The Determination of Iron in an Ore.
Read the background material in the text before beginning this analysis. Dry the
unknown iron ore for at least 3 hr. It is recommended that you carry one sample of about
0.6 g completely through the three-step procedure first. The sample size can then be
adjusted if needed (for a titration volume of about 30 mL). The next two samples can be
prepared at the same time. A blank is not essential in this analysis, but it can be
determined by carrying a 10 mL sample of concentrated HCl through the entire procedure.
Sample preparation. Weigh a sample into a 500 mL conical flask. Add 10 mL of
concentrated HCl and about 3 mL of 0.25 M SnCl2. Cover the flask with a small
watch glass. Heat the solution in the hood at just below boiling until the sample is
dissolved. A white residue of hydrated silica may remain. The tin chloride aids in
the decomposition of the ore by reducing the iron(III) oxides to iron(II). More tin
chloride should be added during the dissolution process to aid in dissolving the
sample and to eliminate the yellow color which is characteristic of iron(III)
chloride complexes. Avoid an excess of tin(II) chloride, however.
Reduction with tin(II) chloride. Take each sample individually through this step to
avoid air-oxidation of iron(II). Adjust the sample size to about 15 mL by dilution
or evaporation. Heat the solution nearly boiling. Add sufficient KMnO4 solution
to impart a faint yellow color to the solution (thus insuring that there is not large
excess of tin(II)). Now add the SnCl2 drop by drop until the color changes from
yellow to colorless or a very light green; then add two more drops. Cool to room
temperature, and rapidly add 10 mL of 5% HgCl2 solution. A small amount of
silky white Hg2Cl2 should precipitate.
An absence of precipitate indicates that insufficient SnCl2 was used and the
reduction of iron(III) was incomplete. A gray residue indicates the presence of