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Volume 43, 1910
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Art. IV.—The Conductivity of Aqueous Solutions of Carbon-dioxide prepared under Pressure at various Temperatures; with Special reference to the Formation of a Hydrate at Low Temperatures.

[Read before the Philosophical Institute of Canterbury, 2nd November, 1910.]

Introductory.

It is well known that at ordinary temperatures carbon-dioxide is soluble in about its own volume of water, whatever be the pressure under which it passes into solution, the volume of the gas being measured at this pressure. Such an aqueous solution shows weak acid properties, giving, for example an acid reaction with phenolphthalein, and turning blue litmus purple. It is assumed in text-books that a compound H2CO3 is formed which ionizes as a weak acid into H+ and HCO3-.

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In order to ascertain what work had been done in connection with the acid properties of CO2 solutions, a thorough search was made in all the available journals—viz., the “Journal of London Chemical Society,” the “Philosophical Magazine,” the “American Journal of Physical Chemistry,” and “Zeitschrift für Physikalische Chemie.” Nearly all the references were found in the first of these. It is very improbable that any valuable work done would escape notice in its abstracts; and, indeed, Walker and Cormack, reviewing in a preface to one of their theses the work known to them as having been done, mention none which is not referred to in this journal.

In searching the journals special attention was paid to the records of conductivity experiments, it being felt that the electrical conductivity method was much the best of approaching the subject of degree of ionization, if not the only practicable one. At the same time attention was paid to every detail concerning CO2 solutions which could possibly bear on the subject, especially to solubility at different pressures and temperatures, and the formation of hydrates.

Bibliography.

The Combination of Carbon-dioxide and Water, Wroblewski, 1882 (Journ. Chem. Soc., vol. 42, p. 692).—The author (who first discovered the existence of a hydrate of CO2) infers that a definite but readily dissociable hydrate exists “capable of existing only under certain pressures, increasing with the temperature, and equal to 12·3 atmospheres at 0° C.”

The Composition of Hydrated Carbonic Acid, Wroblewski, 1882 (Journ. Chem. Soc., 42, 1026).—The author concludes that at a temperature of 0°, and under the pressure of about 16 atmospheres, CO2 unites with water to form a hydrate CO28H2O.

Law of Solubility of Carbon-dioxide in Water at High Pressures, Wroblewski, 1882 (Journ. Chem. Soc., 42, 1021).—From this abstract it seems (inter alia) that the author had been able to form the hydrate only in small quantity, at the free surface of the solution, and the experiment seems to have been complicated by the freezing of the water.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

The Electric Conductivity of Solutions of Carbon-dioxide, Pfeiffer, 1885 (Journ. Chem. Soc., 48, 212; original paper in Ann. Phys. Chem. (2), 23, 625–50, to which there was no access).—Pfeiffer was the first to definitely investigate the conductivity of CO2 solutions under varying pressures. He worked with pressures of from 1 to 25 atmospheres, and apparently at ordinary temperatures. His results, in effect, were—(1.) Conductivity is in every case very small; under normal conditions it equals 1/20 of that of spring water. (2.) If all the dissolved CO2 were converted into ionized H2CO3 its conductivity should be more than 1,000 times as great as observed value. (3.) Change of pressure produces no alteration in the conductivity. (4.) Conductivity increases rapidly with increase of temperature; cf. with acetic and oxalic acids (this statement is not made at all clear in the abstract: temperature seems to be confused with dilution).

Conductivity of Aqueous Solutions of Carbon-dioxide, Knox, 1895 (Journ. Chem. Soc., 68, ii, 100).—Conductivity was determined by Kohlrausch's method, experiments being made at varying pressures, results being recorded for both rising and falling pressures, and at temperatures of 12·5° and 18°. All of his work appears to have been done with comparatively weak solutions, the most concentrated solution mentioned in the table of

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recorded results (Journ. Chem. Soc., 77, 9) having a dilution 12·6, corresponding to less than 2 atmospheres pressure. He worked down to very great dilutions, and obtained a fairly satisfactory dissociation constant under these conditions.

The Dissociation Constant of Carbon-dioxide in Solution, Walker and Cormack, 1900 (Journ. Chem. Soc., 77, 5).—These investigators worked along the same lines as Knox, with apparently more elaborate precautions to insure accuracy. They measured the conductivity of solutions of various concentrations, all at a temperature of 18°, the pressure of CO2 in contact with the solutions never rising above atmospheric. The ordinary Kohlrausch method of determining conductivity was used, measurements being made with induction coil and telephone. The apparatus was of glass, and great care was taken to insure absolute insulation of electrodes, &c. Water for the experiments with a conductivity of 0·7 × 10-6 in Siemens units at 18° was used, being obtained by successive distillations (1) with alkali, (2) with phosphoric acid, and (3) alone, the last being conducted in chemically pure air.

reference is made to Pfeiffer's and Knox's experiments. The solutions in Pfeiffer's experiments were prepared under pressure, and hence were too concentrated to be of service in fixing the dissociation constant of CO2. (The same objection applies to the experiments described in this thesis, and consequently no attempt has been made to determine the dissociation constant.)

Knox's dissociation constant is calculated as 0·380 × 10-6, the authors' as 0·304 × 10-6, corresponding to a difference in actual conductivity of about 10 per cent. This result is considered as being due to the authors having used better conductivity-water and to their more accurate method of determining the concentration of CO2 in the solution—viz., by titration with barium-hydroxide, as against Knox's measurement by means of the pressure of CO2 in the solution. (The above-mentioned discrepancy between the results of good workers would seem to show the experimental difficulty in obtaining accurate results when working with very small conductivities.)

The conductivity imparted to pure water by exposure to atmospheric CO2 is calculated to be 0·65 × 10-6 Siemens units. At this very small concentration 14·4 per cent. of the dissolved CO2 exists in the ionized state.

The remainder of the paper and an addendum (Journ. Chem. Soc., 83, 182) discusses the relative proportions of dissolved CO2 and un-ionized H2CO3 existing in the solution, but without coming to any very definite conclusion.

A Hydrate of Carbon-dioxide, Villard, 1895 (Journ. Chem. Soc., 68, ii, 44).—The author's experiments lead him to take CO26H2O as the composition of the hydrate. It does not decompose below 0° (cf. with Hempel and Seidel, below).

The Absorption Coefficient of Carbon-dioxide in Water at 0°, Prytz and Holst, 1895 (Journ. Chem. Soc., 68, ii, 104).—The freezing-point of a saturated CO2 solution at atmospheric pressure was determined, giving a depression of 0·156°. The calculated depression, assuming all the CO2 to dissolve as such or as H2CO3, was 0·158°. Absorption coefficient at 0° = 1·7308.

Compounds of Carbon-dioxide with Water, Hempel and Seidel (Journ. Chem. Soc. 76, ii, 151).—By sealing solid CO2 and water in a glass tube, allowing temperature to rise to ordinary temperature and then cooling, a CO2 hydrate was crystallized out; at 8° it melted under the vapour-pressure

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of liquid CO2 in the tube, and at -2° under atmospheric pressure. Composition given as CO28H2O or CO29H2O.

Other references which were looked up, but from which no further assistance was gained, were :—

On the Physical Peculiarities of Solutions of Gases in Liquids, Wanklyn (Phil. Mag., 1902, 3, 346).

On the Influence of Pressure on the Electrical Conductivity of Solutions (Journ. Phys. Chem., 1899, 3, 186).

Determination of Electrical Conductivity of Solutions with Direct-current Instruments (Journ. Phys. Chem., 1901, 5, 536).

The Lowering of the Freezing-point of Water produced by Concentrated Solutions of Electrolytes, and the Conductivity of such Solutions (Journ. Phys. Chem., 1903, 7, 311).

Variation of Electric Conductivity at Low Temperatures (Journ. Phys. Chem., 1903, 7, 407).

Summary.

From these abstracts the following summary may be made :—

(1.) Solutions of CO2 in water, at least under certain conditions of temperature and concentration, contain a certain (comparatively small) number of ions, presumably, by analogy with other weak acids, and from the evidence of their chemical behaviour, consisting of H+ and HCO3-, from a molecule of H2CO3.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

(2.) The degree of ionization has been measured by means of the electrical conductivity at ordinary temperature and under pressures varying from 1 to 25 atmospheres (Pfeiffer); also at temperatures of 12·5° and 18°, and concentrations below 1/20 normal, due to atmospheric pressure (Knox, Walker, and Cormack).

(3.) A fairly satisfactory dissociation constant, agreeing with Ostwald's dilution law, has been calculated for small concentrations. The degree of ionization under ordinary conditions is very slight, so that H2CO3 is a very weak acid.

(4.) At 0° and under high pressures a hydrate of CO2 has been crystallized out, stable apparently only under certain corresponding conditions of temperature and pressure. The composition is doubtful—possibly CO28H2O.

Grounds of Research.

It will thus be seen that conductivity-determinations had been made only at the temperatures of 12·5° and 18°, and that high pressures had been employed by only one investigator, Pfeiffer, using from 1 to 25 atmospheres. One statement of Pfeiffer's seemed to call for investigation—namely, that at high pressures the conductivity was constant, being unaffected by change of pressure. This seemed so opposed to the ordinary behaviour of fairly concentrated solutions (a solution of CO2 at ordinary temperatures and under 25 atmospheres pressure being roughly normal) that it was thought worth making a series of conductivity-determinations at 18°, both to settle this point and to check with previous work.

The conductivity of CO2 solutions saturated under various pressures at 0° seemed to call for investigation, especially in view of the fact that at this temperature and high pressures a hydrate of CO2 had been shown to exist, the properties of which, including its composition, had been only vaguely determined (e.g., the discoverer, Wroblewski, stated in one abstract that at 0° the hydrate was formed at 12·3 atmospheres pressure, while in

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another the pressure was given as 16 atmospheres; even the composition was in doubt, being given as having 6, 8, or 9 molecules of water of hydration). It was anticipated that the formation of a hydrate should cause a very noticeable variation in the conductivity, and this anticipation was fully realized.

The work to be attempted consisted in—

(1.) Investigating the conductivities, at 18° C., of saturated solutions of CO2 in water, under pressures ranging from 1 to 30 atmospheres.

(2.) The same, if possible, at 0°—i.e., if formation of a solid hydrate did not interfere with the obtaining of a complete result.

(3.) Investigating, by means of the conductivity, the formation and properties of a CO2 hydrate at various low temperatures and at various pressures.

(4.) Determining the variation in conductivity of CO2 solutions due to a variation in temperature. (This could only be done to a limited extent in the available time.)

(5.) One or two other points of interest in connection with CO2 solutions came up during the course of work, and will be referred to.

Apparatus And Method.

The ordinary Kohlrausch method of determining the electrical conductivity of solutions was adopted, using induction coil and telephone. Bridge-wire and known resistances were calibrated.

Conductivity-Vessel.

A special form of closed conductivity-vessel had to be used in order to stand the high pressures of the experiment. It was made of gun-metal, as it was not considered that glass would stand the pressure used. The only disadvantage attending this was that in the later stages of the experiment the interesting changes taking place within the vessel were not visible to the eye. The diagram on next page shows the details of construction.

The casting consisted of two parts—the vessel B, and the head A, which screwed on to B by the thread C. At D, fitting on to B was a broad lead washer, by screwing the head tight down on to which the apparatus was rendered absolutely gastight, even at the highest pressure. The head was bored with two slightly coned holes E to receive the electrodes F, G, H, L, the tops of which were coned at the same angle. Over the coned tops F of the electrodes pieces of the best rubber tubing were tightly stretched, the electrodes being then pulled home into their sockets; this both made the electrodes perfectly steady and gastight, and also effectively insulated them. The electrodes, which were stout brass rods, tapered off from the shoulders J, and terminated in the platinum electrodes L. The exposed brass parts were encased in hard Jena-glass tubes which fitted tightly on to the shoulder at J. Into the ends H were soldered with silver-solder stout platinum wires, shielded by Jena-glass tubes sealed at K, and terminating in the electrodes L. The latter were made of heavy platinum sheet, measured each 20 mm. by 22 mm., and were placed from 8 mm. to 10 mm. apart, being closer at the bottom than at the top. They could be adjusted to a suitable distance by bending the platinum wires K, L. The ends of the electrodes protruding from the head were threaded to receive the nuts N and P. M was thick fibre washer, acting as an insulator; N a nut which was screwed down tight, thus pulling the electrodes fast into their sockets. P was another nut, which with N formed a binding screw for the lead from the electrode. The whole arrangement of electrodes was found to be quite steady and satisfactory from a conductivity point of view.

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The aperture Q in B received the pipe R, through which CO2 under pressure was forced into the cell. This pipe terminated in the flange S, which could be means of the nut W be forced home on a lead washer at T. The gas passed through a small hole in the washer, then through the passage U into the conductivity-vessel.

The details described made the vessel in practice perfectly gastight. The inside of the vessel was gold-plated, and an inner vessel of special Jena glass, manufactured for conductivity purposes, was placed within the outer plated one, which it fitted closely. As a further precaution, the part of the head near the shoulder J which was exposed above the solution was coated with insoluble enamel. (As a matter of fact, this was needless, as it was found in working that no water condensed on the head and upper part of the electrodes.)

Thus only the best Jena glass (beside the platinum electrodes) was in contact with the solution, and this glass was found to impart a definite but very small conductivity, which could be, and was, allowed for throughout the work.

Regulation of Temperature.

During the experiments conducted at 18° the electrolytic cell was immersed in a thermostat, fitted with a large toluol-mercury thermo-regulator and a centrifugal stirrer worked by a hot-air engine. This gave very satisfactory results, the temperature never varying by 0·05° on either side of 18°. For experiments at 0° the conductivity-vessel was immersed in crushed ice, which was frequently renewed. Probably the temperature of

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the vessel rose to 0·5° at times even when immersed in the ice; but before taking a final reading in any experiment the temperature was always kept (by constant stirring) exactly at 0° till the reading assumed a constant value. The difference in solubility of CO2 at the temperature slightly above 0° would be small, and would make (especially at higher pressures) a negligible difference in conductivity; so that by the above precautions it may be assumed that an accurate value of the conductivity was obtained. For the experiments at low temperatures other than 0°, ranging from—1° to 9°, the observer's continual presence was required to keep the temperature of the bath constant. The thermometer used was graduated to tenths of a degree, and the temperature was kept at the required point by the use below 0° of a salt solution and ice, and above 0° by adding small pieces of ice to the water, with constant stirring and watching the thermometer. In every case a final reading was taken only after keeping the temperature constant for a considerable time (at least thirty minutes), and making sure that readings were no longer varying.

Stirring, Etc.

Owing to the form of the conductivity-vessel, the position of the electrodes, and the high pressure employed, it was deemed inadvisable to attempt to employ a stirrer in the solution; nor could a thermometer be introduced. The large mass of metal in the apparatus, however, insured that the temperature inside was the same as that of the thermostat. The absence of a stirrer was a drawback from a time point of view, as is was found that saturation at any pressure of CO2, though aided by slight shaking of the apparatus and by convection currents, took twenty-four hours to accomplish.

Carbon-dioxide was obtained chemically pure from a bomb of liquid CO2.

Pressure-Gauges.

Two pressure-gauges used to determine the pressure of CO2. The dial gauge was used for higher pressures; it was a new one of 8 in. diameter, and previous to use in the experiments was checked by a standard gauge.

For pressures less than 5 atmospheres an open mercury gauge was used. It could be connected at pleasure with the apparatus by means of a needle valve, and by it pressures could be read off in millimetres of mercury with great accuracy. By means of a screw below the needle valve the pressure in the apparatus could at any time be relieved to the desired extent.

Conductivity-Water.

As the electrical conductivity even of fairly concentrated solutions of CO2 is very small—according to Pfeiffer, “about one-twentieth of that of spring water under normal conditions”—it was necessary to obtain by distillation water of special purity. Walker and Cormack, whose work has been referred to, obtained by three distillations a supply of water with a conductivity at 18° of 0·7 × 10-6 Siemens units, or about 0·75 × 10-6 reciprocal ohms. This appears to have been the best water used by any of the workers who have been referred to. Walker and Cormack worked with very dilute solutions where the conductivity was very small, and the relative effect of impurity in the water consequently great; it was therefore considered satisfactory for the purposes of the present experiments when a supply of water was obtained giving a conductivity of less than 1 × 10-6 ohms at 18° C. The supply was got by distilling in the open air with a modification of the Bousfield still. The distillate condensed on hard

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Jena glass and dropped straight into the special collecting-flask. The water so obtained kept very well, showing a slight improvement after storage in an atmosphere free from carbon-dioxide.

Preparation of Electrodes, Etc.

After the electrodes were in place they were cleansed thoroughly with chromic acid. They were then platinized in a solution of platinum-chloride and lead-acetate, a four-volt current being passed for twelve minutes, reversing every half-minute. To clean from the platinizing solution, they were then used as cathode for twenty minutes in dilute sulphuric acid, and afterwards thoroughly washed with distilled water. A good uniform coating of black was thus given, making the bridge-readings satisfactorily clear and definite.

The cell constant was determined in the ordinary way, using N/50 potassium-chloride solution as standard. (Here, as in the case of all solutions, the glass conductivity-vessel was filled up to a fixed height, sufficient to cover the electrodes half an inch or more.) Several determinations gave a mean value for the cell constant of 0·0833.

Experimental.

Altogether some six hundred readings were made, but of these only the final ones will be noted in this paper, unless for some special reason others leading up to them seem necessary. In making readings care was taken to note time, temperature, and gauge-pressure, as well as the bridge-reading.

Conductivities are expressed throughout in reciprocal ohms.

In most cases it was thought sufficient to determine the conductivity corresponding to every half-atmosphere pressure from 1 to 2 ½ atmospheres, and to every five atmospheres from 5 to 30 inclusive.

Pressures are given as absolute values.

Dial-gauge readings were obtainable to well within a tenth of an atmosphere; mercury-gauge readings to one five-hundredth of an atmosphere.

Series I.

Temperature constant = 18° C. (× 0·05°) in thermostat; stirrer working continuously; cell immersed.

As fluctuations about the mean temperature were very small, producing only negligible variations in conductivity, the temperature throughout taken as 18° C.

Allowance made for half the conductivity of the water in calculating the conductivity of the solution; this done throughout. Calculations made as follows: If R denote the fixed resistance used, R′ the resistance of the solutions, c the conductance (= 1/R), C the specific conductance or conductivity, K the cell constant, x the bridge-reading :— therefore R:R′=x:100-x
c=x/R(100-x)
and C=Kx/R(100-x)

Work done with descending pressures. Air withdrawn through mercury-gauge valve by a Fleuss pump. Valve then closed, and CO2 allowed to fill the apparatus. This in turn withdrawn, and process repeated several times till all air thus swept out. Pressure put up to 30 atmospheres.

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The following is a table of results (temperature 18° C.) :—

Pressure: Atmospheres. Final Bridge-reading. Resistance. Conductivity.
30 40·6 300 188·6 × 10-6
25 40·2 300 185·4 × 10-6
20 38·32 300 171·2 × 10-6
15 35·67 300 152·6 × 10-6
10 32·1 300 129·9 × 10-6
5 37·75 500 99·75 × 10-6
31·85 500 76·59 × 10-6
2 30·35 500 71·35 × 10-6
28·55 500 65·33 × 10-6
1 27·2 500 60·98 × 10-6

The values obtained for the conductivity at different pressures disprove Pfeiffer's statement that conductivity does not vary with increase of pressure. These values lie on a regular curve which ascends very sharply from 0 to 1 atmosphere, and thereafter much more gradually. The influence of concentration on conductivity decreases fairly rapidly as the concentration increases, but does not vanish even at the highest pressure.

Series II.

Temperature = 0° C.

Work was begun with ascending pressure, and a complete range of values obtained. Subsequent work was under very varying conditions of temperature and pressure, such being called for by the observed phenomena.

The following table gives the first results (temperature 0° C.) :—

Pressure: Atmospheres. Final Bridge-reading. Resistance. Conductivity.
1 28·4 1000 32·35
31·6 1000 37·78
2 35·0 1000 44·15
37·4 1000 49·06
5 29·15 500 67·82
10 34·85 500 88·38
15 38·0 500 101·4
20 39·35 500 107·4
25 41·0 500 115·1
30 41·8 500 118·9

So far the behaviour corresponded exactly to that at 18°, the conductivity being, of course, considerably lower. Wroblewski was said to have discovered a hydrate formed at 0° under pressures given as 12·3 or 16 atmospheres of CO2, and it was anticipated that if such a hydrate existed there should be abnormal behaviour of the conductivity at these pressures; but such was not the case.

The pressure was now let down, the intention being to work with descending pressures over the same ground. For all future work the resistance used in the box was 500 ohms.

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The following was the result :—

Date. Time. Pressure: Atmospheres. Bridge-reading. Conductivity.
Sept. 2 11 a.m. 25 41·35 116·7
" 3 6·15 p.m. 20 33·0
" 4 12 noon. 20 32·65 80·04
" 4 10 p.m. 18 32·85*
" 6 4 p.m. 18 31·85 77·14
" 7 12·45 p.m. 16 30·8 73·45
" 8 9·30 p.m. 14 30·9 73·8
" 9 12·10 p.m. 12 30·9 73·8
" 9 10·40 p.m. 10 32·1
" 10 9·30 a.m. 10 31·9
" 10 6·30 p.m. 10 32·4
" 12 1 p.m. 8 33·65 83·8
" 13 10·35 p.m. 5 30·2 71·4

It is evident that somewhere between 25 and 20 atmospheres pressure of CO2 a sudden change occurred in the nature of the solution, the conductivity undergoing a remarkable drop. As other experimenters had obtained evidences of a hydrate of CO2 existing at low temperatures and high pressures, it was assumed that the change in conductivity was due to the formation of the hydrate.

The values of conductivity obtained at 25, 8, and 5 atmospheres pressure show that, apart from the influence of the hydrate, the conductivity of the solution under descending pressures follows closely that under ascending pressures, being in each case somewhat higher.

The remainder of the experiments consisted in an attempt to determine, by varying both pressure and temperature, such properties of the hydrates as capacity for super-cooling, temperature of transition under various pressures, degree of variation with temperature of conductivity of hydrate or of solution, &c. Though consecutive with Series II, they will for convenience be designated Series III, and in most cases details will be omitted.

Series III.

A. An attempt to determine the pressure under which formation of the hydrate would take place. Temperature, 0°.

Pressure very gradually raised to 30 atmospheres, with the following results :—

Time. Pressure: Atmospheres. Bridge-reading. Conductivity.
10 34·13 85·6
11 35·25 90·0
12 35·9 92·6
13 36·45 94·8
14 36·95 96·9
15 37·35 98·6
16 37·9 100·9
17 38·2 102·3
11 a.m. 18 39 (about)

[Footnote] * Temperature, 0·05°

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Hydrate not yet formed. Between 11 a.m. and 12·25 p.m. temperature kept at 0°. At 12·25 p.m. the following reading was obtained :—

Time. Pressure: Atmospheres. Bridge-reading. Conductivity.
12·25 p.m. 18 34·8
12·35 p.m. 34·7
12·45 p.m. 34·6
12·55 p.m. 34·55
1 p.m. 34·4
4·15 p.m. 34·25
10·45 p.m. 34·2 85·9

During the formation of the hydrate the pressure remained constant.

Time. Pressure: Atmospheres. Bridge-reading. Conductivity.
19 34·2 85·9
20 33·9 84·8
21 33·8 84·4
23 33·9 84·8
25 33·2 82·1
30 32·95 81·2

B. Pressure released from 30 to 25 atmospheres, temperature being kept low. A series of determinations of the conductivity with varying temperatures, at the constant pressure of 25 atmospheres. The hydrate decomposed between 4° and 10°, and was not re-formed when the temperature was lowered.

C. Temperature having been kept some hours at 0°, and hydrate apparently re-formed, the following series of readings was made (pressure = 25 atmospheres) :—

Temperature. Bridge-reading. Conductivity.
33·2 82·8
40·15 111·8
42·1 121·2
43·7 129·3
45·5 139·1
47·55 151·0
48·55 157·1
13°* 50·9 172·7
18° 185·3

[Footnote] * Previously determined.

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Each degree makes an average increase of 8·5 × 10-6 in the conductivity, till at about 8·2° a sudden break in the conductivity curve takes place, presumably due to all the hydrate having been decomposed. Thereafter the effect of raising the temperature is much less, on an average about 4 × 10-6 ohms per degree.

D. Pressure = 25 atmospheres. Readings with descending temperatures :—

Temperature. Bridge-reading. Conductivity.
46·7 146·0
42·7 124·2
2·1° 41·05 116·0
37·35 99·3
35·75 92·7
34·0 85·8
32·0 78·4

The hydrate formed suddenly at about 2·1° C.

E. Pressure released from 25 to 20 atmospheres, and a complete and consecutive series of determinations similar to the last made, using both ascending and descending temperatures. The hydrate which had been formed at 25 atmospheres was kept in existence by keeping the temperature low. The following results were obtained (pressure = 20 atmospheres) :—

Temperature. Bridge-reading. Conductivity.
31·2 75·5
32·35 79·7
33·75 84·8
36·5 95·7
37·95 101·9
(a.) 38·8 105·6
42·7 124·2
44·2 131·9
46·5 144·7
47·2 148·9
11° 48·8 158·8
46·4 144·2
43·6 128·8
41·75 119·3
40·8 114·8
39·8 110·1
(b.) 38·75 105·4
33·95 85·7
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(c.) The temperature was now raised again.

Temperature. Bridge-reading. Conductivity.
40·3 112·4
41·8 119·6
43·2 126·7
46·5 144·7

The conductivity increased regularly with the temperature up to about 5 ½°, when a remarkable variation made itself manifest. The conductivity first went down, and then suddenly rose, to the extent of over 2 cm. in the bridge-reading. This was repeated several times, the fluctuations gradually diminishing, the conductivity eventually assuming a constant value, much higher than would have been assumed if following the same law of variation as from 0° to 5°. This is shown in a graph by a vertical line at 5 ½°. (During the fluctuations referred to the sound in the telephone was most interesting; the decrease in bridge-reading was fairly slow and uniform, but the increase was instantaneous, the telephone suddenly sounding a very loud note, and requiring the sliding contact to be moved a good deal higher before again being quiet.)

During the decomposition of the hydrate the indicated gauge-pressure rose by half an atmosphere. This is explained by the fact that the hydrate was formed at a higher pressure (25 atmospheres), and when it decomposed would yield an excess of CO2.

From 5 ½° to 8° the conductivity continued to increase much in the same way as before, but then a break occurred, the conductivity thereafter increasing considerably more slowly with the temperature.

In lowering the temperature from 11° to 0° the conductivity obeyed the latter law of variation. It was therefore assumed that the solution remained as such, no change to hydrate form, with the corresponding phenomena, having taken place.

At (b), about 0°, the conductivity suddenly began to fall at an abnormal rate, eventually assuming the value given for 0°. This was taken to indicate that the formation of the hydrate had taken place.

The remaining part of the experiment (from c) was conducted several hours later. The pressure was in the meantime 20 ¾ atmospheres, not having been relieved after the evolution of the excess of CO2. This will doubtless explain why the conductivity from 5 ½° to 8° was in this case slightly greater than in the former part of the experiment.

It was found here that there was no definite and sudden rise of conductivity, as there had been before. The graph of the conductivity is a continuous curve, almost a straight line, merging at 8° into the ordinary solution conductivity graph. Thus it seems that the abrupt change is characteristic only of those conditions under which a kind of stress exists in the hydrate, owing to its having been formed at a higher pressure than exists when it is about to decompose.

F. Temperature lowered to 0°; and, after bridge-reading had indicated formation of hydrate, pressure lowered to 15 atmospheres. Thus there would be in existence at 15 atmospheres a hydrate formed at

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20 atmospheres, and the conditions would be similar to those of the last experiment. The values of the conductivity are given below, further explanation being unnecessary, the behaviour being quite similar to that under experiment E.

Temperature. Bridge-reading. Conductivity.
33·2 82·8
35·9 93·3
37·2 98·7
* 39·9 110·6
43·9 130·3
45·3 137·9

Conclusions.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

1. At both 0° and 18° solutions of CO2 in water are feebly ionized (about 1/250 in the case of a N/20 solution at 18°); the conductivity increases with the pressure or concentration of CO2 from 1 to 30 atmospheres, the proportional increase, however, becoming less the greater the concentration. Thus at 0° the conductivity at 4 ½ atmospheres is about double, at 14 atmospheres about treble, and at 30 atmospheres less than quadruple its value at 1 atmosphere pressure.

This behaviour disproves the statement ascribed to Pfeiffer, that “change of pressure produces no alteration in the conductivity.”

2. Temperature has a large effect on the conductivity, which at 8° to 9° is increased per degree by about 2·9 per cent. of itself.

3. After a CO2 solution under high pressure has been kept for a few days there is a tendency for the solution to acquire a kind of chemical fixity or supersaturation, as shown by an increased value of the conductivity. Probably the CO2 enters slowly into combination with water, the compound in its turn being only slowly decomposed.

4. A compound of CO2 with water is formed under very varying conditions at low temperatures and high pressures.

(a.) The conductivity of the hydrate-bearing solution is considerably less than that of the free solution under similar conditions.

(b.) It crystallizes out with difficulty, there being a great tendency to a state in the solution resembling supersaturation with respect to the hydrate.

On one occasion the pressure was raised to 30 atmospheres and reduced again to between 25 and 20 atmospheres before the hydrate would form; on another it was formed in an apparently arbitrary way after the pressure had been for some time at 18 atmospheres; it was also formed at 15 atmospheres, and on another occasion could not be induced to form at all.

This tendency to supersaturation, and the element of chance consequent upon it, probably explains Wroblewski's varying observations, giving 12·3 atmospheres in one place and 16 atmospheres in another as the pressure under which the hydrate formed at 0°.

[Footnote] * In this case the decomposition of the hydrate took place at 3°, being accompanied by the same phenomena of fluctation, &c., as in the last experiment, but on a smaller scale. There was also a second break at about 6°, the conductivity thereafter varying much less with temperature.

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(c.) The conductivity of the hydrate-bearing solution, as has been stated, is less than that of the corresponding free solution, but it increases more rapidly than that solution in conductivity as the temperature is increased, eventually becoming equal to it in conductivity at some definite temperature (varying in the experiments performed from about 6° at 15 atmospheres to 8° at 25 atmospheres pressure). Henceforward the conductivity follows the conductivity-temperature law of the ordinary solution.

The similarity between the curve which expresses this behaviour and the vapour-pressure curves of ice and water intersecting at the transition-point is suggestive. Just as water can be supercooled, and its vapour-pressure curve produced below 0°, so the CO2 solution seems capable of supercooling and the solution temperature-conductivity curve can be produced below the true transition-point.

(d.) As far as such evidence as was obtained can be relied on, in conformity with the statements in (b) above, the crystallization-point seems to occur at a lower temperature the lower the pressure: thus, at 25 atmospheres the formation of the hydrate took place at 2°, at 20 atmospheres pressure at 0°.

(e.) A hydrate formed at one pressure can be preserved at a lower pressure if the temperature be kept low; but when it melts, which it does at a definite temperature, it does so completely and rapidly, causing a sudden rise in the conductivity and increase in the pressure, due to evolution of the extra CO2 held. The melting-point rises in such cases with increase of concentration: thus, the hydrate formed at 20 atmospheres and decomposed at 15 atmospheres melted at 3°, while that formed at 25 atmospheres and decomposed at 20 atmospheres melted at 5 ½°. This behaviour agrees fairly well with the observations of Hempel and Seidel, who found that the hydrate formed by them at a very high pressure—i.e., the vapour-pressure of liquid CO2—melted under this pressure at 8° and under atmospheric pressure at—2°.

(f.) At 0° C. the hydrate is decomposed at between 8 and 10 atmospheres pressure.

(g.) Wroblewski's statement that the hydrate is “capable of existing only under certain pressures, increasing with the temperature,” is inaccurate, as the hydrate has been formed or kept in existence at 0° under pressures varying from 9 or 10 to 30 atmospheres. There is apparently a lower pressure-limit, but not a higher.

(h.) It seems certain from the facts mentioned in (e) that the composition of what has been styled “the hydrate” varies. There was no means of determining, however, whether more than one hydrate was formed.