72 QUANTITATIVE AGRICULTURAL ANALYSIS
chloride is used, or if it is added too slowly, free mercury may be
produced:
HgCl2 + SnCl2 -» SnCl4 + Hg.
The indication of such action is the appearance of a gray precipi-
tate of mercury instead of the characteristic white silky crystals
of mercurous chloride. If mercury is so produced the deter-
mination is ruined because this mercury will itself reduce some of
standard oxidizing solution during the process of titration of the
iron.
Standard Solution.—The permanganate solution as used for
calcium is suitable for the iron determination, or a new solution
may be made in the decimal system. In the latter case a con-
centration such that 1 cc is equivalent to 0.005 gm of iron is
conveniently used. As iron is oxidized from a valence of 2 to a
valence of 3 its equivalent weight is 55.84 and 1 cc of a tenth-
normal solution is equivalent to 0.005584 gm of iron. The
equivalent weight of potassium permanganate is one-fifth of its
molecular weight, as was shown on page 67. The concentra-
tion of permanganate solution to be equivalent to any specified
weight of iron may be calculated by the methods illustrated on
page 67.
Determination of Iron in an Ore.—Sample the ore and grind the last selec-
tion to pass the 100-mesh sieve. Weigh three samples of exactly 0.5 gm of
ore on the counterpoised glasses, brushing into each of three porcelain cruci-
bles. Heat the crucibles without covers for 5 minutes, using the ordinary
desk burner, then allow the crucibles to cool, place in casseroles and add to
each 25 cc of concentrated hydrochloric acid. If method (b) is to be used for
reducing the iron add also at this point 0.5 cc of 5-per cent stannoxis chloride
solution. Cover and warm until solution is complete or until no further
action appears to take place. If the residue is not colored, proceed, without
filtration, as directed below. If the residue" is colored it may contain iron.
In this case filter on a small paper and wash the paper free from iron solution
with hot water. Set the filtrate and washings aside and burn the paper at a
low temperature in a porcelain crucible. If the residue is small in amount
and apparently contains little silicious matter it may be decomposed by fus-
ing with potassium pyrosulphate. Cool and dissolve the mass in hot water,
adding the solution to the former filtrate.
Concentrate the iron solution, if necessary, to about 50 cc and transfer to
a 1000-cc Erlenmeyer flask. While the solution is nearly boiling add, drop
by drop from a pipette, a 5-per cent solution of stannous chloride until the
ferric chloride has just been reduced, this being made evident by the disap-