low pH medium for HEK cells

[joesep]

I'm still quite unexperienced in cell culture, so I hope you can help me.
I need to grow HEK cells at a relatively low pH (~6.0 - 6.5). This should kill my transfected cells, whereas normal HEKs should survive (at least for a few days). I normally grow them in RPMI medium (+ 10 % FCS + several other supplements). I tried lowering the pH to 6.0, but within 2 days (37°C, 5% CO2) it rises to about 7.0. I think the medium is HEPES-buffered. Are there other media available, that are un buffered or MES-buffered? Or should I change CO2 %?
Thanks for your suggestions!

[wangleipurdue]

I am not sure whether CO2-independent medium (you can get it from Sigma) will be helpful or not. At least those medium won't be affect by CO2 (the pH should not change).

Or maybe you can put your cells in an incubator without CO2, or use flasks that have no filter in lids that wont' allow CO2 to enter your flasks. (for my understanding, CO2 is just for pH value, maybe I am wrong)