Precipitation of Lead as Chromate Followed by Titration

Following the precipitation of lead chromate, dil. HCI dissolves the precipitate and the dichromate formed oxidises iodide to iodine which can be titrated against thiosulphate. 

Repeat the above precipitation of lead chromate. No need to use a weighed dried crucible. After filtration and washing the precipitate, place it in a 250 cm beaker, add 1 1 HNO3 to dissolve the chromate followed by 20 cm of 10% KI solution, stir to liberate iodine, add water and titrate with standardised 0.05 M thiosulphate solution until the colour of iodine fades. Then add 2 cm of freshly prepared starch solution until the dark iodine/starch adsorption complex disappears, leaving a green Cr(Ill) solution. Alternatively, filter through a GF filter paper, transfer it quantitatively and carefully, after washing, to a conical flask and continue as above. 

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