The membrane made for DO probes works well for a drop checker, and with great effort can be used to make a quick responding drop checker. But, that quick response comes at the price of difficulty in making it, difficulty in reading the color, and short life for it.
CO2 in water is in a constant state of changing between carbonic acid, carbonate and carbon dioxide. It will leave the water as easily as it enters the water, so having to change back to carbon dioxide doesn't slow it down much at all. The reason for this is.....uh.....beyond the scope of this forum. (I refuse to admit a lack of knowledge about this, I just don't want to talk about it.)