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Indian National Chemistry Olympiad Examination 1999

Maximum Points: 80

 

Introduction:

You will be estimating lead and tin content of the given sample by complexometric titration, a technique similar to the neutralisation or oxidation-reduction titrations. The titration which will be performed in this experiment is based upon a particular class of co-ordination compound called as chelates. A chelate is produced when a metal ion coordinates with two or more donor group of a single ligand. Depending upon the number of donor groups available for coordination bonding, chelating reagent is named as bi-, ter-, quadri-, quinque- or hexadentate. In this experiment, you will be using a Ethylenediaminetetraacetic acid (EDTA), a most widely used complexometric hexadentate ligand for titration.

Theory:

EDTA has following structure-



EDTA is a polyaminocarboxylic acid. In addition to four acidic hydrogen, each nitrogen has one unshared pair of electrons. The molecule thus has six potential sites for bonding with metal ion. Valuable property of EDTA as a titrant is that it combines with metal ions in 1:1 ratio regardless of the charge of the cation, and thus gives a sharp endpoint. EDTA in its disodium form is generally used for complexometric titrations which is abbreviated as Na2H2Y.2H2O.

Both Pb2+ and Sn4+ will react with EDTA as follows-

Pb2+ + H2Y-2 ----> PbY-2 + 2H+

Sn4+ + H2Y-2 ----> SnY + 2H+

In the current experiment, a mixture of Sn4+ and Pb2+ ions is complexed by addition of excess EDTA solution. Excess EDTA is then back titrated with standard lead nitrate solution using xylenol orange as an indicator. At the endpoint, slight excess of Pb2+ forms a red complex with xylenol orange and thus the colour of the solution changes from yellow to red. This titration gives the amount of EDTA which is consumed by the total Pb and Sn content of the sample. While conducting the titration, 30% hexamine solution is added to maintain the pH of the sample solution around 6.

When a fixed amount of sodium fluoride is added to the same sample solution, the fluoride ions form a stable complex with Sn4+, but does not react with Pb2+ at pH 6. The reaction of tin with fluoride ions can be represented as:

SnY + nF- + 2H+ ----> SnFn(n-4)- + H2Y2-

where n varies from 4-6.

Thus, in presence of sodium fluoride, EDTA from Sn-EDTA complex is liberated which can be titrated once again with standard lead nitrate solution. EDTA released destroys a small amount of Pb-xylenol orange complex present in the solution which is formed at the endpoint of the earlier titration. Thus, free xylenol orange (yellow in colour) is liberated in the solution which acts as an indicator.

To estimate lead and tin content in the given sample of alloy

Dissolution of supplied sample


You are given a weighed quantity of an alloy. Carefully transfer the sample in a 250 mL beaker. Add 10 mL of conc. HCl & 2-3 mL of conc. nitric acid. Keep the solution at room temperature for 10-15 mins. with intermittent stirring. Then, gently heat the beaker on hot plate (around 40-500C) until the sample dissolves. If burner is used then gently heat the solution using a low flame (if required, discontinue heating after some time). At this stage, the solution is generally dark brown in colour and also has some brown colour spongy mass either floating or adhering to the beaker. The spongy mass is a flux material which will not dissolve in the acids added. Do not try to dissolve or filter the flux material as it does not interfere with the analysis. View the beaker from bottom against light. Presence of any grey particles indicate that your sample is not yet completely dissolved. To ensure complete dissolution of the sample, gently heat the solution further. You may have to add the stated quantities of acids twice or thrice, if the sample does not dissolve. After the sample is totally dissolved, boil the solution to expel the nitric acid from the solution (about 2 to 3 mins). At this stage, the colour of the solution is generally pale yellow. Avoid total drying of the solution. Cool the solution slightly. A white precipitate will appear in the solution.

Concentrated acids are very corrosive. The steps above involve hot solutions. Be careful of vapours!

Preparation of sample solution


To the above solution, add 25 mL of 0.2M EDTA solution supplied to you. Stir well and boil the solution for 1 or 2 minute in order to dissolve the precipitate formed. Dilute the solution with approximately 100 mL of distilled water. Cool the solution for 5 mins and then transfer it to 250 mL standard volumetric flask. Dilute the solution upto the mark with distilled water. This is your sample solution.

Titration with Lead Nitrate


  1. Fill the burette with 0.01 M Pb(NO3)2

  2. Quickly, pipette out 25 mL of the sample solution to a 250 mL conical flask. To this flask, add 15 mL of hexamine solution and 100 mL of distilled water and 5 to 10 drops of xylenol orange indicator. The colour of the solution will be yellow. Simultaneously, prepare three flasks in this manner(# 1, 2, & 3).

  3. Titrate the solution from flask (#1) with standard Pb(NO3)2 solution until the colour of the solution changes from yellow to orangish red (or red). Note down the reading.

  4. To the same flask(#1), add 2g of solid sodium fluoride provided to you. The solution will turn yellow in colour.

  5. Titrate the solution with Pb(NO3)2 till the colour changes from yellow to orangish red (or red). Note down the reading.

  6. Perform titrations for flask #2 and #3 in similar manner.

Blank Titration


  1. Pipette out 25 mL of 0.2M EDTA supplied to you in a 250 mL standard volumetric flask. Dilute the solution upto the mark with distilled water.

  2. Pipette out 25 mL of diluted EDTA solution in a 250 mL conical flask, add 1 mL conc HCl, 15 mL hexamine solution and 100 mL distilled water.

  3. Titrate the solution with standard Pb(NO3)2 using xylenol orange indicator, until the colour changes from yellow to red. Note down this reading. Take at least 3 readings.

Calculate the percentages of Lead and Tin in the sample provided to you. Present your readings for all titrations in appropriate form on the answer sheet provided. Show all the necessary calculations.

Atomic masses: Pb 207.2, Sn 118.7