Experiment 1
The Stabilisation of an Oxidation State of a
Transition Element which is Unknown as a
Simple Ion
Silver(II) Pyridine Complex - 1
Experiment 1: The Stabilisation of an Oxidation State of a
Transition Element which is Unknown as a Simple Ion
Aims
to prepare a silver(II) complex
to confirm the oxidation state by volumetric analysis
to further confirm the oxidation state by magnetic measurements
Introduction
The most common oxidation state of silver is +1. However, by careful choice of ligand system it is
possible to form reasonably stable complex ions in the +2 and +3 oxidation states. Preparation of
such complexes requires a strong oxidising agent and ligands which are able to stabilise the
small, hard, higher oxidation state cation. In this case the oxidising agent is the peroxodisulphate
ion and the stabilising ligand is pyridine, forming a complex which is moderately stable.
Experimental
a) Preparation of [Ag(C5H5N)4]S2O8
- Prepare a solution containing 7.5 g of potassium peroxodisulphate (K2S2O8) in 500 cm3 of ice
cold water.
- Add pyridine (5 cm 3, CARE) to a 5% w/v solution of silver nitrate (12 cm3) with stirring. Add this
mixture to the K2S2O8 solution. A yellow-orange colouration is immediately apparent, precipitation
of the orange product soon begins and is complete in about 30 minutes.
- Filter by suction and wash the solid product with ice-cold water, in which the complex salt is
nearly insoluble.
- As the compound is not completely stable, about one-quarter of the moist product (approx. 0.5 g)
should be transferred to a beaker for immediate analysis (see below) and the remainder dried in a
vacuum desiccator.
- Record % yield based on silver nitrate (NB a 5% w/v solution contains 5 g AgNO3 in 100 cm3).
b) Volumetric Determination of the Composition of the Complex
- Place the complex (0.5 g) in a small beaker and add 2 cm3 of water followed 2 cm3 of dilute
sodium hydroxide with constant stirring. This results in the formation of a jet black precipitate of
silver (II) oxide, AgO.
- Allow to stand for 5 minutes.
- Meanwhile, accurately measure (by pipette) 25 cm 3 of 0.1 M iron(II) ammonium sulphate solution
(provided) and acidify with dilute sulphuric acid (25 cm 3).
- Filter off the silver (II) oxide, making sure it is all transferred to the filter paper (use more water if
necessary) and allow the filtrate to run into the acidified iron (II) ammonium sulphate solution.
Wash the silver(II) oxide with a little water, allowing the washings to run into the solution as well.
- Set the solution aside for later analysis.
Silver(II) Pyridine Complex - 2
Note: The S2O82- ions present in the complex are washed through with the filtrate and oxidise
iron(II) to iron(III) as shown by the equation below:
Fe2+

Fe3+ + e-
and
S2O82- + 2 e- 
2 SO42-
The unoxidised iron(II) will be titrated later allowing the determination of S2O82- ions.
- Accurately measure 25 cm 3 of 0.05 M oxalic acid solution (provided), warm to 60 ºC and without
delay pour over the silver (II) oxide contained in the filter paper. The black silver(II) oxide is
converted to white silver oxalate as shown below.
- Make sure that the solution is all collected and then dissolve the silver oxalate in warm dilute
sulphuric acid. Combine all the acid washings with the oxalate solution.
Note: The resulting solution contains any oxalate that was unreacted during the formation of
the silver oxalate and also contains the silver oxalate. Some of the oxalate is lost as CO2
during the reaction of silver(II) oxide with the oxalic acid.
2AgO + 2 H2C2O4
 Ag2C2O4 + 2CO2
+
2H2O
This will allow us to work out how much silver has reacted with the oxalic acid.
- Titrate the oxalate present in the resulting solution with 0.02 M potassium permanganate
(provided). This allows an indirect determination of silver.
- When the titration is complete just discharge the pink colouration by addition of one drop of 0.05
M oxalic acid.
- Titrate the silver present with 0.1 M ammonium thiocyanate solution (provided) using ammonium
iron (III) sulphate as indicator (the end point is indicated by a permanent orange-red colouration).
- Titrate the iron (II) solution, set aside earlier, with 0.02 M potassium permanganate (provided), to
determine the amount oxidized by the peroxodisulphate ion.
- Determine the magnetic susceptibility and effective magnetic moment of this salt. The method is
given in appendix 1.
- Run an infra-red spectrum of your product as a nujol mull.
Questions
1.
2.
3.
4.
5.
From the volumetric analysis, calculate the ratio of Ag : S2O82- and therefore deduce the
oxidation state of the silver ion.
Does the magnetic moment support your deduction? Why?
Why are higher oxidation states of transition metals generally rather unstable?
Why is the stability of Ag2+ increased by coordination with pyridine?
What other ligands stabilise the Ag2+ ion?
Reference
W. Levason and M.D. Spicer, Coord. Chem. Rev., 1987, 76, 47.
Silver(II) Pyridine Complex - 3