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Re: Alkalinity and Phosphates

> Date: Thu, 2 Jan 2003 21:15:58 -0500
> From: James Purchase
> Cavan asked a question the other day about the possible
> effect Phosphates have on Alkalinity readings. In one form
> or another, this question has been battered around online
> since at least 1994. I've NEVER seen anyone step up to
> the bat to volunteer an answer. Lots of stuff which says that
> things like Borates, Phosphates and Sulphates CAN add
> to Alkalinity readings, but nothing which says by how much,
> or to what degree.
> Is there ANY way to separate out the possible interference
> from Phosphates, Borates and/or Sulphates to an Alkalinity
> reading?...Can you take the concentration of either
> Phosphates and/or Sulphates (both of which are testable
> parameters) and, via some mathematical sleight of hand,
> calculate the contribution of these species to Alkalinity?...

When I first started researching for my original Website, one of the
resources I found was "Standard Methods for the Examination of Water and
Wastewater", Clesceri, Greenberg and Trussell, Editors, American Public
Health Association, Wash., D.C., 1989 (17th Edition). Page 2-43 listed the
concentration of bicarbonates as a result of the difference between Total
Alkalinity and Contributing Alkalinity divided by a water-based constant.
The specific formula, as close as I can get it in ASCII:

[HCO3-] = (Alk(sub)t - Alk(sub)0 + 10(sup)(pH + pf(sub)m - pK(sub)w) / 1 +
0.5x10(sup)(pH - pK(sub)2)

where (sub) and (sup) are sub- and superscripted (ASCII limitation here)


Alk(sub)0 = alkalinity contributed by NH3, H3SiO4-, HPO4--, B(OH)4, CH3COO-
and HS-

so you could isolate the Alk(sub)0 by rearranging and factoring around what
you *do* know.

The necessary constants for water are easily enough found, but I don't
remember off-hand what the term pf(sub)m is, as I wasn't smart enough to
note it back in '97 when I first found the formula. But I understand this
equation is very similar to the ones used by the marine folks, so one of
their sites should delineate it if you don't have access to a library -
almost all of which in the US carry a copy of the above tome.

In order to "back door" the problem from scratch using concentrations and
equilibria, though, you'd better be well- versed in both quadratic equations
and differential calculus - which is most likely why you don't see too many
references scattered among the flock...


David A. Youngker
nestor10 at mindspring_com