Job’s Method of Continuous
Variation
Lab 11
Outline
 Purpose
 Coordination
Complexes
 Reaction
 Determining
“n”
 Graph of Abs vs. Mole Fraction of SCN Procedure
 Safety Concerns
 Waste
 Submit Lab 11 Reports on…
Purpose
Students will become familiar with Job’s
method of Continuous variation, which is
a technique for determining the empirical
formula of a coordination complex.
Coordination Complexes
A
coordination complex consists of a central metal
ion bound by surrounding ligands.



Transition metals are positively charged and will
readily accept electrons.
Ligand molecules contain atoms with non-bonding
electrons, which can be donated to the electron
deficient metal ion in order to form a complex ion.
This can be viewed as Lewis acid – base chemistry
because of the electron involvement.
Reaction
Fe3+ + nSCNiron (III)
thiocyanate
metal ion ligand
[Fe(SCN)n]3-n
iron (III) thiocyanate
complex ion
We will experimentally determine the value of “n.”
Related to the value of “n”:
 The
coordination number of a metal ion is the
number of bonds it can form with ligands –
typically 1, 2, 3, 4, 5, or 6. The coordination
number of Fe3+ is 6.
 Iron
(III) can therefore bond to as many as 6
monodentate ligands or three bidentate
ligands.
XLigand vs. XSCNFor the purpose of this lab:
 Xligand
refers to the mole fraction of SCN- in
each solution (of SCN- + Fe3+).
 XSCN-
refers to the mole fraction of SCN- in
the compound [Fe(SCN)n]+3-n.
To determine “n”:
 We
will prepare a series of solutions, each with a
different mole fraction of ligand:
moles of SCN
Mol Fraction of Ligand 
(moles of SCN )  (moles of Fe3 )
 The
absorbance of the complex at  = 470 nm is
directly proportional to the concentration of the
complex.
Abs vs. Mole Fraction of SCN-
Absorbance 470nm
1.5
1.2
y = -2.5963x + 2.7203
R² = 0.9987
y = 2.6188x + 0.1608
R² = 0.9965
0.9
0.6
0.3
0
0.00
0.20
0.40
0.60
Mole Fraction of Ligand
0.80
1.00
Graph

The point of intersection of the two linear extrapolations
corresponds to the mole fraction of SCN- in the complex
ion.

Remember to limit your graph slopes to the appropriate
significant figures and y-intercepts to the appropriate
digits of precision before attempting any calculations.

Solving for the intersection we are solving for x where:
y1 = y2
m1x + b 1 = m 2x + b 2
b1 – b2 = m2x – m1x = x (m2 – m1)
Graph
b1  b2
  SCN
x=
m2  m1
 Then,
XFe3+ = 1 – XSCN-
 SCN
 And, n =
 Fe3 
Procedure
 Prepare
your solutions following the directions
in the manual.
 Measure
the absorbance of each solution at
a wavelength of 470 nm.
 Apply
Job’s Method to your data to
determine the empirical formula of your
complex.
Safety Concerns

Reagents:
•
•
•
•

Eye Contact:
•

Blurry vision. Severe irritation, redness, pain, burns, conjunctivitis and permanent
corneal damage.
Skin Contact:
•

Ferric Nitrate (Fe(NO3)3
HNO3 (1.0N)
Potassium Thiocyanate (0.1N)
Sulfamic Acid
Severe irritation, burns, redness, pain, stains and ulcers.
Inhalation:
•
Destructive to mucosa and upper respiratory tract. May cause burning,
coughing, choking, wheezing, laryngitis, shortness of breath, headache,
nausea, vomiting, methemoglobinemia, cyanosis, convulsions, tachycardia,
dyspnea, pneumonia, pulmonary edema, asphyxia, chemical pneumonitis
and death.

Ingestion:
•
Pain and burns of the mouth, throat, esophagus and gastrointestinal
tract. Gastrointestinal irritation with nausea, vomiting, diarrhea,
methemoglobinemia, cyanosis, convulsions, systemic toxic effects on the
heart, liver, and kidneys and death.
Waste

Conserve chemicals and distilled water.

If you spill, clean it up.

All solutions in this experiment are acidic.
Dispose of all excess and waste solutions in
provided acid waste containers in the fume
hood(s).

Please DO NOT unplug the hotplates!
Submit Lab 11 Reports:
 Your
instructor will let you know when to
submit your Lab 11 reports for preliminary
grading.