RO/DI water is, by definition, water which is extremely low in or free of buffering agents and acids. That means that there is nothing stopping the pH from shifting as a result of any addition of any product, buffer or not. Without the stabilizing influence of a buffer, pH is prone to rapid changes even if the components being added would not typically change the pH. Equilibrium does not contain any buffering agents, so adding buffers to this solution should bring the pH to your desired level.
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