Desorbable residual moisture content

By integrating the DR-values it is possible to calculate the residual moisture content (RM). The integral is calculated from the last measurement of DR over time up to any other measured DR. The integral is RM at the time up to which the integral has been calculated. The RM calculated in this way is too small by the amount of water which would have been desorbed after the measured DR. Thus a method of calculation can be deduced The straight line of the DR values is extrapolated until the still desorbable RM is small compared with the RM to be measured. 

It should be clear that the RM measured in this way, e. g. 0,1 %, must not be identical with residual moisture contents measured with other methods (see Section 1.3.1) because there will be always some water which cannot be desorbed at the end temperature of the drying. This content of bound water for one product and one temperature is a stable value which can be taken from the measurements of absorption isotherms. 

Fig. 1.97.1. Schema of the Coulometer MeBzelle DL 36 for measurement of residual moisture content (RM) after Karl Fischer. In the titration cell (1) iodine is electrolytically produced (3) from an iodine-containing analyt (2). Water in the titration cell reacts with the iodine. When the water is used up, a small excess of iodine is produced, which is detected by special electrodes, which leads to iodine production being stopped. The amount of water in the cell can be calculated from the reading of the coulometer, and the amount of electrical charge needed. The solids are introduced into the cell either by a lock, or the water is desorbed in an oven and carried by a gas stream into the cell. 10 pg in a sample can be detected with an accuracy of reading of 0.1 pg (KF Coulometer DL36, Mettler-Toledo AG, CH-8603 Schwerzenbach, Switzerland).

The residual moisture content calculated by this method is called desorbable water, dW. 

The residual moisture content, calculated from DR data, measures the amount of water, which can be desorbed at the product temperature. The procedure corresponds therefore to the thermogravimetric method [1.83] with some practical advantages ... 

From DR data the desorbable water (dW) is calculated as an indicator of the residual moisture content (RM) during SD ... 

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