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Re: PO4, Silica and alkalinity

On Sat, 5 Aug 2000, Chuck Gadd wrote:

(I know these questions were aimed at George, but I'll put my two bits in
here anyway.)

> Are there any KH kits that actually measure carbonates?

No, I don't think there are.  All of the kits measure alkalinity.

> George, Do you have any impression of how much phosphate it takes to
> throw off the KH measurements?   

It takes quite a bit.  Even with a pH substantially above 7 (where
phosphate is present mostly as HPO4--) it takes about 17 ppm of PO4 to
make 1 degree of alkalinity.  That's enough of an effect that you can set
up an experiment to measure the effect.  As I recall, George has done

Also on Sat, 5 Aug 2000, Wayne Jones wrote:

> ... I have noticed that the program includes silicates when
> calculating the total alkalinity of seawater. Does anyone know if
> silicates also effect the total alkalinity of freshwater.

Silica is present in water as a very weak acid - H4SiO4.  There is no
effect on alkalinity at pH levels where the acid has all of its hydrogen
ions attached.  That covers the whole pH range we usually see in planted
tanks.  The pKa for the first dissociation (the pH where about half of the
first hydrogen ion drops off) is about 9.8, so there is no effect on
alkalinity if the pH of your water is well below 9.8.

As a formal note, it's a common and well-accepted assumption that silica
is present as H4SiO4; I've seen very few studies that determine the
dissolved species.  But from what I have seen, the silica may be present
as polymers and/or colloidal particles rather than as the acid, so silica
may actually have less effect than if it is present entirely as the acid.

Roger Miller