Assessing Environmental Pollution: Iron Determination in Natural Waters using VISIBLE SpectroPHOTOmetry
science
Description
Practical
3:
Assessing Environmental Pollution: Iron Determination
in Natural Waters using VISIBLE SpectroPHOTOmetry
·
Put your answers into this
sheet and then submit to Turnitin.
·
Refer to the practical sheet
that you were given in the session (also available on Blackboard) for the full
questions and marks allocation.
·
Insert your answers, and use
as many lines as you require to give a full answer.
EXPERIMENTAL PROCEDURE
Part
1. Preparation of an iron standard stock solution:
- Weigh
accurately 0.0393 g of Analar ammonium ferrous sulphate into an empty
beaker and record the actual weight in Table 1.
Table
1. Mass of ferrous ammonium sulphate used
|
Mass of solid |
g |
(1 mark)
Question 1. Calculate the concentration of the ferrous ammonium sulphate
stock solution prepared in mol/dm3.
The molar mass of ferrous ammonium sulphate is 392.14 g/mol.
(1 mark)
Ferrous ammonium sulphate, (NH4)2SO4FeSO46H2O),
contains 1 iron atom per molecule, so this will also be the iron concentration
of the stock solution
Part
2. Preparation of iron working standard solutions (blank + 5 iron reference
solutions):
Using the standard stock solution
prepared above, prepare a set of working standard solutions according to Table 2 using the following guidelines;
- .
Question 2. Complete table 2 on page three by calculating the
concentrations of each of the Fe2+ solutions in mol/dm3),
showing in detail how you calculated the final iron concentration of solution
2.
N.B. 1000 cm3 = 1 dm3 1 cm3 = 1 mL 1dm3 = 1 L (litre)
Ferrous ammonium sulphate = (NH4)2SO4FeSO46H2O);
Mr = 392.14 g mol-1
Molar mass (Mr) of Iron (Fe)
= 55.85 g mol-1
Concentration of iron
(g/dm3) = concentration of FAS (g/dm3) x55.85/392.14
(please ignore this).
At the same time, make up your river water sample in flask 7
Remember to label all solutions, with your group, team and
date, as they will be stored until the
following week, when you will complete part 5.
Table 2. Prepare
the following solutions in a 100 cm3 volumetric flasks
|
Solution number |
Stock
iron solution (cm3) |
Acetate
buffer (cm3) |
Hydroxyl amine solution (cm3) |
AFTER
MIXING FIRST THREE SOLUTIONS: WAIT 5 MINS TO ALLOW
ALL IRON TO REACT |
Phenanth-roline
sol’n (cm3) |
Distilled water (cm3) |
TEST
pH IS APPROX. 4-5. Adjust
only if absolutely necessary MAKE
UP TO EXACTLY 100
cm3 DISTILLED WATER AND MIX
WELL. |
[Fe2+] (mol/dm3) |
|
Measure using: |
PIPETTE |
BURETTE |
PIPETTE |
BURETTE |
MEASURING CYLINDER |
|||
|
1 (Blank reference) |
0.00 |
20 |
5 |
10 |
30 |
|
||
|
2 |
2.00 |
20 |
5 |
10 |
30 |
|
||
|
3 |
4.00 |
20 |
5 |
10 |
30 |
|
||
|
4 |
6.00 |
20 |
5 |
10 |
30 |
|
||
|
5 |
8.00 |
20 |
5 |
10 |
30 |
|
||
|
6 |
10.00 |
20 |
5 |
10 |
30 |
|
||
|
flask 7 |
10.00cm3
of river water |
20 |
5 |
10 |
30 |
xxxxxxxxxxxxxxxxxxxx |
Show
the calculation for solution 2.
. (5 marks)
Question 3. From your UV spectrum, find the wavelength with the most
intense absorption (lmax)
and also the absorbance at this wavelength. Include the spectrum in your report and record the results in the table
below. You will need this for setting the wavelength for the colorimeter.
Table 3. UV spectrum of Fe (II) complex
|
lmax for iron complex (nm) |
|
|
Absorbance
of solution 6 |
|
(1 mark)
Question 4. Use the expression for Beer’s Law, A = ecl to
calculate the molar extinction
coefficient (ε) for the solution. The path length l is 1 cm. Remember that
the units for e are dm3 mol-1cm-1,
so your iron concentration must be in moles/dm3.
(3 marks)
Part 4. Determination of iron concentration in collected
river water sample
Repeat the above procedure with the river water to obtain a
spectrum of the river water (solution 7) as was done for solution 6. Record the
absorbance at λmax in table 4, and then calculate the concentration
of iron, using the Beer Lambert Law and the value of ε calculated above.
RESULTS
Question 5. Complete the table and ensure units are included where
necessary
Table 4
|
Absorbance measured for river
water sample (solution 7), using Perkin Elmer Lambda 25 |
|
|
Using the
Beer-Lambert equation, calculate the concentration of river water solution 7,
measured by Perkin Elmer Lambda 25. Give your answer in mol/dm3. |
|
Because the river
water sample was diluted by a factor of 1/10 to make solution 7, multiply your final iron concentration (in mol/dm3)
by 10 to find the pre-dilution iron concentration in the original sample in
mol/dm3.
Concentration of iron in the original river water sample = …………………… mol/dm3.
Enter this result into the table in Question 6 (µ).
(3 marks)
When you have done part 5 of the experiment
A best-fit
linear regression line will be shown for your five data points. Save this to
your USB and INCLUDE a copy in your
report. (Alternatively, print a copy and include a photo of it in your
report.)
Question 6.
You must then measure and record the absorbance of diluted river water solution
7. Note that this must not be included on linear trendline. You can then read
across the calibration graph to find the iron concentration of the river water.
As before you must then multiply the
answer by 10 to get the iron concentration of the undiluted river water.
Table 5
|
Sample absorbance |
Sample iron concentration (mol/dm3) |
Original undiluted river water
concentration (mol/dm3) |
|
|
|
|
(3 marks)
Question 7.
Convert your iron concentrations, as determined by each method, from
mol/dm3 to mg/dm3 (this is the same as ppm). (. 1 g =
1,000 mg) and enter the results into the table below (¶). (NB. Both results should be
similar, if not identical)
Compare your
final answers with the iron specification limits stated in the EU Water
Framework Directive (see below).
- Environmental Quality Standard
(EQS) for Water Framework Directive UK
- Specific
Pollutants (Scottish Environmental Protection Agency, 2015)
|
Metal |
Environmental Quality Standard (mg L-1) (= mg/dm3) |
||
|
Freshwater |
|
||
|
Iron |
Maximum allowed |
1 mg/dm3 (ppm) |
|
