After being given a solution containing a set of unknown ions, you perform the following sequence of tests to determine the identity of the cations in solution: 1. A flame test produces no color
2. You take a small portion of the solution and add NaOH. A vapor is produced that turns red litmus blue.
3. You add HCl to solution, which results in the formation of a white precipitate. The supernatant is separated and used for the remainder of the tests.
4. The addition of buffered NH4+/NH3 to a portion of the supernatant from step 3 does not produce a precipitate.
5. The addition of ammonium oxalate to a portion of the superntant from step 3 does not produce a precipitate
6. The addition of a strong base to a portion of the supernatant from step 3 generates a precipitate.
7. The supernatant from step 6 is separated from the precipitate. The addition of HCl and K4Fe(CN)6 to the solution produces no reaction.

Based on these tests, identify which of the following ions must be present, can be present, and cannot be present.

a. Na+
b. NH4+
c. Ag+
d. Fe3+
e. Al3+
f. Cr3+
g. Ca2+
h. Mg2+
i. Ni2+
j. Zn2+