Re: pH, Alkalinity

> From: Nang T Bui <ntbui at wwnet_com>
> Subject: Alkalinity vs pH
> 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.

	[H+][OH-] is essentially independent of other species present.
So, if the pH is 8.6 ([H+] =  10^-8.6), then [OH-] is 10^-5.4
(Concentrations in g-ions per litre)

> 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.

	He is wrong.  Buffering (by definition) affects the pH, but
once you have a given pH, this fixes [OH-]. The point is that _all_
the equilibria must be maintained - you can't violate any. 
	The effect of the bicarbonate will be to change the amount
of OH- you have to add to get to the pH you want, but once you get there,
the [OH-] concentration will be the same.

> Therefore, we should always measure the
> alkalinity and keep between 180-300 mg/L CaCO3".  

	_Ask_ where these numbers came from.    :)

> When I asked what kind
> of alkalinity (P alkalinity, M alkalinity, total alkalinity, OH-
> alkalinity,...) do we need to measure?

	Where does your pH start off?  If it is less than 8.3, there
can only _be_ M alkalinity to measure (methyl orange endpoint, pH 4.3).
P alkalinity titrations end at 8.3, going down (phenolphthalein endpoint).
OH- alkalinity ends higher still.

> 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.  

	It sounds as if someone has been on too many management courses
and not enough chemistry courses.

> 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?

	You can do it by adding strong acid to neutralize it.

	I think you _should_ check into the possibility of direct reaction
of HCO3- with Al3+.  I think that is the only way the "alkalinity"
of the solution could have an effect.

Paul Sears    Ottawa, Canada