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I've been following the long thread about the mystery tap water CO2.
What follows is not likely to be the sole cause of the high CO2 readings
but the info may be a useful. The sodium hydroxide titrant (used in the
LaMotte and Hach CO2 test kits, perhaps others too) loses strength over
time because it absorbs atmospheric CO2. The lose in strength is slow.
However, if the bottle was left open for extended periods or if the kit
was old (aged in the vendor's warehouse or home), the effect can be
significant. If the normality of the titrant was half what it should be,
all CO2 readings would be twice actual values. That's why Hach (don't
know about LaMotte) sells a potassium acid phthalate standard solution
to verify the strength of the NaOH titrant (and also to correct for loss
of NaOH strength in doing calculations). Ergo, keep that bottle of NaOH
titrant tightly closed when not in use.
Shiao Y. Wang
University of Southern Mississippi
sywang at whale_st.usm.edu