HILLEBRAND.] FERROUS IKON. *)f) 
applied to the result of titration for total ferrous iron. (See also p. 106, 
under Sulphur.) 
In order to obtain quantitative data regarding the effect of pyrite on 
the ferrous-iron estimation by the hydrofluoric-acid method the follow- 
ing tests were recently made: Pare of a fine crystal of pyrite was 
rather finely powdered and boiled with dilute sulphuric acid, which 
extracted considerable ferrous iron, derived presumably from admixed 
or intergrown pyrrhotite, since a second boiling with fresh acid gave 
a negative test for ferrous iron. After washing by decantation with 
water, followed by alcohol and ether, the powder was dried and 
further pulverized. A quarter of a gram of it when treated with hydro- 
fluoric and sulphuric acids in a large crucible by the Cooke method 
for ferrous iron, then rapidly filtered through a very large perforated 
platinum cone fitted with filter paper, required but 2 drops of a per- 
manganate solution representing only 0.0032 gram FeO to the cubic 
centimeter. 
Since, however, Dr. H. N. Stokes has found in a pending investiga- 
tion that the oxidizing effect of ferric salts on pyrite and other sul- 
phides is vastly greater than seems to have been suspected (see p. 89), 
the following tests were made in order to ascertain the probable error 
due to this action under the conditions prevailing in rock analysis: 
Successive portions of 1 gram each of a hornblende-schist, free from 
sulphur and carrying 10.09 per cent FeO as the mean of several deter- 
minations and 4.00 per cent Fe 2 3 , were mixed in a large (50 cm. 3 ) 
platinum crucible with 0.02, 0.025, and 0.10 gram, respectively, of the 
above purified pyrite powder, and treated with hydrofluoric and sul- 
phuric acids by the Cooke method, the water bath being at boiling heat 
for one hour. The cooled contents of the crucible were poured into 
a platinum dish containing water and titrated rapidly nearly to an 
end. Then, in order to get rid of the pyrite, which would obscure 
the end reaction lyy its reducing effect on the pemanganate, the solu- 
tion was filtered as above and in the clear filtrate the titration was 
carried to completion. The results were 10.02, 10.16, and 10.70. 
Inasmuch as the smallest of these three charges of pyrite was several 
times greater than what may be considered an unusually high amount 
for an igneous rock, it is very evident that for all practical purposes 
the influence of pyrite on the ferrous estimation by the Cooke method 
is negligible. At the same time it is to be borne in mind that with 
increased content in ferric iron an increased amount of pyrite will be 
attacked, and that the extent of this attack is undoubtedly influenced 
by the degree of fineness of the pyrite powder. 
Vanadium. — If vanadium, when present, exists in the trivalent con 
dition, it necessarily affects with an error varying with its amount the 
result of titration for ferrous iron. Knowing the amount of vanadium 
a correction can be applied as follows: One molecule of V 2 O a (150. 8) 
