572 
ting effect on the complete dissolution of copper in 20°/, nitric acid : 
ferrous sulphate, ferrous chloride, ferric sulphate, ferric chloride, 
ferric nitrate, lead sulphate, lead nitrate, lead acetate, copper nitrate, 
copper chloride, barium nitrate, thallium nitrate, lithium nitrate, 
sodium nitrite, manganese chloride, chromic chloride, arsenious 
oxide, strychnine sulphate, ethylene bromide, carbon tetrachloride, 
hexachlorobenzene, phthalic anhydride ete. 
On the other hand, the following substances have a retarding 
effect: hydrogen peroxide, potassium chlorate, potassium permanganate, 
chromic acid, sodium nitrate, ammonium nitrate, manganese nitrate, 
thorium nitrate, sodium sulphite, titanic acid, molybdic acid, ammo- 
nium persulphate, mangenese sulphate, cobalt chloride, copper acetate, 
copper sulphate, calcium nitrate, tartarie acid, ether, urea, acetic 
anhydride, benzoic anhydride ete. 
In a foregoing paper of this series (Trans. Chem. Soc. 1917, 111, 
707) 1 have shown that sulphuric acid in small concentration is an 
accelerator, whilst in large concentrations it is a retarder in the 
oxidation of oxalic acid by chromic acid. Similar results have been 
obtained in the action of nitric acid on copper. The following sub- 
stances in very small concentration exert a slight accelerating effect, 
whilst in large concentrations they have retarding effect: 
Zine chloride, nickel chloride, cobalt nitrate, aluminium nitrate, 
potassium chloride, strontium nitrate, cadmium nitrate, magnesium 
chloride ete. 
When the concentration is very small, the effect of potassium 
nitrate, uranium nitrate, citric acid, potassium dichromate ete. is 
practically nil, but in concentrated solutions they are all retarders. 
The effect of monochloracetic acid is very peculiar. In small con- 
centrations, it is a feeble accelerator and in concentrated solutions 
it has a retarding effect, which instead of increasing, decreases with 
increase of concentration. A similar phenomenon has already been 
observed in the case of the oxidation of formic acid by chromic 
acid in presence of manganese chloride (loc. cit. p. 726). 
It is practically impossible to give a complete explanation of these 
results, they being so diverse. 
Ferrous sulphate and ferrous chloride behave as marked accelerators. 
It would appear that the acid nucleus in this particular instance, plays 
no part. A part of the ferrous ion reduces the nitric acid to nitric 
oxide and passes into the ferric state. The nitric oxide dissolves in 
the ferrous salt solution forming the unstable substance FeNO°°. The 
dissolved nitric oxide then reduces a part of the nitric acid according 
to the following equation: HNO, + 2 NO + H,0 23 HNO. 
