S O L U B I L I T Y O F CALCIUSf C X R B O S A T R I S -\QzC EOUS S O L U T I O N S O F C E R T - U N E L E C T R O L Y T E S I N E Q U I L I B R I U I I W I T H A T X O S P H E R I C -AIR1 BY F R A N K K. C A U E R O S h N D ATHERTON S E I D E L L
T h e experiments on the solubility of calcium carbonate were made in a similar manner to those described in the preceding papers.' Since the solubility ot the carbonates is dependent upon the partial pressure of the carbon dioxide in the gas phase in contact with tlie solution, the effort was made to bring these solutions to equilibrium with ordinary air. Air was therefore drawn through the flasks containing the solutions in contact with the calciuni carbonate in the solid phase. T h e flasks were arranged in series with appropriai e rubber and glass tube connections and a continuous stream of air drawn through by ail aspirator attached to a 1ij.drant. Before entering the solutions the air was drawn through a dilute solution of sulphiiric acid to free it froin any traces of ammonia or other impurities in the laboratory atmosphere. Precipitated calciuin carbonate was used in order to take advantage of tlie large surface exposed by the material in this form, and thus bring the solutions to equilibrium as quickly as possible. ,111 the salts used were obtained from Gaker & Adanison, mere labeled C. P., and were found to be of a satisfactory degree of purity. As siiiall changes of teiiiperature did not produce any marked effect on the soltibility of the calcium carbonate, and as the temperature of the laboratory held quite coiistant during the period when this work was clone, it was not deemed worth while to make use of a constant teinperature bath. Since it was found that the niajor part of the calcium went iiito solution as the hydrogen carbonate rather than as the normal carbonate, the calciuin dissolved is stated in terms of the
'
Contribution from t h e Bureau of Soils, U. S. Department of .lgriculture. Published by permission of t h e Secretary of Agriculture. Jour. Phys. Chem. 5, j 3 7 : 5 , 643 (1901).
hydrogen carbonate. Of course the stateinelits and charts in terms of reacting weights would be the same for either the normal or hydrogen carbonate. I n the accoiiipaiir iiig figure the calcium salt found to be soluble is plotted in all cases 011 the axis of ordinates aiid the amounts of other salt components of the solution oil the axis of abscissas. Solubility of calcium carbonate in sodium chloride solutions a t 25' C Xir was passed through the solutions for twentj--seveii days, the solutions being tested froin time to tiiiie to determine mhen final equilibrium was reached, at Ivliich time they glive no reaction with pheiiolplithaleine ; that is to say. the solutions contained no noriiial carbonate dissolved in them. At first sight this result appeared very surprising, but it is iii line with the observations of Treadwell aiid Renter, atid with the observations made in this laborator) in the study of the ecliiilibriuiii betn eeii the normal carbonate and lij drogen carbonate of calcium, described in a preceding paper. T h e results obtained iii the analyses of the solutions are given in the accompanying table. m hicli is self-esplaiiatorj-. TABLE I. Cdcium hydrogen carbonate ioluble in aqueous qolutions of sodium chloride a t 2 j o C arid in equihbriuni with air -~
~-
~~
- ~-
Grams per liter
__
.__~_____ ~
~
Ca( HCO,):
Sac1
~ - _ _ Reacting weights per liter
~~
Grams per liter
Reacting \reights per liter
0.1046 0.1770
0.0006,j
0.000
0.000
0.001I O
9. 7 2 0
0.205 I 0.2152 0 . 2 2j 2 0.2212 0.2172
0.00128
21.010
0.168 0.362
0.00134
30.301 50.620 69.370
0.19;1 0. I
j69
0. I 2 2 7
0.OoI~o
o.o01$3 O.OOI?, j 0.00123
0.0009j o 00076
98.400
0.522
0.872
1.19j 1.69j 2.540
I4i.400 234.500
4.040
262.300
4. j 2 0
These results are presented graphically in the acconipanj.iiig
figti re.
Reacting Weights o f
Solts p e r Llter
T h e solubilitj- curve presents a well-marked maximum point, corresponding to a solution containing in a liter about 56 grains, or 0.95 reacting weight of calciiini hjdrogen carbonate ; that is to say, at the tnaxiniuni point the corresponding solution would contain about o.oj61 grain of calcium - equivalent to 0.1402 gram of the normal carbonate. Hence it appears that the solubility of calcium carbonate under these conditions is about 83 times what it would be in pure mater free from carbon dioxide. In the series under consideration it was found that in the solution containing no sodium cliloride there was 0.1046 gram per liter of calcium hydrogen carbonate, equivalent to 0.0592 gram per liter of the normal carbonate. Therefore the solubility of the calcium carbonate corresponding to the maximum point of the curve is 2.36 times its solubilit\- in water in equilibrium with the air, but containing no sodium chloride. Solubility of calcium carbonate in sodium sulphate solutions at 24' C In this series of experiments normal carbonates were found
Ci7 /ciu rn Crr r-hmintc 1'11 .4pzieozis S o h t iorzs
53
tcj be present i n the solutions of higher concentrations, although successive exaiiiiiiations at intervals of weeks showed conclu4\ el! that the solutions had reached a final state of equilibrium and reniairied unchanged, even with tlie prolonged passage of air through them. In the accompan) ing table the total amount of calcium in the solutions is g i l e n in terms of the equivalent in calcium li! drogen carbonate, although a part of the calciuin should more properl? be gi\ en as tlie iioriiial carbonate. T h e actual amounts of the bicarbonate or 11) drogeii carbonate found in the solution are also given, and in the figure ciirves are plotted from both series of data. For purposes of comparison this is considered more desirable than gi\ ing tlie amounts of normal carbonate foiind, which could not T\ ell be illustrated in the figures.
TABLEI1 Solubility of calcium carbonate in aqueous 5olutioiis of \odium sulphate a t 24' C , and 111 equilibrium with air - --_
__
-
_
Total calciuin calculated d S Cai HCO, I
~_ - _ _~_ _ Calciuin actu'tllj dissol\ et1 a5 C ~HIC O , ~
~~~
1
~
_
_-
Sodium sulpliate
Grams pel liter
R nts per liter
Grams per liter
0.092 0.1488 0.1729 0.2330 0.3240 0.3960
0.000j7
0.0925
0. COO j7
0.000
o.0000
0.00092
0.14S8
0.o0092
2.800
0.01g1
j.Zgj
0~356 oos31 0.26I 2
0.00IO.;
0.1729
0.0014j
0.2210
o.0020 I
0.3020 0.3440 0.3660 0.3940 o 4060
0.00246
0.45s0
o.002sj
0.j630
0.003,iO
0.j910 0.66j 0
o 00367 0.00413
0.4300
R vts per liter
~
0.00IO.;-
-
'
0.0013; 0
00183
0 . 0 0 2 1 ~
' ~
0 0022s
Grams
per liter
11,730 36.860
74 010 I 16.100
0.0024j
184.200
0.002
213.700 25j.900
2
0.00267
R mts per liter
0.5245
0.8228 I .3054 1.5146
1.8135
It nil1 be ohser\-ed that the curves do not show niaxiiiiiiiii points, but that the solubility of the cnlciuiii carbonate, both in the form of normal carbonate and of h!.clrogen carbonate, steadily increases nit11 increasiiig concentration of bodiuiii sulphate. It is especial1:- north?. of note that tlie solubility of the calcium carboilate is ver!. much greater in solutions of sodium sulphate
than i n solutioiis of sodium cliloride, although in the foriiier c3se tlie slightly soluble coiiipoiiiid, calciuiii sulpliate, is presuinably formed, and in tlie latter tlie very soluble substance, calcium chloride. Even granting that practicall! all the calcium sulphate formed again unites u it11 the sodiuin sulphate to form the soluble double sulphate, and inakiiig the further rather iiiiprobable assumption that this double sulphate d i ~ o c i a t e sentirely in such a wa! as not to j.ield a calciuiii ion, tlie case does not becoiiie clear ; for, at least in the higlier dilutions, it would still be expected that tlie sodium chloride \iould ha1 e a gieater solvent effect than the socliuiii sulphate. I t is true that in the calculation of the data presented i n this paper tlie reacting weight of socliuiii sulphate is based 011 the formula Na,S04 instead of on the forinula ;2KalS04. .in inspection of Fig. I will show that even on this latter assumption the sodiuin sulphate curve 1 1ould lie well above tlie sodium chloride curve. -isstitiiing, therefore, that tlie effect of the sodium ions ~vouldbe tlie same in the t n o cases, it appears that the SO1 ioii has a greater effect than tlie C1 ion in rendering the calcium carbonate more soluble. Above a certain concentratioti (about j o to 60 grains per liter) with respect to sodium sulphate the curves are practicallj straight lines ; that is to sa!, the solubility of calcium carbonate i n solutions above this colicelitration is directly proportional to the amount of sodium sulphate present. Assiiiiiing the iiiass law to liold, this indicates that at these liiglier concentrations the sodium sulphate dissociates a i a binar) electiol! te accoiding to the sclienie + X'aKaSO, 'p, S a
+ SaSO,
Froiii this point of liew it wo~ildbetni that tlie relative effects of the X a S 0 4 ioiic and tlie C1 iotis on tlie solubility of the calcium carbonate are to be compared, aiicl that tlie first-mentioiied ion is the more effective. If it he as~iiiiiediiow that the Ca ion does not unite \\it11 the S a S 0 4 ion to form a more coinples ioii, but does unite
with the C1 ion to form coinplex ions, the greater solvent effect of the sodiuiii sulphate OL er that of the sodium chloride becomes intelligible. I t is ta be frankly adinitted, however. that this train of reasoning involves a number of assumptions, and that, while Sotlie evidences froin other soiirces may be adduced in support, it can be advanced only tentatively. It further accents the very unsatisfactory state of our knowledge of solutions, especially in other than those of high dilution. Solubility of calcium carbonate in sodium chloride solutions a t 25' C in contact with calcium sulphate in the solid phase T h e solutions were found to contain no normal carbonates. T h e effect of the dissolved calcium hydrogen carbonate on the solubility of the calcium sulphate has been discussed in the preceding paper. BJ referring to the figure it will be seen that tlie effect of the dissolved calciuin sulphate on the solubility of ;he carbonate was of the same general nattire, but much greater in degree. n'ith increasing concentration of sodium chloride the solubility of calciuni carbonate gradually increases u p to a inaxitnuin point, then drops snddenl? , and finally decreases very gradually. T h e niaxiiniiin solubility occurs i n a solutioii containing about 40 grams per liter of sodium chloride. I t will be reiiieinbered that the i~iaxiiniiin solubility of the calci~iin5111phate, when calcium carbonate in the solid phase was also prc>sent,was in a solution containing from 80 to 8j grains per liter of soditiiii chloride Tl'hile the effect of the presence of calciuiii carbonate 011 tlie solubility of calcium sulphate was sliowii to b= slight in s3lntions coiitaiiiing less than 80 grains per liter of sodiuin chloride, it will be seen that the effect of the calcium sulphate on the solubility of tlie calciuin carbonate is relatively great in all concentrations with respect to sodium chloride. Furthermore, tlie noriiial carbonate appears to be entirely coni.erted to the hydrogen carbonate, in tlie solutions. Summary T h e solubility curve for calcium carbonate in aqueous soliitions of sodium chloride, and iii equilibrium with ordinary air,
shows a well-marked maxiinurn point, Apparently under such conditions, tlie solutions contain no normal carbonates. Sodiuin sulphate has been s h o ~ v nto have an astonishingly great effect on the solubility of calcium carbonate, tlie resulting solutions containing not only hydrogen carbonate, but normal carbonate as well. Further, there is no necessary precipitation of the lime carbonate with increasing concentration with respect to sodium sulphate ; but the solnbility of the lime carbonate steadily increases, with increasing amounts of sodium sulphate i n the solution, u p to tlie saturation point >f this latter salt. In tlie presence of solid calcium sulphate the solubility of calcinin carbonate is much decreased. With increased concentration with respect to sodium chloride, the solubility of calcium carbonate increases to a iiiaxiinum, then suddenly decreases, showing a break in the solubility curve. I n equilibriuni with ordinary air, the solutions contain no normal carbonates.
