Q1(e). Calibration of the temperature axis in thermogravimetry

Answer in brief

By the Curie-point method: ferromagnetic standards heated in the pan with a small magnet outside the furnace lose their magnetism at their Curie temperatures and give sharp apparent mass steps at known temperatures.

Key points

• Problem: the thermocouple sits near the sample, not in it, so the recorded temperature lags or leads the true sample temperature.
• Curie-point method: the magnetic pull adds an apparent mass; at the Curie temperature the metal turns paramagnetic and a sharp apparent mass step is recorded.
• Several standards cover the range: alumel (about 163 °C), nickel (354 °C), perkalloy (596 °C), iron (780 °C), Hisat-50 (about 1000 °C).
• Fusible-link method: a weight hung on a pure-metal wire drops when the wire melts (In, Sn, Pb, Zn, Al, Ag, Au).
• Calibration is run at the same heating rate, atmosphere and crucible as the analysis; observed against certified values gives the correction curve.