Soal IChO Ke-26

Practical Problem II
(17 blue points, 17 red points)

VOLUMETRIC DETERMINATION OF BROMIDE BY BACK-TITRATION WITH THIOCYANATE AFTER PRECIPITATION WITH SILVER IONS IN EXCESS

Moments worth considering:

* The candidates must consider the number of significant figures that will be reasonable in the results.
* The candidates must be able to carry out the whole analysis by using the delivered portions of silver nitrate and potassium thiocyanate. Supplementation of these two solutions will not be available.
* Only one 25 mL pipette will be at disposal for each candidate.

Principle

Bromide is precipitated as silver bromide after a known amount of silver ions has been added in excess.

(faint yellow-green)

The excess of silver ions is titrated with thiocyanate with a known concentration, after a previous standardization of the thiocyanate solution.

During the titration of the following reaction takes place, resulting in the precipitation of silver thiocyanate:

(white)

Fe(III) is added as indicator producing a red-coloured ion at the equivalence point:

(red)

a)

Procedures
Every candidate has got a 0.5 liter brown bottle with screw cap, containing about 0.08 M of a potassium thiocyanate solution , and also a 0.25 liter brown bottle with screw cap, containing the silver nitrate solution. The concentration of this solution is 0.1000 M. The exact concentration of the KSCN solution is to be determined by the candidates.

i) Determination of bromide in the unknown sample solution

Fill the 250 mL volumetric flask containing the bromide sample solution to the mark with water.

Transfer three 25.00 mL portions (pipette) of the sample solution to three erlenmeyer flasks. Add about 5 mL of 6 M nitric acid (measuring cylinder) to each flask.

Transfer 25.00 mL (pipette) of the accurately known silver solution and about 1 mL of iron(III) indicator (ind.) (measuring cylinder) to each solution.

Titrate the contents of the three aliquotes with the potassium thioscyanate solution. The end-point of the titration is detected when the solution (including the precipitate) becomes permanently very faint brownish. It is important to shake the contents vigorously near the end-point and rinse the walls of the flask with water. The colour should be stable for at least one minute.

ii) Standardization of the potassium thiocyanate solution:

Transfer 25.00 mL (pipette) of the silver nitrate solution to an erlenmeyer flask, add about 5 mL of 6 M nitric acid and about 1 mL of the iron(III) indicator solution and about 25 mL of water (use measuring cylinders for these solutions).

Titrate the contents with the thiocyannate solution and determine the end-point according to the instruction given in the “Determination” procedure.

Atomic mass: Br = 79.90

b)
Exercise

At the equivalent point the solution is saturated with respect to both AgBr and AgSCN. Find the molar consentration of free (unprecipitated) Br^- in this solution:

Ignore the effect of pH and Fe(III) species.

Note: On the answer sheet, not only the required final results shall be given, but also examplifications of how the calculations are carried out.

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