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Re: CO2 tests

> Date: Sat, 18 Oct 1997 14:03:37 -0400
> From: Ed Hengel <hengel at computer_net>
> > I'm unfamiliar with the LaMotte Test Kit but....
> > 
> > If the CO2 reading is obtained by titrating the water sample with
> > Na2CO3, then free mineral acid, if present, will be measured. Also,
> > heavy metals such as iron, chromium, and aluminum salts interfere.
> > This from the Betz Handbook of Industrial Water Conditioning.
> Does anyone know if this is the case with the LaMotte kit?

From the LaMotte Water Test Handbook (free for asking, I think):



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 analysis 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.