Investigating Iron Tablets, A PAG for OCR Chemistry Students PDF

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Summary

OCR Chemistry PAG on how to investigate the content of iron tablets using methodological approaches to achieve accurate,realiable and valid results...

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PAG 12.1 Investigating iron tablets

Research 

What do iron tablets contain? Iron tablets primarily consist of the hydrated form of iron(II) sulphate (also known as ferrous sulphate), which has a chemical formula of FeSO4.xH2O. There are many values for x (the number of moles of water), but the most common one is 7 – heptahydrate. This hydrated form of iron(II) sulphate is used medically to treat iron deficiency.1 All iron sulphates dissolve in water to give the same aquo (a complex ion containing one or more water molecules) complex [M(H 2O)n]z+ - in this case, [Fe(H2O)6]2+.2



The role of iron in the body, and why some people need to take iron tablets. Iron tablets work by replacing body iron. Iron is an essential mineral needed in the body to produce red blood cells (they transport oxygen from your lungs to the organs and tissues throughout your body), as it is the cofactor ion in haemoglobin which carries oxygen around the body. When the body does not get enough iron, it cannot produce the number of normal red blood cells needed to keep you in good health, a condition called iron-deficiency anaemia. Ferrous sulphate tablets are used for the prevention and treatment of iron-deficiency anaemia, by providing a supplement of the mineral iron.3

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The approximate mass of iron present in a standard iron tablet.

1 Wikipedia. (2016). Iron(II) sulfate. [online] Available at: https://en.wikipedia.org/wiki/Iron(II)_sulfate [Accessed 15 Jul. 2016]. 2 Wikipedia. (2016). Metal aquo complex. [online] Available at: https://en.wikipedia.org/wiki/Metal_aquo_complex [Accessed 15 Jul. 2016]. 3 Xpil.medicines.org.uk. (2016). Ferrous Sulfate 200mg tablets (28 pack) - X-PIL. [online] Available at: http://xpil.medicines.org.uk/ViewPil.aspx?DocID=18081 [Accessed 15 Jul. 2016].

Iron supplements usually contain about 50mg per tablet of iron(II). 4 The different forms of iron in supplements contain varying amounts of elemental iron i.e. ferrous fumarate is 33% elemental iron by mass, whereas ferrous sulphate is 20% and ferrous gluconate is 12% elemental iron.5 

The recommended daily intake of iron.

The amount of iron a person needs is 8.7mg a day for men and 14.8mg a day for women.6 Table 1: Recommended Dietary Allowances (RDAs) for Iron Age Male Female Pregnancy Lactation Birth to 6 months 0.27 mg* 0.27 mg* 7–12 months 11 mg 11 mg 1–3 years 7 mg 7 mg 4–8 years 10 mg 10 mg 9–13 years 8 mg 8 mg 14–18 years 11 mg 15 mg 27 mg 10 mg 19–50 years 8 mg 18 mg 27 mg 9 mg 51+ years 8 mg 8 mg * Adequate Intake (AI)7

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Details about a titration method used using potassium manganate (VII) solution that can be used to find the concentration of Fe2+ ions in a solution.

To find the concentration of Fe2+, a redox titration would need to be carried out. This involves titrating a solution of potassium manganate (VII) against a solution of dissolved iron tablets. A mean volume, calculated from the average of two concordant titres, of potassium manganate (VII) will be used to find the moles of 4

Alevelchem.com. (2016). Experimental - Redox titrations - analysis of iron(II). [online] Available at: http://alevelchem.com/aqa_a_level_chemistry/unit3.6/inorg01.htm [Accessed 15 Jul. 2016]. 5 "Office Of Dietary Supplements - Dietary Supplement Fact Sheet: Iron". Ods.od.nih.gov. N.p., 2016. Web. 17 July 2016. 6 Nhs.uk. (2016). Vitamins and minerals - Iron - NHS Choices. [online] Available at: http://www.nhs.uk/Conditions/vitamins-minerals/Pages/Iron.aspx [Accessed 15 Jul. 2016]. 7 "Office Of Dietary Supplements - Dietary Supplement Fact Sheet: Iron". Ods.od.nih.gov. N.p., 2016. Web. 17 July 2016.

manganate (VII) ions being used to oxidise the Fe2+. The moles of Fe2+ can then be calculated (as the molar ratio of MnO 4-:Fe2+ is 1:5), which is then used to work out the mass of Fe2+ in the iron tablet. The iron content in the tablet can be calculated by using the calculated mass of Fe2+ and the mass of the iron tablets measured beforehand to obtain a percentage. 

The chemistry on which this titration is based. Potassium manganate (VII), KMnO4 is a good oxidising agent in acidic solution and is reduced from the deep purple manganate (VII) ions to the nearly colourless manganese(II) ions as it is a self-indicating reagent. Therefore, it can be used to monitor the progress of the reaction as it does not need a chemical indicator for the detection of the end point (the first faint trace of permanent pink in the solution). Iron(II) ions are pale green in solution. They are oxidised by manganate (VII) ions in acidic solution to iron(III), which are pale yellow in solution. MnO4-(aq) 2+

5Fe (aq)

+

8H+(aq) 

+

5e-

 3+

5Fe (aq)

Mn2+(aq)

+ +

MnO4-(aq) + 8H+(aq) + 5Fe2+(aq)  Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)

4H2O(l) 5e-

Plan Equipment list: Stage 1: dissolving of iron tablet 

Five iron tablets



Weighing boat



Weighing scale accurate to 2 decimal places

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50 cm3 of 1 mol dm-3 sulfuric acid

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100 cm3 conical flask (with bung)

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Pestle and mortar

Stage 2: titration of iron tablet 

100 cm3 graduated volumetric flask

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Burette



Retort stand and clamp



250 cm3 beaker



25 cm3 pipette

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Distilled water

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2 x 250 cm3 conical flasks

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25 cm3 measuring cylinder

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150 cm3 of 0.0200 mol dm-3 potassium manganate (VII) solution – it is vital that this needs to be a standard solution of a concentration of exactly 0.0200 mol dm-3

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100 cm3 of 1 mol dm-3 sulfuric acid

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Filter paper and funnel



White tile

Method: Stage 1 1) Using a weighing scale, weigh accurately five iron tablets. 2) Place the iron tablets into a 100 cm 3 conical flask and add approximately 50 cm 3 of the 1 mol dm-3 sulfuric acid provided. 3) Stopper the conical flask, shake its contents well and then leave the tablets to dissolve. Or you can use a pestle and mortar to break up the iron tablets in the acid solution to expose the contents of the tablet as dissolving the outer coating of the tablet is a slow process and may take up to a day. The solution will need filtering before carrying out the titration. Stage 2 1) Without disturbing the residue, which will have settled down to the bottom of the conical flask, carefully filter the solution directly into a 100cm 3 volumetric flask using filter paper and a funnel. 2) Rinse the residue in the filter paper into the volumetric flask using a small volume of distilled water. 3) Add dilute sulfuric acid to make the solution in the volumetric flask up to the mark. 4) Invert the volumetric flask a few times to ensure that the contents of the volumetric flask are thoroughly mixed to produce an acidified solution of iron (II) sulfate. 5) Fill a burette with the 0.200 mol dm-3 potassium manganate (VII) solution. 6) Pour some of the contents of the volumetric flask into a clean 250 cm 3 beaker and, using a 25 cm3 pipette, measure out a 25.0 cm 3 sample of the iron (II) sulfate solution into a clean 250 cm3 conical flask. 7) Using a 25 cm3 measuring cylinder, measure out 25 cm 3 of the 1 mol dm -3 sulfuric acid provided and add this to the contents of the conical flask.

8) Titrate this sample of iron (II) sulfate solution by adding potassium manganate (VII) from the burette until the first permanent pink colour is seen. 9) Repeat steps 6-8 another two times in order to carry out three titrations in total. Record the three values obtained for the volumes of potassium manganate (VII) used. 10) Calculate and record the mean volume of potassium manganate (VII) solution used in the titration (the average titre) using at least two concordant titres.

Mass of iron tablets (g)

Rough titre Initial volume 3

(cm )

Final volume 3

(cm )

Trial 1 Titre (cm3)

Initial volume 3

(cm )

Final volume 3

(cm )

Trial 2 Titre (cm3)

Initial volume 3

(cm )

Final volume 3

(cm )

Trial 3 Titre (cm3)

Initial volume 3

(cm )

Final volume 3

(cm )

Average Titre (cm3)

titre (cm3)

Extension Opportunities 1. Explain any modifications you would make to your experiment to improve it. I would not make many amendments to my method as I believe it was a suitable one and because it has provided me with three concordant titres. However, I could mention the fact that the vessels could be washed out with distilled water into the new vessel that the solution has been transferred into so as to make sure any solution or residue is not left behind as this could affect my results significantly. 2. Compare your results with those your classmates have achieved. Are there any differences? If you used different methods, consider which is better and why. My results are fairly close to those of my other classmates. I have obtained a value of 30mg of iron in each tablet when one of my other classmates has obtained a value of 31mg. In addition, I have determined a value of 90mg iron (II) sulphate per tablet when my classmate’s tablets had 83mg iron (II) sulphate per tablet. 3. How might you investigate how quickly Fe2+ is released from an iron tablet? Think about: 

how you would model the conditions in your stomach



how you would determine the release of Fe2+ over time.

You would need to use hydrochloric acid and put it into a water bath at about 37 degrees to model stomach conditions. Because Fe2+ ions have a green colour in aqueous solutions, you would just measure the change in absorbance over time. This can be measured using a colorimeter and at each time interval you would need to quench the reaction mixture with something so that it stops reacting at that point (in a cuvette separate from the main reaction mixture). A variable could also be the concentration of hydrochloric acid....