Question

How do I determine a mixture solution of Cu, Pb, Mn, Fe by classical method, while not using any solution containing Sulfide?

Answer 1

Copper (Spectrophotometric)
Decomposition of the sample
Take 0.5 gm. of the sample in a 250-ml. beaker, add 25-ml. conc. hydrochloric acid and digest for some time over a hot plate. Add 10 ml. conc. nitric acid and digest, when the reaction subsides, add 5 ml. of 1:1 sulphuric acid. Fume and dry. Remove from the hot plate. When cool, add 5 ml. conc. hydrochloric acid and nearly 100 ml. distilled water and boil and filter through No.40 Whatman filter paper. Wash the residue with hot water acidified with hydrochloric acid. Take the filtrate in a 250 ml. volumetric flask and make up to the volume.
Procedure
Take an aliquot equivalent to 0.1 gm. in a 250-ml. beaker; add 5 ml. 10%
hydroxylamine hydrochloride solution and 5 ml. 10% tartaric acid solution. Adjust the pH at 5 to 5.5 with AR ammonium hydroxide and dilute/AR hydrochloric acid and transfer it to a 250 ml separating funnel. Add 25 ml. of a solution of 2’-2’ biquinoline (100 mg. in 500-ml. amyl alcohol), shake vigorously and discard the aqueous layer. Run the organic layer into the cell of a spectrophotometer. Measure the absorbance against a reagent blank at 545 nm. Compute the copper percentage by reference to the graph prepared with standard copper solutions.
Determination of small amounts of P2O5 in silica sand, carbonate rock
(Spectrophotometry)
Reagents
1. Vanadate reagent – Dissolve 2.5 gm of ammonium vanadate in boiling
water and cool the solution. Add 20-ml. conc. nitric acid, cool and dilute to
one litre.
2. Molybdate reagent – Dissolve 50 gm. ammonium molybdate in hot water
and dilute to 1000 ml.
3. Standard P2O5 solution – Dry potassium dihydrogen phosphate KH2PO4 at 110oC. Weigh 0.9588 gm. and dissolve in water. Add 5-ml. nitric acid and
dilute to 500 ml.
Method
Take 0.1 gm. of finely pulverized sample in a beaker and add 5-ml. conc.
nitric acid and a few drops of HF. Slowly evaporate over a hot plate, then dry and dehydrate. Add 5-ml. conc. nitric acid and 50-ml. water, boil and filter through No.40 Whatman filter paper. Wash the residue with hot water. Take the filtrate in a 100 ml. volumetric flask and make up the volume and mix thoroughly.Take 5 ml. of the solution by pipette from the flask into a 100 ml. volumetric flask and add 10 ml. colourless nitric acid (1:1), 10 ml. ammonium vanadate solutionand 10 ml. ammonium molybdate solution make the volume up to the mark with water and mix thoroughly. After 30 minutes, measure the absorbance at 400 nm. Take a reagent blank. Compute the percentage of P2O5 with Reference to the standard graph prepared using standard solutions containing 0.5, 1.0, 1.5, 2.0 and 2.5
mg per 100 ml. of P2O5 exactly in the manner stated above.

Manganese (Mn) (Volhard’s method)
Pipette out 25 ml solution in 500 ml conical flask. Boil this and remove free
chlorine. Dilute the volume to 150 ml with demineralised water. Add 5-6 g of zinc oxide. Shake vigorously. Heat this on burner to near to a boiling point (avoid boiling). Titrate this against standard potassium permanganate solution to a permanent pink colour. To observe the colour change the flask should be kept in slanting position and the insoluble should be allowed to settle and observe the colour in clear solution. Best is to run a control reading to locate the probable end point and finish the second/parallel titration to an exact end point.
1 ml 1 N KMnO4  0.01648 g of Mn

The reactions involved are
3Mn2+ + 2MnO4 + (X+2) H2O = 5MnO2 XH2O + 4H+
Notes on successful performance of Volhard’s method
There is much criticism about the use of Volhard’s method but in the Ore
Dressing Laboratory of Indian Bureau of Mines, this process has been in practice for many years and the results obtained are quite reliable and satisfactory.
The following precautions are needed for the successful application of the method.
1. The titration is made quickly and the solution is not allowed to cool below
80oC during titration.
2. The permanganate causes formation of precipitate, which obscures the end point. A drop of HNO3 makes the precipitate to settle quickly. The end point can be easily observed if the conical flask is kept in a slanting position on a stand.
3. Avoid boiling of the solution while titrating as it may destroy the
permanganate colouration.
4. A chloride, sulphate or nitrate solution may be used for the determination of manganese but the amount of permanganate used will vary in each case. In fact many factors influence and the method is reported to be only empirical. The permanganate must be standardized against a similar ore or steel of known manganese content.

Iron (Fe) – (Dichromate method)
Pipette out 50 ml from the main stock solution into a 250 ml beaker. Add 2-3
drops of methyl orange (very dilute solution) to impart a very faint red colour to the solution. Add 2 g ammonium chloride. Stirr and dissolve the solids. Precipitate mixed oxide (R2O3) by dropwise addition of dilute ammonia till the colour of indicator turns yellow. Add 2-3 drops in excess. Boil the content on burner. Cool and filter through Whatman 41 filter paper. Wash the precipitate and beaker 5-6 times with hot water. Preserve the filtrate for estimation of CaO and MgO. Transfer the precipitate carefully with jet of hot water into the original beaker. Dissolve the precipitate in concentrated
hydrochloric acid. Wash the sides of the beaker. Now heat the beaker to 80-90oC on burner (avoid boiling) and add 10% stannous chloride solution dropwise with constant stirring to reduce iron. Yellow colour of ferric chloride disappears due to conversion to ferrous ion. Add 1-2 drops of stannous chloride in excess. Cool the solution in cold water. Add 10 ml mercuric chloride solution and stir. A silky white precipitate of mercurous chloride will appear. If the precipitate found is black, discard the solution and repeat the process with new aliquot. The black colour indicates the reduction of mercurous chloride to elemental mercury, which adds inaccuracies to the
determination. Add 10-15 ml sulphuric-phosphoric acid mixture and dilute to about 150 ml. Add 2-3 drops of barium diphenylamine sulphonate indicator solution. Titrate this against standard dichromate solution with constant stirring till a constant stable violet colour appears. Colour changes from dirty green to violet. Avoid use of excess of indicator as it imparts intense dirty green colour near the end point and confuses the actual end point.
1 ml 1 N K2Cr2O7 = 0.05585 g Fe
= 0.07185 g FeO
 0.07985 g Fe2O3

Lead (Pb) - (EDTA–Complexometry)
Separate Pb from other element by conversion to sulphate. Take 50 ml aliquot add 5 ml dil. sulphuric acid fume of the content. Wet the mass left with dil. H2SO4 and add 50 ml. Water. Boil this on burner. Filter this through Whatman 40. Wash the precipitate with hot water containing 5% sulphuric acid. Transfer the filter paper along with precipitate to the original beaker. Add 100 ml, 5.5 pH, ammonium acetate – acetic acid buffer. Boil this, cool this in water bath. Add a pinch of ascorbic acid and a pinch of thiourea. Dissolve the solids. Add 25 ml. Sodium acetate 5.5 pH buffer. Add 2-3 drops of xylenol orange indicator. Titrate this against standard EDTA solution. Colour changes from purple red to golden yellow.
1 ml 1 M EDTA  0.2072 g. of Pb