124 DONALD J. MATTHEWS. 
impossible with the purest chemicals of Merck and Kahlbaum. As an 
example, the artificial sea-water was found to contain 0-0143 me. of 
P.O; in 500 cem.; 0-0234 mg. was added to 500 cem. and 0-0328 me. 
found, a loss of 0-0049 mg. In another experiment made on 250 ccm. the 
loss was only 0-0007 mg. These differences are both of about the same 
magnitude as those found between duplicate analyses of sea-water, so 
the experiments are not conclusive, though it shows that the method is 
at any rate approximately accurate. Comparison was also made with 
the method finally used ; simple precipitation by ammonia gave 0-0312 
mg. per litre, against 0-0318 mg. and 0-0316 mg. by the final method. 
Again, precipitation by ammonia gave 0-057 mg. in duplicate analyses ; 
treatment of the filtrate from one of these by ammonia gave a further 
voluminous precipitate which contained no recognisable phosphates. 
The method is therefore probably accurate to about 0-003 mg. 
In the end, concentration by iron was found to be the most satisfactory 
and quickest method, duplicate determinations taking five hours or less 
when the water had beea previously filtered. The requisites are :— 
Ferrie nitrate or chloride solution, nearly neutral, containing 5 to 6 meg. 
of iron in | cem. 
Nutrie acid, strong and 25 per cent by volume. 
Ammonia, dilute ; 2 N is a convenient strength. 
Ammonium chloride ; about 2 N. 
Pouget and Chouchak’s reagent ; two solutions are required. A: 95 
germs, of molybdic acid and 30 grms. of anhydrous carbonate of soda are 
dissolved in 500-600 cem. of warm water, and after cooling 141 ccm. of 
strong nitric acid are added. The solution is made up to 1000 cem. 
B: a2 per cent solution of strychnine sulphate. For use 1 ccm. of B is 
added to 10 cem. of A and the mixture filtered and used at once. With 
0-03 mg. of P.O; in 50 ccm. of 3-7 per cent nitric acid, this reagent gives 
a strong opalescence at once, while 0-005 mg. will give the reaction in a 
few minutes. This opalescence is yellow when examined in the colori- 
meter; it takes twenty minutes to attain its full strength, and after 
three or four hours a precipitate is thrown down, so the comparisons 
should be made as soon as possible after the twenty minutes have elapsed. 
The colour is proportional to the amount of phosphoric acid when the 
content in P.O; lies between 0-01 mg. and 0-05 mg. in 50 cem.; it is 
aflected by variations in the amount of reagent used and by the amount 
of free nitric acid. Pouget and Chouchak give a number of determina- 
tions in the presence of various oxides, and show that the results are very 
good unless the oxides are present in very large amount ; for instance, 
lime is without influence when there is not more present than 20,000 
