Determination of an Unknown Concentration of Potassium Manganate using Colorimetry Introduction The purpose of this experiment is to conclude the concentration of an unknown solution using colorimetry to plot a calibration curve. Colorimetry measures the absorption of light at wavelengths within the visible region of the electromagnetic spectrum. It can be used to identify the unknown concentration of a solution in reference to a solution of a known concentration. When a solution appears coloured, you see the colour that has not been absorbed by the molecules.
A light beam is passed through a sample inserted into a colorimeter, the light is split into two beams, one being absorbed by the sample, the other beam will pass through to a photocell which measure the amount of light absorbed. This figure is not measured in any unit but is an indication of the amount of light absorbed when referenced against other samples. Equipment Potassium Manganate (100? g/ml), distilled water, colorimeter, cuvettes, test tubes, test tube rack, burette, pipette. Part 1 – Determination of an Absorption Spectrum
An absorption spectrum will be created by measuring a sample of Potassium Manganate (100? g/ml) against a range of wavelengths. The absorption spectrum will show how much light is absorbed at each wavelength. Results Wavelength (nm)| Absorption| 440| 0. 35| 470| 0. 48| 490| 1. 40| 520| 2. 00| 550| 1. 93| 580| 0. 88| 590| 0. 37| 680| 0. 19| Part 2 – Determination of a Calibration Curve The results indicate that the wavelength of maximum absorption is 520nm, this wavelength will be used to measure the absorption of a series of standard solutions.
The standard solutions will be tested at the wavelength of maximum absorption as this wavelength would be the most sensitive to absorbance changes. As the amount of light absorbed is directly related to the concentration of the solution, the results can be used to determine the concentration of the unknown solution. Zero concentration is a measurement of the absorption of distilled water. Results 100 x 0. 5=10ppm100 x 1. 0 = 20ppm100 x 2. 0 = 40ppm 5 5 5 100 x 3. 0 = 60ppm100 x 4. 0 = 80ppm100 x 5. 0 = 100ppm 5 5 5 Solution (ml)| Concentration (ppm)| Absorption| | 0| 0. 00| 0. 5| 10| 0. 29| 1| 20| 0. 54| 2| 40| 0. 85| 3| 60| 1. 36| 4| 80| 1. 89| 5| 100| 2. 00| Part 3 – Determination of the Unknown Concentration of a Solution of Potassium Manganate The unknown concentration will be tested by the colorimeter 3 times. Testing the sample more than once will ensure that the absorption is an accurate result. The unknown solution will be plotted against the calibration curve using the mean average. This will determine the concentration of the solution. Results Unknown Solution| Test| Absorption|
Solution Z| 1| 0. 55| | 2| 0. 53| | 3| 0. 55| Calculation of average absorption of Solution Z: 0. 55 + 0. 53 + 0. 55 = 0. 54 3 Conclusion Based on the calibration curve, the concentration of Unknown Z is 22. 5ppm. When the 20ppm solution was tested against 520nm, an absorption reading of 0. 54 was given. Unknown Z also gave a reading of 0. 54 but when plotted on the calibration curve along the trendline, it is assigned a value of 22. 5ppm. This could mean that Unknown Z has a concentration of 20ppm and that some areas of the experiment were flawed.
Some of the methods used during this experiment were not completely accurate. The biggest source in inaccuracy was during making the standard solutions. When a sample of one solution was transferred by pipette into a cuvette, trace amounts would have been left in the pipette meaning that when the same pipette was used for the next strength solution, the contents were either too diluted or too strong. When making up the standard solutions a burette was used this is also an inaccurate way of measuring liquids.
Either too much or too little solution and distilled water went into the mixture meaning that the standard solutions were not accurate. As the standard solutions were not accurate, the calibration curve will not be exact so when the Unknown Z has been plotted against the curve it will not be against the correct concentration but will be close enough to be given an approximate concentration Other areas of error include not wiping finger prints off of the cuvettes before they were inserted into the colorimeter which would interfere with the absorption of light.
In part 1 of the experiment, the colorimeter gave a reading of 2. 0 at 520nm. This reading is the maximum absorption that the colorimeter could display (shown by the colorimeter display flashing). The reading would suggest that the maximum amount of light possible was absorbed. This does not seem likely as when all light is absorbed, the colour you see is black whereas potassium manganite is coloured purple. Using a colorimeter with a larger range of displays may give more accurate readings of the absorption.
This would not change the wavelength of maximum absorption. Evaluation If this experiment was to be repeated, the absorption of the 100? g/ml solution would be tested 3 times at each frequency and also 3 times for each standard solution at the wavelength of maximum absorption. This would make the experiment as a whole more accurate as the mean average could be used to plot the graph which will be more indicative of the true absorption. Using a measuring cylinder instead of pipettes and burettes would also aid the experiment in being more accurate.