Qf'A.\T/TA 77 VK

chloride*, thus avoiding the appearance of a red color which
would mask the end point of tin* titrafion.1

Reduction of the Iron Solution.— Iron exists in tin* ferric con-
dition iu most ores or other minerals. In order to reduce the
solution of ferric, salt either stannous chloride, zinc, sulphurous
acid or hydrogen sulphide may be employed. The first two are
the only ones now commonly used.

Stannous chloride, in solution, possesses the advantage of
instantaneous action if added to the* hot solution of ferric chloride,
If the* iron is to bo reduced by stannous chloride an addition of
this salt to the ore during the. process of solution will materially
hasten the action. For the* final reduction the stannous chloride
solution may bo added from a pipette, the disappearance of the
red color of basic ferric chloride providing an approximate indica-
tion of the oiid-point.

in the analysis of iron oros there is occasionally trouble at this
point tmloss certain precautions have been taken. In the first
places many iron oros contain appreciable quantities of organic,
matter and this sorvos to produce a yellow color when the ore is
dissolved. As color duo to this cause doe.s not, disappear when
the iron has boon reduced it is not possible1 to determine when the
correct amount: of sfannous chloride has boon added, This
trouble may be avoided by igniting the weighed sample for a
short time in a porcelain crucible, before* dissolving.

The .second cause* of irremovable, color oome*s from fusion of
insoluble renidueH in platinum crucibles. The* pyrosulphute*
which in lined UK a fhix dissolves tntceH of platinum and thin, with
starmouB chloride, forms a yellow solution containing a complex
of tin and platinum. This interference is avoided by the sub-
stitution of porcelain crucible's for t.hejse of platinum*

After a slight e»xcons of Ktannotts chloride* has been added the*
solution is cooled mid a considerable exce*ss of mercuric, chloride
is added, the unused stannoUH chloride being thereby oxidised ;

Mercuric chloride will not oxidize fermtiH rhloiitte and hence wav
bo loft in the, solution. If an insufficient e*xe*e»ss of inere*uric
1 For the fxplftniition of th<!«p pointn sw MAICI.V, "Qunntitntive Ar«iIywt«,M
2nd <*!., pp. 244 '-246.