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
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 !!!
Net Address: ntbui at wwnet_com