Here you go:
Sodium metabisulfite (SMBS) initially reacts with water to form sodium bisulfite (SBS): Na2S2O5 + H2O --> 2 NaHSO3
Next, SBS reacts with hypochlorous acid to form byproducts free from free chlorines: 2 NaHSO3 + 2 HOCl --> H2SO4 + 2 HCl + Na2SO4
You wind up with sulfuric acid, hydrochloric acid and sodium sulfate.
Here's information on the dosing:
While theoretically about 1.5 ppm SMBS will neutralize 1.0 ppm chlorine, typically 1.34 ppm is needed but 3.0 ppm SMBS per 1.0 ppm chlorine is used to account for incomplete mixing.
So if we're using 40 liters of water for a batch (somewhat over 10 gallons), we would need 3 ppm (mg/l) * 40 l, or 120 mg sodium metabisulfite to treat 40 liters of water at 1 ppm chlorine/chloramine. Our water administration adds an average of 1.73 ppm of chloramine and 0.0673 ppm of chlorine dioxide, I'll round to 2 ppm chlorine, so I can safely say that 230 mg of sodium metabisulfite would dechlorinate my 40 liters of water.
Dow Chemical states the reaction is rapid, but be sure and mix thoroughly.
Good stuff, metabisulfite, it gives you a bit of pH drop through sulfuric and hydrochloric acid and a bit of sulfate through the sodium sulfate produced.