My API KH test kit is a one reagent test. You add drops of the reagent, the first drop usually making the solution one color, and the last drop changing the color to another color. Each drop that take equals 1 dKH of carbonate hardness. I have never seen dissolved CO2 affect the KH reading.
I used the API KH test kit too. I'm probably mistaken about it being two bottles, as I stopped buying KH tests many years ago. As rarely as I did KH testing, I never got much use out of them before they expired and failed.
But my memory is absolutely clear that it was affected by CO2. Because I wondered where my KH was going too. I'd test 8°KH in aged tapwater. Do a 50% water change with tapwater, and read 0°KH in the tank the next day. And that was in a tank with gravel. Impossible! But shaking the water sample in a bottle with air first restored the KH.
Just to double-check, I looked up API's MSDS. Unfortunately, it doesn't list any ingredients; as listing anything that presents minimal hazard is optional. Looking for the first MSDS with a full listing, I found this:
Bromocresol Green, Sodium Salt (according to Wikipedia, a pH indicator)
Which is exactly as I originally described, nothing more than a pH test with acid mixed in. And will be affected if another acid is present, like dissolved CO2.
As for why you haven't observed this effect, I'm unsure. Perhaps we performed the test a little differently. From the API directions:
"Cap the test tube and invert several times after each drop."
I did as directed, and inverted
- which is just turning it upside down and then righting it. This does a fair job of mixing the liquids, but without significantly driving air into it and so results in minimal outgassing. Maybe you shook
instead, outgassing most of the CO2 to the airspace in the test vial?