Introduction

Thermogravimetry, TG, measures samples weight change to change of temperature and time function. The sample is exposed to controlled temperature program in controlled atmosphere.^[1] When a compound crystallizes from solution, the crystals may contain solvent of crystallization. The compound is the called a solvate, and if the solvent is water, the compound is a hydrate. The presence of the solvent is revealed by elemental analysis and thermogravimetric analysis shows whether the solvent molecules are loosely or strong bound in the crystal lattice. By heating the material, it shows weight decreasing or increasing. It is used to investigate the thermal degration of inorganic compounds, polymers or the gas uptake of a solid. It is able to simultaneously heat and record the mass of a sample. The gas in TG is inert, reducing, oxidizing or corrosive. Sample is usually heated in air or nitrogen gas. Reactive gas in atmosphere is also used when studying the uptake of a particular gas.^[2] Thermogravimetry temperature working range is from 20 °C to 1000 °C. It can rase to higher temperatures with combination of other apparatus.^[3] In figure 1 there is picture of Renotechs TGA.

Thermogravimetry Apparatus

Thermogravimetry is the apparatus with thermogravimetric analyses are done. The main parts of TG are thermobalance, oven, censors, controlled atmosphere system and computer. The thermobalance is for weighting the sample and it is isolated so that the there is no temperature leak.^[4] For heating TG to wanted temperature it uses oven. Censors are needed to detect the temperature in oven. Sample is placed to the balance and it is heated with the reaction gas. The heat is transferring to the sample by radiation and conduction. To smaller mass of the sample the heat transfer is faster.  The physical and structural shape of sample effects to the speed of products diffusion in the sample. In bigger sample size the reaction first only happens in the surface.^[3][4]

Factors of errors:

• Mass of the sample
• Physical shape of the sample
• Structure of the sample
• Buoyance effect^[3]

Applications

Thermalgravimetry analysis is used for materials and coumpounds thermal stability testing. It is highly used in polymer industry for example determination of elastomer by its pyrolyse temperature. It is also used to analyse the inorganic compounds from polymers. Ceramics have silicon carbone which can take 1300 °C. With TGA the oxidation resistance can be tested. The oxidation or reduction behavior is used to determinate metals or alloys. Oxidation losses are the most common ways to investigate samples. Analysing copper is one of the most important that is done by TG, because copper can be possible used in compustion engines, where engine is heated and the fuel occurs with oxidizer.^[3][5]

Example of Analysis

Example of an TG analysis of Three-stage decomposition of CaC₂O₄·H₂O:

Figure 2. Data of hydrated calcium oxalate (Figure: Paula Kivelä)

There are three steps in the decomposition of CaC₂O₄·H₂O. The first weight change is from 100 % to 87.68 %. The first weight loss is 12.3 %, where it corresponds to a mass of 18.0 g. M(H₂O) = 18.02 gmol^-1, the lowest temperature thermal event is loss of H₂O. The next weight change was from 87.68 % to 68.52 % and the weight loss was 19.2 %, where it corresponds to a mass of: 28.1 g. The second weight loss was carbon oxide because it has molarmass of 28.01 gmol^-1. The third weight change was from 68.52 % to 38.40 %. The weight loss was 30.1 % and the corresponds to a mass of 44.0 g, which correponds to loss of carbon dioxide.The steps:^[2]

References