Instant Notes: Analytical Chemistry

When a complex mass loss occurs, involving several reactions, matters

become more complicated.

Calcium oxalate monohydrate, CaC 2 O 4 .H 2 O, a well-studied example, is

shown in Figure 1. Three separate stages occur, around 150, 500 and 750°C. The

mass losses are about 12, 19 and 30% of the original mass in the three stages.

This can be explained as follows:

CaC 2 O 4 .H 2 O(s) = CaC 2 O 4 (s) + H 2 O (v)

146.1 128.1 18 loss = 12.3%

CaC 2 O 4 (s) = CaCO 3 (s) + CO (g)

100.1 28 loss = 19.2%

CaCO 3 (s) = CaO (s) + CO 2 (g)

56.1 44 loss = 30.1%

These reactions have been confirmed by analyzing the gases evolved (see Topic

G4). Note that, in this example, the mass losses are calculated as a percentage of

the originalsample mass.

In more difficult cases, the reactions may overlap, and then it is difficult to

assess the separate temperature ranges and mass losses. An aid to this is the

derivative thermogravimetric (DTG)curve shown as a dashed line in Figure 1.

This is produced electronically from the TG trace by the computer and

represents the dm/dt, or occasionally the dm/dT,as a function of time or

temperature.

Instrumentation In general, thermal analysis instrumentation consists of four components:

● the furnace, controlled by the computer and a temperature sensor, often with

controlled atmosphere as well;

G1 – Thermogravimetry 307

100

82.5

65

47.5

30

30 130 230 330 430 530 630 730 830 930 1030

Temperature (°C)

Mass %

Loss = 12.4%

Total loss = 31.5%

Total loss = 61.7%

Fig. 1. TG and DTG curves for calcium oxalate monohydrate, 12.85 mg, platinum crucible, 20°C min−^1 , nitrogen,
30 cm^3 min−^1.