AMOUNT OF PHOSPHORIC ACID IN THE SEA-WATER. 123 
The determination of phosphoric acid in sea-water falls into two parts, 
first concentration in small bulk, and then determination of the amount. 
Concentration may be effected either by precipitating, by the addition 
of ammonia, a portion of the alkaline earths, which carries down the 
phosphoric acid, or by adding an iron salt, ammonium chloride, and 
ammonia and precipitating a mixture of ferric phosphate and hydroxide. 
The estimation may be carried out either gravimetrically as ammonium 
phospho-molybdate, or colorimetrically by the very sensitive reaction 
with nitro-molybdate of strychnine described by Pouget and Chou- 
chak (5). The writer finally adopted the colorimetric method following 
concentration with iron, but all four processes have been used. 
From April 21 to September 13, 1915, the method was as follows : 
About 1500 cem. of filtered water was precipitated, after making slightly 
acid and heating, with iron and ammonium chloride and ammonia. 
The precipitate was filtered off, dissolved in warm dilute nitric acid, and 
evaporated to dryness several times with further addition of acid to 
remove silica. The phosphorus was determined by precipitating with 
ammonium molybdate and weighing as anhydride. For such smail 
quantities, the amount of anhydride being only a few milligrams, the 
method cannot be considered satisfactory, as there is danger of loss 
through some of the precipitate dissolving in the wash-waters or being 
carried through the asbestos of the Gooch crucible, and on the other 
hand there is the risk of the results being too high owing to the sim- 
ultaneous precipitation of a trace of molybdic acid. With one exception 
the figures obtained were much higher than most of those by the 
colorimetric method. From April 21 to July 14 about 0-07 mg. per litre 
was found gravimetrically ; there was then a break in the series and the 
next analysis, made on September 13, showed a considerable fall to 
0-04 mg. by the gravimetric method, in good agreement with that for 
September 21, when 0-046 mg. was found by the colorimetric method, 
which was adopted for all following work. 
Samples for the colorimetric method were at first concentrated by 
precipitating 500 ccm. of filtered sea-water with 3 ccm. of pure concen- 
trated ammonia, heating, and filtering off the voluminous precipitate of 
hydroxides, dissolving it in nitric acid, and evaporating on the water- 
bath to dryness, after which it was treated as described later. The 
method had the advantage that only one reagent was used for precipita- 
tion, and as a rule the solution filtered quickly and duplicate analyses 
agreed well, the average difference being 0-0029 mg. per litre. Its accu- 
racy, however, was difficult to establish without making up an artificial 
sea-water free, or nearly so, from phosphates, and this was found to be 
