Determination of SO₄^{2 -} with Pb(NO₃)₂ solution.:

At the potential applied -1.2 V vs SCE only Pb²⁺ is reduced to give diffusion current and is used as a titrant.

Reagents required: 0.01 M AR K₂SO₄ and 0.1M lead nitrate solution, ethyl alcohol, conc. HNO₃ and thymol blue indicator solution.

Procedure: Take 25 cm³  of potassium sulphate into the titration cell, add 2 to 3 drops of thymol blue indicator and few drops of conc. HNO₃  until a red color is acquired (pH is 1.2). To this add 25 cm³ of ethyl alcohol which will reduce the solubility of PbSO₄ precipitate formed. A reaction is

K₂SO₄ + Pb( NO₃ )₂ = PbSO ₄ ↓ + 2 KNO₃

Pass N₂ gas for 15 minutes to remove dissolved O₂ and connect the DME and RE to the respective terminals of the polarograph. Titrate with lead nitrate solution adding in small volumes and pure N₂ for 1 minute after each addition. Note down readings of burette and galvanometer after stopping the passage of N₂. Draw the graph between volume of lead nitrate added and the current recorded from the galvanometer after making volume correction. A reverse L shaped curve will be acquired from which the end point is detected. Calculate the amount of SO₄^2- present in the given solution.

1 cm³ of 0.1 M Pb(NO₃)₂  = 0.009606 g SO₄^{2 -}