1. Job’s Method of Continuous Variation
Lab 11

2. 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.

3. 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.

4. Reaction
Fe3+ + nSCN- [Fe(SCN)n]3-n iron (III) thiocyanate iron (III) thiocyanate metal ion ligand complex ion We will experimentally determine the value of “n.”

5. 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.

6. XLigand vs. XSCN- For the purpose of this lab:
Xligand refers to the mole fraction of SCN- in each solution (of SCN- and Fe3+).
XSCN- refers to the mole fraction of SCN- in the compound [Fe(SCN)n]+3-n.

7. To determine “n”:
We will prepare a series of solutions, each with a different mole fraction of ligand:
The absorbance of the complex at  = 470 nm is directly proportional to the concentration of the complex.

8. Mole Fraction of SCN-
After plotting Abs. vs. Mole Fraction of Ligand, find two linear approximations on the plot, identify their equations, and set them equal.
The point of intersection of the two linear extrapolations corresponds to the mole fraction of SCN- in the complex ion.
If the intersecting point falls on:
Xligand = 0.5, then the ratio of species is 1 SCN- : 1 Fe3+
Xligand = 0.66, then the ratio of species is 2 SCN- : 1 Fe3+
Xligand = 0.75, then the ratio of species is 3 SCN- : 1 Fe3+
Xligand = 0.80, then the ratio of species is 4 SCN- : 1 Fe3+
Xligand = , then the ratio of species is 5 SCN- : 1 Fe3+
Xligand = , then the ratio of species is 6 SCN- : 1 Fe3+
You should see a graph in this experiment that looks like...

9. Abs vs. Mole Fraction of Ligand

10. Analysis
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 + b1 = m2x + b2
b1 – b2 = m2x – m1x = x (m2 – m1)

11. Analysis
x =
Then, XFe3+ = 1 – XSCN-
And, n =

12. 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.

13. Safety Concerns Reagents: Eye Contact: Skin Contact: Inhalation:
Ferric Nitrate (Fe(NO3)3
HNO3 (1.0N)
Potassium Thiocyanate (0.1N)
Sulfamic Acid
Eye Contact:
Blurry vision. Severe irritation, redness, pain, burns, conjunctivitis and permanent corneal damage.
Skin Contact:
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.

14. 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!

15. Submit Lab 11 Reports:
Your instructor will let you know when to submit your Lab 11 reports for preliminary grading.