Factors affecting the measurement results of the melting point
1. Effect of heating rate on the measurement results
No matter which melting point measuring method is used, the temperature can not be obtained directly from the test sample. The classic method is conduct to the mercury thermometer by the heat transfer oil, and modern melting point apparatus obtain the temperature value from the metal furnace. Thus, there is a certain difference between the measured and the actual temperature of the sample. Meanwhile, in theory, in the limited time of the melting process of a pure sample, the temperature should remain constant, but in fact the sample amount is tiny, and the furnace has a great heat capacity, therefore, the temperature of the furnace will continue to rise linearly in the measurement process. The higher heating rate of the furnace is, the higher melting point will be obtained.
Thus, a reasonable choice of heating rate before the melting point measuring is very important. When accuracy can not meet the requirements, don’t select a very high rate of heating.
2. Effect of heating rate on the reproducibility of the melting point measurement
The higher heating rate is, the worse reproducibility of measuring will be. This phenomenon can be explained by the phase diagrams of a binary mixture. The liquid components are different at different temperatures during the melting process of measured samples. At low heating rates, the liquid phase component can be sufficiently diffused, the optical performance is more consistent, and the photodetector result is accessible; at high heating rates, the liquid can not sufficiently diffuse and the performance is inconsistent, making the measured value fluctuates up and down. On the other hand, due to difficulties in thermal equilibrium when the rate is increased, the difference of sample quantity and solidness degrees, as well as the contradiction of different sizes of outer and inner diameter of capillary is more prominent, the discreteness of measurement is increased. Therefore, only at low heating rates can relatively reliable results be got.
3. Effect of purity sample of reproducibility
The reproducibility of the results of melting point measurement is also related to the purity of the sample itself. With the decrease of the purity or the increase of the contaminants, the melting point is obviously decreased (it has been proved that purity within the range of 95% -100%, the melting point and purity are linear, and under special circumstances, when the purity arrives at 90%,it begins to show a linear relationship), consistency of readings will be significantly worsened. Accordingly, there should be some knowing in advance about the purity of the sample before each measurement, or the confidence of the experiment is hard to say.
4. Effect of initial temperature of the melting point
The initial temperature tends to affect the results of melting point measurement, but situations are changing among different samples. A high initial temperature and heating rate often makes it too late to transfer heat, thus leading to a high melting point values. For some erratic samples, a low initial temperature and long warm-up time will produce decomposition and sublimation phenomenon after heating, which makes the tested melting point too low.