354 MR. T. CARNELLEY ON A COLORIMETR1C 



Iron found. Iron calculated, 



milligr. milligr. 



•18 '20 



•18 -19 



•14 16 



•14 14 



•12 -io 



•070 "078 



•053 "062 



•025 '031 



With reference to the delicacy of the method, it was 

 found, as a mean of seven experiments, that 0*0055 

 milligr. of iron gave a very distinct colour on the sur- 

 face, and that 0*015 milligr. give a blue colour on being 

 stirred with 200 cub. cent, of water, and therefore that 1 

 part of iron produces a blue coloration in 1 3,000,000 parts 

 of water containing ferrocyanide of potassium and nitric 

 acid. According to Hartig "*, however, 1 part of iron (in 

 the form of sulphate) only produces a colour in 600,000 

 parts of water containing ferrocyanide ; the difference of 

 these results is due to the effect which the presence of the 

 small quantity of free nitric acid, added in the new method, 

 has in increasing the delicacy of the reaction. 



As to the smallest differences of reading which can be 

 detected, it was found that when any quantity of iron 

 solution below 1 cub. cent, had been added, a difference 

 of 0*05 cub. cent, can be discriminated; above 1 and below 

 2 cub. cent, a difference of o*i cub. cent.; above 2 and 

 below 4 cub. cent, a difference of 0*2 cub. cent.; and above 

 4 and below 5 cub. cent, a difference of 0*3 cub. cent. 



The following are a few samples of different waters in 

 which the iron has been determined as described above. 



* Journ. Pr. Chem. xxii. ci. 



