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Re: KH question

Ole Larsen wrote:

> While I think I understand your explanation, I still don4t see how 
> phosphoric acid can increase the KH in the types of water used for
> aquariums unless H2PO4-1 and HPO4-2 are stronger basic buffers than 
> (bi)carbonates. Any acid that can give off a hydrogen ion ( the definition
> of an acid) will tend to reduce the KH by "using" the (bi)carbonates to 
> form CO2 and water - where am I wrong ?


Having just run the numbers, I have to admit that adding phosphoric acid
to decrease pH will cause KH to drop. 

If someone adds an acid to their tank to decrease pH, they are adding
hydrogen ions.  The hydrogen ions are positively charged.  In order for
the water to remain electrically neutral (the usual condition in aqueous
solutions) there must always be a balancing increase in the
concentration of some negatively charged ion.

The increase in negatively charged ions is actually a net change between
the decline in bicarbonate concentration and the increase in the
negative ion in the acid.

If the negative ions added with the acid aren't measured as part of the
alkalinity, then the measured KH will drop because bicarbonate drops. 
That would be the case with (for example) sulfuric acid, nitric acid or
hydrochloric acid.  If the negative ions added with the acid are
measured as part of the alkalinity then the measured KH can increase,
but the maximum possible increase is very small.

Phosphoric acid will be present in our tanks partly as H2PO4- and partly
as HPO4--.  The H2PO4- won't be measured as part of the alkalinity, so
there should be a net drop in the alkalinity.  The size of the drop can
range from almost nil to essentially complete.

Sorry for the confusion. 

Roger Miller