Bill and Ed,

I would agree with Andy.  Most plant DI units deliver 1 to 2 megaohm quality DI water at best and depending on bed type.  Resistivity is usually measured by an internal sensor with high flow past the probe (air free):  One can even obtain an incorrect resistivity unless high flows are used past a handheld meter due to CO2 contamination.

The ultimate: 18 megaohm water would be found for research or testing purposes and the pH will drift all over due to CO2 dissolution as Andy points out.

At a pH of 8, 0.23 ppm of sodium could be present if the column were bleeding hydroxide; at pH 9, 2.3 ppm sodium; and at pH 10, 23 ppm sodium is possible.  Air/acetylene flame AA has a practical detection limit of 3 ppt and could be used.  A quick litmus test is to aspirate the DI water and a blue flame will go yellow.  Perhaps a better check would be sodium versus internal resistivity.  Note one would expect below 1 megaohm that the impurity level will rise.  Of course, without a known good curve it might be hard to determine whether the bed is behaving normally.  If the bed is bleeding chloride, suggesting failure to the cation resion, simple addition of silver nitrate would show cloudiness as a litmus test.  These observations would lead to broader questions applicable to the vendor.  For example, is regeneration done frequently enough?  Is the minimum acceptible water quality prior to regeneration exhausting one of the beds?  Then, are the beds old?

Paul Wilson
Plant Chemist




----------

> Bill, and Ed
>
> Be very careful trying to use pH as a judge of your DI water. It's
> nearly impossible to measure a sample without picking up CO2 from the
> air and changing the pH. The higher the resistivity the faster the
> change.
>
> Andy Magee
> Flex Guru
> [log in to unmask]
>
> Ed wrote,
>
>          Mon, 20 Jul 1998 11:28:05 -0500
>    From:
>          [log in to unmask]
>
> Bill,
>
> Typically, as D.I. beds begin to fail (exhaust) either the cation or
> anion
> (as a function of your H2O constituents) exhausts first which causes a
> pH
> imbalance.  This occurs increasingly below approx 500K, so 1M builds in
> a
> margin of safety that the water quality will be within reasonable
> boundaries.
>
> If you want to confirm this, simply measure the pH (inexpensive pocket
> meters are available from Fischer Scientific) as yor beds fall below 1M
> and
> plot the curve pH vs. resistivity.
>
> Hope this helps!
>
> Regards,
>
>
>
> Ed Popielarski
> QTA Machine
> 10 Mc Laren, Suite D
> Irvine, Ca. USA 92618
> Ph: (949)581-6601
> Fx: (949)581-2448
> E-Mail: [log in to unmask]
>
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