Adding an alkali to an amino acid solution:
If you increase the pH of a solution of an amino acid by adding hydroxide ions, the hydrogen ion is removed from the -NH3+ group.
You could show that the amino acid now existed as a negative ion using electrophoresis.
Can we use "exists" instead of "existed"? Like this:
You could show that the amino acid now exists as a negative ion using electrophoresis.
Or maybe even:
You can show that the amino acid now exists as a negative ion using electrophoresis.
After all, the sentence which begins with "If you increase" is a so-called Zero Conditional sentence (Present Simple in both protasis and apodosis).
P.S. Electrophoresis cannot show that some chemical species existed as an ion in the past. It can only show that a chemical species is an ion right now: the colored patch moves to the anode (or cathode). A non-ionic species will just stay put, in the middle, during an electrophoresis session - no matter what their charge was in the past.