There are two types of "water" commonly used in laboratories: free water and crystalline water. Free water is ordinary water that evaporates when heated. Crystalline water is hidden inside the crystal and binds more tightly to the substance, only coming out at higher temperatures.Domestic comprehensive thermal analyzerBy placing thermogravimetric (TG), differential thermal analysis (DTA), or differential scanning calorimetry (DSC) in the same machine, both mass and heat changes can be measured simultaneously in one experiment to distinguish the amount of each type of water.
1、 Instrument structure
The main unit of the comprehensive thermal analyzer includes a precision balance, furnace, temperature control module, and gas system.
2. The accuracy of the balance is generally 0.1 μ g, and it can record the weight of the sample in real time.
3. The furnace is heated with a platinum resistor and can be continuously adjusted from room temperature to 1200 ℃.
4. Domestic models commonly use alumina or platinum rhodium crucibles, which are acid and alkali resistant and have low costs.
2、 Experimental steps
1. Sample weighing: Take 10-20 mg of powder and evenly spread it in the crucible, without being too thick.
2. Program setting: Raise the room temperature from 10 ℃/min to 200 ℃, and then to 600 ℃ or higher.
3. Ventilation: Nitrogen is used to measure free water, and air or inert gas can be used to measure crystalline water to avoid oxidation interference.
4. Recording: The software simultaneously draws TG curve (mass temperature) and DSC curve (heat flux temperature).
3、 How to read the results
The TG curve shows a decrease, corresponding to the evaporation of free water, and the temperature range is usually between 30-120 ℃.
The degree of decrease is the quality of free water.
3. If the temperature continues to rise and there are one or more drops in the higher temperature zone, it means that the crystal water is removed step by step.
4. Add up these two quality losses to obtain the total moisture content; Calculate the first paragraph separately, which is the free water content; Subtract the two to obtain the crystal water content.
5. The DSC curve shows an endothermic peak at the dehydration temperature point, and the peak area can be converted into dehydration enthalpy to verify the TG data.
4、 Advantages of Domestic Instruments
1. Low cost: The price is much lower compared to imported machines of the same grade.
2. The software has a Chinese interface that automatically provides the weight loss percentage and interval temperature, without the need for manual integration.
3. Fast after-sales service: Domestic manufacturers can provide short-term on-site after-sales service, with a short replacement cycle.
5、 Precautions
1. The sample size should not be too large. If there is too much, the heat transfer will be slow, the peak shape will be trailing, and it will be difficult to distinguish between the two stages of water loss.
2. The commonly used heating rate is 10 ℃/min; Too fast will mix the peaks of free water and crystalline water together.
3. Drill holes or partially open the crucible cover to allow the water vapor to dissipate in a timely manner, avoiding false peaks caused by secondary condensation.
4. Regularly calibrate with standard substance (calcium oxalate dihydrate) to ensure a quality error of ≤± 1%.
The domestic comprehensive thermal analyzer separates free water and crystalline water according to temperature ranges through a single heating process, and the quality changes are clear at a glance. Easy to operate and reliable in data, it has been widely used in fields such as building materials, minerals, and pharmaceuticals.