I am checking the receptor binding activity of a bacterial toxin in the presence of free sugars (e.g sialic acid) using HNKCM buffer (10mM HEPES (pH 7.4) 140mM NaCl 5mM KCl 2mM CaCl2 and 2mM MgCl2) but I face fall of the reaction mixture pH, which is most likely due to the acidification of buffer by the presence of mild acidic nature of sialic acid.

Can anyone suggest me which buffer should I use to maintain the pH even in the presence of sialic acid, or which component of the current buffer should be changed to get better pH maintenance.