di water

[joey]

is it normal for my di water ph to drop below 6 and still have good resistivity? i have a regular di water generator that uses standard cation and anion resins. after regeneration the ph is around 9 and the resistivity is around 3 meg ohms. as i run water through it the resistivity drops and so does the ph. why does the ph drop?

 

[Nelson]

One way the pH can drop in DI water is if it comes into contact with oxygen. I have seen DI water (not yet treated for pH control) go from 7 pH to 5.7 pH following inadvertant aeration. A lot of applications require that DI water tanks be inerted, as with nitrogen, to prevent this. [sig][/sig]

 

[mgoblue]

There may be several reasons. It depends on the chemistry of your raw water. It may be necessary to have CO2 removal as a step. The first bed can generate a lot of carbonate that appears as CO2 and if not removed, will drop the pH and raise conductivity. In addition if your water contacts air with CO2 in it, it will readily absorb the CO2 due to its highly unbuffered state and the pH can drop quickly and very low (due to formation of carbonic acid). It can in fact become quite corrosive. That is one reason DI water is put into stainless steel piping and vessels.
Hope that helps. [sig]<p>Bob Sander<br><a href=mailto:robert.sander@solvay.com>robert.sander@solvay.com</a><br>[/sig]