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  Thread 469/09

DI water is pH 5.20. Is this right?

Q. I have a ph of my DI water @ 5.20. I go through a Carbon bed, then a separate bed, Mix bed , then polishing bottles. Should the pH be higher, if so why is it dropping?

Terry Dooley
Plating Lab - Chattanooga, Tennessee, USA
2007
publicly reply to Terry Dooley

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A. Topic 25/76 should answer your question. Mark Baker Process Engineer - Syracuse, New York 2007 publicly reply to Mark Baker A. The pH should be 7.0, the ppm will go up and the pH will drop at the same time. This could indicate that one or more of the tanks are spent. Sheldon Taylor supply chain electronics Wake Forest, North Carolina 2007 publicly reply to Sheldon Taylor A. This seems a bit low to me. Is the water held in a tank for any prolonged time? - if so, it is adsorbing carbon dioxide, making it acidic Trevor Crichton R&D practical scientist Chesham, Bucks, UK 2007 publicly reply to Trevor Crichton

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A. Hi It is a difficult thing to measure the pH of di water since there are very few ions in solution to allow the measurement. In fact one could arue that it is meaningless to do pH on it. Having said that, di water will absorb CO2 out of the air and the pH will drift down slightly below neutral. So what you are seeing is not unusual. thanks Terry Tomt - Auburn, Washington 2007 publicly reply to Terry Tomt ? What specification are you trying to meet with your DI water? Christa Lammers -Sylvania, Ohio, USA 2007 publicly reply to Christa Lammers

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Q. Please advise what is the type of resin in the final polishing bottles. Also please check and advise pH after the MB unit. Usually pH after the MB unit will be 6.5 - 7.5, will drop subsequently if the post MB polisher bottles contain only cation resins.

ANIL Y. SHAH
- Mumbai, Maharashtra, INDIA
2007
publicly reply to ANIL Y. SHAH

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A. Lesseee...pH 5 corresponds to a [H+] conc of 10(exp -5)
= 0.00001 M = 0.01 mg/l H+ ion. If I recall chem 101 properly.

That's not much, is it? You could blanket the water with nitrogen, I guess. Or, it also might be a measurement thing, as some other posters have suggested.

Either way, I wouldn't be really concerned.

Dave Wichern
Consultant - The Bronx, New York
2007
publicly reply to Dave Wichern