By H.N. MORSE and A.F. LINN.
The method which we propose consists in the conversion of the nitric acid into nitric oxide; the absorption of the latter in a measured, but excessive, quantity of a standard solution of permanganate of potassium; and the subsequent determination of the excess of the permanganate by means of a standard solution of oxalic acid or sulphate of manganese.
A is an apparatus for the generation of carbon dioxide free from air, which will be explained hereafter.
B is a flask, having a capacity of 125 or 150 c.c., in which the nitrate is decomposed in the usual manner by means of ferrous chloride and hydrochloric acid.
C is a small tube for the condensation of the aqueous hydrochloric acid which distills over from B.
D is a Geissler bulb, containing a concentrated solution of potassium carbonate, to arrest any acid vapors coming from C.
E, E are two pieces of ordinary combustion tubing, having a length of about 650 or 700 mm., in which is placed the permanganate solution employed for the absorption of the nitric oxide. Their open ends are provided with lips in order to facilitate the pouring of liquids from them, care being taken not to so distort the ends that rubber stoppers cannot be made to fit them tightly. They are placed in a nearly horizontal position in order to diminish the pressure required to force the gases through the apparatus and thus lessen the danger of leakage through the rubber joints.
a is a tube through which the ferrous chloride and hydrochloric acid are introduced into B, as in the method of Tiemann-Schulze.
b serves for the introduction of carbon dioxide to expel the air before the decomposition of the nitrate, and the nitric oxide afterward.
c is an unbroken tube ending at the lower surface of the stopper in B, and at the bottom of C.
The rubber joint, d, is furnished with a Mohr and also a screw pinch cock. The joints, e and f, are furnished with Mohr pinch cocks. The rubber tubing upon these should be of the best quality, and must be carefully tied.
DETERMINATION OF NITRIC ACID.
In consequence of the large volume of the permanganate solution required for the complete absorption of the nitric oxide, we have found it advantageous to use three solutions instead of two.
1. A solution of permanganate such that one c.c. is equivalent to about fifteen milligrammes of nitrate of potassium, according to the reaction:
KMnO + NO = KNO + MnO.
This solution is employed for the absorption of the nitric oxide. Its strength need not be exactly known. There is no objection to a more concentrated solution, except that which pertains to all strong standard solutions, namely, that a small error in measurement would then give a larger error in the results. 100 c.c. of this solution are required for each determination, and the measurement is always made in one and the same 100 c.c. measuring flask, which, if necessary, should be labeled to distinguish it from that used for solution No. 2.
2. A solution of oxalic acid which is very slightly stronger than that of the permanganate just described - that is, a solution such that one c.c. of it will somewhat more than decompose one c.c. of the permanganate, according to the reaction:
2KMnO + 3HSO + 5CHO.2HO =
KSO + 2MnSO + 18HO + 10CO.
The exact strength of this solution need not be known, since we only require the difference in value between it and solution No. 1, which is determined by means of solution No. 3. 100 c.c. of this solution are also required for each determination, and the measurement, as in the preceding case, is always made in the same 100 c.c. measuring flask.
3. A dilute, carefully standardized solution of permanganate of potassium.
The method of using these solutions is as follows: 100 c.c. of No. 1 and No. 2 are measured off (each solution in its own measuring flask), brought together in a covered beaker glass, and acidified with dilute sulphuric acid. The excess of oxalic acid is then determined by means of solution No. 3.
When it is desired to make a determination of nitric acid, 100 c.c. of solution No. 1 are measured off, and as much of it as may be convenient is poured into the tubes, E, E, together with about a gramme of zinc sulphate for each tube, which substance appears to considerably facilitate the absorption of the nitric oxide by the permanganate. When the operation is over, the contents of E, E are poured into a beaker glass. 100 c.c. of solution No. 2 are then measured off, and a portion, together with a little sulphuric acid, poured into E, E, to dissolve the oxide of manganese which has separated during the absorption of the nitric oxide. The oxide having been dissolved, the liquid in E, E, and the rinsings of the tubes, also the residues of permanganate and oxalic acid left in the measuring flasks, and the rinsings from these, are all brought together in the same beaker glass. Finally, the amount of solution No. 3 required to decompose the excess of oxalic acid is determined. If we subtract from the amount thus found the quantity of permanganate required to equalize solutions Nos. 1 and 2 (previously ascertained), we shall have the amount of permanganate actually reduced by the nitric oxide, according to the reaction:
6KMnO + 10NO = 3KO + 6MnO + 5NO;
in other words, on the basis that one molecule of potassium permanganate will oxidize one and two-thirds molecules of nitric oxide:
(KMnO = 1-2/3 NO).
The method of using the apparatus is simple. The nitrate is placed in B, and the joints made tight, except that at f, which is left open. A current of carbon dioxide is passed through the apparatus until all of the air has been displaced. Connection is then made at f, and soon afterward the current of carbon dioxide is shut off at d.