Re: GH, KH.

> From: Matt Rhoten <mrhoten at oz_net>
> Date: Sun, 01 Oct 1995 23:40:46 -0700
> Subject: two qs: KH/GH, and CO2 valves
> I understand that GH measures Ca++ and Mg++, while KH measures CO3--
> and HCO3-, and that these values don't necessarily go in sync. As I
> see it, this could happen two ways:
> 1) Some other cation is there instead of Ca or Mg. This could happen
> with the presence of, for example, our friend sodium bicarbonate. If this
> is the case, which ions? Na+? K+?
	The GH test works on Ca++ and Mg++, and will probably show
just about any other ion with two positive charges and similar ionic
radius.  This could include a lot of things, but there is not very
much else that is likely to be there in any significant amount.
	If the KH is higher than the GH then there must be some other
cation there, and once again, while lots of things would do, sodium
and potassium are by far the most likely culprits, in that order.

> 2) What Tetra says is KH is really alkalinity, and some factor other
> than carbonate hardness raises the alkalinity in the collection
> water. If this is the case, which other compounds (presumably
> hydroxides of some metals or other) are present?
	"alkalinity" is a term that is thrown around a lot without a
clear definition being presented.  As far as I can make out, it 
represents the amount of anions of weak acids present, i.e. those
that can be protonated in the pH range that interests us, say 5 to 9.
This includes the reactions:
HCO3-  +   H+   <->   H2CO3       pKa 6.38
HPO4--  +   H+   <->   H2PO4-     pKa 6.21

The pKa is the pH at which the concentrations of the acid and the anion
derived from it are the same.  It is possible that some complex organic
acids might come into this group.

	Hydroxides are not a viable possibility if the water is usable
by fish!  If we had even 1 KH of hydroxide, this would mean a concentration
of about 3 x 10^-4 molar for the hydroxide ion.  (When these things
dissolve, the ions separate.)

	However, we have, in water:   [H+][OH-] = 10^-14
	So, if [OH-] = 3 x 10^-4, [H+] = 3.33 x 10^-11  (molar)
	i.e. the pH = 10.5

This is a bit much even for the african cichlids!

If we had 0.1 KH of hydroxide, the pH would be 9.5, which I seem to recall
is about the limit for the cichlids, so it is just about possible to have
this much.  By the same token, there will not be much CO3-- around at
any useful pH, as I pointed out in my last post here.

> One other more mundane question that came up has to do with setting up
> CO2 from one welding tank to more than one aquarium. Presume for a
> moment that although I am not clueless in the physical sciences I know
> next to nothing about valves or fittings. It is unlikely that I will
> want equivalent amounts of CO2 injection in each aquarium.
> My hypothesis for how I will need to set it up involves having only
> one regulator for the welding tank, but one needle valve per fish
> tank. So there would be a Y-joint 'after' the regulator, and each
> needle valve (one per aquarium) would attach to the Y-joint. Is this
> how it's done?  This doesn't seem to scale well, as needle valves
> aren't cheap. What do people with N (N >> 2) aquaria do about this
> problem? I'm interested in keeping the number of 50psi fittings to a
> minimum.
	Yes, you need a needle valve for each aquarium, all connected
to the regulated supply.  In my experience, good needle valves are the
hardest things to find! 

Paul Sears,     Ottawa, Canada