Spectrophotometric analysis of a two-component mixture
Aim: i. To prepare working standards of dichromate and permanganate ii. To measure the absorbance of the prepared working standards of dichromate and permanganate using a spectrophototometer iii. To determine the concentrations of permanganate in a mixture of unknown.
Working standards of dichromate and permanganate were prepared and absorbance for each found. This was done in order to plot a graph of absorbance versus concentration, from which the concentration of permanganate and dichromate in the unknown sample could be determined. Sulphuric acid was added to the standards in order to force the reaction to completion. Wavelengths of 430 nm and 525 nm were chosen as dichromate absorbs best at 430 nm, while permanganate absorbs best at 525 nm. The concentrations of dichromate and permanganate in the unknown sample were 0.0122 M and - 1.94 x 10-3 M respectively. The negative value obtained for the permanganate ion concentration showed that the concentration was extremely low.
Spectrophotometry is the measurement of how different wavelengths of light are absorbed. Using a spectrophotometer to determine how much of a particular wavelength of light is absorbed by a specimen can yield important insights into its characteristics apart from its mass, crystalline structure and other features. Spectrophotometers are useful because of the relation of intensity of colour in a sample and its relation to the amount of solute within the sample.
A UV-Visible spectrophotometer makes use of the transmission of light through a solution to determine the concentration of a solute within the solution. A spectrophotometer consists of two instruments, namely a spectrometer for producing light of any selected colour (wavelength), and a photometer for measuring the intensity of light.
The visible light can be divided into six principal colours. Therefore, a colour will appear as its complementary colour as it absorbs the colour from the incoming white radiation and transmits the principal colour component unaltered. Solution of coloured compounds is known to absorb light in the UV visible region of the electromagnetic radiation. Transition metal compounds are known for this behaviour. Potassium permanganate (KMn04) in solution is purple / violet colour meaning maximum absorption should be at 500 – 550 nm The orange red coloured dichromate, the maximum absorption (λ max ) is at 440 nm. However, at some point in the spectra, the wavelengths overlap each other, hence dichromate can absorb at 520 nm and permanganate at 440 nm, but only to a small extent.
In simple words, both the species are able to absorb at both wavelengths mentioned. Therefore, the absorption of the solution containing a mixture of both will equal to the sum of the absorbances of both species.
In simultaneous determination of two species it is necessary to generate two equations in order to determine the unknown concentrations. This is developed by Beer-Lambert’s Law:
Absorbance = c ε l
Beer’s Law requires the use of monochromatic radiation and under these restrains, the linear dependence and absorption can occur. A spectrophotometer cannot distinguish between individual species, therefore total absorbance of two or species is as follows:
A total = A1 + A2….λ
A two-component solution will be studied in this experiment. The species being absorbed are permanganate and dichromate ions. Finally, the concentration of both species in an unknown sample will be determined.
Table 2 - CONCENTRATION AND ABSORBANCE OF K2Cr2O7 AND KMnO4 FOR EACH FLASK AT 430 nm |Flask |K2Cr2O7 conc. (M) |K2Cr2O7 Absorbance |KMnO4 conc. (M) |KMnO4 Absorbance | |Blank | |0.000 |...
References: Crouch, S., Holler, F., Skoog, D. & West, D. (8th ed.) (2004). Fundamentals of Analytical Chemistry. (pp. 786-787, 796-797). Nelson Education Ltd.
• Starna Cells. (n.d.). Potassium Dichromate Reference: For assessing the absorbance scale of your UV spectrophotometer. Retrieved March 17, 2011 from file:///F:/Analytical%20Chemistry/PDC.html
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