n e ll U n i v e r s i ty , I t h a c a , N . Y .

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The earliest reported work on the decomposition of barium carbonate is by Abich (1), who stated in 1S31 that it is com­

pletely decomposed at a white heat. In 187S, Isambert (9) used the gas saturation method to determine the pressure of carbon dioxide in equilibrium with barium carbonate and oxide at 1083° C. B y passing nitrogen over the sample at 12 ml. per minute he found the equilibrium pressure was 22 mm. In 1898 Ilerzfeld and Stiepel (7) reported that complete decomposition occurred at about 1450° C., where the compound melted, but Brill ($) found that it melted without decomposition which occurred above 1450° C. P o tt (1 4 ), using the static method, determined the dissociation pressures and found the dissociation complete at 1200° C. However, his dissociation pressures for barium carbonate as well as those for calcium and strontium carbonates are higher than the usually accepted values and his work has been criticized by Johnston (1 0 ).

Finkelstein (4), using the gas saturation method, made a very complete determination of the equilibrium pressures of carbon dioxide with barium carbonate and oxide. He used a gas rate of about 33 ml. per minute. In the light of subsequent work, this rate as well as that used by Isambert is much too fast to ob­

tain equilibrium values. Finkelstein also reported that a basic carbonate of the composition BaO.BaCOj was formed, but Ilack- spill and Wolf (5) have recently proved by x-ray studies that no

1 Present address, Lehigh University, Bethlehem, Penna.

basic carbonate is formed but that the fusion of a eutectic mix­

ture of barium oxide and barium carbonate takes place between 1070° and 1150° C. Iledvall (6) reported the decomposition temperature to be 1361° C., and Hackspill and Wolf (5) using a similar method reported that the decomposition begins about 300° C. above the first decomposition of calcium carbonate.

They also found that barium carbonate undergoes an allotropic transformation from rhombic to hexagonal at 910° C. D utoit (3), using the gas saturation method with a slow gas rate, deter­

mined the dissociation pressures between 1102° and 1236° C.

Nakayama (11) reported that barium oxalate should be heated

Ta b l e I. Dis s o c ia t io n’ Pr e s s u r e o f Ba r i u m Ca r b o n a t e

Investigator Isam bert P o tt Finkelstein D u to it

D a te 1878 1905 1900 1927

M ethod Gas S tatic Gas Gas

saturation saturation saturation

R ate of gas flow,

ml. per min. 12 33 0 . 7 - 0 .8

t , 0 C. p , M m . Hg P P P

915 0 .4

945 0 .8

997 "Ó

1000 2 .7

1017 " 5

1057 45

1083 22

1097 Í2Ó

1102 i s *26

1114 2 1 .7 29

1132 29 27

1137 240

1140 33! 6 ’¿ i

1157 340

1197 675

1256 206 Í 99

1300 381

TableII. DissociationPressures

891 1339 879 1787 1791 1316 871 1325 1757 1751 1747 1310 1295 1299 1304 1751 1743 1745 1759 1755 1728 1749 1749 1747 880 884 1749 1745 1749 1759 1781

3Ü «3

Ju n e 15, 1942 A N A L Y T I C A L E D I T I O N 483

Fi g u r e 1. Ef f e c t o f Te m p e r a t u r e o n Ca l c iu m Ca r b o n a t e

L O G P

Fi g u r e 2 . Ef f e c t o f Te m p e r a t u r e

above 520° C. to change it to the carbonate and Ziemens (15) found that the dissociation of barium carbonate is complete at 1360° C.

T able I gives th e previously determ ined dissociation pres­

sures of barium carbonate.

The apparatus and method used in this work were those pre­

viously described (18, Figure 1) except for the following minor modifications: The micro combustion tube, E, was Vycor glass:

for temperatures below 953° C. a micro absorption tube was used in the absorption train, G, and one was also used in the by-pass, H, to determine the carbon dioxide liberated during the flushing period; the cold junction of the thermocouple was kept at 0° C.

Before starting the work on barium carbonate, the apparatus was checked by several determinations with calcium carbonate.

The purified calcium carbonate was ignited at 500° C. and spcc- trographic analysis showed the absence of strontium and the presence of only very small traces of other elements. Using 0.5 gram of calcium carbonate and a gas rate of 13 ml. per minute at 705° C., a value was obtained which when plotted with the previously obtained results (IS) shows fairly good agreement (Figure 1).

Kahlbaum’s “purest crystal” barium nitrate was recrystallized three times from distilled water. After the third recrystalliza­

tion the crystals were filtered on a sintered-glass funnel and dried overnight at 110° C. One hundred and tw enty grams of the recrystallized barium nitrate were dissolved in 1.5 liters of distilled water. This and a solution of ammonium carbonate (90 grams in 800 ml. of water) were added, slowly and simul­

taneously, to 1.5 liters of hot water. The solution in the reac­

tion vessel was stirred constantly during the addition of the reacting solutions and the barium nitrate W’as always kept in slight excess. After the solutions were mixed, a slight excess of carbonate was added. The barium carbonate was digested over­

night on a steam bath, filtered on a sintered-glass funnel, washed free from ammonia, and ignited at 600° C. for 12 hours. Spectro­

scopic analysis showed that the barium carbonate contained no significant amount of impurity and quantitative analysis by transformation to barium sulfate gave results of 99.94 and 99.95 per cent purity.

The results for th e determ ination of th e dissociation pres­

sures a t various tem peratures and gas rates are tabulated in Table I I and are show n in Figure 2 b y plottin g th e loga­

rithm of th e pressure against th e reciprocal of th e absolute

tem perature. From th is it can b e easily seen th a t th e rate a t w hich th e inert gas is passed over th e sam ple has a pro­

found effect on th e pressure of th e carbon dioxide.

I f the ignition of barium carbonate is carried o u t in a muffle furnace, th e carbon dioxide is in equilibrium w ith th e barium carbonate and oxide; if th e ignition is carried o u t over a b last burner in a covered crucible, th e pressure of th e carbon dioxide also should be v ery close to th e equilibrium valu e.

Therefore, it is desirable to determ ine as closely as possible th e equilibrium values for th e sy stem carbon dioxide-barium carbonate-barium oxide a t various tem peratures.

In Figure 3 th e values for th e logarithm of th e pressure are p lotted against th e rate. B ecause of th e sharp rise in th ese

R A T E

./

Fi g u r e 3 . Ef f e c to f Ra t e o f Ga s Fl o w

484 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 14, No. 6 values for th e logarithm of th e pressure are show n against the reciprocal of the absolute tem perature in Figure 2. T his line should represent approxim ately th e equilibrium values for the pressure of carbon dioxide in th e system carbon d io x id e- barium carbonate-barium oxide. I f W h ite’s d ata (13) from heating calcium carbonate a t various rates a t 636° C. are p lotted b y th e sam e m ethod, th e extrapolated value for the logarithm of the pressure a t zero rate is close to 0.67, w hich is Johnston’s (10) equilibrium value for this tem perature.

In order to m ake sure th a t th e decom position of barium

To verify this conclusion, loss-in-weight experiments were conducted with a platinum-wound muffle furnace of about 700-ml. capacity (12). The same temperature regulator was used as in the determination of the dissociation pressures and the temperature was measured with a Bureau of Standards calibrated platinum-platinum rhodium thermocouple. The temperatures are believed to be accurate to within ± 5 ° C.

Using the same samples this procedure was repeated, varying the time of heating and the temperature. Although the barium carbonate had previously been ignited at 600° C. and was dried overnight at 110° C., a loss in weight of about 0.2 per cent oc­

curred during the first 2 hours’ heating at 845° C. This loss was thought to be due to loss of water and was confirmed in an­

other experiment by first heating the barium carbonate at 300° C.

for 20 hours. In this case approximately the same loss occurred confirms the prediction of the dissociation curve that decom­

position should start at this temperature. At 885° C. the loss,

Each tem pera­ igni­ içn i- tem pera­ hour ign i­ igni­ tem pera­

ignition ture tion tion ture (av.) tion tion ture

June 15, 1942 A N A L Y T I C A L E D I T I O N 485

conditions similar to these. A t 905° C. the loss in weight is appreciable in the course of an hour and an analytical determina­

tion would not be satisfactory if the heating were continued at this temperature for more than a few minutes. To be certain that the loss in weight was not due to the volatilization of the platinum, the em pty crucibles were heated for 113 hours at 905° C. The total loss in weight of the two crucibles was 0.2 and 0.1 mg. experim entation a t carefully controlled pressures, both above and belowr atm ospheric pressure. A search of the literature failed to reveal a n y sim ple device deem ed capable of con­

tinued operation under th e conditions to be encountered. T he glass regulator here described has been found by laboratory trial to m eet satisfactorily the conditions im posed.

The apparatus, the construction of which is shown in the diagram, involves the sealed-chamber principle found in the vacuum regulator of Mc­

pressure differential at the valve seat. Hough calculations show that slightly over 10 grains are required to open a 1 sq. mm. open­

ing against a pressure differential of one atmosphere. As this weight should be displaced by the mercury column in as small a pressure range as possible, it is necessary that the cross section of the float be relatively large. A float made from a 25-mm. Pyrex ig­

nition tube has been found to give satisfactory results. In this case, the lead-loaded float weighs approximately 100 grams, giv­

ing a sizable safetv factor.

pressed, allowing the valve to open and exhaust the accumulated gases until the mercury in rising again closes the needle valve.

If, in operation, the system does not evolve sufficient gas to main­

tain the desired pressure, an inert gas may be supplemented, as indicated in the diagram. The pressure fluctuations necessary to actuate the valve mechanism may be largely eliminated at the regulator precision. U nder proper operating conditions, th e apparatus has been found capable of controlling pressure either above or below atm ospheric pressure, to ab ou t 1 m m .