29 
ferrocyanide to a lead salt is white, ana this, except when 
present in comparatively large quantity with respect to the 
copper, does not interfere with the comparison of the colours. 
In the above experiments the proportion of lead to copper 
was as 5 to 1. 
When copper is to be estimated in a solution containing 
iron the following is the method of procedure to be adopted. 
To the solution a few drops of nitric acid are added in order 
to oxidise the iron, the liquid evaporated to a small bulk 
and the iron precipitated by ammonia,. Even when very 
small quantities of iron are present this can be done easily 
and completely if there is only a very small quantity of 
fluid. The precipitate of ferricoxide is then filtered off, 
washed once, dissolved in nitric acid and reprecipitated by 
ammonia, filtered, and washed. The iron precipitate is now 
free from copper, and in it the iron can be estimated by dis- 
solving in nitric acid, making the solution nearly neutral 
with ammonia and determining the iron by the method 
given in the paper before referred to. The filtrate from the 
iron precipitate is boiled till all the ammonia is completely 
driven off, and the copper estimated in the solution so 
obtained as already described. The following are the results 
obtained with solutions containing known quantities of iron 
and copper : — 
Copper Iron 
Found Calculated Found Calculated 
(1) 0'53 mgrm 0’51 mgrm 0-22 0*20 mgrm. 
(2) 0-69 „ 0-61 „ 2-15 2*40 „ 
(3) 0-79 „ 0-76 „ 2*42 3-00 „ 
(4) 0-66 „ 0-G6 — _ 
When the solution containing copper is too dilute to give 
any coloration directly with potassium ferrocyanide, a 
measured quantity of it must be evaporated to a small bulk 
and filtered if necessary, and if it contains iron, also treated 
as already described. 
