Possible substitutions of HEPES buffer with sodium-free buffers - (Apr/16/2013 )

Hello, I am working with CHO cells expressing the hM2 receptor. Usually, I perform my experiments with this buffer: 12.5 mM HEPES, 12.5 mM MgCl₂; 125 mM NaCl. For my next experiments I need a sodium-free buffer, pH around 7.4
Could anyone suggest me a similar buffer, with the same ionic strenght, but sodium-free?

Thank you!

you could use potassium chloride instead of sodium chloride and potassium hydroxide to adjust the hepes buffer (unless it will cause problems with the cells).

Thank you!!