ESTIMATION OF NITRITES IN THE PRESENCE OF NITRATES. 
203 
of sodium, no orange fumes will be observed, but the coloration of the starch- 
paper is instantaneous and very decided. In the first instance the nitrous 
acid was decomposed, as generated, into nitric acid and nitric oxide, the latter 
being evinced by the orange fumes of pernitric oxide on its coming in con¬ 
tact with the atmosphere. From the foregoing it is evident that to estimate 
a nitrite by an oxidizing reagent, it is better that the two be brought together 
at a temperature sufficiently low, that the oxidizer used may grapple with the 
nitrous acid previously to a reaction being set up between it and the water. 
Experiments have been tried to find the reaction of two of the principal oxi¬ 
dizers at a temperature of 32° Fahr. Permanganic acid was instantly deco¬ 
lorized. Chromic acid had no action at 32° Fahr., but on its being placed on 
one side, so as gradually to attain the temperature of the room, decomposition 
took place without the evolution of any nitrous oxide. 
As stated in a former part of this paper, by a judicious attention to a low 
temperature, and by a manipulation formed upon Feldhaus’s plan, an analysis by 
permanganate of potassium may be performed, but still inferior in accuracy to 
the two processes I am now about to describe. 
The first process is based upon the reduction of chromic acid to chromic oxide 
by nitrous acid. This reaction is not so quick as with permanganic acid, but 
is perfected slowly at ordinary temperatures. Suppose we are analyzing a spe¬ 
cimen of commercial nitrite of sodium, the mode of procedure I adopt is as 
follows :—If the sample contains carbonate of sodium, a weighed quantity, say 
two grammes, is dissolved in a rather considerable quantity of water, and the 
carbonate estimated by a standard solution of sulphuric acid, carefully avoiding 
an excess. To hit the exact point of saturation, I well soak a piece of good 
litmus-paper in the solution after the addition of each quantity of acid from 
the burette, and on drying it the exact state of the solution will be perceived. 
I may here remark that most of the litmus-paper that is bought is not delicate 
enough for this kind of work, as it generally contains some trace of alkali. A 
convenient indicator of this point of saturation in this case will be found in a 
solution of starch and iodide of potassium contained in a test tube: one drop of 
the solution of nitrite added after each addition of acid will, when the carbonate 
is all decomposed, strike a blue shade on falling through the starch solution. 
After noting the amount of carbonate, the solution is in a fit position for the 
estimation of the nitrite; the remainder may practically be noted as nitrate. 
Three grammes of pure bichromate of potassium for every two grammes of 
nitrite taken are dissolved with a little water in a flask fitted with a well- 
ground stopper; an excess of sulphuric acid is then added, and the flask is 
placed in a vessel containing a mixture of sulphate of sodium and hydrochloric 
acid. The solution of the nitrite may be placed also in the same freezing bath for 
a few minutes previously to being poured on the surface of the chromic acid 
without mixing ; the stopper is then inserted, the flask taken out of the freezing- 
mixture, inverted, and left to regain the ordinary temperature of the room ; in 
the course of half an hour or an hour the flask will contain a mixture of chromic 
acid and chromic salt, the chromic oxide representing the nitrite in the sample. 
But here, again, in precipitating the chromic oxide a precaution is necessary. If 
there is any considerable excess of chromic acid left, which is generally the case, 
when examining commercial samples, the ordinary method of precipitating with 
ammonia would not do, as a brown precipitate of a peroxide of chromium (chro¬ 
mate of chromium ?) not decomposable by ammonia is thrown down, although 
this substance is instantly decomposed, upon boiling, by a solution of potash 
into chromic oxide and chromic acid. It is therefore necessary to nearly neutra¬ 
lize with potash, and finish off with a few drops of ammonia, and boil until all 
trace of the latter substance is gone; but if accidentally too much pbtasli is 
added, a few drops of chloride of ammonium and a boiling for a few minutes 
