A few weeks ago, we pickled all RO membranes in our trains with sodium metabisulfite for long term storage. Our membrane manufacturer recommended that we change the solution once the pH drops below 3. We measured a decrease in pH for the first two weeks, but then on the third week, the pH increased. Can you explain why the pH increased instead of decreasing?

When you mix sodium metabisulfite in water, you form a sodium bisulfite solution:

$${Na_2S_2O_5 + H_2O \Leftrightarrow 2NaHSO_3}$$$${NaHSO_3 \Leftrightarrow Na^+ + HSO{_3}{^-}}$$

Bisulfite is in equilibrium with sulfite and sulfurous acid (just like bicarbonate is in equilibrium with carbonate and carbonic acid):

[1] $${HSO{_3}{^-} \Leftrightarrow SO{_3}{^2}{^-} + H^+}$$

[2] $${H_2SO_3 \Leftrightarrow HSO{_3}{^-} + H{^+}}$$

[3] $${SO{_2}{_(}{_a}{_q}{_)} + H_2O \Leftrightarrow H_2SO_3}$$

[4] $${S{_2}{_(}{_g}{_)} \Leftrightarrow SO{_2}{_(}{_a}{_q}{_)}}$$

When H₂SO₃ gasses off as SO₂, the pH rises due to a shift in equilibrium to the left, causing bisulfite to react with H+ to form more sulfurous acid (see Eq.[2],[3],[4]).  This is exactly like what happens when H₂CO₃ gasses off as CO₂ (that’s why the pH of permeate rises after going through a degasifier).  Essentially, since acid is being consumed by the reaction as equilibrium shifts to the left, the pH rises.

Therefore, the pH rise that you are observing is due to SO₂ gas being lost from the solution.  It will have a prominent smell that’s very unpleasant and nauseating.  It is highly likely that the gas is being allowed to vent somewhere.

On the other hand, when sulfite reacts with oxygen, it converts to sulfate:

[5] $${SO{_3}{^2}{^-} + \ ½  O_2 = SO{_4}{^2}{^-}}$$

When that happens, because sulfite is consumed, the bisulfite/sulfite equilibrium reaction shifts to the right (see Eq.[1]).  This causes more bisulfite to break down into sulfite and acid protons, which causes the pH to decrease (higher [H+] means lower pH).

So when your pH declines, it means that your bisulfite solution is getting consumed by oxygen.  On the other hand, when the pH rises, it means that SO₂ gas is being released.  Either way, your total available sulfite/bisulfite is decreasing (either getting converted to sulfate or being lost as a gas), and the solution will eventually need to be changed.

Proprietary preservative solutions, such as AWC C-221, do not gas off SO2, so they are odorless and do not attack the O-rings of the interconnectors.  This allows the solution to last up to a year without needing to be changed.