What are the differences between 50 mM glycine-hydrochloric acid buffer (pH 3) and 100 mM glycine-hydrochloric acid buffer (pH 3), especially regarding osmotic pressure? Why do they have the same pH value but different glycine concentrations.If I need to use it to wash cell , which concentration of glycine buffer is closer to isotonic, thereby ensuring cell survival?
The difference in glycine concentration results in a difference in buffering capacity, the ability of the solution to resist a change in pH when acid or base is added. The higher concentration solution has greater buffering capacity. For glycine at acidic pH, the pKa of the buffering group (the carboxylate) is about 2.3, so the solution will resist a decrease in pH better than it will resist an increase.
The osmotic pressure is proportional to the concentration of molecules or ions. Isotonic saline has a concentration of 0.9 weight % of NaCl, which is 154 mM NaCl. NaCl is fully dissociated into ions in solution, and therefore 154 mM NaCl has an osmolarity of 308 mOsm, due to the presence of 154 mM each Na+ and Cl-.
The osmolarity calculation for the glycine-HCl pH 3 buffer is tricky. Simplistically, the osmolarity of glycine-HCl is 3 times the molarity, since it is composed of 3 species (glycine, H+ and Cl-). However, this isn't exactly true, since some of the H+ is consumed to form protonated glycine. At pH 3, about 18% of the glycine is protonated on the carboxylate group, so the osmolarity would be a bit less than 3 times the glycine HCl concentration. Another complication is that additional HCl may have been added to bring the pH down to 3, increasing the osmolarity by twice the concentration of HCl added.
Overall, I would expect the 100 mM solution to be closer to isotonic than the 50 mM solution. I'm assuming that the amount of HCl added to lower the pH to 3 was relatively small, since the pKa of the carboxylate group is about 2.3. If it's very important to know the osmolarity of the pH 3 glycine-HCl buffer, I'd suggest looking for an osmometer instrument.
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