Redox titration
International College
Background
Since permanganate ion is violet in color and manganese (II) ion is nearly colorless, the endpoint in titrations using permanganate as the titrant can be taken as the first permanent pink color that appears in the solution. In this experiment, potassium permanganate is used to determine the number of water molecules in hydrated iron (II) sulfate. The reaction occurs in acidic medium, and its equation is: MnO4- + 5 Fe2+ + 8H -> Mn2+ + 5 Fe3+ + 4H2O
The pink color of the dilute Mn04 solution indicates the end of the reaction
Procedure
Part I: preparation of the iron (II) sulfate solution (roughly 0.1 mol.dm-3)
1- Admit approximately 2 .78g of the iron (II) sulfate into a 100-cm3 volumetric flask
2- In a 100 cm3 beaker dilute 7.0 cm3 of 6.0 mol.dm-3 sulfuric acid solution in 35.0 cm3 of distilled water
3- Transfer the acidic solution to the volumetric flask and dilute to the mark.
Part II: Titration of the iron (II) sulfate solution
1- Pipette 10.00 cm3 of the iron (II) sulfate solution into a 125 cm3 Erlenmeyer flask, and then add about an equal volume of water.
2- Titrate with permanganate solution to faint golden pink color. Repeat titration for accuracy
Data Collection
Qualitative Data
Color of iron (II) sulfate solution is extremely light green.
H2SO4 solution is colorless.
Iron (II) grains yellow with a tint of green.
Permanganate (MnO4-) is dark purple.
End point of titration: light golden pink color is observable.
After Iron (II) grains were mixed with water, at the bottom of the Erlenmeyer grayish suspensions we observed.
Titration apparatus diagram:
Quantitative Data
Table 1: Vi and Vf (Initial and final volumes) of KMnO4 solution per titration
Trial1*2345
Vi – Initial Volume of KMnO4 (±0.01cm3)0.0113.0526.0018.1018.10
Vf – Final Volume of KMnO4 (±0.01 cm3)13.0526.0038.9631.0531.07
Mass of iron (II) sulfate grains m(FeSO4) (± 0.01 g)2.79g
Volume of iron (II) sulfate solution V(FeSO4) (± 0.01 cm3)10.00
Concentration of permanganate [KMnO4] (mol.dm-3)0.0200
Table 2: Mass of iron (II) sulfate grains, volume of iron (II) sulfate solution, and concentration of permanganate:
Data Processing and Presentation
Sample Calculation of volume of potassium permanganate added:
DVn = Vf – Vi
DV1 = Vf – Vi
= 13.05 – 0.01
= 13.04 cm3
Uncertainty of DV = D(DV)
D(DVn) = 0.01 + 0.01
= 0.02 cm3
= 0.02 x 10-3 dm3
DV = (13.04 ± 0.02) x 10-3 dm3
Table 3: Volumes of Potassium Permanganate Added
Trial1234
V (KMnO4) Added (±0.02 cm3)13.0412.9612.9912.97
Average volume of potassium permanganate (KMnO4) added

∆ =


∆ = 12.96 + 12.95 + 12.97+12.95 / 3
∆ = 12.96 cm3
∆ = 

∆ = 0.02 cm3
∆ = (12.96 ± 0.02 cm3)
Hence, the average volume of potassium permanganate (KMnO4) added is 12.96 cm3
Calculation of molar concentration of Fe2+ solution
MnO4- + 5 Fe2+ + 8 H+ → Mn2+ + 5Fe3+ + 4H2O
At equivalence point: 
[MnO4-] x V (MnO4-) = [Fe2+] x V (Fe2+) /5
[Fe2+] = 0.0200 x 0.01299 x 5
0.01000
[Fe2+] = 0.13 ± 0.02 mol.dm-3
Calculation of number of moles of FeSO4xH2O in total volume solution
n(FeSO4xH2O) = [ Fe2+] x Vsoln
= 0.130 x 100.0 x 10-3
=0.0130 mol
Determination of the molar mass (experimental) of FeSO4xH2O:
M : molar mass, n: number of moles, m: mass
n = m / M
M = m / n
m (FeSO4) = 2.79g
n(FeSO4xH2O) =0.0130 mol
M(FeSO4xH2O) =
x= 
x is an integer, therefore we take it to be 4. The values of the molar masses of the different elements were obtained from the following source:
Tsokos, K. (2005). Physics for the IB Diploma. Cambridge University Press.
Conclusion and Evaluation
In this lab, we are trying to determine the number of water molecules in hydrated Iron (II) Sulfate. We performed redox titration using permanganate as our titrant. When the endpoint of the reaction was reached, the solution turned colorless because of the production of the manganese (I) ion. We calculated the number of water molecules to be 3. However, the theoretical value of water molecules is 7.

%Relative Error =

%Relative Error =

% Relative Error =
% Relative Error = 43%
Our percentage of error was very high; therefore we can say that the experiment is not reliable since our projections 57% off the actual values. This is due to the many sources of error and the inefficiency of the designed experiment.
The human sources of error in the experiment were:
Iron (II) sulfate were exposed to the air for some time before admitting the grains in the water, due to the fact that the groups kept leaving the jar containing the substance open
Weighing errors
Burette reading errors
Inaccurate end point titration volume readings, due to splashing of solutions against walls of Erlenmeyer
Mainly before starting the experiment, we should have prepared some experiments that allow us to determine the actual concentrations of the reactants in our Erlenmeyers and beakers before mixing them together. An acid base titration should have been conducted on the sulfuric acid to determine its exact concentration (i.e. it should have been standardized at the time of the experiment).
Some of the Iron (II) Sulfate grains were left in an exposed jar during the experiment, which may lead to some water molecules being lost. The Iron (II) Sulfate molecule cannot gain more than 7 H20 molecules since Fe2+ has only 7 ligands. However Fe2+ may undergo oxidation and become Fe3+ due to the possible reaction of Fe2+ with the oxygen present in the air. Hence this reaction affects our results
Potassium permanganate is one of the most commonly used oxidizing agent because it extremely powerful. However, this remarkable oxidizing power has its drawbacks. Because it is so powerful, it reacts with practically anything that can be oxidized. This leads to a change in concentration and composition when stored. Hence in this experiment we should have prepared and standardized the potassium permanganate during the time of the experiment. Potassium permanganate solutions are prepared to be an approximate concentration, and then they are standardized against a known primary standard of the same substance that is to be analyzed in the unknown sample. KMnO4should have been titrated against a solution of sodium oxalate with known concentration in order to determine its exact concentration in the solution.
A blank titration between with sulfuric acid and KMnO4 should have been conducted to determine whether or not impurities are present in H2SO4. For example is an impurity which is present can undergo oxidation, the MnO4- will become reduced thus giving a purple color. During the blank titration, when we reach a point were no increase in purple color intensity is observable we can use the information concerning the volume of MnO4- added to calculate in the actual redox titration, the actual volume of MnO4- reacting with the fe2+ ions.
Standardization of potassium permanganate
Material
Magnetic stirrer
Stirring magnet
Dried sodium oxalate
Potassium permanganate
Burette
Erlenmeyer flask
Funnel
Weighing scale
Beaker
Thermometer
Procedure
1.Weigh 3.2g of KMnO4 into a 750ml Erlenmeyer flask and fill 1/3 of it with distilled water
2.Allow the solution to stir for 10 minutes by using a magnetic stirrer
3.Fill the bottle with distilled water and allow stirring to continue until the solution is needed for titration
4.Weigh three samples of 0.30g each of dried sodium oxalate and place into a 500-mL Erlenmeyer flask and dissolve in 50 ml of distilled water
5.Add an additional 200 ml of distilled water followed by 30 ml of 3 mol.dm-3 H2SO4
6.Pour the KMnO4 solution into the burette needed for titration by using a funnel
7.Steadily add by titration permanganate to the sodium oxalate and acid solution and stir slowly until equivalence point is reached and a faint pink color is observed
8.Let the solution stand until the pink color disappears. (This may take 30 to 45 seconds since the reaction is not instantaneous)
9.Heat the solution to 55 to 60 °C.
10.When the required temperature is reached, complete the titration by adding permanganate slowly until a faint pink color is reached and lasts for about 30 seconds
11.The temperature of the solution should be at least 55°C at the end of titration
Blank titration
Material
Magnetic stirrer
Stirring magnet
Potassium permanganate
Electric heater
Burette
Erlenmeyer flask
Funnel
Weighing scale
Beaker
Thermometer
Procedure
1.Add 250 ml of water and 50 ml of 3mol.dm-3 of H2SO4
2.Heating the solution to 55 to 60 °C.
3.Titrate the permanganate solution (prepared the same way as depicted in steps 1,2&3 in the previous procedure) until a faint pink color is reached
4.Titrate three times and calculate the mean volume needed to titrate the acid.
