Re: about KH buffering capacity

[Roger Miller <rgrmill at rt66_com>]

On Thursday 02 May 2002 01:48, BC wrote:

> I am not sure about the measurement of the KH part.  Can you teach me more
> about that or let me know where I can read more about that?

The "KH" test is actually just an alkalinity titration.  There are many 
standard references that describe that test.  It determines the amount of a 
strong acid that must be added to the water to lower the pH of the water to 
an endpoint of 4.5 or so.  The actual end point probably varies a bit from 
kit to kit.

The pH drop is resisted by weak acids in the water that associate between the 
ambient pH of the water and the endpoint pH.  In most cases that is just 
bicarbonate ion; in fact the common standard method for determining 
bicarbonate concentrations is just the results of the alkalinity titration 
converted to mg/l bicarbonate.  The KH-pH-CO2 relationship also assumes that 
the measured alkalinity is caused entirely by bicarbonate.

Bicarbonate is not the only weak acid that can be measued in an alkalinity 
titration.  Many organic acids can also react in the normal pH range covered 
by the test and their presence can be measured in the titration.  In natural 
water that sometimes happens in water from "black water" streams, ponds and 
marshes with a very high organic content.  We can produce similar 
interferences in aquarium water with peat filtration and sometimes with 
driftwood and probably with leaves.

As I pointed out in my last letter, the presence of peat or driftwood in a 
tank does not automatically invalidate the test.  Probably in most cases peat 
and driftwood don't produce enough soluble acids to cause a problem for the 
test.   

The archives have loads of discussion on this topic.


Roger Miller