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Experiment 1: Preparation and Photochemistry of a Complex Ion Salt
This two-week experiment will consist of four parts:
Week one: Preparation from starting materials of the complex ion salt.
Week two: Preparation of a calibration curve
The photochemical reaction
Determination of the number of photons absorbed by the complex.
Preparation from starting materials of the complex ion salt.
Week one
Introduction
Complex Formation: A complex (ion or molecule) is defined as a species composed of
several parts, each of which is capable of independent existence in solution. The complex usually
consists of a positive transition metal ion and a number of electron-rich groups (ligands). The
ligands may be anions, such as CN-, Cl- or OH- and/or molecules with lone pairs of electrons such
as H2O and NH3. The positive metal ion, being electron-deficient, seeks a negative atmosphere.
The central metal atom is said to be electrophilic (electron-loving), and is designated as Lewis
Acids. The ligands, on the other hand, having lone pairs of electrons available, act as electron
donors. Consequently, the ligands are said to be nucleophilic ( in that they seek the positive
charge of the nucleus), and are designated as Lewis Bases. This Lewis Acid-Base theory may be
exemplified in the following reaction between Co3+ and ammonia to form the
hexamminecobalt(III) ion.
Co3+
+
6 NH3
H3N
H3N
NH3
Co
NH3
3+
NH3
NH3
The object of Week one is to prepare a complex salt of iron(III). The complex contains oxalate
ion (C2O42-) as the anionic (negative) ligand, and potassium ion will serve as the co-cation of the
salt. The following reactions will be used to prepare the product, Potassium
tris(oxalato)ferrate(III) trihydrate.
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(1) FeII(NH4)2(SO4)2· 6H2O + H2C2O4 (aq)
H+
FeIIC2O4 (s) + (NH4)2SO4 (aq)
(2) FeIIC2O4 (s) + K2C2O4 (aq) + H2C2O4 (aq) + H2O2 (aq)  K2[FeIII(C2O4)3]·3H2O (s)
+ 2 H2O (l)
In Lab Discussion:
Look at reaction (1) and (2). What is the purpose of the hydrogen peroxide?
What is the limiting reagent in the reaction?
How many grams of product will be formed in this reaction?
Procedure: In a 250 mL beaker, dissolve 10.00 g of ferrous ammonium sulfate hexahydrate,
Fe(NH4)2(SO4)2· 6H2O, in 30 mL of warm deionized water containing 6-8 drops of 6 M H2SO4.
Add, with stirring, 50 mL of 1 M oxalic acid (H2C2O4) solution. A yellow precipitate of iron(II)
oxalate (FeC2O4) should form. [CAUTION: oxalic acid is a poison!]
Carefully heat the mixture to boiling while stirring constantly. CAUTION: The mixture
will tend to splash; do not overheat! The iron(II) oxalate precipitate is finely divided and tends to
be colloidal in nature. Heating the mixture, called digestion, will tens to cause the precipitate to
coagulate, thus making it easier to separate from the solution.
Pour off (Decant) the supernatant liquid and retain the precipitate in the beaker. Use hot
water to wash the precipitate by decantation. To do this, add 30 mL of hot deionized water, stir,
allow the precipitate to settle, and decant the supernatant liquid. Repeat the washing procedure
again.
Add 18 mL of 2 M potassium oxalate (K2C2O4) solution to the beaker containing the
hydrated FeC2O4 precipitate. Heat the mixture to 40 oC. While maintaining a small flame to keep
the temperature constant, SLOWLY add (using your dropper), 17 mL of 6 % H2O2 (hydrogen
peroxide) while stirring continuously.
A rust brown precipitate of iron(III) hydroxide, Fe(OH)3 may be formed. Heat the
mixture to boiling and, maintaining the temperature near the boiling point, add 13 mL of 1 M
H2C2O4 with stirring, the first 9 mL all at once; the last 4 mL drop by drop. Filter the hot solution
if a small amount of residue is present. Discard any precipitate. Transfer the solution to a clean
150 mL beaker; add 20 mL of 95 % ethanol. Allow the solution to cool slowly at first, and then
in an ice bath until no further precipitation occurs.
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Collect the green crystalline product on Büchner funnel via vacuum filtration. Wash the
crystals with small amounts (10 mL) of 1:1 alcohol-water solution. Air-dry the crystals on the
filter by running the vacuum for 5-10 minutes. Place the filter, along with the crystals onto a preweighed watch glass. Allow the crystals to dry in air for a week in your drawer. Next week you
will weigh the watch glass and determine the total weight of the product. Keep in mind when you
are performing your calculations that the product formed is a trihydrate (K 2[Fe(C2O4)3] · 3 H2O).
NOTE: Both the dry solid and aqueous solutions of the product are reactive toward light. Keep
the product away from direct rays of the sun.
Week two
Preparation of a calibration curve
The photochemical reaction
Determination of the number of photons absorbed by the complex.
Introduction
Life on this planet would be impossible without photochemical reactions! This is most
evident if we examine the food we eat. The growth of fruits and vegetables involves direct
utilization of the sun's energy. Even the meat that we eat comes indirectly to us through plant
life.
A flip of the switch or a turn of the key results in a warm house, or a car that starts off for
work or vacation, or a light that goes on so that we can study. We do not think much about where
the energy comes from to perform these simple tasks. If we stop and think, we realize that the oil
and natural gas used to keep us warm during the winter, or the gasoline used to make out cars run,
has its origin in the photochemical reactions responsible for life on this planet millions of years
ago. Although these solar reactions refer to the process of photosynthesis, solar energy is
employed in other important functions such as direct electricity production or heat production,
and most of the information each of us receives from the external world is photochemically
mediated through vision. Even though photochemical reactions perform such an important
function on planet Earth, we know relatively little about how to harness the energy of light.
When light interacts with matter, it can be reflected, refracted, scattered, transmitted or
absorbed. The absorption of light by matter is the first step in photochemistry; only absorbed light
can induce photochemical change, but the absorption of light does not necessarily result in a
photochemical reaction. Only if the absorption of light initiates some chemical event are we
observing photochemistry.
Light can be described in terms of particles called photons. Photons of visible or
ultraviolet light contain a vast amount of energy. The amount of energy may be found by using
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the equation: E = h or hc/ , h = Planck’s constant (6.62 x 10-34 J sec), c = speed of light (3 x
108 m/sec), = frequency of the light in sec-1 (Hz) and  = wavelength of the light in meters (m).
The energy of the photon can also be expressed in terms of its wavelength , since  = c. When
we see a change in colour of light it is because different amounts of the various wavelengths of
photons are involved. When matter absorbs light, it is actually absorbing energy: so in the instant
a molecule absorbs a photon of energy E, it increases its own energy by the same amount E.
Suddenly having all this extra energy makes the molecule M electronically excited:
M
+
photon (h M*
The star on M* indicates that the molecule is excited. We say that the molecule is in an excited
state M*.
M*
Excited State
Photons (h)
M
Ground State
The molecule with all this extra energy can do one of three things:
- relax back to the original state it was in and lose the energy it absorbed by releasing heat
- relax and lose energy by releasing light
- use the energy to undergo a chemical reaction (photochemistry)
The complex you prepared last week, K2[Fe(C2O4)3] · 3 H2O, Potassium
tris(oxalato)ferrate(III) trihydrate has been extensive used in photochemistry to measure the
number of photons emanating from various light sources. When exposed to light, the complex
reacts according to the following equation:
2 [Fe(C2O4)3]3- (aq) + h
 2 Fe2+ (aq) + 5 C2O42-(aq) + 2 CO2 (g)
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The exact number of moles of Fe2+ formed per photon of light is given by the quantum yield ()

, where n
Fe2+
n Fe2 
I abs
is the number of moles Fe2+ produced and Iabs is the absorbed number of photons in
einsteins. An einstein is a mole of photons (6.02 x 1023 photons).
 = 1.2 moles /einstein for K2[Fe(C2O4)3] · 3 H2O
The number of photons absorbed by the sample can be calculated by measuring the
number of moles of Fe2+ generated by the photochemical reaction. Although Fe2+ (aq) is nearly
colorless, we can impart an intense blue color by the reaction of Fe2+ with potassium ferricyanide,
K3[Fe(CN)6] to form the ferroferricyanide complex referred to as Turnbull’s blue.
K3[Fe(CN)6] (aq)
+ Fe2+(aq)
 KFe[Fe(CN)6](aq)
The complex has a maximum in its absorbance spectrum at 700 nm. Measuring the
absorbance of solutions of ferroferricyanide is therefore an accurate way of determining the
concentration of Fe2+. You will prepare both standard solutions of ferroferricyanide and a
solution of ferroferricyanide generated from your photolyzed solution. The absorbance of each
solution will be using the Vernier Spectrometer. Construction of a calibration curve will allow
you to determine the number of moles of Fe2+ formed by the photochemical reaction. Using the
quantum yield () and the number of moles of Fe2+ generated, you will be able to determine the
number of photons of light which were absorbed by your sample.
Procedure:
- Determine the total yield by weighing the watch glass and product from last week.
Preparation of solutions
In Lab Discussion: Using the molecular weight of your produce (K2[FeIII(C2O4)3]·3H2O)
calculate the number of grams needed to make 25 mL of a 0.1M solution.
In a 25 mL volumetric flask add the appropriate number of grams of K2[FeIII(C2O4)3]·3H2O
(calculated in the above In Lab Discussion) and bring to volume with deionized water.
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Pour 10 mL of the prepared solution (test solution) into a beaker and expose it to sunlight/lamp
light for the amount of time (record the exact amount of time the sample was exposed to the light
source) prescribed by the instructor. Pour 10 mL of the remaining solution into another beaker
and leave it on the bench top (reference solution).
Preparation of Fe2+ standard solutions
Obtain the following solutions and place in test tubes [MIX WELL] :
[CAUTION: K3Fe(CN)6 is a poison!]
K3Fe(CN)6 soln.
Fe(NH4)2(SO4)2
soln.
H2O
Calibration 1
Calibration 2
Calibration 3
Calibration 4
Calibration 5
1 mL
1 mL
1 mL
1 mL
1 mL
0 mL
0.5 mL
1 mL
2 mL
3 mL
10 mL
9.5 mL
9 mL
8 mL
7 mL
Record the absorbance of calibration solutions 2-5 using the Spectronic 20 set to 700 nm, using
calibration solution 1 as the blank.
After the test solution has been exposed to the light source, add 1 mL of the K3Fe(CN)6 solution.
Mix the solution and record the absorbance. Record any observations. Repeat this procedure with
the 10 mL of solution which remained on your bench (reference solution).
In Lab Discussion: What is the purpose of the reference solution?
To be included in the report
1. The number of grams of product made and the % yield. Why is your % yield not 100 %?
% yield 
g _ of _ product _ made
 100
g _ of _ product _ exp ected
2. In tabular form indicate the absorbance measurements of each of the calibration solutions
and the molar concentrations of Fe2+. From the absorbance measurements on the
calibration solutions and the concentrations of Fe(II) in each solution construct a
calibration graph of absorbance vs. concentration of Fe2+. Fit the data to a line using the
trendline function in Excel (show the equation of the line).
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3. Determine the molar concentration of Fe2+ in the test and reference solutions. Determine the
number of moles of Fe2+ in each solution. Determine the number of moles formed in the
photochemical reaction by the following equation:
moles Fe2+ formed = moles Fe2+ (test soln) – moles Fe2+ (ref. soln.)
4. Determine the number on photons that were absorbed by your sample using the quantum
yield () of K2[FeIII(C2O4)3]·3H2O
Questions:
A complex of uranium, (UO2)SO4 is also photoreactive. It reacts with oxalic acid in the presence
of light according to the following reaction:
H2C2O4 (aq) + UO22+ (aq) + h  UO2H+ (aq) +
H+(aq) + 2CO2 (aq)
10 mL of a 0.1 M (UO2)SO4 solution produces 2.75 x 10–13 M UO2H+ when 5.0 x 10-13 einstein of
light is shown on the sample. How many photons are in this number of Einstein? What is the
quantum yield (F) of (UO2)SO4? How does it compare with K2[FeIII(C2O4)3]·3H2O?