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A 0.2100 M stock solution of Co ( II ) chloride hexahydrate was analyzed utilizing UV-Vis spectrometry. A series of dilutions of the stock solution were made to analyse the effects of concentration on the optical density values of Co ( II ) chloride hexahydrate utilizing the UV-Vis spectrophotometer. The Cu ( II ) chloride hexahydrate was found to hold the highest optical density value at an mean wavelength of 511.02 nanometers. The mean molar extinction coefficient for Cu ( II ) chloride hexahydrate was found to be 4.5172. Spectroscopic analyses of dilutions of the stock solution were used to make a standardization curve of optical density versus concentration of the Co chloride hexahydrate solution. A solution of unknown concentration was analyzed utilizing the UV-Vis spectrophotometer. The standardization curve was used to find that the unknown had a concentration of 0.1250 M.

Introduction:

Ultraviolet/Visible ( UV-Vis ) spectrometry analyzes electronic passages between atoms and molecules. Spectra are produced when negatrons in molecules or atoms move from one electronic energy degree to another of higher energy. In making so, the captive energy is equal to the difference between to the two degrees. Compounds that absorb visible radiation in the seeable part are colored. Compounds that absorb light merely in the ultraviolet part are colorless.

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Inside a UV-Vis spectrophotometer there are normally two light beginnings, a tungsten lamp for the seeable part ( 380-800 nanometer ) and a heavy hydrogen lamp for the ultraviolet part ( 10-380 nanometer ) . The light beginning produces a white visible radiation beam which contains all wavelengths ( all colourss ) . The light beam directed to a monochromator by a mirror. The monochromator is used to divide visible radiation into specific wavelengths. Each wavelength corresponds to a different colour. The instrument scans through the UV-Vis spectrum, directing different wavelengths of visible radiation through the sample. A individual wavelength passes into the modulator, which consist of a rotor with mirrors that splits the light into two beams. One beam passes through the sample cell, while the other base on ballss through the mention cell. Both sample and mention beams are redirected by mirrors into a sensor. The sensor compares their strengths of the two beams and sends a signal to the computing machine that controls the instrument. The signal is defined as optical density, which is a measuring of how much visible radiation is being absorbed by the sample at that peculiar wavelength.

The Beer-Lambert jurisprudence provinces that optical density ( A ) is relative to concentration of the absorbing species and way length of the medium over a certain clip:

In equation 1, is the molar extinction coefficient and has units of, the way length of the medium or L, is reassured in centimetres or centimeter and the concentration of the absorbing species has units of molar concentration or M.

In this experiment a solution of Co ( II ) chloride hexahydrate was analyzed utilizing UV-Vis soaking up spectrometry. The intent of this experiment is to make a standardization curve of optical density versus concentration by doing series of dilutions of Co ( II ) chloride hexahydrate. The standardization curve will so be used to find the concentration of an unknown sample. The molar extinction coefficient for Co ( II ) chloride hexahydrate will besides be determined utilizing the soaking up at the concentrations of each dilution.

Experimental Procedure:

Using the analytical balance, 2.5072 g of Co ( II ) chloride hexahydrate were weighed and placed into a 50 milliliter beaker. The purple solid was dissolved inside the beaker utilizing 15 milliliter of distilled H2O. The violet liquid was so transferred to a 50 milliliter volumetric flask with the assistance of a funnel. The beaker was so rinsed with another 15 milliliter part of distilled H2O to roll up any staying Co ( II ) chloride hexahydrate left buttocks and so was transferred to the 50 milliliter volumetric flask utilizing the same funnel. Extra 20 milliliter of distilled H2O were added to the 50 milliliter volumetric flask to make the stock solution of Co ( II ) chloride hexahydrate.

Dilutions of the stock solution were made by reassigning 2, 4, 6 and 8 milliliter of the stock solutions to four labeled 10 milliliter volumetric flasks. Distilled H2O was added to make full each flask to the line.

The optical density for each solution was calculated utilizing spectrophotometer. Before any samples were analyzed, a sample incorporating merely H2O was used to blank the instrument. A quartz cuvette was filled with distilled H2O and covered. The clean sample was placed in the sample holder in the dorsum of the spectrophotometer. Using the plan, the spectrophotometer parametric quantities were set to scan the sample from 650 nm to3 90 nanometer. The plan was besides designed to automatically allow the user know which sample to put following into the sample holder.

After the clean sample was analyzed, the cuvette was rinsed with distilled H2O foremost and so with a little part of the stock solution. The cuvette was so filled with a part of the stock solution, covered and analyzed utilizing the spectrophotometer. This process was repeated for all dilutions. After each analysis, the cuvette was foremost rinsed with distilled H2O and so rinsed with a little part of the undermentioned sample.

Consequences:

In order to analyse the sample utilizing the spectrophotometer, the compound needs to be present in the aqueous signifier. The Cu ( II ) chloride hexahydrate appeared violet as a solid. After the 2.507 gms of Cu ( II ) chloride hexahydrate were dissolved in 50 milliliter of distilled H2O, the compound ‘s colour changed from a dark purple to a pink colored solution.

The concentration of the Cu ( II ) chloride hexahydrate stock solution was found utilizing the molecular weight of the compound, the sum of compound used and the sum of distilled H2O used to fade out it. Table # 1 shows the how the concentration of the Cu ( II ) chloride hexahydrate stock solution was found.

Table # 1: Concentration of Cu ( II ) chloride hexahydrate stock solution

Compound

Molecular Weight

( g/mol )

Sum of Compound used

( g )

Sum of H2O used

( L )

Equation

Concentration

( mol/L or M )

237.9300

2.5072

0.0500

0.2100

When the dilutions were made by taking 8, 6, 4 and 2 milliliter of the stock solution, the concentration of each dilution decreased proportionately to the sum of stock being added. The strength of the pink colour of each dilution besides decreased as the sum of millilitres of distilled H2O increased. Table # 2 shows how the concentration for each dilution of the stock solution was calculated.

Table # 2: Concentration of each dilution of the stock solution.

Dilution

Concentration of Stock ( M )

Volume of

Stock used ( L )

Volume of H2O used ( L )

Equation

Concentration of Dilution ( M )

Stock

0.2100

1

0.2100

0.0080

0.0020

0

0.1680

2

0.2100

0.0060

0.0040

0

0.1260

3

0.2100

0.0040

0.0060

0

0.0840

4

0.2100

0.0020

0.0080

0

0.0420

The package was programmed to analyse the solutions in the undermentioned order, the clean sample, the stock solution ( 0.2100 M ) , the 0.1680 M dilution, the 0.1260 M dilution, the 0.0840 M dilution and 0.0420 M dilution. All samples were analyzed in the spectrophotometer, by utilizing quartz cuvettes. The clean sample had no ocular consequences as expected. When the 0.2100 M stock solution was analyzed by the spectrophotometer, the computing machine ‘s proctor displayed the formation of a graph get downing at 650.00 nanometer on the x-axis and 0.00 Optical density on the y-axis. After the graph passed 580.00 nanometer, the graph ‘s optical density values started to increase exponentially. The maximal optical density value was recorded at 0.9993 and it occurred at maximal wavelength of 511.34 nanometers. After the was passed, the graph ‘s optical density values started to exponentially decreased until the graph reached 420.00 nanometer, after 420.00 nm the graphs optical density values displayed a somewhat changeless form until the terminal of the graph at 380.00 nanometers.

Similar consequences were observed for all the dilutions. The 0.16800 M dilution analysis showed a maximal wavelength of 510.92 nanometers and a maximal optical density value of 0.7266. The 0.12600 M dilution analysis showed a maximal wavelength of 511.11 nanometers and a maximal optical density value of 0.5703. The 0.0840 M dilution analysis showed a maximal wavelength of 510.98 nanometers and a maximal optical density value of 0.4024. The 0.0420 M dilution analysis showed a maximal wavelength 510.75 nanometer and a maximal optical density value of 0.1758. Table # 3 summarizes all the maximal wavelengths and optical density values for the stock solutions and all its dilutions. Figure # 1 ( appendix-pg 14 ) illustrates the graph for each solution.

Table # 3: Maximal wavelengths and optical density values for all solutions.

Solution #

Concentration ( M )

Maximal Wavelength ( nanometer )

Optical density

1

0.2100 ( stock )

511.34

0.9993

2

0.1680

510.92

0.7266

3

0.1260

511.11

0.5703

4

0.0840

510.98

0.4024

5

0.0420

510.75

0.1758

Using the information from table # 3, a standardization curve of optical density versus concentration can be created. Figure # 2, the standardization curve can be found in the appendix subdivision, page 13.

The molar extinction coefficient for Cu ( II ) chloride hexahydrate can be found utilizing informations found in Table # 3 and the Beer-Lambert jurisprudence. By algebraically pull stringsing the Beer-Lambert equation ( A = ? * L *c ) , the molar extinction coefficient ( ? ) for Cu ( II ) chloride hexahydrate can be determined by: ? = A / L*c. The molar extinction coefficient for all the solutions can be found in Table # 4. The mean molar extinction coefficient for Cu ( II ) chloride hexahydrate was found to be 30445.

Table # 4: Molar extinction coefficient for all Cu ( II ) chloride hexahydrate solutions

Solution #

Optical density

Length of the medium ( centimeter )

Concentration ( M )

Equation

? = A / L*c

Molar Extinction Coefficient ( ? )

1

0.9993

1

0.2100

4.7586

2

0.7266

1

0.1680

4.3250

3

0.5703

1

0.1260

4.5262

4

0.4024

1

0.0840

4.7905

5

0.1758

1

0.0420

4.1857

*4.5172

*Average

A solution of unknown concentration was analyzed utilizing the spectrophotometer following the same process as all other solutions. The solution of unknown concentration was found to hold a maximal wavelength of 511.49 nanometers and a maximal optical density value of 0.5715. The concentration of the unknown sample was determined utilizing the equation of the line found on the standardization curve ( page 13-Appendix ) . The unknown ‘s optical density value of 0.5715 was used as the y-value and the equation was solved for its analogous x-value or concentration. The unknown ‘s concentration was found to be 0.80 M. Table # 5 shows how the equation of the line from the standardization curve was used to find the concentration of the unknown. Figure # 3 in the appendix section-pg 14, is a graph of all the solutions tested. In figure # 3, the unknown is easier to place because the graph is in a landscape format and the x-axis additions by a factor of 20 nanometers as opposed to a factor of 50 nanometers in Figure # 1.

Table # 5: Determination of the terra incognita ‘s concentration.

Unknown Optical density

Equation of the line

Y = 4.6933x – 0.0165

Unknown Concentration ( M )

0.5715

0.5715=4.6933x – 0.0165

0.5880=4.6933x

0.1253

Decision:

The spectroscopic analysis of Cu ( II ) chloride hexahydrate made the pupils familiar with runing a spectrophotometer. Dilutions to a stock solution of Cu ( II ) chloride hexahydrate were made to analyze how different concentrations of the compound affected the optical density values of each sample. The Cu ( II ) chloride hexahydrate was found to hold the highest optical density value at an mean wavelength of 511.02 nanometers. A standardization curve for the concentration versus optical density of Cu ( II ) chloride hexahydrate was created utilizing the informations obtained from stock solution and dilutions utilizing the spectrophotometer. A unknown sample was found to hold a concentration of 0.1250 M. The concentration of the terra incognita was determined by utilizing the standardization curve along with the informations obtained from the spectrophotometer. The mean molar extinction coefficient for Cu ( II ) chloride hexahydrate was found to be 4.5172. The value for the molar extinction coefficient was determined utilizing the theory behind Beer-Lambert jurisprudence and maximal optical density values from the spectrophotometer.

Discussion

A different attack to find the concentration of the unknown involves utilizing the mean molar extinction coefficient for Cu ( II ) chloride hexahydrate found in table # 4. By algebraically pull stringsing the Beer-Lambert equation a expression for concentration can be derived: degree Celsius =

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