Determination of Chromium (Vi) by Direct Visible Spectrophotometry (External Calibration Method)

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Experiment No. 3
Determination of Chromium (VI) by Direct Visible Spectrophotometry (External Calibration Method)

Group Members: Cabahug, Elisha Niña M.Date Performed: November 20 & 22, 2012
Mejia, Helen Mae N.Date Submitted: November 29, 2012 Score:

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)...
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