In the Laboratory
Reflectance Spectroscopy Using Wine Bottle Glass An Undergraduate Experiment María Gabriela Lagorio INQUIMAE and Departamento de Química Inorgánica, Analítica y Química Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires, Argentina;
[email protected] The determination of the concentration of light-absorbing species in heterogeneous media is increasingly relevant nowadays. Examples include photocatalysis (1) and solid-state photochemistry and photophysics (2, 3), where the actual number of absorbed photons makes the calculation of quantum yields possible. These determinations can be made by diffuse reflectance spectroscopy. Just as undergraduate students are routinely taught how to obtain an absorbance spectrum of a transparent homogeneous sample, they now also need to be taught how to evaluate absorbance in solids or suspensions where light scattering takes place. A few experiments using diffuse reflectance have been published for the undergraduate laboratory (4–9). Mostly, they have been concerned with instrumentation or with examples of applications. None of them have presented the theoretical basis of the technique. This paper describes an inexpensive diffuse reflectance experiment that illustrates how the absorbance or the number of absorbing species may be obtained for a solid or a suspension. The theoretical basis is established first; then the application is demonstrated by means of disposable glasses. Here, a piece of green glass from a wine bottle and any piece of colorless glass have been used. No toxic materials are involved in the experiment.
going in the opposite direction. For J, then we can write:
Theoretical Background
The left side of eq 5 is known as the remission function, F (R), and is linearly dependent on the number of absorbing chromophores in any sample (i.e., on the concentration of the absorbing species) (2, 7 ). For a sample where there are homogeneous absorbers distributed in a solid substrate (for example dyes adsorbed or linked to solid supports), the experimentally obtained F (R) may be expressed as:
Absorption in the Solid Phase: Diffuse Reflectance and Remission Function The radiation reflected by any solid surface has two distinct parts: (i) the mirror or specular reflectance and (ii) the diffuse reflectance. The diffuse reflectance comes about through penetration of a portion of the incident flux into the interior of the sample. A part of this radiation is absorbed and the rest is returned to the surface after multiple scattering at the boundaries of the particles in the sample. The approach usually adopted for describing the interaction of light with diffusing media is the Kubelka–Munk theory (10). This theory does not take into account specular reflection and so it is only applicable to diffuse reflection. The Kubelka–Munk theory assumes that the medium comprises randomly distributed, uniformly absorbing scattering particles. Two light fluxes, I (the flux in the direction of the incident light) and J (the flux in the direction of the reflected light), are considered to travel in opposite directions perpendicular to the irradiated surface at x = 0 (see Fig. 1). The attenuation of the flux I is dI (x) = {I (x)(K + S )dx + J (x) × Sdx
{dJ (x) = {J (x)(K + S )dx + I (x) × Sdx
(2)
Equation 1 shows that the absorbed and the scattered part of I decreases I, while the scattered part of J augments I (see Fig. 1). For J, conversely: the absorbed and the scattered part of J decreases J, and the scattered part of I increases J (see eq 2). It should be noted that the signs in eqs 1 and 2 have been chosen consistent with the fact that as x increases I decreases, while J increases. The diffuse reflectance is R = J0 /I0 (3) where I0 is the incident flux and J0 the reflected flux at the surface (x = 0). Solving eqs 1 and 2 for a layer so thick that any further increase in thickness does not affect R and
J(x) = R(x) = R I0exp[{(K 2 + 2SK )1/2Sx]
1–R 2R
F(R)total =
2
= F(R) = K S
Σi Ki + Kg
/S = Σ F (R)i + F (R)g i
(4)
(5)
(6)
where i refers to the absorbing species and g to the substrate.
(1)
where K and S are absorption and scattering coefficients, respectively. Coefficients K and S are linearly dependent on the fraction of the light absorbed and scattered, respectively, per unit path length in the sample. J is a generated light flux
Figure 1. Representation of a layer of light-absorbing and lightscattering particles.
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Reference beam
M3
(a)
(b)
M3
Reference beam
M2
M2 0.20 mm cuvette
Powder sample Sample beam
Standard reflecting plate
Sample beam Standard reflecting plates
M1
M1
Figure 2. Geometry for measuring (a) diffuse reflectance of a solid and (b) suspension absorbance.
Absorbance Measurement in Suspensions of Solids in Liquids Using a modified integrating sphere assembly, the absorbance of suspensions may be determined (1, 11). A 0.20-mm cell is placed in the sample position of the integrating sphere with a BaSO 4 standard reflecting plate behind. When the cell is filled with the solvent (blank), the intensity of the signal coming from the sample I1′ will be I1′ = I0 – 2Iabs
(7)
where I0 is the incident light and Iabs is the light absorbed by the solvent. The factor 2 takes into account that the incident beam is reflected by the BaSO4 plate, passing through the sample twice. It is important to use a thin cell to avoid losses from the sides. When the cell is filled with the suspension, the intensity of the beam coming from the sample I2′ will be I2′ = I0 – 2Iabs – 2Iabs(sus)
(8)
where Iabs(sus) is the light absorbed by the particles in the suspension. The instrument response when the measuring mode is set to “Absorbance” will then be A1 = {log[(I0 – 2Iabs)/I0]
(9)
A2 = {log[(I0 – 2Iabs – 2Iabs(sus))/I0]
(10)
for the solvent and for the suspension, respectively. The real absorbance spectrum of the particles in suspension is by definition
A = { log I 0 – I abs(sus) /I 0 = { log 1 –
I abs(sus) I0
{A 1
10
(11)
{A 2
– 10 2
(12)
For colored materials suspended in liquids (such as dyes or pigments adsorbed or linked to solid substrates), A1 may be obtained with a suspension of the support (free of dye) instead of pure solvent. The absorbance value calculated from 1552
Experimental Procedure The absorbance spectrum for a piece of green glass and another for a colorless glass (standard optically transparent glass) were obtained with a Shimadzu UV-3101 PC spectrophotometer in the transmittance mode. Air was used in the pass of the reference beam. The pieces of glass were separately ground (10 µ m) and reflectance spectra of the powders were obtained. Diffuse reflectance measurements were carried out with the same spectrophotometer but it was equipped with the Shimadzu integrating sphere assembly1 (see Fig. 2a for the measuring geometry). BaSO4 was used as the 100% reflectance standard. The green glass powder was then suspended in water2 (50 mg/mL) and the suspension was placed in a 0.2-mm quartz cell. Instrument response in the absorbance mode (A2) was recorded using the reflectance attachment and following the geometry depicted in Figure 2b. The procedure was repeated to measure the instrument response for the suspension of colorless glass (A1). Any other kind of colored glass including optical filters may be used for the demonstration. Grinding was performed in a porcelain mortar. CAUTION: Protecting goggles for the eyes are recommended during this operation to prevent accidents. Results
and it may be calculated from eqs 9 and 10 to yield
A = { log 1 –
eq 12 will then represent the dye or pigment absorbance without taking into account the contribution of the solid support. In this case care should be taken to assure that the same mass of solid is suspended in a known volume of liquid both for the colored and for the uncolored suspension.
Absorbance spectra for the transparent pieces of colorless and green glass are shown in Figure 3. Reflectance spectra of the powdered glasses were mathematically treated to obtain the remission function, F(R), as a function of wavelength according to eq 5. F (R) for the colored material in the glass was obtained by subtracting to the total value, F(R) for the colorless glass according to eq 6. The results are shown in Figure 4. From the instrument response of the suspension measurements (A 1 and A 2) the particles’ absorbance was calculated from eq 12 and the real absorbance spectrum for the colored material in the suspension was obtained.
Journal of Chemical Education • Vol. 76 No. 11 November 1999 • JChemEd.chem.wisc.edu
In the Laboratory
Figure 3. Absorbance spectra for the transparent pieces of (- - -) colorless and (––) green glass and (––) difference spectrum (green minus colorless).
Figure 4. Remission function as a function of wavelength for the solid powdered glasses: (- - -) colorless glass, (––) green glass, (––) difference spectrum (green minus colorless).
Figure 5. Suspension absorbance calculated by using eq 12.
Figure 6. (s ) Remission function for the powdered green glass at given wavelengths (obtained from spectrum in bold in Fig. 4) and (u) suspension absorbance at given wavelengths (obtained from Fig. 5) as a function of the absorbance of the transparent green glass at the same wavelengths (obtained from spectrum in bold in Fig. 3).
Discussion
the glass (Fig. 3) if the magnitude calculated by eq 12 is actually proportional to the number of absorbing species. In Figure 6, the remission function of the solid optically thick sample (obtained from eq 6) and the suspension absorbance (calculated from eq 12) are plotted as a function of absorbance of the green glass (obtained by transmission). Because it was desired to plot all the data in the same graph and the scale ranges differed, the data were normalized to the value at 450 nm. In both cases, the data are fitted to a straight line passing through the origin, in good agreement with theory. It should be emphasized that in the three cases the absorbance of the colored material without the contribution of the solid support is analyzed. Using any colorless glass to subtract the solid support contribution is clearly an approxi-
From comparison of Figures 3, 4, and 5 students will note that absorption maxima appear at the same wavelengths for the three cases. They should also correlate the observed absorption minimum around 550 nm with the green color of the glass. As stated by eq 5, the remission function obtained by the Kubelka–Munk theory for an optically thick sample is expected to be linearly dependent on the absorption coefficient. So, for a given concentration of absorbing species, a linear dependence between F(R) (Fig. 4) and the absorbance spectrum of the glass obtained by transmission (Fig. 3) should be obtained. An analogous linear dependence is expected between the suspension absorbance (Fig. 5) and the absorbance spectrum of
JChemEd.chem.wisc.edu • Vol. 76 No. 11 November 1999 • Journal of Chemical Education
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mation but it works well to illustrate the objective of this lab experiment. Instead of the presentation in Figure 6, students may represent the three normalized spectra obtained for whole glass, powdered glass, and suspension in the same graph. Complete superposition of the spectra will confirm that both the remission function (calculated for the powder) and the suspension absorbance (from eq 12) are directly proportional to the chromophore absorption coefficient. Notes 1. The integrating sphere assembly for the standard Shimadzu UV3101 PC spectrophotometer may be provided as an accessory by Shimadzu at a cost of approximately $2300 U.S. 2. Other solvents may be used for the suspensions and similar results are expected. High-viscosity solvents such as Nujol or a
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poly(vinylalcohol) solution in water (10% w/w) may be good alternatives to obtain more stable suspensions.
Literature Cited 1. Serpone, N. J. Photochem. Photobiol. A: Chemistry 1997, 104, 1. 2. Vieira Ferreira, L. F., Freixo, M. R.; García, A. R.; Wilkinson, F. J. Chem. Soc. Faraday Trans. 1992, 88, 15. 3. Lagorio, M. G.; Dicelio, L. E.; Litter, M. I.; San Román, E J. Chem. Soc., Faraday Trans. 1998, 94, 419. 4. Scamehorn, R. J. Chem. Educ. 1995, 72, 566. 5. Raymond, K. W.; Corkill, J. A. J. Chem. Educ. 1994, 71, A204. 6. Frodyma, M. M.; Frei, R. W. J. Chem. Educ. 1969, 46, 522. 7. Wendlandt, W. W. J. Chem. Educ. 1968, 45, A861, A947. 8. Clark, R. J. H. J. Chem. Educ. 1964, 41, 488. 9. Lermond, C. A.; Rogers, L. B. J. Chem. Educ. 1955, 32, 92. 10. Wendlandt, W.; Hecht, H. G. Reflectance Spectroscopy ; Interscience: New York, 1966; Chapter III. 11. Sun, L.; Bolton J. R. J. Phys. Chem. 1995, 100, 4127.
Journal of Chemical Education • Vol. 76 No. 11 November 1999 • JChemEd.chem.wisc.edu
