Mean_Chlorine wrote:
Thusly Elaine T Spake Unto All:


You've got me curious because I've not seen pH stabilize around 7.5 when
I fill a tank calcium carbonate. I've always had hard, high pH
tapwater, though. When I stuff a tank with carbonates, it's usually for
marine fish or Tanganyikan cichlids and with hard water to start with,
the pH generally ends up above 8.0. If you have time to elaborate on
the equilibrium chemistry for soft water or have a link, I'd enjoy
taking a look.


If you start with very hard water, it'll take very long for the acids
and CO2 produced by your fish & plants to drive pH down to where the
limestone will start reacting; quite possibly it'll never happen if
you top up with hard water or do regular water changes - the buffering
capacity of the water itself is such that the buffering capacity of
the limestone never comes in to play.

That is, you start with so hard water that it doesn't matter what
rocks you put in. If you start with very soft water, you'll tend to
end up around 7.5.

Very hard natural water will at equilibrium have a pH of 8.3. This is
the highest pH you can get with bicarbonate (the product of limestone
dissolution and chief buffering component of natural waters).
Natural waters with a pH significantly higher than that (e.g.
Tanganyika, with a pH of 9 - 9.2) get that pH either because 'soda'
(hydroxide) is leaching into the water from the surrounding sediments,
or because there is so heavy plant growth that bicarbonate becomes
depleted (the CO2 content of the water is not at equilibrium with the
atmosphere).

Also, and don't take this the wrong way, but unless you're using a
recently calibrated electronic pH meter, ie you use
drip-titration-kits or, worse, pH paper or multisticks, you don't
actually know your pH with greater accuracy than +/- 0.5 to 1 unit,
regardless of what it says on the box.

Thanks - that makes sense. Basically you're saying that bicarbonate
from calcium carbonate only reaches its 8.3 pKa in natural waters where
the amount of limestone is huge and the water has years of slow contact,
right?

I mostly use liquid bromthymol or bromphenol blue for measuring pH
around neutral. I'm not sure what the indicator is for my liquid high
range kit. However, you reminded me of the classic science lab
experiment where you put a drop of BTB in distilled water. Gently blow
on it for a bit and it goes yellow. Shake hard for a little while and
it goes blue. Similarly, it's wickedly hard to get a reproducible pH
measurement on a CO2 injected plant tank with a liquid kit.

I actualy don't have too much trouble with my liqid kits getting
reproducible measurements within a given day in my hardwater non-CO2
tanks (I've tried this), but you're correct that I don't know the accuracy.

--
Elaine T __
http://eethomp.com/fish.html '__
rec.aquaria.* FAQ http://faq.thekrib.com