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7 Analysis steps
The crystal tube, thermometer, and stirrer used in the measurement must be clean and dry
For the determination of the crystallization point of a liquid sample, the measuring device using a Dewar flask as a cooling bath is shown in Figure 2
For the determination of the crystallization point of a solid sample, a measuring device using a beaker as a cooling bath is shown in Figure 3
Pour the sample into the crystallization tube, adjust the sample liquid level, about 15mm above the upper edge of the middle bubble of the main thermometer, and the sample filling height is about 60mm
When the temperature of the sample drops to 3°C higher than the crystallization temperature, start the stopwatch, record the time and temperature, and start stirring up and down at the same time, moving up and down 30mm, the stirring speed is about 60 times/min
When measuring the sample with supercooling phenomenon, the temperature begins to drop below the crystallization temperature, and then quickly rises naturally to reach a certain maximum temperature (stop stirring at this time), and stay at this temperature for a period of time (see Figure 4), and then the temperature again Descent again
Before the temperature rises naturally, the minimum temperature should not be 3°C lower than the measured junction point
When measuring a sample without supercooling, during the temperature drop, the temperature remains constant for a certain period of time (the stirring should be stopped at this time) and no longer rises, and then drops again (see Figure 5).
Figure 5 Cooling curve of samples without overcooling
During the measurement process, when the temperature drops to 3°C lower than the crystallization point and the sample is still in a liquid state, it means that overcooling has occurred.
8 Expression and calculation of analysis results
8.
The temperature recorded during the measurement is the ordinate and the time is the abscissa, and the cooling curve is drawn
8.
It is not necessary to record the temperature and time during the measurement process, and it is directly observed that the highest temperature in the constant phase is the crystallization point of the sample
8.
3 When there is a dispute over the measurement result, the cooling curve method shall prevail
.
8.
4 Calculation of analysis results:
The crystallization point (t) is calculated as follows:
t=t 1 +△t 1 +0.
00016h(t 1 -t 2 )
In the formula: t-reading of the main thermometer, depending on the crystallization point, ℃;
△t 1 —The correction value of the main thermometer, ℃;
t 2 —The reading of the auxiliary thermometer, ℃;
h—The difference between the reading of the main thermometer exposed to the liquid level of the sample and the reading of the apparent crystallization point;
0.
00016—The apparent expansion coefficient of mercury
.
Take the arithmetic mean of the two parallel determination results as the determination result, and the difference between the two parallel determination results shall not exceed 0.
1°C
.