A Jones reductor is a device which can be used to reduce a metal ion in aqueous solution to a very low oxidation state. The active component is a zinc amalgam. It can be used to prepare solutions of ions, such as chromium(II), Cr2+, and uranium(III), U3+, which are immediately oxidized on contact with air.
Contents
Preparation and use
Amalgamated zinc is first prepared by treating zinc metal with a 2% solution of mercury(II) chloride, in a beaker. The metal may be in the granulated form or as shavings, wool, or 20-30 mesh powder. The mercury ions are able to penetrate the passive layer and are reduced to elemental mercury, forming the amalgam, which is a kind of alloy of both metals, on the metal surface. The amalgam is thoroughly washed by decantation and placed in a long glass tube, similar to a chromatography column, equipped with a stopcock and a means to support the amalgam, such as a sintered-glass disk. The outlet of the tube is connected to a collection flask by an air-tight seal and the collection flask is connected with a vacuum source.
To use the reductor, the solution to be reduced is placed at the top of the tube, and then is drawn through it. If the column is loosely packed, the solution may pass though without assistance, but if the tube is tightly packed, the pressure in the collecting flask may need to be reduced. In effect this configuration is similar to that used for column chromatography as the extent of reduction increases to 100% as the solution passes down the tube and the product is completely separated from the starting material.
Applications
Amalgamated zinc is a powerful reducing agent, with a standard redox potential of −0.76 V. The amalgam does not have a passive layer so it is a more effective reducing agent than zinc metal. The Jones reductor is useful for preparing solutions of titanium(III), vanadium(II), chromium(II), molybdenum(III), niobium(III) and uranium(III). All these ions are extremely susceptible to oxidation. For example, the reductor can be used in the preparation of crystalline chromous acetate. This solid is moderately stable in air and is a good starting material for the preparation of other chromium(II) compounds.