946 COPPER ASSAYING 



cess. The whole of tho ores belonging to the different classes may bo estimated in 

 this way. 



A weighed quantity of tho pulverised ore is introduced into a long-necked flask of 

 hard German glass. Nitric acid is now added, and the flask exposed to tho heat of a 

 sand-bath. A little hydrochloric acid is subsequently introduced, and tho attack con- 

 tinued until the residue, if any remains, appears to bo free from all metallic stains. 

 The contents of the flask are transferred to a porcelain evaporating dish, and evapo- 

 rated to dryness, taking care, by means of frequent stirring, to prevent the mass from 

 spirting. The whole must now bo removed from tho sand-bath and allowed to cool, 

 a little hydrochloric acid subsequently added, and afterwards, some distilled water. 

 The contents of the basin must then be made to boil, and, whilst still hot, filtered into 

 a beaker. A piece of bright wrought iron, about two inches in length, three-quarters 

 of an inch in width, and a quarter of an inch in thickness, is now introduced, and tho 

 liquor gently heated on the sand-bath until the whole of the copper has been thrown 

 down. The liquor is now removed by means of a glass siphon, and the metallic copper 

 freed from all adhering chlorides, by means of repeated washings with hot water, and 

 then dried in a water-bath, and weighed. 



In case the mineral operated on should contain tin or antimony, very minute traces 

 only of these metals will be found with the precipitated copper. When lead is pre- 

 sent, it is best to add a few drops of sulphuric acid during the process of the attack ; 

 by this means the lead will be precipitated as sulphate of load, and be removed by 

 filtration. 



Assay of Commercial Varieties of Copper, Brass, Bronze, cf e. 



By Hyposulphite of Soda. This is a very accurate method of estimating copper : it 

 was first made known by Mr. E. 0. Brown. The reagents required are, 1. A standard 

 solution of hyposulphite of soda. This is made by dissolving 1,300 grains of tho 

 crystals in 4 pints of water ; tho solution is standardised by means of weighed pieces 

 of electrotype copper by the process afterwards described, and tho mean result taken 

 as standard. 2. Iodide of potassium. The crystals should be free from iodate. About 

 six times the weight of the copper present is required for each assay. 3. Solution of 

 Starch. Prepared by boiling from 1 to 2 grains of starch in about half a pint of 

 water. When cold the clear solution is decanted off for use. The estimation of the 

 copper by this method depends on tho following reactions. When iodide of potassium 

 is added to acetate of protoxide of copper, subiodide of copper is formed, and free 

 iodine is set free, which dissolves in the excess of iodide of potassium. By means of a 

 standard solution of hyposulphite of soda, the free iodine is converted into iodide of 

 sodium with the formation of tetrathionate of soda. The solution of starch serves to 

 indicate the completion of the reaction, by the bleaching of the blue iodide of starch at 

 first formed. The reactions may be expressed by the following equations : 



2(CuO, A) + 2KI = Cu'I + 1 + 2(KO, A) 

 I + 2(NaO,S'0 2 ) = Nal + NaO.S'O 5 . 



Process. From 8 to 10 grains of the copper or alloy are dissolved in dilute nitric 

 acid, and the nitrous acid expelled. The solution diluted witli water, and carbonate 

 of soda added until carbonate of protoxide of copper remains precipitated. Acetic acid 

 is added in excess, and afterwards about 60 grains or less of iodide of potassium. 

 The solution of hyposulphite of soda is now run in from a burette, until tho solution 

 acquires a yellow colour. A little of the starch-solution is now added, and the ad- 

 dition of the hyposulphite of soda cautiously continued until the solution becomes 

 colourless. The number of divisions on the burette are read off, and the amount of 

 copper calculated therefrom. Example : If 1 ,000 grains of the standard solution are 

 equal to 10 grains of copper, and 10 grains of the alloy require 600 grains of the same 

 solution, then 



1,000 : 600:: 10 : 6 and 



6 x 10 = 60 per cent, of copper. 



When lead is present, a little sulphuric acid should be added during the solution of tho 

 metal in nitric acid, to convert it into insoluble sulphate of protoxide of lead. To 

 render it applicable to copper ores, after the solution of the ore and dilution, tho 

 copper must first be separated by means of sulphuretted hydrogen, hyposulphite of 

 soda, or by iron or zinc, and the precipitate redissolved in nitric acid, taking care to 

 expel all trace of nitrous acid before proceeding with tho assay. 



Assay of Slags and other Substances containing Small Quantities of Copper. 



Coloration Test. This method depends on the fact that the intensity of the blue 

 colour of an ammonia cal solution of copper is proportionate to the quantity of metal 

 present. There are two plans which may bo followed: a. A standard coloration 



