Roberl Delhez lnstitut d e Chimie
Lisge, Belgium
A Thermogravimetric Balance for Student Experiments
Laboratory courses in inorganic preparations often include the verification of products by analysis. I n many cases, a single thermogravimetric determination may be substituted for tedious and timeconsuming analyses. Such experiments also can be used to demonstrate the structure of complex compounds containing volatile or thermolabile ligands. The range of temperature over which salt hydrates are stable can be studied as well as the behavior of hydrated salts which will form basic salts on heating. A complete treatment of the theory of thermogravimetry is available in the reference work by Duval.' The accompanying .figure shows schematically the essential features of a simple, robust, inexpensive homemade thermobalance. Our four years of experience
Homemade thermo balance.
' DUVAL,"Inorganic Themogravirnetric Andysis," New York, Elsevier Publishing Co., Inc., 1953.
with the device convince us of its utility for students' use. The spring is made from 4-mm diameter steel wire, coated with plastic to prevent rusting. I t consists of 20 1-cm turns and has a sensitivity of 5 mm/g. Any desired sensitivity can be obtained easily, in a few trials, by varying the tension applied to the wire when winding it. The screen is cut out from an aluminum sheet; the screening is effective enough to allow direct calibration of the spring a t room temperature. The suspension wire must be either of stainless steel or of nickelchrome alloy. In running the experiment, a stream of nitrogen is passed into the apparatus through the inlets (a). In the upper part, it serves to cool the spring and to prevent the introduction of water and acid vapors; in the lower one, it hinders secondary reactions with the other air components and carries away the gaseous products. If wanted, these gaseous products may be easily collected at the outlet (b) and analyzed. In other experiments, e.g., reduction or oxidation reactions, the suitable gas is substituted for nitrogen. The stretching of the spring is read from a millimeter scale placed behind the tube. Measurement of temperature may be carried out either by means of a thermocouple, the hot junction of which is located inside the tube, or by means of a thermometer inserted in the space between the tube and the inner wall of the electric furnace. In the later instance, a systematic error as high as 30" C i s to be expected; the general shape of the curve weight versus temperature, however, remains the same, as does the ordinate of the horizontal parts. There are two important precautions to be taken in operating. Half-filling of the crucible minimizes the risk of loss of substance by "climbing" and the breaking up of small crystals on heating, and a slow but constant rate of heating is important. One degree per minute is a convenient rate, but a maximal one. Although the theoretical accuracy of the apparatus fs low, the results found in many operations are in good agreement with the calculated ones, and with those obtained with research-type apparatus as reported in the literature. Thanks are due to Professor L. D'Or for a useful discussion of the initial design and to M. Haesen for technical help and advice.
Volume 37. Number 3, March 1960
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