24 
compared with it. It would he thus possible to compare to- 
gether the indications of various instruments working in 
different places provided that these before being issued had 
their co-efficients determined at the central observatory. 
'‘ On a Colorimetric Method for determining small quan- 
tities of Copper/’ by Thomas Caknelley, B.Sc., F.C.S., 
Demonstrator in the Chemical Laboratory of Owens College. 
Communicated by Professor H. E. Koscoe, E.RS., &c., &c. 
Last year I brought before this Society a paper (Proceed- 
ings Vol. XIV,, 2) on a colorimetric method for detennining 
iron in waters, and as this method has been found convenient 
for estimating small quantities of iron in substances other 
than water, I thought it would likewise be useful to have a 
delicate and easy method of a similar kind for copper, and 
it is the description of such a method that forms the subject 
of the present paper. 
The reagent used is the same as in the case of iron, viz., 
potassium ferrocyanide, which gives a purple-brown colour 
with very dilute solutions of copper. This reaction, how- 
ever, is not so delicate as it is with iron, for 1 part of the 
latter in 13,000,000 parts of water can be detected by means 
of potassium ferrocyanide, while 1 part of copper in a neutral 
solution, containing ammonium nitrate, can be easily detected 
in only 2,500,000 parts of water. Of the coloured reactions 
which copper gives with different reagents, those with sul- 
phuretted hydrogen and potassium ferrocyanide are by far 
the most delicate, and as a preliminary the comparitive 
values of these two reagents were tested with the following 
results, the determination being made in each case in loOcc. 
of water : — 
(1) With HaS. 1 part of copper produces a colour in 
2,500,000 parts of water. 
* Among others I may mention that nse has been made of this method 
by Wanklyn, in the indirect determination of Alum in Bread . — Chemical 
Neivs, Vol. XXXI., p. 67. 
