Experiment 9

EXPERIMENT 9

Spectrophotometric analysis of copper (II)

Batuy, Charmaine; Esteban, Ramiel; Farinas, Donna Rose; Lacdao , May; Mallari, Meghan

Department of Physical Sciences College of Science University of the Philippines Baguio Gov. Pack Road, Baguio City

ABSTRACT In this experiment, tests were performed to measure th e absorbance of a sample metal. First, the pr eparation of the standard solutions was made by dissolving 9.5g Cu(NO3)2.3H2O in a 1L volumetric flask. Series of Cu(II) standard solutions were made. The second step was to determine the analytical wavelength by measuring the absorbance of the most concentrated standard Cu(II) solution and from the data obtained, an absorbance vs. wavelength plot was made. After determining the analytical wavelength, wavelength, the preparation of the calibration curve was executed by measuring the absorbance of the solutions prepared in the first step at the analytical wavelength obtained in the second step. The experiment involved the determination of the Cu(II) concentration of an unknown solution. Beer¶s Law was used for the formula in getting the light absorbance of th e sample. Since solutions of pure substances do not absorb the energy of all wavelengths wavelengths of light equally, a substance ma y be identified by the unique  pattern of wavelengths absorbed.

INTRODUCTION

Spectrophotometric methods of analysis rely on the fact that electromagnetic radiation may be absorbed by matter. It is basically the use of electromagnetic radiation to measure chemical concentrations. It is a fast, relatively simple process. The device used in spectrophotometry is called spectrophotometer. A sample of a certain solution is contained in a cuvet²a transparent vessel. The cuvet is   placed in the spectrophotometer. A beam of monochromatic light would pass through the solution and that light is said to be absorbed.

METHODOLOGY

A. Preparation of the Standard Solutions A 2500 Copper (II) stock solution was prepared by dissolving 9.5536g Cu(NO3)23H2O in a 1 liter volumetric flask and was filled to the mark with distilled water. Five 50-ml volumetric flasks were filled with increasing volume (2mL) of the standard solution; first flask contains 2 mL, second flask with 4 mL and the fifth flask with 10 mL. A volume of 10 mL concentrated  NH4OH solution was added to each flask containing the prescribed volume of stock solution and was diluted to the mark with distilled water. B. Determination of the Analytical Wavelength The absorbance of the most concentrated standard Cu (II) solution was measured in 20 nm steps from 500-700 nm. From the data obtained, a plot of absorbance versus wavelength was

made.

Then, from the curve, the wavelength at which the solution absorbs strongly was

determined and was used for the analysis. C. Preparation of the Calibration Curve The slope and the y-intercept for the obtained calibration curve were determined using the leasts squares method. The working equation for the determination of the concentration of  the unknown was expressed in terms of these parameters. D. Determination of the Cu(II) Concentration of the Unknown Solution The unknown sample solution was treated in the same manner as the standard solutions. At least three absorbance readings of the solution were made. The concentration of the unknown solution is to be determined from the equation obtained in step C. RESULTS AND DISCUSSION

DATA 0 1 2 3 4 5 Absorbance: 1.058

0

2 ml 4 ml 6 ml 8 ml 10 ml

Standard Deviation of Y: 0.03903

CALCULATIONS

     Where:

     

CONCENTRATION 0 373.6 747.2 1120.8 1494.4 1868

Y 0.999 1.023 1.042 1.067 1.084 1.124

         





           

          





  

      

                  

Using the methods previously described, a record of measured absorbance for the five samples of the unknown solution is shown in the table above. The tabulated date is also used to develop the graph shown in Figure 1. It simply plots the measured absorption of a monochromatic light in the spectrophotometer relative to the known concentration of each sample. A straight line was approximated through the data points using a feature of MS Excel. The equation of the approximated line is given by the calculations done above. Furthermore, given that the calculated standard deviation of the absorbances of the samples was 0.03903,

which is relatively small, one can conclude that the data set is precise, but not necessarily  precise.

12

10

8

6

4

2

0 1

1.02

1.04

1.06

1.08

1.1

1.12

1.14

Figure 1. Standard Curve for Analysis of Unknown

Basically, what happened during the experiment is that a sample of the unknown solution was placed in the cuvet, which was later placed in the spectrophotometer. Inside the spectrophotometer, a light beam passes through the substance in the cuvet. The sample absorbs the light that passes through it. The transmission of light is measured by the spectrophotometer  and is displayed in its read-out. This exercise was helpful in orienting the students about spectrophotometric techniques and its significance, primarily in determining concentrations of unknown samples which could  be helpful in the identification of certain chemical species present in a substance.

POSTLAB QUESTIONS

1.

Why is it important to take absorbance readings at the proper wavelength? Since the concentration of colored solute in a solution is directly proportional to the intensity

of its color, which in turn is proportional to the amount of absorbance of light at the wavelength that the color absorbs, it is important to take the absorbance readings at the proper wavelength to avoid inaccuracy in the results or readings. The color or absorbance, of a solution is also  proportional to the path length that the light passes through. This is expressed as the Beer¶s Law.

2. A 1.0000 g sample of commercial steel was analyzed colorimetrically for chromium (Cr) and 2-

manganese (Mn) after proper treatment and oxidation of chromium to dichromate (Cr 2O7 ). The following absorption readings were obtained: Solution

KMnO4 (mol/L)

K 2Cr 2O7 (mol/L)

ABSORBANCE At 440 nm

At 545 nm

-4

0.079

0.854

2.070 x 10

-4

0.040

0.430

3

1.354 x 10-4

0.027

0.284

4

-

1.649 x 10-3

0.668

0.027

5

-

5.497 x 10-4

0.225

0.011

-4

0.335

0.14

0.130

0.550

1

4.133 x 10

2

6

8.245 x 10

SAMPLE SOLUTION (1.0000 g in 500 mL solution) Assume that the absorbance for the two colored species are additive at the given wavelengths. Calculate a) the average molar absorptivities of each spec ies at the given wavelength.  b) the % Cr and Mn in the sample.

A) Calculate the average molar absorptivities of each species at the given wavelength. a) Molar absorptivity of K MnO4 at 440 nm e=A cl e = molar absorptivity A = Absorbance c = concentration l = path length e= 0.079 . -4 (4.133 x 10 M)(1.0cm) -1

-1

= 191.14 M cm

e= 0.027 . -4 (8.245 x 10 M)(1.0cm) = 199.41 M-1cm-1 Ave. molar absorptivity = 194.60 M-1cm-1

e= 0.040 . -4 (2.070 x 10 M) (1.0cm) -1

-1

= 193.24 M cm

 b) Molar absorptivity of K MnO4 at 545 nm e= 0.854 . -4 (4.133 x 10 M)(1.0cm) -1

-1

e= 0.430 . -4 (2.070 x 10 M) (1.0cm) -1

= 2066.3 M cm

-1

= 2077.3 M cm

e= 0.284 . -4 (1.354 x 10 M)(1.0cm) = 2097.5 M-1cm-1 Ave. molar absorptivity = 2080.4 M-1cm-1

c) Molar absorptivity of K 2Cr2O7 at 440 nm e= 0.668 . -3 (1.649 x 10 M)(1.0cm) -1

-1

e= 0.225 . -4 (5.497 x 10 M) (1.0cm) -1

= 405.09 M cm

-1

= 409.31 M cm

e= 0.335 . -4 (8.245 x 10 M)(1.0cm) -1

-1

= 406.31 M cm

Ave. molar absorptivity = 406.90 M-1cm-1

d) Molar absorptivity of K 2Cr2O7 at 545 nm e= 0.027 . -3 (1.649 x 10 M)(1.0cm) = 16.37 M-1cm-1 e=

0.14

e= 0.011 . -4 (5.497 x 10 M) (1.0cm) = 20.01 M-1cm-1

.

-4

(8.245 x 10 M)(1.0cm) -1

-1

= 169.80 M cm

Ave. molar absorptivity = 68.73 M-1cm-1

B) Calculate the % Cr and Mn in the sample. % Mn

0.5 L

x

-4

4.133 x 10 mol/L

x

1mol Mn 1 mol KMnO4 1.000g

x 54.938 g/mol

1mol Mn 1 mol KMnO4 1.000g

x 54.938 g/mol

1mol Mn 1 mol KMnO4 1.000g

x 54.938 g/mol

x 100 .

= 1.14 %

0.5 L

x

-4

2.070 x 10 mol/L

x

x 100 .

= 0.569 %

0.5 L

x

-4

1.354 x 10 mol/L

x

x 100 .

= 0.372 % % Cr 

0.5 L

=

x

17.15%

-3

1.649 x 10 mol/L

x

2mol Cr x 103.99 g/mol 1 mol K 2Cr 2O7 1.000g

x 100 .

0.5 L

=

-3

5.497 x 10 mol/L

x

2mol Cr x 103.99 g/mol 1 mol K 2Cr 2O7 1.000g

x 100

2mol Cr x 103.99 g/mol 1 mol K 2Cr 2O7 1.000g

x 100

.

57.16%

0.5 L

=

x

x

85.74%

-3

8.245 x 10 mol/L

x

.

REFERENCES