Why neutralizer dose is an equivalents problem, not a pH problem
pH is a logarithmic scale, so a stream at pH 2 carries ten times the acid of a stream at pH 3. Dosing cannot be prorated across pH units. This calculator converts the starting and target pH into hydrogen and hydroxide ion concentrations, multiplies the difference by flow to get equivalents per day, then divides by the equivalent weight and strength of the chemical you feed.
Equivalent weights applied here are 40.00 g/eq for caustic soda, 37.05 g/eq for hydrated lime, 53.00 g/eq for soda ash, 49.04 g/eq for sulfuric acid and 36.46 g/eq for muriatic acid. As an example, a 20 GPM stream moved from pH 2 to pH 7 needs about 1,090 equivalents per day, which is roughly 192 lb/day of 50 % caustic soda solution, or about 231 lb/day once a 20 % margin is added.
Treat the output as a floor, not a design value. It assumes a clean strong acid or strong base with no buffering. Real streams carry organic acids, phosphates, fluoride and metal ions that resist pH change, and actual demand often runs 1.5 to 3 times the theoretical figure. Precipitating metal hydroxides usually calls for pH 9 to 10, which pushes consumption higher again.
Size feed pumps and day tanks from a titration curve run on an actual sample, using this number only to set the range. Discharge pH limits and reporting sit under your NPDES permit and local pretreatment ordinance, so confirm the requirements with the permitting authority and a qualified engineer.
Frequently Asked Questions
Buffering. Organic acids, phosphate and dissolved metals hold the pH steady until far more equivalents are added. A bench titration on a real sample tells you the multiplier to apply.
Lime costs less per equivalent and helps precipitate metals, but it generates much more sludge. Caustic soda is cleaner and easier to control automatically at a higher chemical cost. Compare them with sludge hauling included.