Different formulation are used depending on pH and osmotic pressure,eg
Krebs-Ringer buffer (115 mM NaCl, 5.9 mM KCl, 1.2 mM MgCl2, 1.2 mM NaH2PO4, 1.2 mM Na2SO4, 2.5 mM CaCl2, 25 mM NaHCO3, 10 mM glucose, pH 7.4)
http://www.protocol-online.org/biology-forums/posts/25841.htmlKrebs buffer (118 mM NaCl, 4.7 mM KCl, 1.2 mM KH2PO4, 1.2 mM MgSO4, 4.2 mM NaHCO3, 2 mM CaCl2, 10 mM glucose, 200 mM sulphinpyrazone and 10 mM Hepes, pH 7.4)
http://www.med.upenn.edu/kaestnerlab/documents/Krebs.pdfand so on