648 
Proceedings of the Royal Society 
ing 50 c.c. of the permanganate and 10 c.c. of the sulphuric acid 
solutions, were taken — to one a considerable amount of sulphate 
of ammonium w T as added (as, of course, any ammonia or ammonium 
salt present would assume this form), and then to both an equal 
amount of nitrite of sodium solution. In each case precisely the 
same amount of ferrous ammonium sulphate was reduced. More- 
over, solutions of nitrite of ammonium gave concordant results, and 
results which indicated that no secondary decomposition had 
occurred. Besides any nitrogen given off would almost certainly 
have been manifested by hubbies of gas. 
These questions having been considered, we may now look to the 
details of the method. A tentative experiment, by the simple addi- 
tion of sulphuric acid and permanganate solutions in Feldhaus’ 
original way,* is done to roughly ascertain how much permanganate 
solution is required to oxidise the nitrous acid in 50 c.c. of the 
nitrite solution under examination, or the amount which would be 
required, were the nitrite pure, is calculated, which is the simpler 
plan, and one proceeds as follows : — Into a beaker of not less than 
500 c.c. capacity, 200 c.c. or so of distilled water is poured. The 
required number of c.c. of permanganate for 50 c.c. nitrite solution 
being approximately known, twice or thereabouts that number is 
run into the beaker, and then 10 c.c. of the 10 per cent, sulphuric 
acid (or 12 c.c. of the pharmacopceial diluted acid) is mixed with 
this. Now slowly add 50 c.c. of the nitrite solution, the addition 
extending over about one minute, and the nozzle of the pipette 
being passed well below the surface of the liquid, and moved round 
so as to agitate the contents of the beaker. The nozzle is drawn 
above the surface when almost empty, and a little distilled water 
run over its exterior to wash off any adhering fluid from the beaker. 
After waiting two or three minutes (less time is really sufficient, as 
I find after one minute no trace of nitrite, but it is, as a matter of 
precaution, well to wait about three minutes) titrate back with the 
ammonio-ferrous solution to ascertain the amount of unchanged per- 
manganate. The exact point of decolorisation is best obtained by 
zig-zag titration, a method of obtaining the end-point used with 
excellent results by Dittmar for his “ Challenger ” reports. f Now 
* Vide supra. 
t “Challenger” Reports, “Physics and Chemistry,” vol. i. pt. i. p. 4. 
