Part A: Titration with standard Fe2+ solution.
We used a solution of Ammonium Iron (II) sulphate to obtain our Fe2+ for the experiment; we used this compound because it is much more stable to air oxidation than most other iron (II) salts.
Permanganate ions oxidize Fe2+ (aq) to Fe3+ (aq), permanganate ions (MnO4-) are similarly reduced to Mn2+ (aq). This is illustrated by the half equations and redox equation below:
Reduction: MnO4- + 8H+ + 5e- Mn2+ + 4H2O
Oxidation: 5Fe2+ 5Fe3+ + 5e-
Redox: MnO4- + 8H+ + 5Fe2+ Mn2+ + 4H2O + 5Fe3+
Results of titration
* Ammonium Iron (II) sulphate in solution (NH4)2Fe (SO4)2.6H2O
* M. (NH4)2Fe (SO4)2.6H2O = 392.14 gmol-1
* m. (NH4)2Fe (SO4)2.6H2O = 9.9626 g
* n. (NH4)2Fe (SO4)2.6H2O = m/M = 0.0254 mol
* Therefore n.Fe2+ = 0.0254 mol
* n.Fe2+ = c x v therefore c = n/v = 0.0254/250x10-3 = 0.1016 moldm-3
* 4 samples from 250cm3 each sample = 25cm3
o n. Fe2+ in 25cm3 = 0.1016 x 25x10-3 = 0.00254 mol
* Mole ratio = 5Fe2+ : MnO4-
o Therefore n. MnO4- = n.Fe2+/5 = 0.00254/5 = 5.0812x10-4 mol
* Therefore c. MnO4- = n/v = 5.0812x10-4/35.05x10-3 = 0.0145 moldm-3
Part B: Colorimetric Determination of Permanganate ion Concentration
Permanganate ions are coloured, the greater the concentration of permanganate ions the darker the solution and therefore the greater amount of light absorbed. Because of this we were able to use a colorimeter to measure the absorbance of 4 solutions of permanganate of known concentration.
We were then subsequently able calculate the concentration from the absorbance of the unknown permanganate ions using the data produced by the known concentrations.
From a standard solution of permanganate: 0.0020moldm-3 we diluted:20, 12, 10, and 5cm3 of the standard to 100cm3 in 4 separate volumetric flasks; labelling accordingly.
We then set a UV spectrometer to record only absorbencies at 525nm, after zeroing the spectrometer we proceeded to measure the absorbance values of a sample of each solution. We diluted 1cm3 of the unknown to 100cm3 and recorded its absorbance:
We plotted these results in a graph in excel and used the line equation (y=mx+c) of the graph to calculate the concentration of the unknown.
* Y=mx+c , from our graph the equation is; y=2385x - 0.003
* Using this to calculate c. MnO4- we get: 0.360=2385x - 0.003
* x=(0.360+0.003)/2385 = 1.5220x10-4
* multiply by 100 for undiluted concentration= 0.01522 moldm-3
* Therefore according to this method c. MnO4- = 0.01522 moldm-3
My results: 0.01522 moldm-3 for the spectrometer and 0.0145 moldm-3 for the titration method show a difference of only 0.0072 moldm-3 a discrepancy of only 4.6%, this suggests that the two methods do produce relatively accurate results and that the true concentration of permanganate ion lies closely to the two results.
I expected the results to be similar as the 2 methods are known to be accurate ways of finding the concentration of compounds. The true concentration is more likely to lie closer to the uv/vis spectrometer result because it is the more accurate method of experiment. When titrating there remains a lot of room for human error because the method involves so much human approximation such as the end point and measuring the volume of titre used. The spectrometer removes these risks as the wavelength is fixed and absorbance is measured digitally removing human error.
There is possible error in both methods due to the possibility of human error and measurement error dilution of the permanganate in method 2 (not all graph point precisely on the trend line) and making the solution of ammonium iron (II) sulphate in method 1 which explains why it is unlikely that either method has produced a 100% accurate result.