Experiment 1

Title: Standardization of potassium permanganate solution by ammonium iron (II) sulphate

Name: Toh Zi Xin
Name of partner: Wong Jing Hui, Gan Chun Yiang, Wong Teck Jun
Date: 17.6.2015
Lecturer: Dr. Neo Kian Eang
Practical class: P4 Introduction: Potassium permanganate solution can be standardized by titration against a standard solution of ammonium iron(II) sulphate solution. This is an example of standardization, which is a process to determine the concentration of a solution by using it to titrate another solution which have a known concentration. This titration is known as redox titration as the titrant, which is the potassium permanganate is a strong oxidizing agent. The ammonium iron (II) sulphate solution is measured by using a pipette and transfer it into a conical flask while the potassium permanganate solution is placed in a burette. The ammonium iron (II) sulphate solution is made acidic by adding dilute sulphuric acid. Potassium permanganate solution is dark purple colour because of the presence of permanganate ions, MnO4- . Since potassium permanganate is a strong oxidizing agent, it can oxidizes iron(II) ions to iron(III) ions.
Fe2+ Fe3+ + e- On the other hand, the Mn7+ ions of the dark purple colour permanganate ion, MnO4- are reduced to colourless Mn2+ ion. MnO4- + 8H+ + 5e- Mn2+ +4H2O As a result, the overall ionic equation is: MnO4- + 8 H+ + 5 Fe2+ → Mn2+ + 5 Fe3+ + 4 H2O At the end point of the titration, the colourless solution in the conical flask will change to pale pink colour. As a result, no indicator is needed. Iron (II) sulphate heptahydrate crystals, FeSO4 . 7H2O, which is a common reducing agent is not suitable to be used in this titration because it will easily oxidizes with air to form Fe3+ ions. The equation is:

As a result, ammonium iron (II) sulphate, Fe(NH4)2 (SO4)2·6H2O is used. This is because it is much less easier to be oxidizes in air to Fe3+, can be obtain in high state of purity and has a high molar mass. The oxidation of Fe2+ is depend on pH and it will be more readily occur when the pH is high. Thus, sulphuric acid is added to reduce the pH of the solution in order to reduce the oxidation of Fe2+. Objectives: To standardize potassium permanganate solution by ammonium iron (II) sulphate. Procedure: 1. About 3.9g of ammonium iron (II) sulphate was weighed accurately in 4 decimal places with a clean and dried 100mL beaker using analytical balance. 2. The salt was dissolved in distilled water ( ̴ 50 mL) and the solution was transferred to a 100mL volumetric flask. 3. The beaker was rinsed with small quantity of distilled water. 4. The flask was stoppered and shook well. 5. Distilled water was added to make up the volume to the mark by using a dropper. 6. The flask was stoppered and shook well by inverted the flask repeatedly. The molarity of ammonium iron (II) sulphate was calculated. 7. 25cm3 of the ammonium iron (II) sulphate was pipetted into a conical flask. 8. 15cm3 of 1M sulphuric acid was added. 9. The potassium permanganate solution was titrated until the first permanent pink colouration is observed. 10. The first conical flask was kept as reference and the titration were repeated twice.

Result: a) Calculation of result Titration number | 1 | 2 | 3 | Initial volume of burette (cm3) | 0.00 | 0.00 | 0.00 | Final volume of burette (cm3) | 25.00 | 24.90 | 25.00 | Total volume of burette (cm3) | 25.00 | 24.90 | 25.00 | Average volume of KMnO4 required for titration = 24.97cm3 Mass of ammonium iron (II) sulphate, Fe(NH4)2 (SO4)2·6H2O used = 3.9027g Molar mass of ammonium iron (II) sulphate, Fe(NH4)2 (SO4)2·6H2O = 392.05 g/mol No. of mole of ammonium iron (II) sulphate = Mass of ammonium iron (II) sulphate Molar mass of ammonium iron (II) sulphate = 3.9027g392.05 g/mol = 0.01 mol Molarity of ammonium iron (II) sulphate solution = No. of mole of ammonium iron ( = 2 \* ROMAN II) sulphateVolume = 0.01mol0.1 L = 0.1 M ∴ Thus, the molarity of ammonium iron (II) sulphate solution in the volumetric flask is 0.1M.

Molarity = No. of moleVolume No. of mole = Molarity × Volume No. of mole of ammonium iron (II) sulphate in conical flask = 0.1 M × 0.025 L = 0.0025 mol The overall equation is : MnO4- + 8 H+ + 5 Fe2+ → Mn2+ + 5 Fe3+ + 4 H2O From this equation, 1 mole of MnO4- ions react with 5 moles of Fe2+ . No. of mole of KMnO4 required to reach the end point = No. of mole of ammonium iron = 2 \* ROMAN IIsulphate in conical flask 5 = 0.0025 mol5 = 0.0005 mol Average volume of KMnO4 required for titration = 24.97cm3 = 0.02497 L Molarity of KMnO4 = No. of mole of KMnO4Average volume of KMnO4 = 0.0005 mol0.02497 L = 0.02002 M ∴ The molarity of KMnO4 that we get from this experiment is 0.02002M. b) Result interpretation

(x – mean ); First result >> 25.00-24.97 = 0.03 Second result >> 24.90-24.97 = -0.07 Third result >> 25.00-24.97 = 0.03 (x – mean )2; First result >> (0.03)2 = 0.0009 Second result >> (-0.07)2 = 0.0049 Third result >> (0.03)2 = 0.0009 Ʃ( x – mean)2 = 0.0009 + 0.0049 + 0.0009 = 0.0067 = 0.04726 ∴ The value of standard deviation indicate that the 3 data collected are very close to the mean value and data collected are very precised. Percentage error = | Actual value-Experimental value |Actual value × 100% = | 0.02000M-0.02002M |0.02000M × 100% = 0.1% ∴ The molarity of KMnO4 determined in this experiment is 0.02002 M which has a percentage error of 0.1%.

Discussion In this redox titration, potassium permanganate is used as the titrant while ammonium iron (II) sulphate is used as the analyte. Ammonium iron (II) sulphate is used as a primary standard to standardize potassium permanganate and this process is known as standardization. A standard solution is a reagent with a known concentration that is used in volumetric titration. A primary standard is a highly purified compound that used as a reference material in titration and also can be used in other analytical methods. A primary standard must be very pure, atmospheric stability, absence of hydrate water, large molar mass to minimize error during weighing and be soluble. (Skoog et al, 2014) Potassium permanganate solution is not a primary standard because distilled water that used to dissolve the solid potassium permanganate may contain traces of reducing organic substances, which will react with potassium permanganate to form manganese dioxide, MnO2 and this manganese dioxide will catalyzes the auto-decomposition of potassium permanganate. Thus, potassium permanganate is not pure enough to act as a primary standard. (Mani, 2014) The ammonium iron (II) sulphate is much more stable towards oxidation both as the solid and in solution, have high molar mass and so, this makes ammonium iron (II) sulphate suitable for use as a primary standard in this volumetric analysis. (Experiment 4 Preparation of Ammonium Iron (II) Sulfate Hexahydrate, (NH4 ) 2Fe(SO4 ) 2 . 6 H2O, from Iron Metal, 2013) Potassium permanganate is a strong oxidizing agent and is dark purple colour due to the presence of Mn7+ ion in MnO4-. Thus, it will oxidize iron (II) ions into iron (III) ions. The ionic half equation is: Fe2+ Fe3+ + e- On the other hand, the Mn7+ ions of the intense purple colour permanganate ion, MnO4- are reduced to colourless Mn2+ ion. MnO4- + 8H+ + 5e- Mn2+ +4H2O As a result, when each drop of dark purple potassium permanganate solution drop into the solution in the conical flask, it is reduced and decolourised. (How to Balance Redox Reactions, n.d.) The overall ionic equation is: MnO4- + 8 H+ + 5 Fe2+ → Mn2+ + 5 Fe3+ + 4 H2O
Indicator is not used in this titration because the dark purple colour of the potassium permanganate act as an indicator. At the end point of the titration, the colourless solution in the conical flask will change to pale pink colour. This is because when one drop excess of potassium permanganate is added to the solution, there will be no more Fe2+ ions left in the conical flask to convert the purple MnO4- ions into colourless Mn2+ ions. The solution at the end point will be pale pink in colour even though the concentration of excess MnO4- ions is very low since MnO4- ions have an intense purple colour. (Potassium Permanganate Solutions, n.d.)
After the titration, we found out that the pale pink colour solution in the conical flask turned into pale brown after left it for a short period. This is because the unreacted excess MnO4- ions react with the reduced Mn2+ ions to form brown product, which is MnO2. (Mandatory Experiment 4.5, n.d.)
2MnO4- + 3Mn2+ + 2H2O 5MnO2 + 4H+

Dilute sulphuric acid is added to provide an acidic condition for titration. This is necessary for titration that involving potassium permanganate. The reason is to ensure that the Mn7+ ions of the dark purple colour permanganate ion, MnO4- are reduced to colourless Mn2+ ions instead of brown Mn4+ ions. If the condition is neutral or alkaline, the Mn7+ ions will reduced only to Mn4+ ions, forming brown precipitate in the solution. So, it is impossible to determine the end point of the titration. Hydrochloric acid is not suitable to be used in order to provide acidic condition for this titration because it can react with potassium permanganate and produce Cl2 gas since potassium permanganate is a very strong oxidizing agent that can oxidizes Cl- into Cl2. The equation is:
2MnO4- + 10Cl- + 16H+ 2Mn2+ + 5Cl2 + 8H2O
Nitric acid is also not suitable because nitric acid is a very strong oxidizing agent. So, iron(II) ions will be both oxidises by nitric acid and potassium permanganate and this causes the titration to be not accurate. (Mandatory Experiment 4.5, n.d.)
Dilute sulphuric acid is also added to prevent iron(II) ions air oxidizes into iron(III) ions. This is because iron(II) can undergo air oxidation to form iron(III) ions under neutral or alkaline condition. So, acid is added to lower the pH of the condition and to prevent oxidation of iron(II) ions. (Chapter 15 – Volumetric analysis (Redox titrations), n.d.) In these 3 titrations, we found out that when the first drop of potassium permanganate is added, the purple colour of the MnO4- remain briefly but when further drops of potassium permanganate are added, it decolourise immediately. This is because the Mn2+ ion act as an catalyst to catalyse the reaction and this is an example of auto-catalysis. Thus, when the first drop of potassium permanganate is added, the purple colour of potassium permanganate do not decolourise immediately because there are no Mn2+ ions in the solution to catalyse the reaction and so, the reaction proceeds with a slow rate. However, when further drop are added, the purple colour of MnO4- decolourise more faster due to presence of Mn2+ ions as a catalyst. (A Level Appendix 6, n.d.) Several precautions have been done to minimise the error occur in this titration. Firstly, after transfer the ammonium iron (II) sulphate solution form the beaker to the volumetric flask, the beaker is rinsed with distilled water and transfer the water into the volumetric flask. This is to prevent some of the ammonium iron (II) sulphate from stick to the wall of the beaker and to ensure that all of the mass of ammonium iron (II) sulphate has been transferred to volumetric flask. The glass rod that has been used to stir the solution also rinsed with distilled water and the water is transferred into the volumetric flask. Secondly, a dropper is used to slowly add the distilled water up to the mark of the volumetric flask because this will be more accurate and safer. This can prevent overshoot of the volume. Thirdly, while doing the titration, the burette is ensure to place at vertical position to avoid parallax error so that the reading observed will be more accurate. Moreover, a piece of white paper also place at the bottom of the conical flask while doing the titration so that the change of colour will be more easily to observe and the reading obtained will be more accurate.

Conclusion:
From this titration, the molarity of potassium permanganate solution is 0.02002M with a percentage error of 0.1% which is very close to the molarity of the prepared potassium permanganate. The 3 value collected from the 3 titration are very close to the mean value. So, the molarity of the potassium permanganate obtained can be considered as accurate.

References:
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