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Re: KH-pH-CO2 charts
This subject has been thrashed recently, but I'd like to add a few
comments.
As others have pointed out, you unfortunately can't assume a given rate
of CO2 addition will give the same CO2 level under different conditions.
The CO2 dissolution rate varies with the pH, surface tension etc. After
a while you achieve saturation, but what this concentration will be
depends on a number of equilibrium reactions, chiefly the bicarbonate
system. Thats where pH and carbonate hardness come in.
In some situations, like when a non-bicarbonate buffer is present, the
actual carbonate hardness can't be easily determined. If we could
measure it accurately however, the chart _should_ still be accurate. You
can largely account for the contributions of say phosphate buffers to
alkalinty and so calculate the 'true' carbonate hardness. To be honest
it simply isn't worth the effort :).
The chart makes extensive use of equilibrium constants for the
bicarbonate system and with them come a few assumptions. The most
obvious is that they only (strictly) apply under equilibrium conditions.
Few things ever are truly in equilibrium, this allows life to exist, but
gives process engineers nightmares. We with our aquaria can assume the
next best thing- a 'steady state' so this wont affect the chart
noticably.
The equations assume an 'activity coefficient' of one. I'm afraid
chemists have found many cases where 'constants' based on concentrations
aren't strictly constant. Rather than dismiss a concept as usefull as
equilibrium constants all the 'concentration' terms get replaced with
ones for 'activity'. This gives us a new fiddle factor to make things
work :). Mostly the terms differ with highly charged ions like
aluminium, or in solutions of high ionic strength- in these situations
simple formulae exist relating activity to concentration. There are
other exceptions but only in relatively esoteric situations like
non-aqueous solutions. In aquaria all the concentrations are dilute, so
I doubt this assumption has any effect.
The equations assume there are no other equilibria _involving_ CO2 apart
from the carbonic acid/bicarbonate/carbonate system. I cant easily see
how say phosphate buffers could involve themselves here. My peat
extracts do appear to involve themselves with CO2 equilibria- Ive a few
guesses but to be honest I'm not exactly sure how :(.
Whatever the chart gives you about the best estimate of CO2 levels you
can easily obtain. More importantly I reckon it gives you a feel for
what affects the CO2 levels- for aquarists thats really the bottom line.
Toado.
BTW- Dupla add a root growth (hormone?) tablet to the (extortionist)
laterite kits they market here. Anyone know what the active ingredient
is?.