Neutralized lanthanum solution: a largely noncolloidal ultrastructural tracer.

Lanthanum nitrate solution adjusted to pH 7.4 and pH 7.7 was subjected to column chromatography, ultrafiltration and conductivity measurements. Lanthanum concentration was measured by a colorimetric method employing eriochrome cyanine RC. Under these conditions, lanthanum was not excluded from the column by a packing with an exclusion limit of 1800 daltons. Ultrafiltration through a membrane with a filter limit of 500 daltons allowed approximately 75% of the lanthanum to pass. Conductivity measurements showed a decrease of charge of about 20% on adding sodium hydroxide to a solution of lanthanum nitrate up to a pH of 7.7. It is concluded that approximately 80% of the lanthanum exists as a charged particle of less than 500 daltons at pH 7.7; the other 20% consists of larger, possibly colloidal particles. Nonfiltered and ultrafiltered lanthanum have equally good staining and tracer properties in the electron microscope, suggesting that staining depends largely on the ultrafiltrable noncolloidal lanthanum ion.