Assessing Environmental Pollution: Iron Determination in Natural Waters using VISIBLE SpectroPHOTOmetry

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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:

  1. 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;

 

  1. .

 

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).

 

    1. Environmental Quality Standard (EQS) for Water Framework Directive UK
    2. Specific Pollutants (Scottish Environmental Protection Agency, 2015)

Metal

Environmental Quality Standard

(mg L-1) (= mg/dm3)

Freshwater

 

Iron

Maximum allowed

1 mg/dm3 (ppm)

Instruction Files


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