2KMnO_{4} + 3H_{2}SO_{4} + 5C_{2}H_{2}O_{4}.2H_{2}O =
  K_{2}SO_{4} + 2MnSO_{4} + 18H_{2}O + 10CO_{2}.

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_{4} + 10NO = 3K_{2}O + 6MnO + 5N_{2}O_{5};

in other words, on the basis that one molecule of potassium permanganate will oxidize one and two-thirds molecules of nitric oxide: 

  (KMnO_{4} = 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.

The flask, B, is now heated as long as may be necessary in order to produce, on cooling, the diminished pressure required for the introduction of the ferrous chloride and hydrochloric acid.  Before removing the flame, the joint at f is closed to prevent the return of the permanganate solution.