ACTION OF CONCENTRATED SULPHURIC ACID ON IRON. 61 



action of a large number of metals on sulphuric acid and 

 has found that the concentrated acid in the cold acts very 

 slowly on iron, the rate of gas evolution being still slow at 

 130°, only commencing to increase appreciably at 150°, 

 when the gas consists of a mixture of hydrogen and sulphur 

 dioxide. When the dilution of the acid reaches about 1:3, 

 hydrogen alone is evolved whatever the temperature. At 

 all dilutions with iron he detected sulphuretted hydrogen 

 together with sulphide of iron, the latter accounting for 

 the darkening of the acid. With concentrated acid the 

 sulphuretted hydrogen was decomposed with precipitation 

 of sulphur. Thomsen pointed out the formation of H 2 S in 

 connection with zinc, stating that if the dilute acid was 

 evaporated, an intermediate reaction took place when a 

 certain concentration was reached according to theequation, 

 4 Zn + 5 H 2 S0 4 Aq = 4 ZnS0 4 Aq + BUS + 4 H 2 0. 



It is the custom in commerce to store sulphuric acid in 

 iron drums and in iron tanks, and as the present view is 

 that the concentrated acid is practically without action on 

 iron in the cold, the practice is generally considered to be 

 quite safe. With regard to the manufacture of Nordhausen 

 sulphuric acid, Roscoe and Schorlemmer 1 mention that 

 certain absorbers are used " containing concentrated sul- 

 phuric acid, being made of wrought iron which is not 

 attacked by acid of this strength (97 - 98%), whereas cast 

 iron although not corroded rapidly cracks under these 

 conditions." 



The present investigation was undertaken specially in 

 connection with iron, and the author finds that this metal 

 has a definite action on concentrated sulphuric acid in the 

 cold, and that the products of the reaction are mainly 

 ferrous sulphate and hydrogen. There is also a small 

 quantity of sulphur dioxide gas formed. 



1 Treatise on Chemistry, Vol. i, see also Lunge, Sulphuric Acid and 

 Alkali, Vol. i, p. 39. 



