EXPERIMENT 9

Spectrophotometric analysis of copper (II)

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

Department of Physical SciencesCollege of Science

University of the Philippines BaguioGov. Pack Road, Baguio City

ABSTRACT

In this experiment, tests were performed to measure the absorbance of a sample metal. First, the preparation 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, 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 the sample. Since solutions of pure substances do not absorb the energy of all wavelengths of light equally, a substance may 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 cuvet—a 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 0 CONCENTRATION0

Y0.999

1 2 ml 373.6 1.0232 4 ml 747.2 1.0423 6 ml 1120.8 1.0674 8 ml 1494.4 1.0845 10 ml 1868 1.124

Absorbance: 1.058

Standard Deviation of Y: 0.03903

CALCULATIONS

y=mx+b

Where:

y=absorbance of solution

m=slopeof line

x=c=concentration of the solution

b= y−intercept

m=n∑ (xy¿)−∑ (x )

n∑ (x2 )−(∑ x)2 ¿

m=6 (6076.2 )− (6.339 )(5604)

6 (7676732.8 )−¿¿

m=6.37×10−5

b=∑ y−m∑ x

n

b=6.339−(6.40 x 10−5 ) (5604 )

6

b=0.997

y=6.37×10−5 x+0.997

1.058=6.37×10−5 x+0.997

x=957.61

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.

1 1.02 1.04 1.06 1.08 1.1 1.12 1.140

2

4

6

8

10

12

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

manganese (Mn) after proper treatment and oxidation of chromium to dichromate (Cr2O72-). The

following absorption readings were obtained:

Solution KMnO4 (mol/L) K2Cr2O7 (mol/L) ABSORBANCE

At 440 nm At 545 nm

1 4.133 x 10-4 0.079 0.854

2 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

6 8.245 x 10-4 0.335 0.14

0.130 0.550

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 species 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 KMnO4 at 440 nm

e = A cl

e = molar absorptivityA = Absorbancec = concentrationl = path length

e = 0.079 . e = 0.040 . (4.133 x 10-4M)(1.0cm) (2.070 x 10-4M) (1.0cm)

= 191.14 M-1cm-1 = 193.24 M-1cm-1

e = 0.027 . (8.245 x 10-4M)(1.0cm)

= 199.41 M-1cm-1

Ave. molar absorptivity = 194.60 M-1cm-1

b) Molar absorptivity of KMnO4 at 545 nm

e = 0.854 . e = 0.430 . (4.133 x 10-4M)(1.0cm) (2.070 x 10-4M) (1.0cm)

= 2066.3 M-1cm-1 = 2077.3 M-1cm-1

e = 0.284 . (1.354 x 10-4M)(1.0cm)

= 2097.5 M-1cm-1

Ave. molar absorptivity = 2080.4 M -1 cm -1

c) Molar absorptivity of K2Cr2O7 at 440 nm

e = 0.668 . e = 0.225 . (1.649 x 10-3 M)(1.0cm) (5.497 x 10-4 M) (1.0cm)

= 405.09 M-1cm-1 = 409.31 M-1cm-1

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

= 406.31 M-1cm-1

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

d) Molar absorptivity of K2Cr2O7 at 545 nm

e = 0.027 . e = 0.011 . (1.649 x 10-3 M)(1.0cm) (5.497 x 10-4 M) (1.0cm)

= 16.37 M-1cm-1 = 20.01 M-1cm-1

e = 0.14 . (8.245 x 10-4M)(1.0cm)

= 169.80 M-1cm-1

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

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

% Mn

0.5 L x 4.133 x 10-4 mol/L x 1mol Mn x 54.938 g/mol x 100 1 mol KMnO 4 .

1.000g

= 1.14 %

0.5 L x 2.070 x 10-4 mol/L x 1mol Mn x 54.938 g/mol x 100 1 mol KMnO 4 .

1.000g

= 0.569 %

0.5 L x 1.354 x 10-4 mol/L x 1mol Mn x 54.938 g/mol x 100 1 mol KMnO 4 .

1.000g

= 0.372 %

% Cr

0.5 L x 1.649 x 10-3 mol/L x 2mol Cr x 103.99 g/mol x 100 1 mol K 2Cr2O7 .

1.000g= 17.15%

0.5 L x 5.497 x 10-3 mol/L x 2mol Cr x 103.99 g/mol x 100 1 mol K 2Cr2O7 .

1.000g= 57.16%

0.5 L x 8.245 x 10-3 mol/L x 2mol Cr x 103.99 g/mol x 100 1 mol K 2Cr2O7 .

1.000g= 85.74%

REFERENCES