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Volume 43, 1910
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Art. VI.—Depression of the Freezing-point of Water by Carbon-dioxide in Solution.

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

This paper contains a brief account of the results of two series of experiments on the freezing-point of water containing carbon-dioxide in solution. The only work done in the same direction previously appears to be that of Garelli and Falciola.*

In the experiments now described the freezing was carried out in the thick glass tube A of the diagram, 28 mm. external diameter, widened slightly at the top, and sealed into the brass head B by means of a linen-tape collar and marine glue. B was connected by a copper tube D with the brass chamber E, and so with the pressure-gauge P, the carbon dioxide bomb, the atmosphere through H, and an open mercury mano-meter M through the needle valve N [not shown in figure].

The water in A was stirred by a silver-wire stirrer soldered to a short cylinder of sheet iron 22 × 20 mm., which, after silver-plating, was covered with cycleenamel and baked. This stirrer-ring enclosed the thermometer, and was raised and dropped by an intermittent current flowing in a coil of No. 18 copper wire, in 15 layers of 41 turns each, fitting loosely round the top of the glass freezing-tube. The current, 2–3 amperes, was adjusted by a rheostat, and made and broken (flowing about one-third of the time) by a wire dipping into a mercury-cup, and attached to a pendulum making 32 swings per minute.

[Footnote] * Atti. R. Accad. Lincei, 1904 (v), 13, 1, 110–18. Abstract in Journ. Chem. Soc., 1904, vol. 86, ii, p. 312.

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The temperature of tube and contents was controlled by a weak ice-and-salt solution in a felt-covered glass jar. The Beckmann thermometer was fitted by a stretched rubber covering-tube into a tapered hole, 13–15 mm. diameter, in a circular brass cap C, 62 mm. diameter, having a square top and screwing into a female screw on the head B, provided also with a recess containing a lead washer engaging with the flange projecting from the top of B.

The zero-point of the thermometer was found by inserting the bulb in distilled water contained in an ice sheath prepared by freezing distilled water round a Jena-glass tube and releasing the latter by warm water. A normal thermometer reading to 0·02° C., and known to be without appreciable error throughout the range employed, was treated similarly, and the two carefully compared up to about 5° C. in a continuously stirred water-bath of rising temperature.

The pressure-effect on the thermometer was considerable, and for the first thermometer used was not satisfactorily found, owing to its accidental destruction. Hence the first series of twenty experiments, otherwise perhaps the best, are not given here. For the second thermometer the pressure-effect was determined by filling the experimental tube with mercury, raising the pressure to the maximum, and taking simultaneous readings on thermometer and manometer as the pressure was progressively reduced. The curve plotted from the results is nearly a straight line. It is assumed that this line can be continued backwards to a point of no pressure.

On setting up the apparatus all air was driven out of the connecting tubes by a stream of carbon-dioxide, and out of the freezing-tube by saturating the contained water at 80 lb. pressure and removing the pressure. The gas evolved from the water in four or five such operations completely expelled the last traces of air. In each experiment, to insure saturation, the freezing-tube A was surrounded by ice-water, and the contained water stirred continuously some 15 to 30 minutes at a pressure about 10 lb. above that at which readings were to be taken. The gas was then allowed to escape until bubbles were evolved from the water. When, under continuous stirring, the manometer no longer rose, owing to gas evolved from solution, the temperature was reduced by a freezing-bath some 5° to 7° below final freezing-point. The formation of ice (in flakes throughout the solution) was followed by a sudden change in the click of the stirrer and by a rapid rise in the Beckmann thermometer. With the first thermometer used the temperature usually came to a final steady value, while the pressure slowly rose after freezing had progressed a little, owing to the gas frozen out of solution.

In the experiments taken as trustworthy the rise of pressure during freezing never amounts to more than 1 cm. of mercury. The pressure used in calculation is that shown during the steady period of thermometer, or an average of the pressures recorded if they varied for that period.

Series A, with Original Thermometer (20 Experiments).

Results are of qualitative value only, as, owing to an explosion, the pressure-effect on thermometer was not accurately determined.

Details of one complete experiment are given to show the method employed throughout the work. In Series B and Series C tables of corrected results only are given.

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Freezing-point No. 2, Series A.

The freezing-tube surrounded with ice, and stirred under a slight excess of pressure for 15 minutes. Pressure then released by opening H. Gas given off from the water showing solution to be saturated. Stirring continued for 10 minutes, when rise of manometer no longer evident, and then cooling-bath brought under. Thermometer-readings every 15 seconds. Stirrer in motion throughout.

Thermometer. Manometer. Pressure: b-a Thermometer. Manometer. Pressure: b-a
a. b. a. b.
0·665 214 621 407 0·632
0·540 0·634
0·459 0·637
0·421* 215 621 406 0·638 212 623 411
0·460 0·639
0·530 0·639
0·565 0·639 212 623 411
0·590 213 622 409 0·639
0·609 0·639
0·617 0·639
0·621 0·639 212 623 411
0·629 213 623 410
Atmospheric pressure 763 mm
CO2 pressure 411 mm
     Total 1174 mm
Atmospheric temperature 10° C
Pressure corrected for room-temperature 1172 mm
Freezing-point 0·63
Correction of thermometer for pressure 0·22
Corrected freezing-point 0·41
Thermometer zero 0·58
Depression in degrees Beckmann 0·16
Depression in degrees Cent. 0·15
Depression calculated 0·142

Series B, With New Thermometer (19 Experiments).

This thermometer being rather short, the top of bulb came within 5 mm. of the lower plane of the stirrer-solenoid, while the water of the freezing-tube came up to this plane and was subject to some heating from the coil. The freezing-points were, in consequence, less definite than in Series A, the heating due to the stirrer-coil keeping the upper part of water always free from ice, and the temperature always showing a tendency to creep up.

Nineteen experiments were made, with total pressure ranging from 3277 mm. to 1432 mm. of mercury.

[Footnote] * Freezing began at this point by formation of ice in flakes.

[Footnote] † After 3 minutes' stirring the temperature was again read. Two successive readings of 0·639 were obtained.

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Table of Results, Series B, with Stirring.
Total Pressure of Carbon-dioxide in Millimetres of Mercury, corrected for Room-temperature. Observed Depression/Calculated Depression=i.
1432 0·88
1563
1622 0·70
1681 0·70
1688 0·84
1822 0·87
1897 0·85
2185 0·88
2189 0·76
2198 0·79
2218 0·87
2290 0·76
2342 0·76
2423 0·92
2670 1·14
2792 0·90
2794 1·13
2831 1·01
3271 1·16

The results, on the whole, show that the depression of the freezing-point for a stirred solution is about that which would be expected were all the dissolved gas present as carbon-dioxide molecules, and that at the higher concentrations a measurable amount of dissociation takes place.

Series C.

To avoid heating by the coil, now recognized as a disturbing factor, the latter was removed after saturation and the experiment was continued without further stirring. In all the experiments of this series (13) little gas was given off during freezing, even when this was complete. When the freezing-bath was removed gas was given off copiously from the solid ice, which appeared either to hold the gas in a solid solution or to be associated with a solid hydrate which decomposes on melting.

Table of Results, Series C, without Stirring.
Total Pressure of Carbon-dioxide in Millimetres of Mercury, corrected for Room-temperature. Observed Depression/Calculated Depression=i.
752 1·19
762 1·05
1853 1·05
1979 1·32
2017 1·29
2195 1·21
2300 1·25
2320 1·07
2337 1·25
2412 1·10
2545 1·09
2696 1·29
2965 1·32
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Conclusions.

The results seem to indicate—

(1.) That for a solution which is stirred while freezing the coefficient (i) is less than 1 for the lower pressures, and, increasing with rise of pressure, becomes greater than 1 at the higher pressures.

(2.) That for solutions not stirred the coefficient is always greater than 1, and increases with increasing pressure.

The only quantitative result found amongst earlier work is given by Garelli. He found that 0·35 grammes carbon-dioxide per 100 grammes water gave a depression of 0·165° C. The author's value for the depression at this concentration is 0·169° C., which agrees fairly well with that of Garelli.