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Alkalinity vs pH
---------- Forwarded message ----------
Date: Thu, 01 Aug 1996 00:33:05 -0400
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
pH.
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. Therefore, we should always measure the
alkalinity and keep between 180-300 mg/L CaCO3". When I asked what kind
of alkalinity (P alkalinity, M alkalinity, total alkalinity, OH-
alkalinity,...) do we need to measure? He could not answer my question,
but still demanded that I should come up with some answers and action
plans regarding to the alkalinity issue. Where is the 180-300 mg/L
CaCO3 alkalinity come from? Some water hardness could have alkalinity up
300 mg/L. How can someone uses those values to correlate to the [OH-],
especially for the AL(OH)3 formation? I understand that the alkalinity
measurement help to determine the buffer capacity while the pH help to
determine the [H+] as well as [OH-]. I told him that as long as I run
the program at pH = 8.6 there is no need to measure alkalinity because
we have enough [OH-]. Why wasting time to create more work by measuring
the alkalinity? Am I wrong?
Another questions: When the total alkalinity is too high, can we be
able to reduce it without adding more fresh water?
Your reply is really greatful. Thanks !!!
Sincerely,
Nang Bui
Net Address: ntbui at wwnet_com