Determination of lead and copper in carbon steels:
Within the application of the polarographic method of analysis to steel, ferric ion interferes due to its reduction near zero potential in sodium tartrate supporting electrolyte.
Fe (III) has to be reduced to Fe (II) along with hydrazinium chloride in a hydrochloric acid medium so in which Fe (II) is reduced at much negative potential of -1.4 V vs SCE that is more negative than copper and lead.
Procedure: Dissolve accurately weighed 5.0 g of the steel within a mixture of 25 cm3 of water and 25 cm3 of concentrated hydrochloric acid and heat gently. Add few drops of standard potassium chlorate solution to dissolve carbides and boil the mixture until the solution is clear. Dilute and cool to 50 ml with water in a volumetric flask. Pipette 2 ml of this solution into a polarographic cell and add 1.0 cm3 of 20% hydrazine hydrochloric solution to reduce Fe (III) to Fe (II) ion, 1.0 cm3 of 0.2% methyl cellulose to act as maximum suppressor and 5.5 cm3 of 2.0 M sodium formate solution. Keep the cell in a nearly boiling water bath for 10 minutes in order to complete the reduction. Cool and take the polarogram using SCE. The polarogram will have two steps. The first one at E1/2 of - 0.25 V vs SCE is due to the reduction of cuprous to copper metal. The second step at E1/2 of - 0.45 V vs SCE is because of lead. Use standard addition method for determination. For this carry out a calibration by adding known amounts of copper and lead ions to a solution of steel of low copper and lead content and determine the increase in wave heights due to addition. Calculate the percentage of copper and lead in the sample.