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Determination of Manganese content of Steel using Colorimetry...

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LAB 7: Determination of Manganese content of Steel using Colorimetry Aim: To determine the absorbance of Manganese in steel using colorimetry.

Introduction: Most kinds of steel contain a small amount of manganese, it is added to add strength. The percentage of manganese can be detected by colorimetry, by oxidising it to the permanganate ion, and measuring the absorbance of permanganate solution . Which has an intensely purple colour (Chemteach university of Canterbury). The absorbance of permanganate can be determined from a standard curve by applying the Beer-Lamberts Law (Chemistry Libretexts). Manganese is a strong oxidising agent. For example in a hot acid solution the manganese ion will oxidise to permanganate, demonstrated in the equation below. 3H2O + 2Mn2+ + 5IO4-  2MnO4- + 6H

Procedure Part A: Preparation of Standard Solutions for Calibration Curve In this experiment the standard permanganate solution was (100 mg L -1 KMnO4). Using a prepared set of standard KMnO 4 solutions containing 1, 2, 3, 4, 5 mgL 1 KMnO4 in a 100 mL volumetric flasks. Then add a small spatula end of potassium periodate to stabilise the permanganate in each of the diluted standards. Plug in the spectrophotometer and Calibrate it to 545 nm. Stabilise the Spectrophotometer using water as a blank solution, Fill the cuvette ¾ full. Use the water’s absorbance as a zero reference. Measure each of the absorbance of each of the standard solutions; 1,2,3,4,5mgL-. Record results in the table below. Label each solution with initials, and the volume of solution it contains and store until sample solution is prepared.

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Part B: Preparation of Permanganate Solution from Steel Sample. Hazards

WEAR gloves!

Concentrated HNO3 Class 8 Corrosive Concentrated H3PO4 Class 8 Corrosive Ammonium Per Sulphate (NH4)2S2O8 Class 5.1 Oxidising agent

Steps 1-3 should be done in the FUME HOOD 1 2 3 4 5

Accurately weigh out 0.23g of steel sample into a conical flask. Dissolve the 0.23g of sample steel in 30mL of 1:3 nitric acid Boil the sample for 1-2 minutes to get rid of oxides of nitrogen. Remove the substance from the heat and add 0.5g of AR ammonium per sulphate. Once you’ve done that Boil the sample for 10minutes to oxidise any carbon compounds and destroy excess per sulphate. 6 Remove the sample from the heat and dilute the sample with distilled water to 100mL. 7 Will still off of the heat, add 10mL of AR concentrated phosphoric acid to the sample. 8 Add 0.5g of potassium periodate. 9 Add back onto the heat and boil for 1 minute 10 Turn the heat off, keep the sample on the hot plate to keep hot for 10 minutes. 11 Remove the sample from the heat completely and allow to cool. 12 Pour 250mL of the sample into a standard volumetric flask (the sample solution is the unknown concentration of permanganate). 13 Plug in the spectrophotometer and Calibrate it to 545 nm. 14 Stabilise the Spectrophotometer using water as a blank solution, Fill the cuvette ¾ full. Use the water’s absorbance as a zero reference.

15 Measure the sample solutions, and record the result in the table below. 16 Re measure the Standard solutions to double check they are correct and change any data in the table below.

Results Below table 1 displays the recorded results conducted from the lab. Table 1. Concentration of standard solution

1 mgL-1

2 mgL-1

3 mgL-1

4 mgL-1

5 mgL-1

Sample solution

Volume of permanganate required (mL)

1mL

2mL

3mL

4mL

5mL

N/A

Absorbance

0.035

0.081

0.124

0.166

0.208

0.205

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Graph and Calculations Below is the calculation that was used to determine the volume of permanganate solution required to make up the standards.

C1V1 = C2V2 C1 = 100mgL-1 KMnO4 V1 = ? C2 = 5mgL-1 V2 = 100mL V1 = 5 x 100/100 V1 = 5mL Below figure 1 demonstrates the results conducted put into a graph. Figure 1.

Absorbance vs Concentration 0.25

Concentration mL

0.2

f(x) = 0.04 x

0.15

ABS Linear (ABS) Linear (ABS)

0.1

0.05

0 0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

Absorbance

Calculations Below I used the graph of absorbance vs concentration to fond the concentration of Manganese in the sample solution I have Displayed the calculation used to determine the percentage of absorbance of sample steel in the manganese. 0.0413

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ABS Y =0.205 0.0413 =4.96mgL-1 4.960.25 =1.24 1.24 230 100= 0.53% =0.53%

Discussion Questions 1

What is the most likely source of error in constructing the standard graph? mark)

(1

The most likely error that would occur when putting together the graph displaying the results in the Lab would be, entering the data obtained from the experiment in wrong. This would affect the results because it will not show an accurate reading of the experiments. It will also make the graph wrong and display incorrect results. 2

Explain why 545 nm has been selected for the absorbance measurement (1 mark)

Spectroscopy works by correlating the concentration of a species in solution to the amount of light it absorbs. In a solution containing colour, in this experiment the colour of the solution is purple. The intensity of the colour is used to measure the concentration of the solution; the more intense the colour the higher the concentration. The light absorption at that wavelength is expressed as a numerical value that can be related directly to the concentration of coloured compound in the solution. It is important to choose a wavelength where the solution strongly absorbs light. The stronger the absorption at a particular wavelength the more sensitive the instrument will be at that wavelength and the more accurate your results. The wavelength of 545nm is the best unit to use to measure the absorbance of light through a purple substance, which is a direct measure of the intensity of the solutions colour (Chemteach. University of Canterbury). 3

Explain how the Beer-Lambert law is used to determine the concentration of your solution from measuring its absorbance. (4 marks)

“The Beer-Lambert law states that the quantity of light absorbed by a substance dissolved in a fully transmitting solvent is directly proportional to the concentration of the substance and the path length of the light through the solution.” Therefore the amount if light absorbed by the Spectroscopy is the same as the concentration of manganese in the sample solution.

The Beer-Lambert law relates the depletion of light to the properties of the material through which the light is traveling. For each wave length of light passing through the spectrometer, and the intensity of each wave length of light passing through is measured, then the sample has absorbed some of the light and the absorbance is

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proportional to the concentration of the solution of the sample steel. It is also the same as the length of light path through the cuvette (chemistry libretexts). 4

Provide a balanced REDOX equation for the reaction showing half equations (6 marks)

Half Equations: Oxidation: Mn2+ MnO44H2O+Mn2+MnO4-+8H+ 2+ 7+ 2+ 4H2O+Mn MnO4 +8H++5e 4H2O+Mn2+MnO4-+8H 2 8H2O+2Mn2+2MnO4-+16H Reduction: IO4  IO3 2H++IO4  IO3+H2O 1+ 12H++IO4 +2e IO3+H2O 2H++IO4 IO3+H2O5 10H++5IO4 5IO3+5H2O Full Equation: 3H2O + 2Mn2+ + 5IO4-  2MnO4- + 6H Conclusions In this lab it was discovered that there was 0.53% of manganese in the sample steel. These results were conducted from the experiment conducted using colourimetry. Above are multiple explanations as to how colourimetry works and was utilised throughout this experiment.

References Chemteach. (n.d). Determination of manganese in steel by permanganate colourimetry. University of Canterbury. College of science. Pg.1-8 Retrieved from: http://www.chemteach.ac.nz/investigations/documents/manganese_ste el_colorimeter.pdf Clark, J. & Gunawardena, G. (2019). The Beer-Lambert Law. Chemistry LibreTexts. Retrieved from: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry _Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemis try)/Spectroscopy/Electronic_Spectroscopy/Electronic_Spectroscopy_Basics/ The_Beer-Lambert_Law Chemistry 11. (n.d.). Measuring Manganese Concentration using Spectrophotometry. Santa Monica College. Pg. 1-8. Retrieved from: https://www.smc.edu/AcademicPrograms/PhysicalSciences/Documents/Chem istry_11_Experiments/Mn_Lab.pdf

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Marking schedule Lab attended and health and safety rules observed

Pas s or Fail

Aim and Introduction 6 Procedure summarised using past tense and impersonal language. 6 Results neatly tabulated and correct calibration curve supplied 6 Calculations correct with working clearly shown 6 Discussion questions answered correctly and in full 12 Conclusions and references given 4 Total 40

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