Iron(III) Nitrate and Potassium Thiocyanate

Iron (111) Nitrate and Potassium Thiocyanate
Author’s Name:
Partner’s Name: Date of Experiment
Abstract
The purpose of the experiment was to verify Beer’s law which states that Absorbance is
directly proportional to Concentration of a given solution. To verify this,Fe3+ were made to react
with SCN- where they formed a dark red colored solution whose absorbance was determined in a
spectrophotometer. The complex formed absorbs a blue light at an absorbance of 459.4 which
was the highest before the solutions were diluted. The solutions containing the same ions were
made to react but their concentrations were lowered and at the same time, the Fe3+were made to
be the limiting agent.The equilibrium was still attained despite the change in concentration.This
is according to the law of equilibrium. The absorbance of each of the solutions was then
determined by setting the photo-spectrometer at 459.4 i.e. the highest wavelength attained by the
initial solution. It was observed that as the concentration decreased, the absorbance also
decreased as equilibrium is attained which now verified the Beer Lambert’s law.
Absorbance; a light diffusion
Spectrophotometer; measuring instrument.
Equilibrium; Reaction endpoint.
Introduction
It is difficult to determine the concentration of a solution in a mixture of two solutions and
especially if they are not colored.

This can be simplified by use of a spectrophotometer which is
an instrument that measures the amount of light absorbed by a substance which is known as
absorbance and has no units. This absorbance reflects a certain wavelength which now
determines the concentration of the substance. This is according to Beer’s law which states that
wavelength is directly proportional to a certain concentration of a substance. Beer’s Law
equation is given as; C = A/E*L where A = Absorbance
E = Molar absorptivity
L = Wavelength.
Molar absorptivity refers to how many wavelengths of light that can be absorbed by a substance
and is unique for each substance.
Experimental:
A spatula tip of Iron(111) Nitrate (Fe(NO3)3.9H2O) was dissolved in 20mls of distilled
water in a small beaker. The same was done with Potassium Thiocyanate(KSCN) and the
beakers labeled with the appearance of the solutions recorded.10mls of each of the prepared
solutions were mixed in a different beaker and the observations recorded. Using the available
solutions, tests were carried out to determine the ions available in the solutions.
Other solutions containing 5mls of KSCN and 15mls of Fe(NO3)3 were prepared as stock
solutions. They were then mixed thoroughly and allowed to stand for 5-10 minutes. Their
absorbance and wavelengths were then measured and recorded. Two clean burettes were
obtained and rinsed with the stock solution. The burettes were then filled with the stock solution
ensuring that they do not have air bubbles. Eight solutions were prepared such that they
contained 5mls KSCN, 0.5-0.8mls Fe(NO3)3 and the rest water such that the whole solution was
20mls.The solutions were thoroughly mixed and allowed to react completely for 5-10 minutes.
The spectrum of each solution at the maximum absorbance wavelength determined in part 11A
was determined.
Results and discussion:
PART 1
Fe(NO3)3 formed a clear yellowish colored solution on dissolving it in water while
KSCN also dissolved in water to form a colorless solution.
On mixing 10mls each of the two solutions, a dark red colored solution was formed.
This was due to the presence of the Fe3+ ions as shown in the equation below;
30956255016500 Fe3+(aq) +3NO33-(aq)+3 K+(aq)+3 SCN-(aq)Fe(SCN)3(aq) + 3KNO3(aq)
PART 2
The table below shows the absorbance and wavelength obtained when 5mls KSCN
and 15mls Fe(NO3)3 were made to react for 5-10minutes.
solution wavelength Absorbance
1 0.293 390.9
2 0.361 391.8
3 0.352 458.5
4 0.308 459.4
Calculating the value of E*l of in Beer’s law according to by taking one of the
wavelengths gives;

A=E*L*C C= 1.24*10-3

0.308= E*L*1.24*10-3

E*L = 2.48*102 moles of Fe3+
Since in this trial one mole of Fe3+requires 4 moles of SCN- i.e.
Fe3+ + 4SCN- = [Fe(SCN)4]- then moles of SCN- will be given by;

( 4.95×10-3)x1/4=1.23×10-3.
PART(111)
The table below represents the composition of eight solutions prepared;

solution Vol. of KSCN(MLS) Vol. of Fe(NO3)3(MLS) Vol. of water(MLS) Total vol. (MLS)
1 5 1.1 13.2 19.3
2 5 12.9 12.9 19.9
3 5 13.1 13.1 21.2
4 5 10.7 10.7 19.9
5 5 10.2 10.2 20.1
6 5 9.1 9.1 20
7
8 5
5 7.9
6.9 7.9
6.9 19.9
20.1
The table below gives the Absorbance and the concentration of the above eight
solutions.

solution Absorbance Concentration(
1 454 0.159
2 450 0.216
3 448.6 0.215
4 383.7 0.230
5 451.3 0.237
6 449.9 0.259
7
8 384.6
448.5 0.259
0.275
If a graph of absorbance against concentration was to be drawn, it would be a straight line
as shown below(“The cytochemical staining measurement of protein with mercuric bromophenol
blue” Mazia et al 60).

CONCLUSION
From the graph above, it is clear that the higher the concentration the higher the
absorbance. This verifies Beer’s law which states that ‘Concentration is directly proportional to
absorbance.

Notes
1. The equilibrium will always be achieved even in the presence of a limiting agent.
2. Le chatelier’s principle applies in the reaction by varying the concentration of the
reactants.

Work cited
Mazia, Daniel, Philip A. Brewer, and Max Alfert. “The cytochemical staining and
measurement of protein with mercuric bromphenol blue.” The Biological
Bulletin 104.1 (1953): 57-67.