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1 October 2019
Spectrometric Determination of the Acid Dissociation Constant of an Acid-base Indicator
The primary goal of this experiment is to determine the dissociation constant Ka of a particular
acid-base indicator through the use of a spectrometer. Through this, a better understanding of
beer’s law is achieved. Using U-V spectroscopy the absorbance is measured of the light passing
through the solution with a wavelength between 200 and 800nm which can then be used for
qualitative analysis for the determination of alpha fractions as well as the Ka species. The
indicator that was chosen in this case was the bromophenol blue which was analyzed both for
high absorbance and low absorbance at 430 and 565 nm.
With the main aim being to study the equilibrium constant the following equation is responsible
in this case.
Equation 1 main reaction
The absorbance is calculated through the use of beers formula as shown below
A = εbc
ε = molar absorptivity coefficient in M-1 cm-1
b = path length
c = analyte molar concentration
having the concentrated calculations, the equilibrium constant can be computed from this.
To start with, the standard solutions required for the experiment were prepared. A master
standard Hln solution is created with the concentration of 2.985.10^5 I 25 Ml. From this, 10 mL
was set aside to be used in the experiments. In addition, a std2 solution is prepared with the use
of 3ML of standard one solution together with 1ML of 0.10M HCL with the use of micropipette.
Then a std3 was prepared that involved mixing 2ML of HCL, 0.1M HCL and 1ML of std1
solution. Then std4 solution was prepared that involved mixing of 3ML, 0.1M HCl with 1.00 Ml
std1 solution. Finally, std5 was prepared that involved mixing 3ML, 0.1M HCl with 1ml of std1
In a similar manner a master solution for Ln- was created with a solution of 25ML with a
concentration of 2.985.10^5M. To start with, about 10ML of the master solution was set aside to
be used for the other parts of the experiments (PH experiment). Std2 was prepared through
mixing of 1ML of diluted buffer 7 with 3.0ML of std1 with the help of a micropipette. Std3 was
then prepared through mixing of 2 ml of diluted buffer 4 and 2 Ml of std1. In addition, std4 was
prepared through a mix of 3Ml of diluted buffer 4 and 1 Ml of std1. Finally, std5 was prepared
through mixing of 3Ml of diluted buffer 4 and 1 Ml std1 solution.
After this the experiment was conducted with the use of PH probe and a UV spectrometer. In this
case, the absorbance of 10 solutions was measured with the use of the spectrometer. In addition,
the intensive of the particular wavelengths at 565 nm. After this std1 solution is used for the next
potions whose PH was recorded after the solution had stabilized in addition to its absorbance
being recorded. Then the PH was adjusted with about 0.1M HCl units with absorbance measured
at 430 and 565 nm again. This was then repeated until a total of 12 sets were obtained.
Obtained results and discussion
With the use of Beer’s law plot, the concentration of the deprotonated and protonated species
was determined. A system of two equations is produced from the absorbance of the two species
with their respective wavelengths calculated. The data and graph used can be seen from the
figure and table below respectively.
From these results te beer plots are obtained.
Beers Law Plot
y = 74954x - 0.201
R² = 0.9775
y = 136...
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