25 
(2) TO/iK,FeCyc 
(a) In acid solutions, the colour produced being earthy 
brown, 1 part of copper produces a colour in 1,000,000 parts 
of water. 
(5) In neutral solutions, the colour being purple-brown, 
1 part of copper produces a colour in 1,500,000 parts of 
water. 
(c) In neutral solutions containing ammonium nitrate, the 
colour being purple-brown, I part of copper produces a 
colour in 2,500,000 parts of water. 
From the above it will be seen that of the two reagents, 
sulphuretted hydrogen is the more delicate, except in the 
latter case when they are of equal value. But potassium 
ferrocyanide has a decided advantage over sulphuretted 
hydrogen in the fact that lead, when not present in too 
large quantity does not interfere with the depth of colour 
obtained, whereas to sulphuretted hydrogen it is, as is well 
known, very sensitive. 
And though iron if present would, without special pre- 
caution being taken, prevent the determination of copper 
by means of pota.ssium ferrocyanide, yet by the method 
as described below the amounts of these metals contained 
together in a solution can be estimated by this reagent. 
As the above results show ammonium nitrate renders the 
reaction much more delicate ; other salts, as ammonium 
chloride and potassium nitrate, have likewise the same effect. 
The method of analysis consists in the comparison of the 
purple-brown colours produced by adding to a solution of 
potassium ferrocyanide — first, a solution of copper of known 
strength, and secondly, the solution in which the copper is 
to be determined. 
The solutions and materials required are as follows : — 
(I) Standard copper solution . — Prepared by dissolving 
0*893 grm. of pure CuSo4'5H20 in one litre of water. Icc. 
is then equivalent to 0*1 mgrm. Cu. 
