The estimation of Copper in Ores by the Iodide method.—The standard solution is again used, and needs no further checking. The student may take for analysis a sample of copper pyrites, portion of which he may reserve (after reduction and sampling) for subsequent estimation by the Cyanide method.

The Analysis.—Powder the sample to pass an 80 sieve, and from this by quartering obtain a sample of a few grams. Grind this further in an agate mortar, and from this sample weigh out .5 gm. The subsequent treatment as modified by A. H. Low (Jour. Amer. Chem. Soc., xviii., No. 5, and contribution to Peters’ Copper) is as follows:—

Transfer the ore to a 250 c.c. flask. Add 10 c.cs. 16E. HNO3 and boil nearly to dryness. Add 10 c.cs. 16E. HCl and again boil. After 2 or 3 minutes add 10 c.cs. 36E. H2SO4 and heat strongly till white fumes rise freely from the liquid. Cool. Add 40 c.cs. water and heat to boiling. Filter through a 7.5 cm. paper. Wash, and endeavour to keep the washings under 75 c.cs.

Proceed now with the aluminium precipitation as before described, except that in the case of ores Low strongly recommends the addition of about 1 gm. KClO3 before evaporating down to between 1 and 2 c.cs. This oxidises any arsenic present to arsenic acid, which is inert. The titration is performed as usual. The student may later on check the results obtained here by means of the Cyanide method or the Electrolytic method. According to Low the Iodide method is equal in accuracy to the almost perfect Electrolytic method.

VOLUMETRIC ESTIMATION OF COPPER BY IODINE AND STANDARD SODIUM THIOSULPHATE.

1. In CuSO4,5H2O.
Apparatus, Reagents.—For the preparation of the standard solution pure crystalline Na2S2O3 5H2O and pure electrolytic copper are necessary, also a solution of starch in boiling water.

Method. Reactions.-When a solution of KI is added in excess to a solution of a cupric salt in either slightly acid or an acetic acid solution Cu2I2 is formed and I liberated thus,—

2CuSO4 + 4KI = 2K2SO4 + Cu2I2 +12

This liberated iodine is now titrated with standard thiosulphate, using starch as an indicator,

2Na2S2O3 + I2 = Na2S4O6 + 2NaI

As soon as this reaction is completed the dark blue colour given by the iodine and starch immediately disappears.

Knowing, then, that a certain weight of copper will liberate from excess of potassium iodide a certain weight of iodine, then, by measuring this quantity of iodine with a solution of thiosulphate of sodium, the volume and value of which are known, the quantity of copper present may be accurately calculated. Care must be taken to follow exactly the instructions given regarding the quantity of acid used, its expulsion and neutralisation.

Preparation of the Standard Solution —A normal solution of Na2S2O3,5H2O should contain 46 + 64 + 48 + 90 = 248 gms. per litre and a N/10 solution 24.8 gms. per litre.

Weigh out 24.8 gms. of the pure salt, which has been carefully powdered in a wedgewood mortar. Dissolve in a little distilled water and make up to 1 litre at 16° C.

If exactly N/10 one c.c. of this solution is equivalent, to .0127 gm. iodine, which is equivalent to .0063 gm. copper.

Standardising the Solution. —Weigh out two portions of clean copper foil each of about .2 gms. and transfer to 200 c.c. flasks. To each add 5 c.cs. 16E. HNO3. When the copper is dissolved boil down each lot to between one and two c.cs. Cool and add to each 5 c.cs. distilled water to dissolve the copper nitrate. Add to each 5 c.cs. 20 E. NH4HO. The solution should now be strongly alkaline. Heat to boiling and boil for a few minutes to perfectly neutralise the acid in the nitrate of copper.

Remove and add to each 5 c.cs. 17 E. HC2H3O2 and then 50 c.cs. of distilled water.
Fill the burette with the thiosulphate and titrate the brown solution till the brown colour becomes faint, and then add enough starch solution to produce a strong blue coloration. Titrate slowly till the blue colour vanishes, leaving a cream-coloured precipitate. This change takes place very suddenly, and at the first decided change the student should pause a few moments, as the colour generally completely disappears on standing a moment. Note the number of c.cs. used in each determination and calculate the value, in copper, of each c.c.
Duplicates should agree within .00003 gm. of copper.

The Actual Analysis.—Weigh out about 1 gm. of the pure sulphate of copper. Transfer to a 200 c.c. beaker; dissolve in about 20 c.cs. distilled water; add 10 c.cs. 36E. H2SO4, and 30 c.cs. distilled water. In the solution place a strip of sheet aluminium about 7.5 cm. x 3.75 cm. x .15 cm., turned up at right angles for about an inch at each end, the body of the strip resting flat on the bottom of the beaker. Cover the beaker and heat to boiling, and boil from five to seven minutes till the copper is all precipitated in the metallic form (test a little of the clear solution for copper). Now transfer the liquid into a 300 c.c. flask and as much of the copper as convenient, leaving the aluminium and a little adhering copper behind after rinsing well with the wash bottle. Set the beaker and aluminium aside. Let the copper in the flask settle and decant the liquid through a white filter paper. Wash the copper several times by decantation. Wash the paper once or twice, bringing any particles of copper down to the point. Place the beaker with the aluminium under the funnel. On the paper, drop by drop, add 4 c.cs. 16E. HNO3 to dissolve any copper on the paper. When this has drained through, wash with three or four very small washes of hot water. When all the copper that may have been loft in the beaker is dissolved, rinse the contents of the beaker into the flask, leaving the aluminium behind. Heat the flask and contents till all the copper is dissolved. Evaporate down to 1 to 2, c.cs., and proceed with the remainder of the analysis as in the standardisation of the thiosulphate by metallic copper. Repeat the operation on a duplicate sample.

Calculation.—Assume, for example, that 40 c.cs. Na2S2O3 are required for 1.0012 gm. of the salt, and that 1 c.c. Na2S2O3 equals .00632 gm. copper. Then the quantity of copper present is 40 x .00632 = .2528 gm. The percentage is calculated as usual, and the results may be compared with those found by gravimetric methods. The standard solution may be relied on to keep for several weeks, but it is better for the student to make a point of re-checking a solution that has not been in use for some days.