[kwright9]

The beginning questions for my most recent chemistry lab, in which we were to create a 1M standard cation solution, was "Why do we add acid to a standard cation solution?" and "if two cations have the same result for a test, how do we differentiate between them?"

Obviously, "Because the lab manual said so" is not an acceptable answer.  I'm sure that at one point I knew why, chemically, this was necessary, but to be honest, I can't remember and I have few resources to find out.  Also, the lab I am in is for Gen. Chem 1, and my current course is in Gen. Chem 2 and we haven't covered acids and bases at all yet.  So does the pH of the aforementioned solution affect the results of solubility and confirmation tests for cations?  If so, what is the optimum pH for a standard cation solution?

As to part 2, if two cations give the same result for a given test, the easiest way to confirm one or the other would be to perform a test that gives a different result.  For example, Na+ and Ca2+ both give a clear, colorless result when the DMG reaction is performed.  However, only Ca2+ precipitates when the hydroxide reaction is used, and they give yellow and brown results, respectively, when Alizarin S. is used.  Is there another acceptable answer to this question that I do not see? 

[Borek]

Many hydroxides are so weakly soluble, that they can precipitate even in neutral solutions.