Lead and silver are often both present in alloys (such as tin-lead-silver or lead-silver), which are — Physical Chemistry — Thermodynamics Chemistry Question
Titrimetric determination of lead and silver in their mixture
Lead and silver are often both present in alloys (such as tin-lead-silver or lead-silver), which are successfully applied in bearing assembly, ballast, casting, step soldering, and radiation shielding. The alloys usually contain 30 – 90 % of lead and 1 – 5 % of silver. Redox titration was found to be a precise standardless method allowing determination of these metals.
In this work, you will determine lead and silver in a solution by redox titration.
### Chemicals and reagents
* A sample of lead and silver containing alloy, or test solution simulating a dissolved alloy (a standard solution containing about 500 – 1000 mg Pb and 70 – 190 mg Ag in 0.1 dm3),
* Ammonia aqueous solution (25% ammonium solution and water, 1 : 1 v/v),
* Oxalic acid, saturated solution at room temperature,
* Potassium permanganate, 0.0100 mol dm-3 standard solution,
* Sulfuric acid, solution c = 1 mol dm-3,
* Nitric acid, solution c = 4 mol dm-3,
* Ammonium iron(III) sulfate, saturated solution,
* Ammonium thiocyanate, 0.0100 mol dm-3 standard solution.
### Equipment and glassware
* Analytical balance (± 0.0001 g),
* Hot plate,
* Filter paper or glass filter,
* Burette, 25 cm3 (2 ea.),
* Funnels (to fill the burettes),
* Volumetric pipette, 10.00 cm3,
* Pipette filler,
* Erlenmeyer flask, 100 cm3,
* Volumetric flask, 100 cm3,
* Glass beaker, 100 and 250 cm3,
* Graduated cylinders,
* Waste bottle for oxalate solution
### Procedure
A. Decomposition of the alloy sample
(Optional and may be omitted; if so, a model solution of metal salts is to be prepared; see Chemicals and reagents for the solution composition)
Take a precise weight of the metal (~250 mg) and place it in a beaker. Carefully add 5 cm3 of concentrated nitric acid (to be done under a fume hood because of gaseous NO2 evolvement). Heat the beaker slightly on the hot plate to provide for an effective dissolution. When the digestion is complete evaporate the solution to near dryness to remove the major part of the acid (avoid evaporating to dry salts, since hydrolysis may occur. If still so, add a minimal amount of HNO3 to dissolve the residue). Allow the beaker cooling down to room temperature.
*ATTENTION! Nitric acid is very corrosive! You will have to deal with hot solutions in the above and subsequent steps. Be careful and beware of steam!*\n
B. Separation of lead
Using the hot plate, remove the excess of the acid by evaporating the solution obtained at stage A to dryness and dissolve the residue in water (skip this step if a model test solution is used rather than a real alloy solution). Bring the solution to boiling, then add about 10 cm3 of the saturated solution of oxalic acid and observe a precipitate formation. Avoid large excess of oxalic acid. To partially dissolve the precipitate, add aqueous ammonia solution (1:1 v/v) dropwise.
*ATTENTION! The ammonia solution is corrosive and has a very strong smell! Keep the bottle stoppered when not in use.*
C. Determination of lead
Heat the solution above the precipitate on the hot plate to remove the excess of ammonia, and cool the mixture quickly under the running tap water. Filter the slurry through the glass filter. Keep the filtrate for next step. Wash the filter cake with cold water and then dissolve it in hot solution of HNO3 (0.5 mol dm-3) adding the acid in small portions. Collect the obtained solution in the 100 cm3 volumetric flask and make it up to the mark with water. Titrate oxalate in the prepared solution (take 10.00 cm3 aliquots) with 0.0100 mol dm-3 solution of potassium permanganate.
*ATTENTION! Oxalate solutions are toxic. Do not pour the solutions down a sink. Instead, dispose these in a special waste bottle.*
D. Determination of silver
Add 10 cm3 of nitric acid solution (4 mol dm-3) and 1 – 2 cm3 of saturated iron(III) ammonium sulfate solution to the filtrate (from step C). Use burette to add the standard solution of ammonium thiocyanate until vanishing reddish-brown color is observed. Shake the flask and continue titrating until the color is stable.
Write down balanced chemical equations for the reactions that take place upon:
a) formation of the precipitate (step B),
b) partial dissolution of the precipitate in ammonia (step B),
c) dissolution of lead oxalate (step C),
d) oxalate titration with permanganate (step C).
Model Answer
a) Pb2+ + C2O4 2– → PbC2O4↓,
PbC2O4↓ + C2O4 2– → [Pb(C2O4)2]2– (excess of the precipitant),
2 Ag+ + C2O4 2– → Ag2C2O4↓
b) Ag2C2O4↓ + 4 NH3 → 2 [Ag(NH3)2]+ + C2O42–
c) PbC2O4↓ + 2 H+ → H2C2O4 + Pb2+
d) 5 H2C2O4 + 2 KMnO4 → 10 CO2 + 2 MnSO4 + K2SO4 + 8 H2O
Explain the role of iron(III) at step D.
Model Answer
Step D describes the direct titrimetric determination of silver with ammonium thiocyanate. Iron(III) is a very sensitive indicator of the excess of thiocyanate ion:
Fe3+ + SCN– → FeSCN2+ (reddish-brown; higher complexes are also formed)
Calculate the lead and silver content in the sample (starting alloy or test solution).