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Volume 54, 1923
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Art. 49—The Solubility and Hydrolysis of Calcium Carbonate.

[Read before the Philosophical Institute of Canterbury, 6th July, 1921; received by Editor, 31st December, 1921; issued separately, 22nd June, 1923.]

From an examination of the literature of the solubility of calcium carbonate it was found that the effects brought about by the addition of salts to carbonate solutions had been only partially investigated, mostly in comparatively strong solutions, and chiefly in the presence of air. It seemed, therefore, that interesting results might be obtained by examination of the effects produced by very dilute solutions, air being excluded—more especially as certain work carried out by E. A. Rowe (1) tended to show that the effects were uniform—that is, certain types of salts gave similar effects, but each salt had a definite influence, in some cases giving apparent breaks in the solubility-curve. (Private communication.)

The work to be described was carried out for the following purposes: (i) To attempt to verify Rowe's results; (ii) to determine the degree of hydrolysis of calcium carbonate; (iii) to determine, if possible, the effect on the solubility product of calcium carbonate.

The solubility of calcium carbonate has been determined by a number of workers, but the results differ considerably.

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Holleman (2) found 1 part of carbonate in 80,040 of water—i.e., approximately 0.0125 gram per litre. He quotes (l.c.) the following: Fresenius, 1 part in 16,600 of water; and Bineau, 1 part in 50,000 of water.

Kendall (3) found the value of 0.01433 gram per litre at 25° C.

Gothe (4) found the solubility slightly variable, his final value being 31.0 mg. per litre. This result appears anomalous, but may be explained when the great effects of traces of carbon dioxide, taken up from the air or elsewhere, are considered.

Seyler and Lloyd (5) determined the solubility product of calcium carbonate, obtaining

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[Ca] [CO″3] = 71.9 × 10−10 at 25° C.;

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they also obtained the equation x2/1-x.Ca=1.92×10−4

where x is the fraction of carbonate hydrolysed. By solving these equations, the value 14.6 × 10−5 grm. mols. per litre is obtained. This result agrees closely with that of Kendall.

McCoy and Smith (6) calculated from the data of Kohlrausch (7) for the conductivity of saturated solutions of calcium carbonate a solubility of 12 mg. per litre. Their calculation allowed for hydrolysis to the extent of 66 per cent. They themselves obtained the value 16.6 mg. per litre. They say no explanation of the discrepancy is forthcoming.

An important matter in relation to all these determinations is the partial pressure of the carbon dioxide in contact with the solution. For each pressure there is a definite concentration of ions derivable from calcium carbonate. For a discussion of this point see especially a paper by Johnston and Williamson (8).

In the presence of salts of the alkalis the solubility of calcium carbonate is greatly altered, perhaps by formation of complex ions, thus reducing the concentration of the calcium ions.

Gothe (l.c.) states that the solubility is increased by chlorides, nitrates, and sulphates; while a decrease occurs in the presence of alkali carbonates and salts of the alkaline earths.

Rowe (l.c.) came to somewhat the same conclusions, and added, “as regards the alkalis, that calcium carbonate appears most soluble in the ammonium salt of any given acid, and in solutions of the sulphate of any given base.” He also found that there is apparently a discontinuity in the curve with solutions of sodium chloride of strengths about M/500. (Private communication.)

In this present case the apparent solubility of calcium carbonate, in the calcite form, was determined in gas-free water, in presence of varying amounts of alkali salts; while the degree of hydrolysis was deduced from electromotive-force measurements.

Solubility Measurements.

These were carried out in Jena glass vessels fitted with mercury-sealed stirrers and side tubes leading to a series of guard-bottles. This form was adopted after two other types of solution-vessels had been used. A multiple apparatus was set up. A large rectangular copper bath was used as a thermostat, the sides being protected with asbestos and wood. A large toluol regulator was used to maintain the temperature at 25.0° C.; while a spiral tube stirrer driven by a hot-air engine was employed to keep the

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water in circulation. The solution-vessels with their stirrers were supported in the bath. The pulleys from the stirrers were connected by means of solid rubber belting with a central pulley, which in turn connected with a small electric motor. It was thus possible to remove any one or more of the vessels without causing a stoppage of the motor.

In the solution-vessel were placed 3.000 grams of finely powdered calcite crystals and 250 c.c. of salt solution (freshly made from gas-free water). The contents were kept stirred at a rate sufficiently high to keep the carbonate in suspension. The solutions were left for several days, the period depending on the concentration of the salt solution; the period was determined by a series of experiments, sodium chloride being taken as standard. The calcium in solution was estimated as oxide or sulphate.

The effect of traces of carbon dioxide is very considerable, since ordinary distilled water gave a much larger value than the freshly boiled water: for example—

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Ordinary distilled water 19.6 × 10−5 grm. mols. per litre.
Gas-free water 13.4 × 10−6 grm. mols. per litre.

and also with sodium-chloride solutions. The figures are—Gram molecules of carbonate per litre × 105.

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Concentration of NaCl M/50 M/100 M/150 M/200 M/400 M/500 M/600 M/800 M/1000
Ordinary distilled water 69.8 35.9 32.1 28.4 22.2 19.1 16.5 16.1 16.3
Gas-free water 52.0 28.6 25.0 24.1 17.9 17.9 17.9 16.1 16.1

From this it appears that a slight minimum occurs in presence of traces of carbon dioxide at about M/800; but this disappears from the second set.

The following table summarizes the results of the solubility measurements (gram molecules of carbonate × 105):—

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Salt M/50 M/100 M/150 M/200 M/400 M/500 M/600 M/800 M/1000
NaCl 52.0 28.6 25.0 24.1 17.9 17.9 17.9 16.1 16.1
NaBr 40.9 25.0 23.4 20.5 17.1 16.1 15.2 12.5 11.6
NaClO3 63.4 32.0 27.7 25.0 36.1 16.1 16.1 17.9 17.0
NaNO3 67.8 34.5 30.4 28.6 19.3 17.9 17.9 16.1 16.1
KCl 32.0 26.8 25.0 24.1 17.0 15.0 14.3
KBr 41.7 24.1 21.4 16.1 15.2

From these figures it is seen that a similar type of curve is obtained in each case, but the effect is not proportional to the concentration of the salt solution. Sodium chlorate and nitrate give very large increases.

Measurement of Hydrolysis.

The velocity-constant method for the determination hydroxyl ions was not successful in this case, so recourse was had to electromotive-force measurements.

By measurement of the electromotive force developed in a cell one element of which contains the solution under consideration, the concentration of the hydroxyl ions may be obtained. The actual determination is that of the hydrogen-ion concentration, but the hydroxyl-ion concentration is then given by the relation

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[H.][OH′] = 1 × 10−14 at 25° C.

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The formula used for the calculation of the hydrogen-ion concentration is E = Eo + 0.059 log10C at 25° C, where E = E.M.F. of the element in volts, Eo = 0.277 volt, C = concentration of hydrogen ions.

When a normal calomel electrode is used as the other half-element, if x is the observed E.M.F., then the potential of the other element is

(0.564 — x) volt at 25° C.,

which is E of the formula above.

Also, according to theory, the observed E.M.F. will increase with decrease of hydrogen-ion concentration; and, since the carbonate solutions are all akaline to phenolphthalein, the hydrogen-ion concentration will be small, and hence the observed E.M.F. will be great.

This difference of potential may be deduced approximately as follows: The concentration of the calcium ions in the solution is of the order 10−4 grm. mols. per litre. Now, assuming that the calcium exists solely as carbonate or the products of hydrolysis of the carbonate, the hydrogen ion is present at about 10−10 grm. mols. per litre.

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Since the hydrolysis of the carbonate proceeds according to the equation 2CaCO3 + 2H2O = Ca(OH)2 + Ca(HCO3)2

it follows that, knowing the concentration of the calcium, and assuming that all exists in the ionic form, the greatest possible concentration of hydroxyl ions is determined. Then, knowing the hydrogen-ion concentration, the degree of hydrolysis of the carbonate may be deduced at once; and from the hydroxyl-ion concentration the concentration of the carbonate ion is determined, from the relation found by Seyler and Lloyd (5):—

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2CaCO3+2H2O=Ca(OH)2+Ca(HCO3)2

whence the value of the product [Ca] [CO3″] may be calculated.

The E.M.F. produced was measured by the ordinary potentiometer method. The usual wire was replaced by a pair of resistance-boxes and a high-resistance galvanometer employed as zero instrument. A cadmium cell was used as standard. Two types of hydrogen electrode were used— (i) Liuther-Brislee, and (ii) platinized wire sealed into glass tubing, as recommended by C. J. J. Fox (9). Both electrodes gave identical results.

The hydrogen used was prepared by electrolysis, the gas being passed through a hot tube containing platinized asbestos to remove the traces of oxygen. The hydrogen was generated under a pressure sufficient to keep the gas bubbling through the solution. The solutions were filtered through cotton-wool out of contact with air, the first portions being rejected. The electrode vessel was then left in the thermostat till the liquid was saturated with hydrogen.

In some cases a maximum E.M.F. was found; the observed E.M.F. in no case reached the theoretical, the defect being some hundredths of a volt. It appeared that some unknown factor was at work. Nevertheless, the data obtained yielded some useful information: this will be considered in the general discussion.

Discussion of Results.

The solubility-curves obtained were not straight lines, and the deviation was greater than the experimental error. Apparently a new effect appears at about M/50, since a sudden change of direction occurs near this point.

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The apparatus was airtight, and the pressure of the carbon dioxide under these conditions may be considered about 3.7 × 10−7 atmosphere. (Johnston and Williamson (8).)

Johnston and Williamson obtained a minimum concentration of calcium carbonate at this pressure, their value being 16 × 10−5 grm. mols. per litre at 16° C. On either side of this value the concentration increased rapidly, thus again showing the great influence of the carbon dioxide. For this minimum concentration they found that the hydrolysis was about 54 per cent. Against this value is that of Seyler and Lloyd (5)—namely, 66.7 per cent.

In discussing the electromotive-force experiments it was shown that theoretically, and in part practically, the concentration of the hydroxylion in the solution can be determined.

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Now, the ions present in the solution are H., Ca, HCO3′, OH′, and CO3″,

if the presence of the neutral salts be neglected. And between these ions the following relations exist:—

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Ca=CO3″+HCO3″,

H.×OH′=1×10−14

H.×CO3′=4.15″×10−11

The last equation is due to Kendall (10).

Hence if y is the concentration of the calcium ion and c that of the hydrogen ion, then

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y=y(1-x)+1×10−14/2c+Cy(1-z)/2×4.15×10−11

where x is the degree of hydrolysis of the carbonate.

This more complicated relation takes the place of that deducible approximately, viz.,

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x=1×10114/Cy

The following table contains some of the more reliable data:—

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Salt. Concentration of Salt. Ca × 104. H. × 1010. OH′ × 105. Per Cent. Hydrolysis.
NaCl M/400 1.98 4.1 2.44 84
NaCl M/500 1.62 2.3 4.35 79
NaCl M/1000 1.44 1.95 5.00 75
NaNO3 M/100 3.00 4.7 2.1 83

Owing to the extremely high resistance of the pure-water solutions, reliable values could not be obtained.

For the purpose of the above calculations the degree of ionization of the carbonate was taken as 0.9.

From the above table the product [Ca] [CO3″] may be calculated.

Values are obtained ranging from 52 to 64 × 10−10, and in a few cases as low as 26 × 10−10, using Seyler and Lloyd's equation, [Ca]2 (1 — x) = constant.

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By considering the effect of the neutral salts in the solutions the following equation is obtained:— x2 (1— 4b — 4a) + x (b2 + 4ab) — ab2 = 0.

a = total concentration of calcium ions; b = original concentration of alkali salt; x = concentration of alkali carbonate formed.

From this equation the following results may be deduced:—

  • (i.) The concentration of calcium as carbonate in the solution is nearly constant, being that of the carbonate in pure water.

  • (ii.) The concentration of alkali carbonate is proportional to the concentrationof the added salt.

  • (iii.) The value of the ratio CaCO3/M2CO3, where M2CO3 is the alkali carbonate, for a given dilution is independent of the salt used. The values for a series of dilutions lie on a straight line in each case.

As an example, some of the results obtained with sodium bromide may be quoted:—

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Concentration. CaCO3 × 105. Na2CO3 × 105. CaCO3/Na2CO3.
M/50 15.7 24.5 0.6
M/100 13.5 12.4 1.1
M/200 14.7 5.8 2.5
M/400 14.2 2.9 4.9
M/500 13.8 2.3 6.0
M/1000 11.5 1.05 11.0

The following conclusions were drawn from the work done:—

1. The solubility of calcium carbonate (calcite for in) in gas-free water at 25° C. in a closed vessel is 13.4 × 10−5 grm. mols. per litre. The mean value in ordinary distilled water is 19.6 × 10−5 grm. mols. per litre.

2. The presence of traces of carbon dioxide exerts a considerable influence on the solubility.

3. The degree of hydrolysis of calcium carbonate may be calculated by electrometric means. Similarly, the value of the solubility product may be calculated.

4. The addition of known quantities of neutral salts increases the solubility, but not regularly.

5. It is doubtful if there exists a minimum concentration of the carbonate in presence of sodium chloride except in presence of small quantities of carbon dioxide.

Literature cited.

(1.) E. A. Rowe, Trans. N.Z. Inst., vol. 52, p. 192, 1920.

(2.) A. F. Hollman, Zeit. physik. Chem., vol. 12, p. 135, 1893.

(3.) J. Kendall, Phil. Mag. (vi), vol. 23, pp. 958–76, 1921.

(4.) F. Gothe, Chem. Zeit., vol. 39, pp. 305–7, 1915.

(5.) C. A. Seyler and P. V. Lloyd, J.C.S., Trans., vol. 111, p. 996, 1917.

(6.) H. N. McCoy and H. J. Smith, J. Am. C.S., vol. 33, p. 473, 1911.

(7.) F. Kohlrausch, Zeit. physik. Chem., vol. 54, p. 236, 1903.

(8.) J. Johnston and D. Williamson, J. Am. C.S., vol. 38, p. 975 et seq., 1916.

(9.) C. J. J. Fox, B. A. Rep., p. 457, 1909.

(10.) J. Kendall, J. Am. C.S., vol. 38, p. 1480, 1916.