18 
bulk is taken, the amount being regulated according to the 
quantity of iron contained in the water, which is judged 
by a previously made qualitative experiment of adding Icc. 
of the ferrocyanide to a portion of the oxidised water. One 
cubic centimetre of the sulphuric acid is added and then the 
permanganate from a burette till a permanent faint pink 
colour is obtained ; the vvhole is made up to one litre, when 
it forms what may be called the “water test solution.” 
Into each of the cylinders Icc. of the potassium ferrocyanide 
is added and then a measured quantity of the water test 
solution put into one of them (x), both are next filled with 
water up to the mark and Icc. of the standard nitric acid 
added to each. After (x) has been well stirred the standard 
iron solution is gradully run into {y), the liquid being stirred 
after each addition, and the colours in the two cylinders 
compared by placing them side by side over a sheet of white 
paper in front of a window ; this is repeated till the colours 
in each of the cylinders appear to be equal, which point 
completes the operation. 
Every cubic centimetre of iron solution used corresponds 
to O’l mgrm. of iron, from which the amount of iron added 
to cylinder (y) can be calculated. Then assuming that equal 
shades of colour are, ceteris 'paribus, produced by equal 
weights of iron, the amount of the latter in cylinder (x) is 
equal to that added to (y), and since the volume of the 
original sample of water in the test solution is known, and 
also the volume of the latter put into {x), the amount of 
iron in a measured quantity of water can therefore be cab 
culated. The volume of the test solution put into cylinder 
(x) should be such as not to require more than 5cc. of the iron 
solution to be added to (y) to produce an equal shade, for if 
more be added the colour obtained would be too dark to 
compare with ease and accuracy. 
If the sample of water contains such a small amount of 
iron as, after oxidation, not to give a coloration directly 
with the ferrocyanide and nitric acid, a sufficient quantity 
of it must be evaporated with a cubic centimetre of the 
sulphuric acid till it occupies from 100 to 200cc. The 
