Measuring pH properly in pure and ultrapure water is difficult. pH is critical for process control and for preventing corrosion for example.
pH of Ultrapure Water
pH of Ultrapure water is 7.00. The only ions present are H+ and OH-, and since these must be in balance, the pH is 7. Don’t try to measure the pH of Ultrapure Water! If you are above 16 Megohm, pH is between 6.95 and 7.05.
pH of Pure Water (RO Permeate)
pH of Pure water (even RO permeate) is difficult and requires special probes, procedures, grounding, etc. You need the right probe, and you need conductivity to make the pH probe work well. We recommend MT Thornton “pHure Sensor” which is a grounded, shielded vessel/probe system using a slip-stream with controlled flow. The flow is low, and the measured stream goes to waste. The probe must be one of two special probes for this application.
pH is especially important for 2-Pass RO with NaOH injection. For the 2nd pass RO to work well the pH must be controlled between pH 8.4-8.7. Below 8.4, some uncharged CO2 will pass the membrane. Above 8.7, RO-2 permeate conductivity will increase as you get NaOH passage.