Standardization of Ceric Ammonium Sulphate

Preparation and Standardization of Ceric Ammonium Sulphate

Aim: To perform the standardization of ceric ammonium sulphate. 

Requirements: 

• Apparatus: Beaker, Funnel, Pipette, Burette. 
• Chemicals: Ceric ammonium sulphate, Sulphuric acid, Arsenic trioxide, Ferrroin sulphate. 

Principle: 

Cerimetry or cerimetric titration is also known as cerate oximetry. It is a redox titration in which a Fe²⁺-1, 10-phenanthroline complex (ferroin) colour change indicates the endpoint.  Ferroin can be reversibly discoloured in its oxidized form upon titration with a Ce⁴⁺ solution.  Since cerimetry is linked to the Fe³⁺/Fe²⁺ redox pair, it can be used for the analysis of nonstoichiometric levels that either oxidize Fe²⁺ or reduce Fe³⁺. For the case of oxidation, a precise excess of high-purity crystalline Mohr’s salt is added upon the oxide digestion in aqueous hydrogen chloride (HCl), while for the case of reduction, an excess of 1 M iron trichloride (FeCl₃) is added. In both cases, Fe²⁺ ions will be titrated subsequently. Because the Ce⁴⁺ solution is prone to hydrolysis, the titration is done in a strongly HCl-acidic solution into which some phosphoric acid (H₃PO₄) is added to obtain a less coloured phosphato complex of Fe³⁺. 

Procedure: 

(A) Ceric ammonium sulphate, 0.1 M: Dissolve 65 g of ceric ammonium sulphate, with the aid of gentle heat, in a mixture of 30 ml of sulphuric acid and 500 ml of water.  Cool, filter the solution, if turbid, and dilute to 1000 ml with water. Standardise the solution in the following manner. 

(B) Standardisation: Weigh accurately about 0.2 g of arsenic trioxide, previously dried at 105°C for 1 hour, and transferred to a 500 ml conical flask. Wash down the inner walls of the flask with 25 ml of an 8.0 per cent w/v solution of sodium hydroxide, swirl to dissolve, add 100 ml of water and mix. Add 30 ml of dilute sulphuric acid, 0.15 ml of osmic acid solution, 0.1 ml of ferrroin sulphate solution and slowly titrate with the ceric ammonium sulphate solution until the pink colour is changed to a very pale blue, adding the titrant slowly towards the end-point. 

Observation Table of Preparation and Standardization of Ceric Ammonium Sulphate

Sr. No.	Parameters	Reading
1.	Burette solution	Ceric ammonium sulphate
2.	Conical flask solution	Arsenic trioxide, sodium hydroxide
3.	Indicator	Ferroin sulphate
4.	Endpoint	Pink to blue

Burette reading B.R (ml)	Trial 1	Trial 2	Trial 3	Average (BR)

Factor: 1 ml of 0.1 M ceric ammonium sulphate is equivalent to 0.004946 g of As₂O₃. 

Result: The molarity of ceric ammonium sulphate is …… M.

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