Spectrophotometric measurings with UV or seeable light radiation are utile in observing passage metal ions and extremely conjugated organic compounds. In UV and seeable light parts. energy infinites molecules undergo electronic passages. By comparing the spectrum of an analyte with those of sample molecules. you can acquire an thought as to the individuality of the absorbing groups. Solutions of passage metal ions can be colored ( i. e. . absorb seeable visible radiation ) because vitamin D negatrons within the metal atoms can be excited from one electronic province to another. The colour of metal ion solutions is strongly affected by the presence of other species. such as certain anions or ligands. an illustration of this is the Cr ( VI ) . Cr ( VI ) coinage. The Cr ( VI ) nowadays in K bichromate will be determined utilizing direct seeable spectrophotometry. The standardization procedure is employed in this experiment since it is indispensable in every analytical process. The external standardization method will be done in the experiment.
a. To find the wavelength with maximal optical density of Cr ( VI ) coinage. B. To cipher the molar absorption factor of the different concentrations of K bichromate by using the Beer’s Law. c. To use the external standardization method in determing an unknown concentration of K bichromate solution.
Note: Remember to put the OA or 100 % T every clip the wavelength scene is changed utilizing the clean solution. Besides take the optical density reading of your solution 10 nanometer increases within the 50 or 100 nm scopes where the optical density is at upper limit. IV. Results and Discussion
Chromium ( VI ) is extremely engrossing coinage. It can easy be determined through direct spectrophotometry. The tabular array below shows the optical density of K bichromate solution ( contains the Cr ( VI ) coinage ) . Table 3. 1. Optical densities of K2Cr2O7 sol’n in 0. 05 M H2SO4 @ 340-700 nanometer. Concentration ( in M ) | 0. 01| 0. 001| 0. 0002| 0. 0001| 0. 00001| Wavelength ( in nanometer )
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 soaking up curve K2Cr2O7 sol’n in 0. 05 M H2SO4 at 340-700 nanometer.
The solutions of K bichromate of different concentrations varied in their optical densities. However they show the same behavior over a scope of wavelengths that their maximal wavelengths ( ? ) were at about 340 nanometers. Their wavelengths besides peaked at about 600 nanometers. The maximal optical density at 340 nm part is so used to build the standardization curve. Through the usage of the standardization curve an unknown concentration of a K bichromate solution was determined.
Table 3. 2 Optical density of K2Cr2O7 sol’n in 0. 05 M H2SO4 @ ?max Absorbance ( at ?max ) | [ 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 finding of Cr ( VI ) coinage.
The concentration of the unknown sample can so be predicted utilizing the above arrested development line. Rearranging the equation of the line gives the expression for happening the concentration which is equal to x. The equation of the line:
y=39. 818x+0. 0738 combining weight. 1
To work out for ten:
x=y-0. 073839. 818 combining weight. 2
Table 3. 3 Optical density 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 mean optical density value of the informations above for the unknown. its concentration can be calculated as follows: Using combining weight. 2:
[ x ] =y-0. 073839. 818
Therefore the concentration of the unknown bichromate solution is 0. 016 M. Using the Beer’s jurisprudence. the molar absorption factor of the K bichromate can be calculated. Beer’s jurisprudence provinces that for monochromatic radiation. optical density is straight relative to the way length B through the medium and the concentration degree Celsius of the absorbing species. The relationship is given
where a is a proportionality invariable called the absorption factor. The magnitude of a depends on the units used for set c. For solutions of an engrossing species. B is frequently given thousand centimetres and degree Celsiuss in gms per litre. Absorptivity so has units of L g-I cm-I. When the concentration in the above equation is expresses in moles per liter and the cell length is in centimeters. the absorption factor is called molar absorption factor and is given the particular symbol ? . Thus B is in centimeters and degree Celsiuss in moles per liter.
where ? has the units L mol-1 cm-1.
? . To calculate for the molar absorption factor. ? :
?1=Abc=0. 4621?0. 01=46. 2
?2=Abc=0. 2141?0. 001=214
?3=Abc=0. 0951?0. 0002=475
?4=Abc=0. 0371?0. 0001=370
?5=Abc=0. 0121?0. 00001=1200
From the experiment the wavelength with at which the solution of K bichromate. which contains the Cr ( VI ) coinage. produces a maximal optical density is at approximately 340 nanometers. All concentrations of the bichromate tested have similar wavelengths with maximal optical density. Based on literature values. the Cr ( VI ) specie exhibits two soaking up sets. one with ?max at 348 nanometers and a 2nd broader set with a ?max at 435 nanometer. The Beer’s jurisprudence is applied to cipher the molar absorption factor. ?. of each of the concentrations of the K bichromate solution. It can be summarized through the tabular array below: molar absorption factor. ? ( Fifty mol-1 cm-1 ) | Concentration|
46. 2| 0. 01|
214| 0. 001|
475| 0. 0002|
370| 0. 0001|
1200| 0. 00001|
The external standardization method was used to foretell the concentration of an unknown bichromate solution. It was accomplished by obtaining the response signal ( optical density ) as a map of the known analyte concentration. The curve is prepared by plotting the information or by suiting them in a suited mathematical equation. The following measure is the prediction measure. where the response signal is ohtained for the sample and used to foretell the unknown analyte concentration. c. from the standardization curve or best-fit equation. The concentration of the unknown solution based from computations is 0. 016 M.
* Skoog. D. A. . F. J. Holler and S. R. Crouch. 2007. Principles of instrumental analysis 6th erectile dysfunction. Thomson Brooks/Cole. Canada.
* Skoog. D. A. . D. M. West. F. J. Holler and S. R. Crouch. 2008. Fundamentalss of analytical chemical science 8th erectile dysfunction. . 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.