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From Chemguide:

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.

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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.

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    I want to answer this question, but 'amino' position to do so.
    – Varun Nair
    Apr 21 '16 at 7:13
  • @VarunKN - well, let's then wait for a native speaker to provide us with a 'solution'. Maybe even an 'ideal solution'. Apr 21 '16 at 7:46
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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.

In the Original Poster's example, the second sentence follows on directly from the first. The first sentence uses the present simple to represent general time. The second sentence is presented as a hypothetical conditional. However, it is a bit odd to use this conditional like this here. Why? Because what they are trying to show is a general fact. This would be far better expressed by a clause using the present simple:

  • 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 exists as a negative ion using electrophoresis.

However, this is a matter of opinion and style not grammar!

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Existed is correct.

Though your grammar points are understandable in terms of "during that moment in time the amino acid had" either a positive or negative charge depending on the pH of the overall solution, the point of the article is to discuss the isoelectric point of amino acids. That is, the shifting back and forth in molecular polarity depending on the solution's acidity.

It is also the case that the natural state of amino acids are not net positive or negative but a mixture: zwitterion. By changing the solution's pH, and then using electrophoresis, it can be shown what the pH of the solution was at that time.

As the article points out later, and is the true case, the amino acid solution will tend to move to an equilibrium, it will act as a buffering agent, meaning the pH will shift over time, so the pH an hour ago may be different than the pH now. The electrophoretic measurement was an indication of how things were, but possibly not how they are now.

It's kind of like Heisenberg's Uncertainty Principle, except the measurement is not the causing the state change. So the author is saying that it can only be shown what the state was.

While it is true that at the extremes, the buffering capacity could be overwhelmed, that is not be point of the article.

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