Cerium primary standard

The standardisation of thiosulphate solutions may be effected with potassium iodate, potassium dichromate, copper and iodine as primary standards, or with potassium permanganate or cerium)IV) sulphate as secondary standards. Owing to the volatility of iodine and the difficulty of preparation of perfectly pure iodine, this method is not a suitable one for beginners. If, however, a standard solution of iodine (see Sections 10.112 and 10.113) is available, this maybe used for the standardisation of thiosulphate solutions. 

Ceric ammonium nitrate is used as a volumetric oxidizing reagent in many oxidation-reduction titrations. Cerium(IV) ion is a strong oxidant simdar to permanganate ion. It is the most widely-used primary standard among all Ce(IV) compounds. Other apphcations of this compound are in organic oxidation reactions and as a catalyst in polymerization of olefins. 

A sample of pure (NH4)2Ce(N03)g indicates a purity (for cerium) of 99.98% when it reacts with primary standard arsenic(III). Examination of the (NH4)2 Ce(N03)g by emission spectroscopy indicates that it contains 0.06% of the element thorium [presumably as (NH4)2Th(N03)e]. Such results can be rationalized by assuming a deficiency of NH3 and that the sample contains H2Ce(N03)6. (a) What is the percentage of the theoretical cerium content (6) Calculate the percentage of H2Ce(N03)e in the sample, (c) Calculate the percentage of the theoretical NH3 content. 

The most widely used compounds for the preparation of solutions of cerium(IV) are listed in Table 20-4. Primary-standard grade cerium aimnonium nitrate is available commercially and can be used to prepare standard solutions of the cation directly by weight. More commonly, less expensive reagent-grade cerium(IV)... 

Potassium dichromate solutions are indefinitely stable, can be boiled without decomposition, and do not react with hydrochloric acid. Moreover, primary-standard reagent is available commercially and at a modest cost. The disadvantages of potassium dichromate compared with cerium(IV) and permanganate ion are its lower electrode potential and the slowness of its reaction with certain reducing agents. 

Cerium(IV) solutions can be standardized against primary standard AS2O3, Na2C204, or electrolytic iron. The reaction with arsenic(III) is slow, and it must be catalyzed by adding either osmium tetroxide (OSO4) or iodine monochloride (ICl). Ferroin is used as the indicator. The reaction with oxalate is also slow at room temperature, and the same catalyst can be used. The reaction is rapid, however, at room temperature in the presence of 2 M perchloric acid. Nitroferroin is used as the indicator. 

Cerium(IV) solutions to be standardized are usually prepared from ammonium sulfatocerate, (NH4)4Ce(S04)4 2H2O ammonium nitratocerate, (NH4)2Ce(N03)6 not the high-purity primary standard variety, ttiough) or hydrous ceric oxide, Ce02 4H2O. Primary standard ammonium nitratocerate is used only if the solution is not to be standardized, because of its increased expense. 

Other primary standards for the standardization of cerium(IV) solutions are pure iron or sodium oxalate. For the standardization of cerium(IV) against sodium oxalate, a hot solution... 

Oxalohydroxamic acid is a strong reducing agent. It has been used as a primary standard. Stoichiometry of the redox reaction i.e. OHA and Cerium (IV) is established earlier. The course of reaction is described by the scheme and end product analyses. These facts are implemented simply and successfully for the determination of 0/M in nuclear fuel. Analysis of U3O8 sample is precisely reported in the proposed method. 

This shows that Ce(IV) will oxidise Fe(II) but unlike dichromate, Ce(IV) compounds are not usually available as highly pure solids. However, technical grade ammonium cerium (IV) nitrate can be reciystallised and the purified crystals dissolved to give a solution. This, however, has to be standardised by titration against K4[Fe(CN)6]. The purest salt can be considered as a primary standard. The value of E" depends on the nature of the anion. 

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