Casting a Film on a Salt Plate

Sometimes, a polymer is made which is too brittle to cast a film which can be handled. In that case, a film may be cast on a salt plate. This is simply a disc of sodium chloride (table salt) which is usually optically clear and invisible to IR light. However, there are a few more considerations for casting a film this way.

1. DO EVERYTHING IN THE HOOD
2. Always wear gloves when handling salt plates (Why?)
3. Clean the salt plate with methylene chloride
4. Disolve polymer in CH₂Cl₂, or other organic solvent which is not an acid or alcohol (Why?). Other good candidates are chloroform, ether, and benzene.
5. Using a disposable pipette, place some of your solution on the salt plate (don't let it run off the edges)
6. Let dry (solvent must be volatile, which is why DMSO, DMF, NMP, and substituted aromatics should not be used)

Salt plate are not cheap, so if you use this method, be careful. NaCl is soluble in water (isn't it?)Don't get it wet! It also will be damaged by alcohols and all organic and inorganic acids. It is also brittle. Don't drop it!