Re: Alkalinity vs. pH
> From: Nang T Bui <ntbui at wwnet_com>
> To: Aquatic-Plants at actwin_com
> Subject: Alkalinity vs pH
> Can someone help me out?
> Actually I've been working in the water waste treatment area. In my
> application, I am dealing with the Al(OH)3 formation by using caustic
> (OH-) to neutralize the Al3+. I could use dissociation constant to
> calculate the pH where most Al(OH)3 could be formed. However to
> conserve the activities of other chemicals in my application, I have to
> run the pH between 8.5 and 8.8 (preferable 8.6) where I found that the
> most and right Al(OH)3 floc sizes could form. Also, in my application I
> have found that my treatment process works well at the above pH range.
> At pH = 8.6, I believe that I have enough OH- to interact with Al3+ to
> form mostly Al(OH)3 with a fraction of Al(OH)4-, so I only monitor the
> My supervisor overheard about alkalinity. He didn't believe that
> measuring pH alone is adequate. He said: "The presence of bicarbone
> (HCO3-) could buffer the water, so pH at 8.6 doesn't mean we have the
> same [OH-] at all time.
Yes, it does. pH is a measure of the activity of H+ ion (potentially
other ions like Na+ at low pH, that is an aside that doesn't affect you)
and the H+ and OH- activities are related through the autoionization
constant of water. So if you hold constant pH, you have constant pOH
as well, unless you suddenly do something that dilutes the amount of
Water in the samples. Like if they suddenly became 50% ethanol, you
would be screwed up if you used the usual form of Kw which is Kw=[H+][OH-]
rather than the complete form, which is
Kw = a[H+]a[OH-]
where I'm doing two things... explicitly showing the concentration of
water, and making everything activities rather than concentration, which
is most correct. I should also note that Kw is a function of both
temperature and pressure.
Fundamentally, the only way to alter the first simple Kw relationship
between pH and pOH is to change the amount of water, via dilution with
something else that doesn't autoionize the same as water, i.e. massive
amounts of salt, or a miscible organic modifier (methanol, ethanol, other
low alcohols, glycerol, sugars, acetonitrile, etc.)
Alkalinity doesn't affect the relationship between [H+] and [OH-].
I'm not sure of how you can express this to your supervisor in a way that
won't get you shot.