Prepare an ammonium iron(III) sulphate solution by dissolving 10.0
g of the salt in about 80 mL of 3 M sulphuric acid and dilute to 100 mL with
acid of the same strength. Weigh out accurately about 0.3 g of the sample of
copper(I) chloride into a dry 250 mL conical flask and add 25.0 mL of the
iron(III) solution. Swirl the contents of the flask until the copper(I)
chloride dissolves, add a drop or two of ferroin indicator, and titrate with
standard 0.1M
Evaporate 55.0 g of ammonium cerium(IV) nitrate almost to dryness with excess (48 mL) of concentrated sulphuric acid in a Pyrex evaporating dish (FUME CUPBOARD). Dissolve the resulting cerium (IV) sulphate in 1M sulphuric acid (56 mL concentrated sulphuric acid to 1000 mL water), transfer to a 1 L graduated flask, add 1 M sulphuric acid until near the graduation mark, and make up to the mark with distilled water. Shake well.
Weigh out 70-72 g of pure cerium(IV) sulphate into a 1000 mL beaker, add 56 mL of 1:1-sulphuric acid/water and stir, with frequent additions of water and gentle warming, until the salt is dissolved. Transfer to a 1 L glass-stoppered graduated flask, and when cold, dilute to the mark with distilled water. Shake well.
Alternatively, weigh out 64-66 g of ammonium cerium(IV) sulphate into a solution prepared by adding 28 mL of concentrated sulphuric acid to 500 mL of water: stir the mixture until the solid has dissolved. Transfer to a 1 L graduated flask, and make up to the mark with distilled water.
The relative molecular masses of cerium(IV) sulphate Ce(SO4)2 and ammonium cerium(IV) sulphate (NH4)4[Ce(SO4)4],2H2O are 333.25 and 632.56 respectively.
The most trustworthy method for standardising cerium(IV) sulphate solutions is with pure arsenic(III) oxide. The reaction between cerium(IV) sulphate solution and arsenic(III) oxide is very slow at the ambient temperature; it is necessary to add a trace of osmium tetroxide as catalyst. The arsenic(III) oxide is dissolved in sodium hydroxide solution, the solution acidified with dilute sulphuric acid, and after adding 2 drops of an 'osmic acid' solution prepared by dissolving 0.1 g osmium tetroxide in 40 mL of 0.05 M sulphuric acid, and the indicator (1-2 drops ferroin or 0.5 mL N-phenylanthranilic acid), it is titrated with the cerium(IV) sulphate solution to the first sharp colour change: orange-red to very pale blue or yellowish-green to purple respectively.
2Ce4++H3AsO3 + H2O = 2Ce3++
H3AsO4 + 2H +
Procedure: Weigh out accurately about 0.2 g of arsenic(III) oxide, previously dried at 105-110 °C for 1-2 hours, and transfer to a 500 mL beaker or to a 500 mL conical flask. Add 20 mL of approx. 2M sodium hydroxide solution, and warm the mixture gently until the arsenic(III) oxide has completely dissolved. Cool to room temperature, and add l00mL water, followed by 25 mL 2.5M sulphuric acid. Then add 3 drops 0.01 M osmium tetroxide solution (0.25 g osmium tetroxide (CARE! FUME CUPBOARD) dissolved in l00mL 0.05M sulphuric acid) and 0.5 mL N-phenylanthranilic acid indicator (or 1-2 drops of ferroin). Titrate with the 0.1 M cerium(IV) sulphate solution until the first sharp colour change occurs. Repeat with two other samples of approximately equal weight of arsenic(III) oxide.
Standardisation may also be carried out with sodium oxalate; in this case, an indirect procedure must be used as the redox indicators are themselves oxidised at the elevated temperatures which are necessary. The procedure, therefore, is to add an excess of the cerium(IV) solution, and then, after cooling, the excess is determined by back-titration with an iron(II) solution. It is possible to carry out a direct titration of the sodium oxalate if a potentiometric method is used.
Prepare an approximately 0.1 M solution of ammonium iron(II) sulphate in dilute sulphuric acid and titrate with the cerium(IV) sulphate solution using ferroin indicator.
Weigh out accurately about 0.2 g sodium oxalate into a 250 mL conical flask and add 25-30mL 1M sulphuric acid. Heat the solution to about 60 °C and then add about 30 mL of the cerium(IV) solution to be standardised dropwise, adding the solution as rapidly as possible consistent with drop formation. Re-heat the solution to 60 °C, and then add a further 10 mL of the cerium(IV) solution. Allow to stand for three minutes, then cool and back-titrate the excess cerium(IV) with the iron(II) solution using ferroin as indicator.
Practically all the determinations described under potassium permanganate and potassium dichromate may be carried out with cerium(IV) sulphate. Use is made of the various indicators already detailed and also, in some cases where great accuracy is not required, of the pale yellow colour produced by the cerium(IV) sulphate itself.
cerium(IV) sulphate.
Repeat the titration with 25.0 mL of the iron solution, omitting the addition of the copper(I) chloride. The difference in the two titrations gives the volume of 0.1 M cerium(IV) sulphate which has reacted with the known weight of copper (I) chloride.