Letter 27073

pH of incoming rinse water [Connecticut] 

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I am the production unit leader for a process line/ paint shop. We use DI water for all incoming water to the process line. We have our own deionizer and maintain a purity of < 1 micro siemin. The problem I have is we are looking at some new work coming in but the customer requires that we maintain a ph of 5.5 to 8.0. Our incoming water from the deionizer is only around 4.0 to 4.5. Is there a unit which can be installed on the deionizer which will raise, monitor and maintain the ph of the water?

Michael Dalleo
Production Unit Leader - Bloomfield, CT, USA

Measuring the pH of deionized water (especially as low as 1 microsiemen)is inaccurate at best. Theoretically, the pH should be 7, but it will never measure that way. Rest assured that ANY amount of contamination from the previous process will affect the pH wildly. If the upstream process (dragin)is alkaline then you will most likely meet the pH requirement unless your rinse flow is too slow. If it is acid you will not. Adjusting the pH requires adding ions. De-ionizeing removes them. You can't have it both ways.

Paul Morkovsky
- Shiner, Texas, USA

Paul in the previous response is correct. It is very difficult to measure the pH of high purity water and commercial meters don't work very well. At a resistivity of 1 uS, theoretically, you must be between 6 and 8 pH. The pH of water leaving a separate bed DI will vary some, however, depending upon how long it has been since the last regeneration and other factors. If you really feel that you have to add something, you could add a tank of mixed-bed resin after the deionizer. This will boost the resistivity to over 10 Meg-Ohm-cm. You still won't be able to accurately measure the pH, but it will be between 6.8 and 7.2.

Lyle Kirman
water treatment systems - Cleveland, Ohio

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