Just measure the pH of the media after incubation in the CO2 incubator, remembering that CO2+H2O= H+  and HCO3-....10 mM Hepes pH 7.4 should be able to easily maintain this pH in 5% CO2...if you like you can raise or lower the % CO2

What is a buffer? It is a mixture of acid (or base) with its salt. So HEPES really can not be buffered in a 5% CO2 environment. HEPES solution can still maintain the pH but if your system continuously produce CO2 then its buffering capacity may not be ideal.

For Ru-Jeng

HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid ) is a zwitterionic organic chemical buffering agent; one of the twenty Good's buffers. HEPES is widely used in cell culture, largely because it is better at maintaining physiological pH despite changes in carbon dioxide concentration (produced by cellular respiration) when compared to bicarbonate buffers, which are also commonly used in cell culture.