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Determination of Chromium (Vi) by Direct Visible Spectrophotometry

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I. Introduction
Spectrophotometric measurements with UV or visible light radiation are useful in detecting transition metal ions and highly conjugated organic compounds. In UV and visible light regions, energy spaces molecules undergo electronic transitions. By comparing the spectrum of an analyte with those of sample molecules, you can get an idea as to the identity of the absorbing groups. Solutions of transition metal ions can be colored (i.e., absorb visible light) because d electrons within the metal atoms can be excited from one electronic state to another. The color of metal ion solutions is strongly affected by the presence of other species, such as certain anions or ligands, an example of this is the chromium (VI), Cr (VI) specie. The Cr (VI) present in potassium dichromate will be determined using direct visible spectrophotometry. The calibration process is employed in this experiment since it is essential in every analytical procedure. The external calibration method will be done in the experiment.

II. Objectives

a. To determine the wavelength with maximum absorbance of chromium (VI) specie. b. To calculate the molar absorptivity of the different concentrations of potassium dichromate by applying the Beer’s Law. c. To apply the external calibration method in determing an unknown concentration of potassium dichromate solution.

III. Procedure

NOTE: Remember to set the OA or 100% T every time the wavelength setting is changed using the blank solution. Also take the absorbance reading of your solution 10 nm increments within the 50 or 100 nm ranges where the absorbance is at maximum. IV. Results and Discussion

Chromium (VI) is highly absorbing specie. It can easily be determined through direct spectrophotometry. The table below shows the absorbance of potassium dichromate solution (contains the Cr (VI) specie). Table 3.1. Absorbances of K2Cr2O7 sol’n in 0.05 M H2SO4 @ 340-700 nm. Concentration (in M)| 0.01| 0.001| 0.0002| 0.0001| 0.00001| Wavelength (in nm)

340| 0.462| 0.214| 0.095| 0.037| 0.012|
400| 0.388| 0.140| 0.039| 0.035| 0.008|
450| 0.099| 0.037| 0.038| 0.035| 0.007|
500| 0.049| 0.033| 0.036| 0.034| 0.004|
550| 0.032| 0.028| 0.034| 0.027| 0.011|
600| 0.053| 0.053| 0.058| 0.036| 0.008|
650| 0.031| 0.032| 0.033| 0.032| 0.007|
700| 0.026| 0.032| 0.031| 0.032| 0.010|

Figure 3.1. Spectral absorption curve K2Cr2O7 sol’n in 0.05 M H2SO4 at 340-700 nm.

The solutions of potassium dichromate of different concentrations varied in their absorbances. However they show the same behaviour over a range of wavelengths that their maximum wavelengths (λ) were at about 340 nm. Their wavelengths also peaked at about 600 nm. The maximum absorbance at 340 nm region is then used to construct the calibration curve. Through the use of the calibration curve an unknown concentration of a potassium dichromate solution was determined.

Table 3.2 Absorbance of K2Cr2O7 sol’n in 0.05 M H2SO4 @ λmax Absorbance (at λmax)| [K2Cr2O7] (in M)|
0.462| 0.01|
0.214| 0.001|
0.095| 0.0002|
0.037| 0.0001|
0.012| 0.00001|

Figure 3.2. Calibration curve for the determination of Cr (VI) specie.

The concentration of the unknown sample can then be predicted using the above regression line. Rearranging the equation of the line gives the formula for finding the concentration which is equal to x. The equation of the line:

y=39.818x+0.0738 eq.1
To solve for x:
x=y-0.073839.818 eq.2
Table 3.3 Absorbance for unknown [] of K2Cr2O7 sol’n.
Trial| %T| T| A (y)|
1| 17.9| 0.179| 0.747147|
2| 18.2| 0.182| 0.739929|
3| 18| 0.18| 0.744727|
Average| 18.03333| 0.180333| 0.743934|

Using the average absorbance value of the data above for the unknown, its concentration can be calculated as follows: Using eq. 2:


[x]=0.016 M
Therefore the concentration of the unknown dichromate solution is 0.016 M. Applying the Beer’s law, the molar absorptivity of the potassium dichromate can be calculated. Beer’s law states that for monochromatic radiation, absorbance is directly proportional to the path length b through the medium and the concentration c of the absorbing species. The relationship is given
by: A=abc

where a is a proportionality constant called the absorptivity. The magnitude of a depends on the units used for band c. For solutions of an absorbing species, b is often given m centimeters and c in grams per liter. Absorptivity then has units of L g-I cm-I. When the concentration in the above equation is expresses in moles per litre and the cell length is in centimetres, the absorptivity is called molar absorptivity and is given the special symbol ξ. Thus b is in centimetres and c in moles per litre,

where ξ has the units L mol-1 cm-1.

∴, To compute for the molar absorptivity, ξ:

V. Conclusion
From the experiment the wavelength with at which the solution of potassium dichromate, which contains the Cr (VI) specie, produces a maximum absorbance is at about 340 nm. All concentrations of the dichromate tested have similar wavelengths with maximum absorbance. Based on literature values, the chromium (VI) specie exhibits two absorption bands, one with λmax at 348 nm and a second broader band with a λmax at 435 nm. The Beer’s law is applied to calculate the molar absorptivity, ξ, of each of the concentrations of the potassium dichromate solution. It can be summarized through the table below: molar absorptivity, ξ (L mol-1 cm-1)| Concentration|

46.2| 0.01|
214| 0.001|
475| 0.0002|
370| 0.0001|
1200| 0.00001|

The external calibration method was used to predict the concentration of an unknown dichromate solution. It was accomplished by obtaining the response signal (absorbance) as a function of the known analyte concentration. The curve is prepared by plotting the data or by fitting them in a suitable mathematical equation. The next step is the prediction step, where the response signal is ohtained for the sample and used to predict the unknown analyte concentration, c, from the calibration curve or best-fit equation. The concentration of the unknown solution based from calculations is 0.016 M.

VI. References

* Skoog, D. A., F. J. Holler and S. R. Crouch. 2007. Principles of instrumental analysis 6th ed. Thomson Brooks/Cole. Canada.

* Skoog, D. A., D. M. West, F. J. Holler and S. R. Crouch. 2008. Fundamentals of analytical chemistry 8th ed.. Thomson Brooks/Cole. Singapore

* Chirium Laboratory site.htm

* Bobby Stanton,Lin Zhu,Bobby Stanton, Lin Zhu, Charles “Butch” Atwood,Charles H. Atwood. Experiments in General Chemistry: Featuring MeasureNet.

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