!Method C!

Procedure.—­Weigh out into 500 cc. beakers two portions of 0.175-0.200 gram each of pure arsenious oxide.  Dissolve each of these in 10 cc. of sodium hydroxide solution, with stirring.  Dilute the solutions to 150 cc. and add dilute hydrochloric acid until the solutions contain a few drops in excess, and finally add to each a concentrated solution of 5 grams of pure sodium bicarbonate (NaHCO_{3}) in water.  Cover the beakers before adding the bicarbonate, to avoid loss.  Add the starch solution and titrate with the iodine to the appearance of the blue of the iodo-starch, taking care not to pass the end-point by more than a few drops (Note 1).

From the corrected volume of the iodine solution used to oxidize the arsenious oxide, calculate its relation to the normal.  From the ratio between the solutions, calculate the similar value for the thiosulphate solution.

[Note 1:  Arsenious oxide dissolves more readily in caustic alkali than in a bicarbonate solution, but the presence of caustic alkali during the titration is not admissible.  It is therefore destroyed by the addition of acid, and the solution is then made neutral with the solution of bicarbonate, part of which reacts with the acid, the excess remaining in solution.

The reaction during titration is the following: 

Na_{3}AsO_{3} + I_{2} + 2NaHCO_{3} —­> Na_{3}AsO_{4} + 2NaI + 2Co_{2} + H_{2}O

As the reaction between sodium thiosulphate and iodine is not always free from secondary reactions in the presence of even the weakly alkaline bicarbonate, it is best to avoid the addition of any considerable excess of iodine.  Should the end-point be passed by a few drops, the thiosulphate may be used to correct it.]

DETERMINATION OF COPPER IN ORES

Copper ores vary widely in composition from the nearly pure copper minerals, such as malachite and copper sulphide, to very low grade materials which contain such impurities as silica, lead, iron, silver, sulphur, arsenic, and antimony.  In nearly all varieties there will be found a siliceous residue insoluble in acids.  The method here given, which is a modification of that described by A.H.  Low (!J.  Am.  Chem.  Soc.! (1902), 24, 1082), provides for the extraction of the copper from commonly occurring ores, and for the presence of their common impurities.  For practice analyses it is advisable to select an ore of a fair degree of purity.

Procedure.—­ Weigh out two portions of about 0.5 gram each of the ore (which should be ground until no grit is detected) into 250 cc.  Erlenmeyer flasks or small beakers.  Add 10 cc. of concentrated nitric acid (sp. gr. 1.42) and heat very gently until the ore is decomposed and the acid evaporated nearly to dryness (Note 1).  Add 5 cc. of concentrated hydrochloric acid (sp. gr. 1.2) and warm gently.  Then add about 7 cc. of concentrated sulphuric acid (sp. gr. 1.84) and evaporate over a free flame until the sulphuric acid fumes freely (Note 2).  It has then displaced nitric and hydrochloric acid from their compounds.