Indirect determinations with dichromate

The reaction of dichromate with iron(II) has been widely used for the indirect determination of a variety of oxidizing agents. In these applications, a measured excess of an iron(II) solution is added to an acidic solution of the analyte. The... 

Zinc metal can be determined in zinc oxide by means of indirect methods. Zinc is oxidized with dichromate (in an H2SO4 or H3PO4 medium), and then the unreacted part of Cr(VI) is determined with 1,5-diphenylcarbazide [106]. In another version, the sample is dissolved in dil. sulphuric acid in the presence of Fe(III) and 1,10-phenanthroline. The red Fe(n)-phen complex is equivalent to the content of Zn in the zinc oxide studied [107]. 

Lead nitrate can be used for the titration of tartrates > or glycerol phosphates in addition to sulphates already mentioned. A standard solution of lead nitrate is also used for the final titration in an indirect determination of morphine. > The method consists of treatment of the sample containing morphine with an excess of a standard solution of potassium dichromate in acid solution. Morphine is oxidized and the chromate reduced to the chromic state. The excess of dichromate is determined by titration with a standard solution of a lead salt at pH 4-2. The method was used for the analysis of Tinctura Opii. 

K2Cr207 is not as strong an oxidant as KMn04 or Ce4+. It is employed chiefly for the determination of Fe2+ and, indirectly, for species that will oxidize Fe2+ to Fe3+. For indirect analyses, the unknown is treated with a measured excess of Fe2+. Then unreacted Fe2+ is titrated with K2Cr207. For example. CIO7, NOj, Mn04, and organic peroxides can be analyzed this way. Box 16-1 describes the use of dichromate in water pollution analysis. 

Determination of iron The most important applications of dichromate involve either directly or indirectly the titration of Fe(II). An excess of standard Fe(II) can be added to determine oxidants, or an excess of Fe(III) to determine reductants. These determinations usually can be carried out equally well with Ce(IV). For routine applications, however, the low cost and ease of preparation of standard solutions and the great stability of dichromate offer some advantages. Permanganate is at a disadvantage, expecially if hydrochloric acid solutions are to be used. 

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