about testing carbon dioxide 2

good question.

but even if you bubble in excess SO2, the H2SO3 rpoduced would turn insoluble CaSO3 into more soluble Ca(HSO3)2.

you can bubble the gas into acidified permanganate solution to differentiate SO2 and CO2 : SO2 is a reductant, which turns purple permanganate to colorless; CO2 cannot do this.


2011-07-25 13:41:54 補充：
code PLUS:
don't agree.
sodium hydrogen carbonate indicator is NOT a specific indicator or CO2.
in fact, it's just a sensitive pH indicator (composed of cresol red and thymol blue), which changes color when there's only a slight increase in acidic gas content in water.

2011-07-25 13:43:17 補充：
CO2 is acidic gas, SO2 too.
if the dye molecules are not bleached by SO2, it'd change color as same as CO2 does.

of course SO2 may bleach the dye, but then there's no need to specifically use this indicator, right?

You can try to use sodium hydrogencarbonate indicator to test carbon dioxide.