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Re: Aquatic Plants Digest V4 #216

At 03:48 AM 4/13/2000 -0400,  Jennifer Glover asked:
>why water could have a PH of
>7.4 and no measurable KH.  I tried two differnt test kits, but I
>haven't ruled out a bad reading.  If I add CO2 to this, will that be
>really bad?

Speaking very roughly, pH is determined by the balance between the two
ionic products of water, H+ and OH-.  An excess of H+ is referred to as an
acid condition, an excess of OH- is called basic.  Adding a tiny bit of a
strong base (like lye) to pure water could easily result in a pH of 7.4.  

But this solution would not readily resist the addition of H+ to it causing
a lowering of pH.  So it's KH, which measures the ability to resist pH
changes, would be very low.

This is in contrast to a solution of lots of baking soda, to which some
acid had been added to adjust the pH to 7.4.   In this example, the
bicarbonate ion (HCO3-) would resist the lowering of the pH by adding H+.
The bicarbonate ion would accept one of the incoming H+ to become carbonic
acid (H2CO3).   This acid can even leave the solution by escaping as CO2.
So this solution would have a high KH because you have to add LOTS of acid
(H+) to lower the pH because the bicarbonate eats incoming acid.  You have
to add enough acid to satiate the bicarbonate, then the pH will finally
start dropping.

This phenomenon (resisting pH change that would occur in pure water by the
addition of acid or base) is called buffering and the agent that absorbs
the H+ or OH- is called a buffer.   Carbonate is a buffer with two
buffering pHs, phosphate has 3.  These substances are also called weak
acids, because they lose a proton (H+) only reluctantly (at relatively high
pH values).

I hope this has made it a bit clearer.   Watch for even more of this stuff
in the third issue of Planted Aquaria Magazine.
Dave Gomberg, San Francisco            mailto:gomberg at wcf_com
Jobe's Fern and Palm Spikes FREE        http://www.wcf.com/pam