Use the practical evidence carefully: know when to add starch, why sulfuric acid is used and how manganate(VII) or iodine-thiosulfate endpoints appear.
Brass is an alloy of copper. The percentage by mass of copper in a sample of brass can be determined by a three-stage process.
Stage 1 Oxidation of the copper to copper(II) ions with excess concentrated nitric acid.
Stage 2 Reduction of the copper(II) ions to copper(I) ions with excess iodide ions.
Stage 3 Titration of the iodine produced in Stage 2 against a standard solution of sodium thiosulfate.
The procedure in Stage 3 is shown:
- the iodine produced in each flask from Stage 2 is titrated with 0.095 moldm−3 sodium thiosulfate solution until the iodine colour is pale yellow
- a few drops of starch indicator are added
- sodium thiosulfate is added drop by drop with swirling until the end-point is reached
- the titration is repeated until two concordant titres are obtained.
Give a possible reason why the starch indicator is added when the iodine colour is pale yellow rather than at the start of the titration.
an (insoluble) complex/solid will be formed
Allow iodine binds to the starch and
makes the end point more difficult to see
Give the colour change at the end-point of the reaction.
blue-black to colourless (at the end point)
Allow blue or black
Ignore clear