Determination of Iron in Water Lab Report
10/12/11
Molly Brinser, Cara Lucas, Ryan Miller, Maya Evanitsky
Purpose
The purpose of this experiment was to determine the concentration of iron (II) in an unknown
sample.
Theory
Spectrophotometers are used to measure the amount of light that is absorbed by a certain
sample. A spectrophotometer contains two parts. One is the spectrometer which is used to
produce light and the other is the photometer which measures the intensity of light.
Spectrophotometers are designed for a cuvette filled with liquid to be placed between the
photometer and spectrometer. A portion of light is passed through the sample and is absorbed by
the photometer which calculates the amount of light that was absorbed. The light is measured in
nanometers and the machine can release different wavelengths of light to pass through samples.
Since this machine measure the absorbance of a certain wavelength, the term absorbance
must be understood. Absorbance, in spectroscopy, is the amount of light a sample absorbs, or
takes in. All objects absorb light and differ in absorbencies. The absorbance is found by placing a
substance in the spectrophotometer. It releases light through the substance and calculates the
amount of light that was absorbed. Absorbance is directly proportional to the concentration of
the substance. The two create a linear graph that is used to display Beer’s Law.
Beer’s Law is A = abc where A is absorbance, a is molar absorpivity (a constant), b is
the path length (constant in cm.), and c is the concentration’s molarity. Multiple solutions with
different concentrations can construct the graph and create the linear equation. An unknown
solution’s concentration can be found by finding the absorbency of the solution and plugging it
into the given linear equation.
Procedure
First, 5 volumetric flasks of 0.0ppm, 1.0ppm, 2.0ppm, 4.0ppm, and 6.0ppm of Fe2+ were made
using a 100ppm stock solution. Five milliliters of 0.25% ortho-phenanthroline solution was
placed in each flask and then each flask was diluted with distilled water. The spectrophotometer
was connected to the computer by a USB cable and was calibrated by zeroing out a cuvette
containing the 0.0ppm solution. The 6.0ppm cuvette sample was used to find the wavelength of
maximum absorbency and that was then used to find the absorbance of each solution. After all
the results were found, a Beer’s Law graph was constructed. After an unknown solution’s
absorbance was found, this graph was used to find its concentration.
Data
Table 1. Absorbency Data
*wavelength of maximum absorption was 510 nm.
Concentration
Absorbance
0.0ppm (blank)
0
1.0ppm
.095
2.0ppm
.162
4.0ppm
.269
6.0ppm
Unknown B
.416
.160
Calculations
-finding concentration of unknown B in ppm
y=mx +b
y=A
A=mx + b
A=.160
m=.06302
b=.03069
.160= (.06302)x + .03069
x= 2.1ppm
-finding concentration in terms of molarity
𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑚𝑔/𝐿
𝑀 = 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 𝑥 1,000
1ppm = 1mg/L
2.1ppm=2.1mg/L
Fe 2+ amu = 55.85g/mol
2.1𝑚𝑔/𝐿
𝑀 = 55.85𝑔/𝑚𝑜𝑙 𝑥 1,000
M = 3.8 x 10-5
-finding the percent error
|𝑎𝑐𝑡𝑢𝑎𝑙−𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙|
% 𝑒𝑟𝑟𝑜𝑟 =
𝑋 100
𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙
% 𝑒𝑟𝑟𝑜𝑟 =
|2.1−2.3|
2.3
𝑋 100 = 8.7 %
Error Analysis
The percent error for this experiment was 8.7%. An error that occurred in the lab was the when
the computer went into sleep mode. Between calibrating the spectrophotometer and preparing the
standard iron solutions, a lot of time passed by resulting in the computer going into sleep mode.
When trying to get the absorbance values, the computer was not synced one-hundred percent
with the spectrophotometer and the Logger-Pro program. This caused a slight error to occur.
Conclusion
The purpose of this experiment was met because the concentration of the unknown iron (II)
solution was discovered. The concentration in ppm was 2.1 and the concentration in terms of
molarity was 3.8 x 10-5.
Questions
1. Ortho-phenanthroline was added to the solution to give it an orange color. Iron (II) ions are
colorless and when ortho-phenanthroline is added, the 2 create a visible orange color.
2. The purpose of preparing and analyzing standard iron solutions was to get enough information
to create an almost perfect Beer’s Law graph. The absorbencies of the standard solution would
create a linear graph. The absorbency of the unknown solution would be plugged into the Beer’s
Law equation given from the graph to find the concentration of the unknown.
3. Other items that could be analyzed using spectrophotometry are biological samples such as
DNA, RNA, and proteins.