we have problem to determine water content of hygroscopic substances. the result tends too high. IMO, this problem is due to humidity contamination from environment. could you give any suggestion about how to create dry micro-environment in our lab that suitable for this purpose? and what is the adequate humidity level or range? thanks in advance.
note: I live in tropical country with humid environment
Use dry glove box. You can make one yourself from plexiglass or buy one. To dry the air inside, it is best to let dry nitrogen or other dry gas flow through the box. I managed to decrease humidity in our self-made box to 5 %. But it is still very hard to tell if its enough. If the substance is very hygroscopic, then the humidity must be even lower.
I have read USP <921> and found this:
Where the monograph specifies that the specimen under test is hygroscopic, use a dry syringe to inject an appropriate volume of methanol, or other suitable solvent, accurately measured, into a tared container, and shake to dissolve the specimen. Using the same syringe, remove the solution from the container and transfer it to a titration vessel prepared as directed for Procedure. Repeat the procedure with a second portion of methanol, or other suitable solvent, accurately measured, add this washing to the titration vessel, and immediately titrate. Determine the water content, in mg, of a portion of solvent of the same total volume as that used to dissolve the specimen and to wash the container and syringe, as directed for Standardization of Water Solution for Residual Titrations, and subtract this value from the water content, in mg, obtained in the titration of the specimen under test. Dry the container and its closure at 100 for 3 hours, allow to cool in a desiccator, and weigh. Determine the weight of specimen tested from the difference in weight from the initial weight of the container.
Could you tell me what kind of substance it is? I suppose it is solid. How much substance do you have that can be used for analysis?
hygroscopic substances, for example povidone and other superdisintegrant, lyophilized products, etc.
quantity of samples can be determine as mentioned in Test Preparation section of USP <921>.
We do use Karl Fischer for micro-determination (samples contain usually 300-1000 ug of water). Since you probably have larger amount of sample (~100 mg of water? in each sample) the method proposed in USP is I think more suitable for you than for us. But anyway, its still better to protect the sample from moisture even when it is in methanol.