Relationships Among pH, Osmolality, and Concentration of Fixative Solutions

Abstract

This report contains graphs that relate molarity with pH and osmolality (as measured by freezing point depression) for 5 buffer systems that are commonly used in preparing fixatives for electron microscopy, namely: cacodylate-HCl, s-collidine-HCl, Millonig's phosphate, Sorensen's phosphate, and veronal-acetate-HCl. In addition, osmolality is shown as a function of concentration for glutaraldehyde, OsO₄, NaCl, glucose, sucrose, and CaCl₂. These data provide simple guides for the design of buffered fixatives of any desired pH and tonicity. Osmolality cannot generally be calculated accurately from theory because of the partial ionization and aggregation of the concentrated solutions used. Commercially available glutaraldehyde often contains impurities which may increase its osmolality.