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 reac-tion involved may be expressed as follows :
It is interesting to observe that the solutions of ammoniurn ceric sulphate possess a number of advantages over permanganate and dichromate methods discussed earlier, viz.,
(i) solutions remain fairly stable even when boiled,
(ii) solutions quantitatively react with either arsenite (AsO33–) or oxalate [(COO)2]2– ion, and there-fore, either arsenic trioxide or sodium oxalate may be employed as a primary standard,
(iii) cerous ion Ce3+ is colourless and hence offers no interference with the indicator end-point,
(i) Ce3+ always solely results on reduction of Ce4+, whereas permanganate (MaO4– ) can be reduced to any of several oxidation states,
(ii) ammonium ceric sulphate unlike potassium permanganate, may be conveniently employed as an oxidizing agent in the presence of high concentrations of HCl, thereby facilitating determinations of Fe2+ in the presence of Cl–, and
(iii) ferrous phenanthrolone ion (ferroin) has proved to be a very successful indicator in titrations with ceric salts. Thus, we have :
Orthophenanthroline (base) dissolves rapidly in aqueous solutions of ferrous salts, thereby three moles combine with one Fe2+ ion to give a complex termed as ‘ferroin’ having an, intense red colour. Now, any strong oxidizing agent converts the ferrous to a corresponding ferric complex having a slight blue colour.
Materials Required : Ceric ammonium sulphate : 66 g ; sulphuric acid (conc.) : 30 ml.
Procedure : Dissolve 66 g of ceric ammonium sulphate, with the help of gentle heat, in a mixture of ml of sulphuric acid and 500 ml DW. Cool, filter the solution through a fine-porosity sintered-glass crucible, dilute to 1 litre mark in a volumetric flask and mix thoroughly.
Since the oxidation reaction is given by :
Materials Required : Arsenic trioxide : 0.2 g ; sodium hydroxide solution (8.0% w/v) : 25 ml ; diluted sulphuric acid (10% w/v) : 30 ml ; osmic acid solution (1.0% w/v in water) : 0.15 ml ; ferroin sulphate solution (dissolve 0.7 g of ferrous sulphate in 70 ml of DW and add 1.5 g of 1, 10-phenanthroline and sufficient water to produce 100 ml) : 0.1 ml.
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(SO4)2. 2(NH4)2SO4 . 2H2O.
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 H2SO4 (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 sulphu-ric 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 C4H2FeO4.
Equations and Calculations :
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.
Procedure : Weigh accurately about 0.2 g of acetomenaphthone and boil it with 15 ml of glacial acetic acid and 15 ml of dilute hydrochloric acid under a reflux condenser for 15 minutes. Cool the contents carefully and taking adequate precautions to avoid any atmospheric oxidation. Add 0.1 ml of ferroin sulphate solution as indicator and titrate with 0.05 N ammonium ceric sulphate. Repeat the assay without the substance being examined (blank determination) and incorporate the correction, if any. Each ml of 0.05 N ammonium ceric sulphate is equivalent to 0.006457 g of C15H14O4.
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).
Thus, we have :
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.