Part 29 (1/2)
GULDBERG-WAAGE CURVE AT 100.
----------------------+-----------------------+-----------------+-------
100 moles of water
K2CO3 Solid phases.
contain, in moles,
[Sigma]_k__{2}
-----
K2SO4
K_{2}CO_{3}
K_{2}SO_{4}
----------------------+-----------+-----------+-----------------+------- BaCO_{3} + K_{2}SO_{4}
23.9
12.65
35.65
1.82 + BaSO_{4}
BaCO_{3} + BaSO_{4}
6.28
2.02
8.3
3.1 ” ”
3.17
0.851
4.025
3.7 ----------------------+-----------+-----------+-----------------+-------
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APPENDIX
EXPERIMENTAL DETERMINATION OF THE TRANSITION POINT
For the purpose of determining the transition temperature, a number of methods have been employed, and the most important of these will be briefly described here. In any given case it is sometimes possible to employ more than one method, but all are not equally suitable, and the values of the transition point obtained by the different methods are not always identical. Indeed, a difference of several degrees in the value found may quite well occur.[398] In each case, therefore, some care must be taken to select the method most suitable for the purpose.
I. The Dilatometric Method.--Since, in the majority of cases, transformation at the transition point is accompanied by an appreciable change of volume, it is only necessary to ascertain the temperature at which this change of volume occurs, in order to determine the transition point. For this purpose the _dilatometer_ is employed, an apparatus which consists of a bulb with capillary tube attached, and which const.i.tutes a sort of large thermometer (Fig. 129). Some of the substance to be examined is pa.s.sed into the bulb A through the tube B, which is then sealed off. The rest of the bulb and a small portion of the capillary tube is then filled with some liquid, which, of course, must be without chemical action on the substance under investigation. A liquid, however, may be employed which dissolves the substance, for, as we have seen (p. 70), the transformation at the transition point is, as a rule, accelerated by the presence of a solvent. On the other hand, the liquid must not dissolve in the substance under examination, for the temperature of transformation would be thereby altered.
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In using the dilatometer, two methods of procedure may be followed.
According to the first method, the dilatometer containing the form stable at lower temperatures is placed in a thermostat, maintained at a constant temperature, until it has taken the temperature of the bath. The height of the meniscus is then read on a millimetre scale attached to the capillary.
The temperature of the thermostat is then raised degree by degree, and the height of the meniscus at each point ascertained. If, now, no change takes place in the solid, the expansion will be practically uniform, or the rise in the level of the meniscus per degree of temperature will be practically the same at the different temperatures, as represented diagrammatically by the line AB in Fig. 130. On pa.s.sing through the transition point, however, there will be a more or less sudden increase in the rise of the meniscus per degree (line BC) if the specific volume of the form stable at higher temperatures is greater than that of the original modification; thereafter, the expansion will again be uniform (line CD). Similarly, on cooling, contraction will at first be uniform and then at the transition point there will be a relatively large diminution of volume.
[Ill.u.s.tration: FIG. 129.]
[Ill.u.s.tration: FIG. 130.]
If, now, transformation occurred immediately the transition point was reached, the sudden expansion and contraction would take place at the same temperature. It is, however, generally found that there is a lag, and that with rising temperature the relatively large expansion does not take place until a temperature somewhat higher than the transition point; and with falling temperature the contraction occurs at a temperature somewhat below the transition point. This is represented in Fig. 130 by the lines BC and EF. The amount of lag will vary from case to case, and will {332} also depend on the length of time during which the dilatometer is maintained at constant temperature.
As an example, there may be given the results obtained in the determination of the transition point at which sodium sulphate and magnesium sulphate form astracanite (p. 268).[399] The dilatometer was charged with a mixture of the two sulphates.
-------------------------------------------------------- Temperature.
Level of oil in capillary.
Rise per 1.
-------------------------------------------------------- 15.6
134
16.6
141
7 17.6
148
7 18.6
154
6 19.6
161
7 20.6
168
7 21.6
241
73 22.6
243
2 23.6