Re: CO2 test kit recommendations

> From: rrs0062 at ibm_net (Mark A. Thompson)
> One warning, though.  When I was using tap water the kit read
> artifically high (due to something that George and others can explain,
> I'm sure).  

That's a hint, right?

> A few months ago I started using a RO unit and my tests
> show CO2 to be much closer to what I expect using the pH/KH/CO2 chart
> in The Optimum Aquarium.

A couple of reasons for high CO2 readings:

1) Dissolved CO2 in the tap water 
2) KH was not the predominate component of alkalinity
3) Mineral acids in the water

The RO unit should negate all of these causes. 

Here's some info from the Hach Water Testing Handbook (free for the

  Hach Company
  PO Box 608
  Loveland, CO

  phone (800) 227-4224
  in Colorado (303) 669-3050



Carbon dioxide is present in all surface waters, generally in amounts
less than 10 mg/L.  However, higher concentrations are not uncommon in
ground waters.  Dissolved carbon dioxide has no harmful physiological
effects on humans and is used to recarbonate water during the final
stages of water softening processes and to carbonate soft drinks.
High concentrations of carbon dioxide are corrosive and have been
known to kill fish.

The anlysis for carbon dioxide is similar to that for acidity.  A
water sample is titrated to a phenophthalein end point with Sodium
Hydroxide Standard Solution. Strong mineral acids are assumed to be
absent or to be negligible in effect.  Care must be taken during the
anlysis to minimize the loss of carbon dioxide from the water sample
as a result of aeration when collecting and swirling the sample.

Chemical Reactions

The reaction of sodium hydroxide with carbon dioxide (as carbonic
acid) occurs essentially in two steps, first a reaction from carbonic
acid to bicarbonate and then to carbonate:

  CO2 + H2O -> H2CO3

  H2CO3 + NaOH -> NaHCO3 + H2O

  NaHCO3 + NaOH -> Na2CO3 + H2O

Because the conversion of carbon dioxide to bicarbonate is complete at
pH 8.3, phenophthalein can be used as a color indicator for the
titration.  The sodium hydroxide titrant must be of high quality and
free from sodium carbonate.