[ 251 } 
On§the Amount of Phosphoric Acid in the Sea-Water 
off Plymouth Sound. II. 
By 
Donald J. Matthews. 
With one Figure in the Text. 
In a previous paper* the writer gave the results of the determination 
of the phosphates in sea-water, by means of Pouget and Chouchak’s 
reagent, on a number of samples collected between September 13, 1915, 
and February 5, 1916, at the Knap Buoy, half a mile outside the break- 
water at Plymouth. 
The analyses have been continued so as to cover a period of sixteen 
months and show a large seasonal variation. 
The method is described in detail in the previous paper, and consists 
in throwing down the phosphoric acid with iron and an alkali, treating 
with nitric acid, and determining the amount colorimetrically in the 
Dubosq apparatus after adding nitromolybdate of strychnine. A few 
modifications have been made and are described below. 
In the first place, as the pressure of other work made it impossible 
to examine all samples immediately after collection, they were sterilised 
as soon as taken with toluol or chloroform. Toluol was perfectly satis- 
factory, but the chloroform in some cases threw down a precipitate on 
standing, and the absence of figures for July, August, November and 
December, 1916, is due to the loss of samples from this cause. If the 
sample is allowed to stand without previous sterilisation the phosphates 
decrease and may be entirely removed in a few weeks. 
The standard phosphate solution contained 0-003 mg. of P,O; in one 
cubic centimetre, and was made up in approximately decinormal nitric 
acid to prevent the growth of moulds. 
For the precipitation of the iron, sodium carbonate was used instead 
of ammonia and ammonium chloride ; 1 ccm. of 2 N solution for 1 cem. 
of iron solution was sufficient to more than neutralise the excess acid of 
the latter. 
In some instances the iron precipitate was greasy and was difficult 
* Matthews, D. J., ‘‘On the Amount of Phosphoric Acid in the Sea- Water off Plymouth 
Sound.” This Journal, xi., No. 1, p. 122, March, 1916. 
