STO’CH’OMETRY OF/INTRODUCTION A METAL COMPLEX
The mole ratio of Fez+
metal complex (in aquéocgsasnd’ OfthO-Phenanthroune, and, thus, the formula of this Fe(ll)
o ution) Will be determined in this eXperiment.
F62+ + “n”
(aq) (OTThO-phenanthrolmehfl) -> Fe(o-phenanthroline),,2+
This
Visiblglllilgbhi (all: :3: :EELOEIHQ the fact that the metal complex produced strongly absorbs
the orthO-lghenanthr l’ r ance maxrmurn at 51onm, while the aqueous ferrous ion and
amount of li ht t th 0 me (o-phenanthroline) molecule absorb only an msrgnificant
l’ h 9 a at wavelength. Beers law (A = ebC) indicates that the absorption of
9 t by the metal complex, A, IS directly proportional to the molar concentration, C, of that
complex (where the molar absorptivity, a, and the pathlength, b, are constant).
A series of solutions will be prepared in which the number of millimoles of iron will be the
same, but the number of millimoles of o-phenanthroline will change stepwise from zero
in the first solution to a large excess in the final solution. As the number of millimoles of
o-phenanthroline increases from solution to solution, more is available to form the desired
complex with Fe(ll), and the absorbance will increase proportionally. When all the iron
has been fully complexed, an increase in the number of millimoles of o-phenanthroline
will have no effect on the absorbance reading, since no more metal complex can form.
By plotting the absorbance of each solution on the y-axis vs. the number of millimoles of
o-phenanthroline in that solution on the x-axis, two regions of the graph may be observed:
a region of positive slope (where absorbance increases with an increasing number of
millimoles of o-phenanthroline) and a region of zero slope (a horizontal region where the
value for the absorbance does not significantly change with the number of millimoles of
o-phenanthroline). The value of the x-axis at the intersection of the two lines formed by
the data in these regions is the number of millimoles of o-phenanthroline needed to fully
complex the iron present in these solutions. The ratio of the number of millimoles of
o-phenanthroline to the number of millimoles of iron gives the value of “n” in the formula
Fe(o-phenanthrolineh”.
ortho-phenanthroline
(l,lO-phenanthroline)
Q Q C,2H3N2‘HZO MM = 198.2 g/mole
it it
0/
39
