Mn(3+) is stable at very low pH. I made this myself and could keep it around in a test tube for several days. After that I used it for further
experiments.
V(2+) might be unstable. I can imagine that it reduces water and that hydrogen is formed.
For many anions, sulfate is suitable as well. Sulfate does not coordinate to copper(II), cobalt(II), nickel(II). It does coordinate to Cr(3+) though.
With iron(III) I never tried, but I expect that in dilute sulphuric acid the iron(III) ion also looks nearly colorless.
Dilute nitrate (less than 1 M) certainly does not tend to form NO2 when no suitable reductor is present. Dilute solutions of nitrates of nearly all
metal ions, even the somewhat reducing ones, are stable. |