350 MR. T. CARNELLEY ON A COLORIMETR1C 



excess of the latter is very detrimental to the accuracy of 

 the method ; for from experiments made in relation to this 

 point it was found that when the amount of free acid pre- 

 sent in 200 cub. cent, is more than 0*0025 cub. cent, of 

 the strong acid, it renders the colour deeper than it other- 

 wise would be. 



One cubic centimetre of the standard nitric acid is added 

 to each of the cylinders, (1) because it renders the reaction 

 much more delicate ; (2) because the colours produced 

 in the presence of this amount of free acid are almost al- 

 ways of the same tint, being of a pure blue, whilst, when 

 no free acid is present the colour varies, even when ap- 

 parently of the same depth, from a blue to a bluish green, 

 which renders them less easy to compare ; (3) because it 

 destroys the effect which the presence of a small quantity 

 of any free acid previously existing in the liquid might 

 have in altering the shade of colour produced ; for, with 

 reference to this point also, from a series of experiments 

 made it was found that when the amount of the free acid 

 present, in addition to the 1 cub. cent, of standard nitric 

 acid added, is only small, i. e. less than 0*05 cub. cent, of 

 the strong acid in 200 cub. cent, of water, it has no effect 

 on the depth of colour produced. When any free acid 

 exists in the water to be examined, it must, before being 

 oxidized, be made as nearly neutral as possible with am- 

 monia, and the iron then determined. 



In order to test the method as a whole, the experiments 

 given below were made, and were carried out as follows : — 



A known weight of ammonio -ferrous sulphate was taken, 

 dissolved in water, 1 cub. cent, of the sulphuric acid added, 

 and then oxidized with permanganate, and the whole made 

 up to 1 litre. The iron in the solution thus obtained, was 

 determined as described above. One of a number of series 

 of experiments thus made is given below. 



