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Spectrophotometric Determination of pKa Lab Report

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Experiment No. 5
Spectrophotometric Determination of pKa
Introduction
Spectrophotometry is the measurement of absorption of radiation in a particular
spectral region [1]. A spectrophotometer consists of spectrometer for producing light of
any selected wavelength and a photometer for measuring light intensity (amount of
photons). Spectrophotometry is used in this experiment to measure the amount of light
absorbed as a function of wavelength [2].
The Beer-Lambert law or Beer’s law states that the amount of attenuation depends
on the concentration of the absorbing molecules and the path length which absorption
occurs. [3] According to this law, absorbance is directly proportional to the concentration
of the solution (c), and to the path length (b), which is equal to the length of the cuvette.
[4]
Acid/base indicators are weak acids or bases. When dissolved in water they
dissociate and form ions. Consider an acid type indicator, HIn. Its behavior can be
described by the equilibrium equation:
HIn + H
2
O In
-
+ H
3
O
+
equation (1)
The equilibrium-constant expression for the dissociation of an acid-type indicator,
K
a
, is expressed as:



equation (2)
Getting the logarithm of both sides of equation (2) and using the definition of pH
results to:
   


equation (3)

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The ratio of [In
-
] and [HIn] may be determined spectrophotometrically because
these two forms of indicator absorb strongly in the visible range. [3]
This experiment aims to determine the equilibrium constant of the dissociation of
an indicator in water experimentally using spectroscopic method and to demonstrate the
principle of equilibrium and the interaction of matter with light.
Methodology
The following were the materials used in this experiment: spectrophotometer, 50
mL 0.1 M NaH
2
PO
4
solution, 50 mL 0.1 M Na
2
HPO
4
solution, 50 mL 0.1 M NaOH solution,
concentrated HCl, 0.05% methyl red indicator, distilled water, 25-mL beaker (12 pcs), 1-
mL graduated pipette (2 pcs), 10-mL graduated pipette, 25-mL volumetric flask, (12)
potentiometer, suction bulb, graduated cylinder, wash bottle, and tissue.
For this experiment, the 0.1 M Na
2
HPO
4
solution, 0.1 M NaH
2
PO
4
solution and 0.1
M NaOH solution were prepared first using the calculations done before the experiment
proper. Afterwards, ten solutions covering the pH range from 1 to 13 were prepared using
the quantities indicated in Table 1.
Solution 1 and 10 were prepared first. Solution 1 was prepared by mixing 1.0 mL
of indicator, 4.0 mL of distilled water, and 0.2 mL of concentrated HCl. The mixture was
diluted to 25 mL. To prepare solution 10, 1.0 mL of indicator and 24.0 mL of NaOH
solution were mixed. Solutions 2 to 9 were prepared by mixing volumes of 0.1 M NaH
2
PO
4
solution and 0.1 M Na
2
HPO
4
solution shown in Table 1, then they were diluted to 25.0
mL. Figure 1 shows solutions 1 to 10.
Table 1. Composition of the solutions
Solution
No.
Vol. of Indicator
(mL)
Vol. of NaH
2
PO
4
(mL)
Vol. of Na
2
HPO
4
(mL)
pH
1
1
0
0
1
2
1
5
0
2.92

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Experiment No. 5 Spectrophotometric Determination of pKa Introduction Spectrophotometry is the measurement of absorption of radiation in a particular spectral region [1]. A spectrophotometer consists of spectrometer for producing light of any selected wavelength and a photometer for measuring light intensity (amount of photons). Spectrophotometry is used in this experiment to measure the amount of light absorbed as a function of wavelength [2]. The Beer-Lambert law or Beer’s law states that the amount of attenuation depends on the concentration of the absorbing molecules and the path length which absorption occurs. [3] According to this law, absorbance is directly proportional to the concentration of the solution (c), and to the path length (b), which is equal to the length of the cuvette. [4] Acid/base indicators are weak acids or bases. When dissolved in water they dissociate and form ions. Consider an acid type indicator, HIn. Its behavior can be described by the equilibrium equation: HIn + H2O ⇄ In- + H3O+ equation (1) The equilibrium-constant expression for the dissociation of an acid-type indicator, Ka, is expressed as: 𝐾𝑎 = [𝐻3 𝑂 + ][𝐼𝑛− ] [𝐻𝐼𝑛] equation (2) Getting the logarithm of both sides of equation (2) and using the definition of pH results to: 𝐼𝑛− 𝑝𝐾 = 𝑝𝐻 − 𝑙𝑜𝑔 𝐻𝑙𝑛 equation (3) The ratio of [In-] and [HIn] may be determined spectrophotometrically because these two forms of indic ...
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