The medium is already a buffered (bicarbonate buffer) solution which maintains a proper pH when in the incubator with 3-5% CO2. But the pH rapidly changes when working with the medium outside the incubator However, you can add HEPES (final concentration 10-20 mM) to the medium gives more buffering capacity which maintains pH between 7.2-7.6 even outside the incubator. The other alternatives to HEPES are MOPS and DIPSO.

and my medium needs to be more acid, what y should add to it?

Hi,

You can use 0.1 M HCL. Other options would be citric acid, malic acid or tartaric acid.

thank you santosh, my doubt was about if hcl makes something precipitated, but if you are sure thet no...