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process lias been unanimously condemned. Lang says it is bad, and I eidhaus 
also condemns it. As regards my own experiments, they were so unpromising 
that X did not carry on my examination of this method to any extent, for to 
attempt to estimate quantitatively free nitrous acid by a boiling solution is evi¬ 
dently radically wrong in principle.* 
Estimation by permanganic acid we shall dwell upon at some length, for two 
reasons : first, because the process has been strongly recommended by authori¬ 
ties ; and, secondly, from the fact that, at first sight, this, of all the available 
oxidizing substances, seems the most plausible. Permanganic acid is instantly 
decomposed, even at 32° Fahr., by nitrous acid f and, from the quickness of the 
reaction, many plans have been devised to remedy the evils attendant upon this 
mode of estimation. From the gaseous nature of nitrous acid, it is not desir¬ 
able to decompose the nitrites first by acids, preparatory to estimating them by 
a standard solution. To avoid this source of error, Peau de Saint-Gilles has 
recommended that the nitrite be decomposed by adding very diluted sulphuric or 
nitric acid. “This prevents a loss of nitrous acid but that this is not prac¬ 
tically correct is evident from the fact, that iodized starch-paper becomes 
coloured on holding it in the orifice of the flask in which the nitrite is decom¬ 
posed. Feldhaus says that the salt must be diluted with 2000 parts of water, 
and the reaction commenced in a very slightly acidulated liquid; when it is 
nearly completed, more acid may be added if necessary, so as to ensure the 
marking of the exact point. Lang condemns the process. There can be no 
doubt that by attention to the instructions given by Feldhaus, and by working 
at a very low temperature, an approximation may be attained. One of my ex¬ 
periments was to add the nitrite under examination to a known excess of per¬ 
manganate of potassium, and, as nitric acid was present, to work off the excess 
by a volumetric solution of oxalic acid. It is, however, impossible to determine 
the analysis, as, towards the end of the process, a reaction takes place between 
the permanganic acid added, the manganous oxide resulting in a copious deposit 
of peroxide of manganese— 
2 HMn 2 0 4 + 3 Mn 2 O - H 2 O = 5 Mn 2 0 2 . 
This reaction seems to take place even more readily in the presence of nitric 
acid. 
I attempted to estimate the nitrites by ignition with oxide of copper, intend¬ 
ing to wash the oxide, and, on dissolving it in hydrochloric acid and chloride of 
sodium, to estimate the amount of cuprous oxide formed volumetrically; but I 
found, even after prolonged ignition, the nitrite was not decomposed,—or, at 
least, that the nitrite present underwent decomposition. 
The most successful methods are those which I am now about to describe. 
But X must premise that in all processes of estimating nitrites by oxidation, one 
thing must be borne in mind, that it is imperatively necessary to set free the 
nitrous anhydride at a low temperature in the presence of an excess of the oxi¬ 
dizing material; for it must be remembered that nitrous anhydride or nitrous 
acid, when liberated in the presence of water, is decomposed at slightly elevated 
temperatures. 
The following experiments will illustrate this fact:—If, at an ordinary tem¬ 
perature, a small portion of nitrite of sodium be dropped into diluted sulphuric 
acid in a test tube, the following reaction will be observed:—The tube will 
be filled with orange-coloured vapour, whilst a piece of iodized starch-paper, 
held over the orifice, will be slowly coloured. If, on the other hand, the test- 
tube be placed in a freezing mixture, previously to addition of the nitrite 
* A standard solution of nitrate of urea is heated to boiling, and the solution of nitrite, 
acidulated, is added drop by drop until the solution produces a blue coloration with starch 
paste and iodide of potassium. The decomposition is CH 4 N 2 0 + 2HN0 2 = C0 2 + N 4 + 3H 2 0 
—carbonic anhydride and nitrogen being evolved. 
