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 !!!


Nang Bui
Net Address:  ntbui at wwnet_com