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.
As soon as the flask, B, has become sufficiently cool, the ferrous chloride and hydrochloric acid are introduced through the tube, a (which has been full of water from the first), in the same manner and quantities as in the well-known Tiemann-Schulze method.
The pinch cock at d is then opened, and the apparatus allowed to fill with carbon dioxide. When the pressure has become sufficient to force the gas through the solution of permanganate, the pinch cock at f is removed. It should be opened only slightly and with great caution at first, unless one is certain that the pressure is sufficient. If the pressure is insufficient, the fact will be made apparent by a rise of the permanganate in the small internal tube.
The flow of carbon dioxide is now reduced to a very slow current, or entirely cut off. The contents of B are slowly heated, until the decomposition of the nitrate is complete and the greater part of the nitric oxide has been expelled, when the apparatus is again closed at f and d, and allowed to cool. The tube, a, is then washed out, by the introduction through it into B of a few cubic centimeters of strong hydrochloric acid.
The process of filling the apparatus with carbon dioxide, and of heating the contents of B, is repeated. When it becomes apparent, from the light color of the liquid in B, that all of the nitric oxide has been expelled from it, the current of carbon dioxide is increased and the heating discontinued. Care must be taken, however, not to admit too strong a current of carbon dioxide, lest some of the nitric oxide should be forced unabsorbed through the permanganate solution. It is also necessary, for the same reason, to avoid too rapid heating during the decomposition of the nitrate.
When all of the nitric oxide has been forced into the solution of permanganate, the determination is made in the manner already described.
To test the method, nine determinations were made with quantities of pure nitrate of potassium varying from 100 to 200 milligrammes. The maximum difference between the volumes of permanganate actually used and those calculated was 0.05 c.c., while the main difference was 0.036 c.c. The measurements of the permanganate were made from a burette which had been carefully calibrated. We also made a number of determinations, using a solution of manganous sulphate in the place of the oxalic acid. The advantage of this method lies in the fact that it is not necessary to dissolve the oxide which is precipitated upon the glass within the tubes, E, E, since, in the presence of an excess of permanganate, the reduction by nitric oxide extends only to the formation of MnO; also in the fact that the solution of manganous sulphate is more stable than that of oxalic acid. A solution of the sulphate having been once carefully standardized, can be used for a long time to determine the value of permanganate solutions.
The details of the method are as follows: A solution of manganous sulphate slightly stronger than No. 1 is prepared.
The difference between 100 c.c. of it and 100 c.c. of No. 1 is ascertained, according to the method of Volhard, by means of solution No. 3.
The contents of E, E, together with the rinsings from the tubes, are poured into a capacious flask. 100 c.c. of the manganous sulphate and a few drops of nitric acid are then added, and the whole boiled. Finally, the excess of manganous sulphate is determined, in the manner described by Volhard, by means of solution No. 3. Subtracting from the total amount of permanganate thus used the quantity required to equalize the 100 c.c. of solution No. 1 and the 100 c.c. of the manganous sulphate, we shall have the quantity of permanganate reduced by the nitric oxide.
It must, however, be remembered that the value of solution No. 3 is now to be calculated on the basis of the equation KMnO + NO = KNO + MnO. One molecule of permanganate equals one molecule of nitric oxide when manganous sulphate is used, since no part of the permanganate employed in this method is reduced below the superoxide condition. In other words, solution No. 3 now represents only three-fifths as much nitric acid as it does when oxalic acid is used.
The results obtained by this method were moderately satisfactory, but not quite so exact as those obtained when oxalic acid was used. A series of four determinations gave differences, between the volumes of permanganate calculated and used, of 0.05 to 0.15 c.c.
The principal objection to the method lies in the difficulty of determining, in the presence of the brown oxide of manganese, the exact point at which the oxidation is complete.
The carbon dioxide generator, A, was devised by us to take the place of the ordinary generators, in which marble is used. We have found that a submersion of twenty hours in boiling water does not suffice to completely remove the air which, as is well known, is contained in ordinary marble; hence some other substance must be employed as a source of the gas. In the apparatus which we are about to describe, the acid carbonate of sodium is used.
It consists of a long, narrow cylinder (450 x 60 mm.); a tightly fitting rubber stopper, through which three tubes pass, as shown in the figure; a small cylinder, F, containing mercury; and a sulphuric acid reservoir, G.
The tube, g, is drawn out to a fine point at the end and curved, so that the acid which is delivered into A falls upon and runs down the outside of the tube. The tube, h, dips under the mercury in F. G and g are connected by means of a long piece of rubber tubing which is supplied with a screw pinch cock.
The apparatus is made to give any required pressure by raising or lowering G and F; but the elevation of G, as compared with that of F, should always be such that the gas will force its way through h rather than g. The upper part of the cylinder, F, is filled with cotton wool to prevent loss of mercury by spattering.
The material placed in A consists of a saturated solution of acid carbonate of sodium, to which an excess of the solid salt has been added. The sulphuric acid is the ordinary dilute. The apparatus, if properly regulated, serves its purpose very well. The principal precaution to be observed in using it is to avoid a too sudden relieving of the pressure, which would, of course, result in the introduction of an unnecessarily large quantity of sulphuric acid into A.