(b) Nitric oxide unites directly with oxygen at ordinary tem peratures to form nitrogen peroxide, 2N0+02=2NO2.
(c) Nitrogen peroxide reacts with water to form nitric acid and nitric oxide, 3NO2+H20 = If excess of oxygen be present, the nitric oxide is again oxidized to nitrogen peroxide, The square brackets indicate the concentrations of the respective gases and the expression involving x represents the values if atmospheric air be taken as initial material. It is known that k is related to the heat of formation (q) thus, so that if x be determined at an absolute temperature T, k may be calculated for that temperature and a value of q obtained. W. Nernst used the heat of formation of nitric oxide determined by J. Thomsen, viz.-43,200 cal., and compared theoretical and practical values of x. Thus at 1,811°, x was observed to be 0.37 and calculated as 0.35, whilst at 2,675° the respective values were 2.23 and 2.35. Nernst and his fellow-workers used external elec tric heating, but in 1903 W. Muthmann and H. Hofer had ob tained 3.6 to 6.7% of nitric oxide when the air was exposed directly to the arc. 7% would correspond to a temperature of 3,75o°, but experiments by Haber and A. Koenig point to a specific influence of the arc ; there seems to be an "electric equi librium" in addition to the thermal equilibrium.
Patents for the synthesis of nitric acid were taken by Mme. Lefebre (1859) and McDougall (1899), but manufacturing on a commercial scale was first started by C. S. Bradley and R. Love joy (Brit. Pat. 8,230/1901; U.S.PP. 709,867; 709,869). The air was driven through a vertical iron cylinder containing a rotating steel shaft, both tube and shaft were provided with electrodes and between them, 6,900 arcs were struck and broken per second. The process was not operated after 1904, more efficient processes having made an appearance.
Another method of exposing air to the arc is due to 0. Schon herr (1905). An arc (which may be from direct current) of 16 to 23 ft. in length is set up in a vertical tube : the lower electrode is an iron rod which is gradually worn away, the wastage is small and the expense of renewal trifling. A water-jacket surrounding
the upper part of the tube serves as the other electrode ; since the arc varies in length, it does not always strike the same area and wear and tear are small. Air passes upwards along this tube, re turns downwards through a concentric outer tube and serves partially to heat the incoming air.
After leaving the electric furnace, the gases are cooled (much heat may be usefully recovered) and pass on to an oxidation tower. By treatment in a succession of towers with water in counter-current, a weak nitric acid is recovered which may be concentrated or, more usually, neutralized with lime or ammonia for the production of calcium or ammonium nitrate. The former is utilized as a fertilizer, the latter finds a use in explosives. The washing with water does not remove all the oxides of nitrogen; more complete exhaustion is effected by spraying the remaining gas with solution of soda ash, sodium nitrate (fertilizer) or sodium nitrite (dyestuff industry) being recovered.
The Birkeland and Eyde process was established in 190 and utilized largely by the Norsk-Hydro Co. at Notodden and later at Rjukan. The Schonherr process was largely developed by the Badische Co. and introduced at the above mentioned Norwegian works. The German capital was afterwards withdrawn from the Norsk-Hydro owing to the development of the Haber process. The Pauling process was used in France, Austria and Italy, whilst another arc process, the invention of Mosicki, was established in Switzerland.
Several methods for the recovery of oxides of nitrogen formed at high temperatures in combustion processes have been sug gested ; of these, that due to F. Musser with respect to internal combustion engines has received most publicity.