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Re: KH questions
>Date: Wed, 9 May 2001 15:21:09 -0600 (MDT)
>From: "Roger S. Miller" <rgrmill at rt66_com>
>Subject: Re: KH questions
>
>I think "pH Down" used to be phosphoric acid. Perhaps they changed the
>product. It's been about 15 years since I last used it. Whether or not
>the product is acidic makes little difference, because (for common
>conditions) lowering the pH does little to the change KH.
>
>Whether or not the product is still phosphoric acid makes a difference
>here only in my example; other acidic buffers will behave the same. Over
>the range of pH that we keep in aquariums the phosphate will exist as
>HPO4-- and H2PO4-. These two species are present in equal amounts at a pH
>of about 7.2; at lower pH H2PO4- predominates and at higher pH HPO4--
>predominates.
>
>A KH test is an acid titration that measures the amount of acid needed to
>drop the pH of the sample to a level of about 4.5. Usually the reaction
>from bicarbonate to carbonic acid is the only reaction that works
>against the titration, so only the concentration of bicarbonate is measured by
>the KH test. If phosphate is present then the KH test will also measure
>the amount of HPO4-- that reacts to H2PO4-; so some of the phosphate added
>with the products gets measured as part of the KH.
>
>The only acids that can be added to an aquarium to lower pH without also
>raising the KH are relatively strong acids, including hydrochloric acid,
>sulfuric acid and nitric acid -- acids that remain completely dissociated
>above a pH of 4.5.
>
>*Any* base that associates (gains a hydrogen atom) in the range of pH
>between the starting pH of the sample and the endpoint pH of about 4.5 is
>measured as part of the KH. That includes not only HCO3- and HPO4--, but
>also NH3, HS-, HSO3-, other inorganic complexes and a host of organic
>molecules. HCO3- is the only base on the list that commonly occurs in
>signficant concentrations in natural water. If there's something other
>than HCO3- that changed the KH in an aquarium then probably the aquarist
>added it.
>
>
>Roger Miller
>
While I think I understand your explanation, I still donīt 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 ?
Ole