Metallic reductors

Although as already stated the use of metal amalgams, and in particular use of the Jones reductor or of the related silver reductor, is the best method of reducing solutions in preparation for titration with an oxidant, it may happen that for occasional use there is no Jones reductor available, and a simpler procedure will commend itself. In practical terms, the need is most likely to arise in connection with the determination of iron, for which the reduction of iron(III) to iron(II) may be necessary. 

The reduction of aqueous chromium(III) solutions can be carried out electrolytically o chemically with zinc amalgam, zinc and acid or a Jones reductor.2,24 Electrolytic procedures ca be cumbersome, and with chemical reductants contamination with other products can occur Chromium metal and acid can be used to reduce chromium(III) salts, and this requires less c the metal than in the method described in Section 35.3.1.1.i. 

Numerous versions of the Belousov-Zhabotinsky system differ by chemical compounds used. The typical reaction involves oxidation of some organic compound by bromate ion (BrOj ) occurring in acid medium with metal catalyst (Ce3+, Mn2+, as well as complexes of Fe2+, Ru2+). As an example, a particular reaction [4] could be mentioned, where an organic reductor is malonic acid CH2(COOH)2 and Ce3+ ions serve as a catalyst. In this reaction a solution changes periodically its colour due to oscillations in Ce3+ concentration. Generally speaking, the reaction consists of two stages. At the first one metal is oxidized... 

It is alloyed with about 4% A1 and 0.02% Mg. The aluminum strengthens the zinc and also prevents the molten alloy from attacking the steel pressure casting dies. Zinc readily reacts with mercury or will displace mercury from a mercury(II) salt to form an amalgam that is usefril for reductions, as in the preparation of compounds of the lower oxidation states of transition metals and lanthanides (e.g. Cr , V , Eu°, dimeric Mo ) and in analytical chemistry (e.g. in the Jones reductor see Analytical Chemistry of the Transition Elements). 

A sample of alkali metal chlorides was analyzed for sodium by dissolving a 0.800-g sample in water and diluting to exactly 500 mL. A 25.0-mL aliquot of this was treated in such a way as to precipitate the sodium as NaZn(U02)3(0Ac)9 6H2O. The precipitate was filtered, dissolved in acid, and passed through a lead reductor, which converted the uranium to U. Oxidation of this to required 19.9 mL of 0.100 M K2Cr207. Calculate the percent NaCl in the sample. 

Reduction The process whereby a species acquires electrons. Reduction potential The potential of an electrode process expressed as a reduction synonymous with electrode potential. Reductor A column packed with a granular metal through which a sample is passed to prereduce an analyte. 

The optimal acidity corresponds to pH 3.0 0.2. Only Au, Hg, and Pt interfere in determination of palladium. The reaction of TMK with Hg(ll) is slower (about 2 h). Oxidants also interfere the reagent behaves as a reductor. Chloride masks silver ions. EDTA can be used to prevent hydrolysis of some metal ions. 

Strongly reducing agents such as Cr(II), V(II) and U(III) are generally formed from metallic powders in a solid-phase reactor and, to this end, the Jones reductor is often used [84]. Their use in batchwise analytical procedures is however limited by the need to work under oxygen-free conditions, which makes storage difficult [74]. 

In aqueous solution many metal ions are reduced by sodium borohydride to borides [for example, Ni(II) and Co(II)], to the metal [for example, Ag(I)], or to a lower oxidation state [for example, Ce(IV) converted to Ce(III)]. Some of these reactions have found analytical application for example, analysis of iron, wherein NaBFL replaces SnCL or the Jones Reductor. 

The mercury(l) chloride is unaffected by oxidants during the subsequent titration. Sulfite (or SO2) and hydrogen sulfide are alternative reductants. Metals may also be used. Small pieces of metal (zinc - a Jones reductor, silver - a Walden reductor) are used to fill a column, through which the analyte solution is passed. The effluent is titrated with oxidant. A comparison of the reduction products of the two reductor columns is given in Table 3. 

Every chemist who has performed a reaction with a metal has observed that the spontaneous reaction is not the general rule. Even relatively strong reductors, (alkali metals, magnesium, aluminum) frequently react sluggishly after an induction time because of the presence of surface impurities which inhibit the reaction. This phenomenon is known as "passivation". 

Jones redactor. A Jones reductor is a deviee whieh ean be used to reduee a metal ion in aqueous solution to a very low oxidation state. 

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