Measuring CO2 if using Phosphate Buffer?

[Tin Pusher]

I am using a phosphate buffer to regulate my Ph. I have heard that these buffers make the GH reading unreliable. If this is true how can I measure CO2 (I have been using the GH/Ph method)? ???

[Ralph]

The phosphate replaces the carbonates as the buffering agent and invalidates the CO2 test you are using, just like you've heard. I only know of one test for CO2 with phosphates present. It requires very accurate pH testing though, within .1 at least.
This stuff is at the edge of my expertise, it's getting into the area of the Dutch style tanks where they use maximum CO2 to get the fastest, biggest, greenest plant growth possible. If you are talking about a discus tank, you are talking about too much CO2 (fish must come first). Usually for a discus tank, they use the low tech method of counting the number of seconds between CO2 bubbles injected (and you don't even need a calculator). I can explain the other method if you are still interested though.

[Tin Pusher]

I have never understood counting bubbles. It varies too much depending on how you difuse the bubbles.
Thanks

[Ralph]

Take a water sample from the tank and measure pH (call this pHTank). Take another water sample and run an air stone in it for a minute, then measure the pH (call this pHAerated). Then plug into the following formula:

log(CO2 in ppm) = pHAerated - pHTank - .3

The idea is that the aerated sample has a CO2 level equal to the CO2 level in the air, which is usually a constant. Then the difference in pH is used to give the CO2 in the tank. I don't know why there is the .3 adjustment.
I've never used it (never needed to) but it is supposed to be reasonably accurate if your pH readings are accurate.

[Tin Pusher]

Thanks Ralph, I will give it a try. I think I better get a more accurate Ph test kit first.