A simple titration using iodide was turned into a method for assaying copper and obtain reliable analysis.
- To .5 grams of sample in a 250 cc beaker add 8 cc’s of HN03 and after violent action has ceased place on low temperature plate and decompose slowly until all brown fumes of nitrous oxide are expelled.
- Add 10 cc’s of H2SO4 and fume to dryness.
- Take up with 5 cc’s of H2SO4 and 60 cc’s H2O and heat until all soluble salts are in solution then filter, washing well with hot water into a 250 cc beaker.
- Place a strip of aluminium strip in filtrate and boil until all the copper has been precipitated. (Indicated by the aluminium strip being clean and the copious froth on the solution while boiling).
- Remove (the) from the hot plate and was the watch glass and side of breaker with saturated H2S water.
- Filter by decantation washing well with hot water (6 to 8 times).
- Place the beaker containing the aluminium strip and copper under the filter funnel and wash the filter paper with dilute HNO3 2-3 times; then wash with hot water twice.
- Wash the filter paper with saturated bromine water (once is sufficient) which dissolves any copper which has been precipitated by the H2S water.
- Wash 2-3 times with hot water. Place on hot plate and boil to expel the bromine.
- Take to dryness carefully.
- Add from a pipette graduate 2 cc’s HNO3.
- Dilute with 20 cc’s H2O and heat until boiling.
- Add 3 cc’s acetic acid, boil one minute, and cool.
- Add 2 grams potassium iodide and titrate the liverated iodine with a standard solution of sodium thiosulphate.
- Use starch as indicating solution when the titration has become a pale yellow.
- Proceed slowly, adding a drop at a time until titration is finished.
Note about this Iodide Copper Analysis Method:
- After the acetic acid has been added, cool and neutralize with NH4OH. Boil until all odour of ammonia is dispelled.
- Dilute with water if necessary to 20 cc.
Preparation of Solution:
- Sodium thiosulphate: dissolve 19.5 grams of the salt and 4 grams of the NaOH in water and dilute to 1 litre. The NaOH prevents decomposition by CO2 and keeps the solution very stable.
- Standardize with 1 gram of pure copper foil. Dissolve the copper with 3 cc’s HNO3 and boil to expel the nitrous oxide fumes. Take to dryness carefully and proceed as in the method for the assay.
Starch Solution
Make a cold saturated solution of NaCl and filter it. Measure out 500 cc’s of the saturated solution, add 100 cc’s of glacial acetic acid, and bring to a boil. Add 3 grams of soluble starch dissolved in a little cold water. Stir well and boil 1 minute. Add 25 cc’s of water and cool. The solution is now ready for use.
Notes on this Method:
- Decompose slowly as the action of the HNO3 on the sulphides is very violent and a loss may occur at this stage of the game.
- In the fuming to dryness and then adding a definite amount of H2S04 a standard concentration of acid in all determination is maintained.
- In the precipitation of the copper, pro-longed boiling will use up all of the free acid so that in washing the sides of the beaker with the H2S water copper will go into the solution as CuS.
(Presumably the copper on the sides of the beaker becomes oxidized not being under the influence of a reducing agent) - If all the free acid is used up by the action on the aluminium. Care should be taken not to inhale any of the bromine fumes in dissolving the CuS with bromine water.
- The bromine oxidises any of the arsenic or antimony present to the higher oxides, so that it has no action on the liberated iodine. Otherwise, low results will occur.
- In taking the copper nitrate solution to dryness, it should be removed from the high temperature plate before all the acid is expelled and finished on the low temperature plate. Otherwise, the copper nitrate residue in the beaker will scorch to a black copper oxide and low results will be obtained.
- In boiling off the excess NH3, boil until the copper nitrate starts to precipitate from the copper ammonium nitrate.
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