Jones Reductor
Definition and meaning of Jones Reductor in chemistry.
A Jones reductor is a glass column packed with zinc-mercury amalgam used in analytical chemistry to reduce metal ions to lower oxidation states before quantitative redox titration.
In more detail
The device consists of a vertical glass tube filled with zinc granules coated with a thin layer of mercury. When an acidic sample solution is passed through the column, metallic zinc acts as a powerful reducing agent, being oxidized to Zn2+ while reducing the analyte species. The mercury coating serves a critical function: it raises the hydrogen overvoltage at the zinc surface, suppressing hydrogen gas evolution from the acidic solution, which would otherwise waste the reducing capacity and dilute the analyte. Common applications include reducing ferric ions (Fe3+) to ferrous ions (Fe2+) and dichromate ions (Cr2O7 2-) to chromium(II) before subsequent titration with permanganate or other oxidizing titrants. Because the Jones reductor is a strong, non-selective reductant, it drives reductions further than milder alternatives such as the silver-based Walden reductor, which typically stops at chromium(III).
Key facts
| Field | Analytical Chemistry |
|---|---|
| Primary composition | Zinc-mercury amalgam (Zn-Hg) in a glass column |
| Reducing agent | Metallic zinc (oxidized to Zn2+) |
| Common analytes | Fe3+, Cr(VI), vanadium compounds |
A sample containing iron(III) can be passed through a Jones reductor to convert all Fe3+ to Fe2+, which is then back-titrated with potassium permanganate to accurately determine total iron content in the original sample.
Frequently asked questions
Why is mercury amalgam necessary instead of pure zinc?
Mercury raises the hydrogen overvoltage on the zinc surface, preventing hydrogen gas formation when zinc contacts the acidic sample solution. This preserves reducing capacity and prevents solution dilution that would compromise analytical accuracy.
What oxidation state changes occur in a Jones reductor?
High oxidation states are reduced to lower ones, such as Fe3+ to Fe2+, Cr(VI) to Cr(II) (passing through Cr3+), and V(V) to V(II), enabling consistent redox titrations.