BEHAVIOUR OF IRON IN CONTACT WITH SULPEURIC ACID. 399 
The slow action of concentrated sulphuric acid on iron 
or steel, whereby hydrogen is produced, is dependent on all 
the usual factors governing the action of acids on metals. 
The peculiarities noticed in the case of iron and concen- 
trated sulphuric acid are due partly to the protective coat- 
ing of FeSO,, H2O which tends to cover the iron soon after 
the action has begun; partly also, we think, to the fact 
that iron is capable of absorbing hydrogen; and partly to 
impurities in the iron. Concentrated sulphuric acid does 
not of itself passidify iron. 
Part I].—Variation in the rate of action according to the 
Concentration of Acid. 
It would probably be expected that dilution of the acid 
would greatly increase the velocity of action. Our experi- 
ments have shown, however, that the increase in action is 
not in any way proportional to the increase in the per- 
centage of water in the acid, but rather that no great 
increase in the rate of action is obtained when the acid is 
reduced from 94 per cent. to a concentration of 85 per cent. 
H,SO, (154 water). Below a concentration of 85% sul- 
phuric acid, the velocity increases somewhat more notice- 
ably as the concentration of H,SO, diminishes. A further 
decidedly greater increase in velocity is noticed in proceed- 
ing from 70% acid to 64°5{% acid. Some investigations into 
the rate of action of concentrated sulphuric acid on iron 
have been made by Knietsch.* Broadly speaking, it might 
be said that his experiments give somewhat similar results 
to those we have obtained. 
An exceptionally slight action was obtained in our experi- 
ments with 89°3°4 H.SO.. Knietsch, on the other hand, 
obtained a minimum action with 94° H.SO.. This may 
possibly be accounted for by the different kind of iron used 
in the two researches. 
+ Journ. Soc. Cham. Ind., Xx1, p. 343, 1902. 
