MES, Sodium Salt (2-(N-Morpholino)ethanesulfonic acid sodium salt) is a zwitterionic N-substituted aminosulfonic acid known as a Good's buffer, active in the pH range 6-8 with a pKa of 6.10 at 25° C. With a pKa value near physiological pH, it is not absorbed through cell membranes, and it is essentially transparent to UV light. MES is used as a running buffer for resolving very small proteins (ex: 13 kDa) on bis-tris gels. Also used in cation-exchange chromatography and as a biological buffer in plant cell cultures. MES solutions can be prepared to the desired pH by titrating the free acid with sodium hydroxide.