estimating Phosphoric Acid and its Compounds. 183 
This liquid, which contains the perchloride and peracetate of 
iron, together with the chloride of ammonium, the acetate of 
ammonia, and a little free acetic acid, I find to be well adapted 
for the estimation of phosphoric acid ; and as it appears from my 
experiments that it may be kept for a considerable time without 
undergoing any change, it is therefore preferable to the acetate 
and other salts of iron hitherto employed, which, from their 
proneness to decompose, require to be freshly prepared before 
using them. Having made the standard solution of iron, the 
next step is to prepare the phosphate, and if it is (as is ge- 
nerally the case) an insoluble one dissolved by an acid, am- 
monia is first added to the solution till it is distinctly alkaline 
to turmeric paper*, then acetic acid to redissolve completely 
the phosphate precipitated by the ammonia, and leave that acid 
in slight excess; and finally the standard solution of iron is 
carefully added from a Mohr’s alkalimeter, or any other con- 
venient form of volumetric apparatus, till the iron begins to be 
in slight excess. 
I ascertain this point by taking out of the mixture (after it 
has remained a few minutes with occasional stirrig to effect the 
complete combination of the oxide of iron with the phosphoric 
acid) a drop of the solution on the end of a glass rod, and 
touching with it a piece of thick and close-textured filtermg 
paper, under which is placed some ordinary filtering paper, 
which has been previously saturated with a strong solution of 
gallic acid, and then dried. 
By this arrangement we avoid filtering: the insoluble phos- 
phate of iron formed in the process being retained by the upper 
paper, and the solution passing down to the lower, at once shows, 
by the light-purplish stain produced, the point when sufficient 
iron has been added to combine with all the phosphoric acid 
present, and a very minute excess exists in the mixture. 
This experiment being repeated a second or third time, by 
haying the phosphate under examination dissolved in a given 
quantity of solution and taking a certain amount of it for each 
* A large excess of ammonia should be avoided, for by its afterwards 
combining with the acetic acid to form the acetate of ammonia, which dis- — 
solves to a slight extent the phosphate of iron, the amount of phosphoric 
acid estimated in that case by ie process is somewhat diminished. A 
large excess of acetic acid, however, appears to affect but very slightly the 
results obtained. 
+ The form which I have used and found most convenient is Mohr’s 
alkalimeter with the addition of Professor Erdmann’s float, which affords 
great facilities for the accurate reading of the volume of liquid employed. 
t When the excess of iron added is very minute, I have found that by 
gently drying the wet spot on the gallic acid paper, the stain becomes 
more visible. 
02 
