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Re: CO2 measurements
Paul Sears wrote:
> What do we mean here by "KH", anyway? I understood it to mean
> the (bi)carbonate present, but it seems that people have of late been
> using it to mean _all_ the weak acid anions, i.e., "alkalinity".
I think most of us are referring to bicarbonate concentration when we use
the term "KH". However, the quantity that we measure is actually
alkalinity.
The USGS (and USEPA) determines bicarbonate in water using the alkalinity
titration with results expressed as mg/l HCO3-. Other weak acids in the
sample are simply regarded as "interference". The weak organic and
inorganic acids that can interfere with the bicarbonate value are
negligible in most natural water. The hobby kits are just simplified
versions of the same titration.
It's possible to detect the presence of other acids by recording pH as a
function of acid volume added during titration. If the sample contains no
interfering acids then the pH will drop rather abruptly as the last of the
bicarbonate reacts. If the sample contains other acids then the pH will
decline to the pH 4.5 endpoint gradually or in multiple small steps.
I think the method is purely qualitative and can't be used to distinguish
quantitatively between bicarbonate and other acids.
Roger Miller