As a general rule do not premix any chemical reagents. The reasons are numerous why you shouldn't ranging from their reactivity deteriorates with storage, damage to certain storage containers, to forming deadly byproducts (eg: gases) if stored under the improper conditions. Reagents are best stored separately and mixed on an as needed basis. Furthermore, I advise against adding the entirety of the required calculated amount of an oxidizer at the time of use. Oxidizers should only be added in small increments through the course of the reaction. This allows for controlled additions of the oxidizer minimizes the risk of adding excessive amounts of the oxidizer, which leads to problems later in the refining process.
The only exceptions I can think of off the top of my head to premixing reagents is test solutions and standards. You can premix stannous chloride as long the test solution is kept in an airtight bottle that does not deteriorate over time when exposed to HCl (eg: glass bottle with glass/Teflon stopper). DMG for testing palladium would be a second example. One other example that comes to mind is the premixing of concentrated potassium or ammonium chloride salt solutions for precipitation of platinum group metals. Single salt saturated solutions in general should be safe to premix and store, as long as the storage container is rated to withstand the salt solution for long durations. Failure to seal salt solutions when stored can result in the salt growing out of the container as the water in the solution evaporates and can lead to contamination of the precipitant (eg: bugs and dust).
Steve
