• March 3rd, 2016

Spectrophotometric analysis of commercial aspirin

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Week 6 Assignment Reading Material
In this experiment we will use visible electromagnetic radiation, or white light, as a means to analyze the percent composition of commercial grade aspirin. The visible region usually is the region from 380 to 750 nm. Within this region light is composed of the various spectral colors. White light or sunlight is composed of all these wavelengths and is known as polychromatic light. Light of a single wavelength is known as monochromatic light.

When white light comes into contact with an object, it may be reflected by, absorbed by, or transmitted through the object. If a portion (certain wavelengths) of the incident radiation is absorbed by the object, the transmitted or reflected light will appear as the complementary color of those wavelengths, as shown in Table 1.

Table 1: Correlation between wavelength, color, and complementary color in the visible region

Wavelength, nm Color Complimentary color
380 – 435
435 – 480
480 – 490
490 – 500
500 – 560
560 – 580
580 – 595
595 – 650
650 – 750 Violet
Red Yellow-green

Since many substances form colored solutions, a comparison of the intensity of the color of a known concentration with that of a solution of unknown concentration offers a convenient way for the quantitative estimation of the concentration of the colored substance in the unknown solution. If the comparison is done visually, the method is called colorimetry. If a photoelectric cell is used in place of the human eye, the method is called photometry. In the technique used in this experiment, when monochromatic light is used as the source and a photoelectric cell is used as the detector, the method is known as spectrophotometry. Figure 1 shows the optics of a spectrophotometer such as the Spectronic 20. The white light emanating from the tungsten lamp passes through the entrance slit and is reflected by a diffraction grating. The grating is a dispersing element, acting like a prism to separate the white light into its component wavelengths. In setting the wavelength, the grating is rotated by means of a cam, so that the desired wavelength passes through the exit slit. The monochromatic light that passes through the exit slit goes on through the sample and finally strikes the measuring photoelectric cell, where the light energy is converted to an electric signal.

Figure 1: Optics of a Spectrophotometer

When light of intensity I0 impinges on a solution contained in a transparent cell, some of the light is scattered, or reflected, by the walls of the container and by the particles in solution IS; some is absorbed, IA; and the rest is transmitted through the solution. Overall, I0 = IS + IA + IT.
Under most experimental conditions, IS is small and can be kept constant by using matched cells and by always positioning the cells in the instrument in the same way. Thus, I0 –IS can be approximated as I0. The absorbed light, IA, is related to the concentration of the absorbing material in the solution. Thus, by measuring I0 and IT, the value of IA can be calculated, and the concentration of the absorbing substance can be determined. IA cannot be measured directly but can be related to the percent transmittance, %T, which is measured by the phototube.
The absorbance A, defined as

A = 2-log%T (2)

is proportional to the concentration, c, of the absorbing substance in the solution.

bCeA =  (3)

The Beer-Lambert law provides the mathematical correlation between  and b are constants thate absorbance and concentration. In equation (3) allow the proportionality A – C to be converted to an equation. The  is known as the molar absorptivity and is characteristic ofeconstant  the absorbing solute. The pathlength of the radiation through the cell is identical with the cell thickness b. Since cells of constant thickness are used, the absorbance A will depend linearly on the  will be constant.econcentration of the absorbing solute for which

The relationship between absorbance and concentration serves as the basis for the quantitative analysis of many substances. In practice, the absorbances of a series of solutions of known concentrations are measured and a plot of absorbance vs. concentrations is constructed. Such a plot is known as a Beer – Lambert graph and also represents a calibration curve for the particular system being studied. An unknown solution containing the same absorbing substance may then be analyzed by measuring its absorbance, locating this A value on the calibration curve, and reading the corresponding concentration. In Figure 2, the dotted line shows how the curve is used to find the concentration of an unknown solution with an absorbance of 0.82 (C = 4.3 x 10-4 mol/L).

Figure 2: A Typical Beer – Lambert’s Law Plot

Similarly, an Excel spreadsheet can be used and the absorbance-concentration data points plotted. A linear regression of the plot that goes through zero is then performed. The slope of the b. The concentrationestraight line is therefore equal with the product  of the unknown is determined from the linear equation A = (slope) x C as


The absorbance has no units while the concentration is expressed in mol/L.

In recording measurements with the Spectronic 20, the linear transmittance scale is more conveniently and precisely read than the logarithmic absorbance scale. However, since it is the absorbance that is linearly dependent on concentration, %T readings must be converted to the corresponding A values. These can be readily calculated with equation (2).

Determination of Acetylsalicylic Acid

Acetylsalicylic acid, commonly known as aspirin, is hydrolyzed rapidly in basic medium to yield salicylate dianion, as shown in the chemical equation below (5):

Acidification, followed by addition of iron(III) ion, Fe3+, yields the soluble tetraaquasalicylate iron(III) complex shown in equation (6).


This intensely violet solution exhibits the transmittance spectrum shown in Figure 3. The complex in solution has a very high molar absorptivity, which allows it to be used as a sensitive indicator for the presence of salicylate species.

Figure 3: Transmittance / Absorbance Spectrum of a Solution of Iron(III)-Salycilate
Complex Ion

The wavelength of minimum % T just above 500 nm corresponds to the  is at its maximum value and where A versus C wille, where lwavelength,  have its widest linear range. In addition, maximum sensitivity is .lrealized at this


Obtain a stock solution of acetyl salicylic acid with the concentration of 1.6 g/L treated with sodium hydroxide to ensure formation of acetyl salicylate dianion (equation 5). Obtain a stock solution of iron(III) chloride, FeCl3, with the concentration of 0.01 mol/L. Prepare the following five solutions and a blank to be used to construct the Beer – Lambert plot as outlined in Table 3

Table 2: Preparation of the solutions to be used to obtain Beer – Lambert plot

Solution ID Volume of stock solution of acetyl salicylic acid, mL Volume of HCl (6 mol/L)
drops Volume of FeCl3 solution
Blank 0.0 1 10.0
A 1.0 1 9.0
B 2.0 1 8.0
C 3.0 1 7.0
D 4.0 1 6.0
E 5.0 1 5.0

Consider the final volume of each solution as 10.0 mL. Measure the percent transmittance, %T, of each violet solution on Spectronic 20 at 510 nm against the blank. Record all measurements.

Obtain an unknown solution and prepare the following dilution: 3.0 mL unknown, 1 drop concentrated HCl and 7.0 mL of iron(III) chloride solution (total volume 10.0 mL). Measure the %T of the unknown solution.


By utilizing the assumptions implicit in the discussion, that is, quantitative hydrolysis of the acetylsalicylic acid and a ratio of one-to-one salicylate ion to iron in the complex (equation 6), the concentration of the acetyl salicylic acid in each reference solution can be calculated.

The concentration of acetylsalicylic acid is 1.6 g/L. The molar mass of acetylsalicylic acid (CH3COOC6H4COOH) is 180.2 g/mol. From this, the molar concentration of acetylsalicylic acid can be determined:

The stock solution is diluted in varying proportions to yield the solutions A, B, C, D, and E. For example, the concentration of acetylsalicylic acid in solution E will be:

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