Ceric sulphate

Karrer and Schmid have noted that the dimeride of C-curarine I does not give the characteristic colour reactions of this alkaloid with acids, but still gives that with ceric sulphate and potassium dichromate (p. 384). 

Alkaloid B. This was isolated as the chloride, C20H25ON2CI, which in 2N-sulphuric acid gives a deep, reddish-violet colour with either potassium dichromate or ceric sulphate. The picrate has m.p. 195-6°. 

C-Alkaloid UB forms a picrate, CjgH2303N2, CgHjO N, m.p. 238-240°, which gives an intense, but unstable, carrtiine colour with ceric sulphate in strong sulphuric acid and is convertible into a crystalline iodide. 

C-Alkaloid X obtained in minute quantity as a crystalline chloride, which with nitric acid gives an intense red colour with a violet fluorescence, and orange slowly becoming red, with ceric sulphate in sulphuric acid. 

In a sulphate ion medium, ceric sulphate oxidises Br to Br2 according to the rate expression ... 

The compound is employed inter alia as an indicator in titrations with potassium dichromate and ceric sulphate solutions. 

The work on the electrochemical generation of a solution of ceric sulphate from slurry of cerous sulphate in 1-2 M sulphuric acid was abandoned by BCR due to difficulties encountered in handling slurried reactants. A 6kW pilot reactor operated at 50 °C using a Ti plate anode and a tungsten wire cathode (electrolyte velocity about 2ms 1) produced 0.5 M Ce(S04)2 on the anode with a current efficiency of 60%. The usefulness of Ce(IV) has been limited by the counter anions [131,132], Problems include instability to oxidation, reactivity with organic substrates and low solubility. Grace found that use of cerium salts of methane sulfonate avoids the above problems. Walsh has summarized the process history, Scheme 6 [133],... 

Explanation A reaction usually takes place by the combination of oxidizing and reducing agents and this may be considered as the basis for the quantitative measurement of one of the reactants. For instance, FeS04 can be determined quantitatively by its reaction with ceric sulphate [Ce(S04)2] as expressed by the following equation ... 

PERMANGANATE, DICHROMATE AND CERIC SULPHATE TITRATION METHODS... 

Ammonium ceric sulphate serves as a powerful oxidizing agent in an acidic medium. The salt has a bright yellow colour and so its solution. On reduction, the resulting cerous salt obtained is colourless in appearance and, therefore, strong solutions may be considered as self-indicating. In general practice, 0.05 N solutions are employed invariably for estimations. As this concentration is very dilute for observation of the respective end-point, hence the inclusion of an appropriate indicator becomes necessary. The oxidation reaction involved may be expressed as follows ... 

It is interesting to observe that the solutions of ammonium ceric sulphate possess a number of advantages over permanganate and dichromate methods discussed earlier in this chapter, viz.,... 

Procedure Weigh accurately about 0.2 g of arsenic trioxide previously dried at 105°C for 1 hour and transfer to a 500 ml conical flask. Wash down the inner walls of the flask with 25 ml of sodium hydroxide solution, swirl to dissolve, add 100 ml of water and mix. Add 30 ml of diluted sulphuric acid, 0.15 ml of osmic acid solution, 0.1 ml of ferroin sulphate solution and slowly titrate with ceric ammonium sulphate solution until the pink colour is changed to a very pale blue. Each 4.946 mg of arsenic trioxide is equivalent to 1 ml of 0.1 N ammonium ceric sulphate or 0.06326 g of Ce(S04)2. 2(NH4)2S04.2H20. 

It is evident from the above equations that 4 equivalents of ceric sulphate is required to oxidise 1 mole of arsenic trioxide, hence, 1 equivalent weight of arsenic trioxide is 1/4 mole or 197.84/4 or 49.46 g and 1 milliequivalent shall contain 49.46 mg or 0.04946 g. 

Calculations Therefore, the normality of ammonium ceric sulphate solution may be expressed as follows ... 

Materials Required Ferrous fumarate 0.3 g diluted H2S04 (10% w/v) 15 ml ferroin sulphate solution 0.1 N ammonium ceric sulphate solution. 

Procedure Weigh accurately about 0.3 g of ferrous fumarate and dissolve in 15 ml of dilute sulphuric acid by the help of gentle heating. Cool, add 50 ml of water and titrate immediately with 0.1 N ammonium ceric sulphate, employing ferroin sulphate solution as indicator. Each ml of 0.1 N ammonium ceric sulphate is equivalent to 0.01699 g of C4H2Fe04. 

Materials Required Acetomenaphthone 0.2 g glacial acetic acid 15 ml dilute hydrochloric acid (10% w/v) 15 ml ammonium ceric sulphate 0.05 N ferroin sulphate solution. 

First, acetamenaphthone (I) undergoes hydrolysis in acidic medium to yield the corresponding phenol and secondly, this phenol is oxidised quantitatively with ammonium ceric sulphate to give the resulting 1, 4-dione derivative (II). 

A number of pharmaceutical substances and dosage forms may be determined by the help of ceric sulphate titration methods as given in Table 6.1. 

Table 6.1 Redox Titrations Ceric Sulphate Titration Method...

Ferrous Gluconate 15 g Ferroin sulphate Each ml of 0.1 N ammonium ceric sulphate = 0.04461 g of C12H22Fe014... 

Discuss the various theoretical aspects involved in the assay of permanganate, dichromate and ceric sulphate titration methods. Give equations to explain your logical stand. 

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