Re: [APD] KH question and pH crash

[gbooth at frii_com]

>From an old, old FAQ which I think covers this issue:

  Carbonate hardness (KH) is the measure of bicarbonate (HCO3-) and
carbonate (CO3--) ions in the water.  In freshwater aquariums of
neutral pH, bicarbonate ions predominate and in saltwater aquariums,
carbonate ions begin to play a role.  Alkalinity is the measure of the
total acid binding capacity (all the anions which can bind with free
H+) but is comprised mostly of carbonate hardness in freshwater
systems.  Thus, in practical freshwater usage, the terms carbonate
hardness, acid binding, acid buffering capacity and alkalinity are
used interchangeably.  In an aquarium, KH acts as a chemical buffering
agent, helping to stabilize pH.  KH is generally referred to in degrees
hardness and is expressed in CaCO3 equivalents just like GH.

  In simple terms, pH is determined by the negative log of the
concentration of free hydrogen ions (H+) in the water.  If you add a
strong acid such as nitric acid to water, it completely dissociates
into hydrogen ions (H+) and its "conjugate base" or "salt", NO3- or
nitrate.  The hydrogen ions freed in the reaction then increase the
concentration of hydrogen ions and reduce the pH.  Since nitric acid
is the end product of the nitrogen cycle, this explains why aquarium
pH tends to decrease and nitrates tend to increase over time.

  When the aquarium has some carbonate buffering in it, the
bicarbonate ions will combine with the excess hydrogen ions to form
carbonic acid (H2CO3) which then slowly breaks down into CO2 and
water.  Since the excess hydrogen ions are used in the reaction, the
pH does not change very much.  Over time, as the carbonate ions are
used up, the buffering capacity will drop and larger pH changes will
be noted.  From this it is clear why aquariums with low KH seem
unstable - as acid is produced by biological action, the KH is used
up; when it is gone, the pH is free to drop rapidly as H+ ions are
generated.


George from Ft. Collins, CO

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