Cool the flask or beaker, add 25 cc. of water, heat the solution to boiling, and boil for two minutes.  Filter to remove insoluble sulphates, silica and any silver that may have been precipitated as silver chloride, and receive the filtrate in a small beaker, washing the precipitate and filter paper with warm water until the filtrate and washings amount to 75 cc.  Bend a strip of aluminium foil (5 cm. x 12 cm.) into triangular form and place it on edge in the beaker.  Cover the beaker and boil the solution (being careful to avoid loss of liquid by spattering) for ten minutes, but do not evaporate to small volume.

Wash the cover glass and sides of the beaker.  The copper should now be in the form of a precipitate at the bottom of the beaker or adhering loosely to the aluminium sheet.  Remove the sheet, wash it carefully with hydrogen sulphide water and place it in a small beaker.  Decant the solution through a filter, wash the precipitated copper twice by decantation with hydrogen sulphide water, and finally transfer the copper to the filter paper, where it is again washed thoroughly, being careful at all times to keep the precipitated copper covered with the wash water.  Remove and discard the filtrate and place an Erlenmeyer flask under the funnel.  Pour 15 cc. of dilute nitric acid (sp. gr. 1.20) over the aluminium foil in the beaker, thus dissolving any adhering copper.  Wash the foil with hot water and remove it.  Warm this nitric acid solution and pour it slowly through the filter paper, thereby dissolving the copper on the paper, receiving the acid solution in the Erlenmeyer flask.  Before washing the paper, pour 5 cc. of saturated bromine water (Note 3) through it and finally wash the paper carefully with hot water and transfer any particles of copper which may be left on it to the Erlenmeyer flask.  Boil to expel the bromine.  Add concentrated ammonia drop by drop until the appearance of a deep blue coloration indicates an excess.  Boil until the deep blue is displaced by a light bluish green coloration, or until brown stains form on the sides of the flask.  Add 10 cc. of strong acetic acid (Note 4) and cool under the water tap.  Add a solution containing about 3 grams of potassium iodide, as in the standardization, and titrate with thiosulphate solution until the yellow of the liberated iodine is nearly discharged.  Add 1-2 cc. of freshly prepared starch solution and titrate to the disappearance of the blue color.

From the data obtained, calculate the percentage of copper (Cu) in the ore.

[Note 1:  Nitric acid, because of its oxidizing power, is used as a solvent for the sulphide ores.  As a strong acid it will also dissolve the copper from carbonate ores.  The hydrochloric acid is added to dissolve oxides of iron and to precipitate silver and lead.  The sulphuric acid displaces the other acids, leaving a solution containing sulphates only.  It also, by its dehydrating action, renders silica from silicates insoluble.]