Mutual Metal Separation System with Enrichment Using pH‐Peak Focusing Countercurrent Chromatography

Abstract

A highly efficient enrichment of metal ions was achieved by pH‐peak‐focusing high‐speed countercurrent chromatography (CCC) that is developed by the variation of the pH‐zone refining high‐speed CCC. In this method, Ca, Cd, Cu, Mg, Mn, and Zn are chromatographically extracted in a basic organic stationary phase containing ammonia and a complex‐forming reagent such as di(2‐ethylhexyl) phosphoric acid, by introducing the sample solution into the column rotated at 800–1200 rpm. When the column is eluted with the acidified mobile phase containing hydrochloric acid, metal ions are trapped and concentrated around the sharp pH border formed in between the acidic and the basic zones, moving toward the outlet of the column. Enriched metal ions are finally eluted with the sharp pH‐border as a highly concentrated peak into a small amount of eluent. The pH of the border is determined by the kinds of base and acid, and it shows the neutralization point. At this pH, trapped solute is moved towards the outlet of the column, based on its distribution ratio in each phase. The travel of this pH border through the column is determined by the molar ratio between the base in the stationary phase and acid in the mobile phase. By selecting appropriate experimental conditions, such as an acidity of the sample, bore of the column, and volume of the sample, each metal separation was achieved due to a longitudinally pH gradient on both sides of the pH border. Metal ions were concentrated at different positions in the moving pH border. When the pH border was sufficiently wide in the final stage of the elution, each concentrated metal could be separated mutually. The present method can be successfully applied to trace determination of cadmium in tap water, with coexsisting relatively large amounts of magnesium.

Keywords:

• Mutual metal separation
• pH-peak focusing countercurrent chromatography
• Enrichment
• Metal ions