Acid Ionization Constant (K a ) of an Indicator - Quantum - East ...

Acid Ionization Constant (K a ) of an Indicator
2014, Sharmaine S. Cady
East Stroudsburg University
Skills to build:
Using a micropipette
Using acid-base equilbria principles
Preparing buffers with specific pH values
Using spectrophotometry to determine K a
Introduction
Indicators are usually weak organic acids and bases that are partially ionized in
water. If HIn represents the general formula for an indicator, then the ionization reaction
is
HIn(aq) + H 2 O(l)
H 3 O + (aq) + In - (aq)
According to Le Châtelier's principle, the above equilibrium shifts from left to right (or
right to left) as the pH of the solution changes. If the pH of a solution containing HIn is
increased (increase in base concentration), the H 3 O + concentration decreases and the
equilibrium shifts to the right. The solution will change from the color of HIn to the color
of In - . The opposite is true if the pH of the solution is decreased.
Bromocresol purple (1) and its sodium salts (2, 3) are indicator dyes that are
yellow below a pH of 5.2 (In - pure present) and purple above a pH of 6.8 (pure In 2-
present). Between 5.2 and 6.8 (transition range), both HIn and In - are present at various
concentrations. The wavelength of maximum absorbance for bromocresol purple at pH
5.2 is 427-431 nm and 588-590 nm at pH 6.8.
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Acid Ionization Constant
The ionization reaction in aqueous solution can be written
Br
O
Br
O
CH 3
CH 3
H 3 C
H 3 C
HO
–
O
S
O
+ H2O H3O + +
–
O
–
O
S
O
Br
O
Br
O
yellow
purple
The equilibrium constant, or acid ionization constant (K a ), for an indicator is given
by the following expression
[ ][ ]
[ ]
This equation for the above reaction is
[ ][ ]
[ ]
The [H 3 O + ] can easily be measured with a pH meter. The ratio of [In - ]/[HIn] can be
determined spectrophotmetrically as long as the amount of total indicator remains
constant in all solutions used to measure absorbance. Under these conditions, the
following is true
[total indicator]=[HIn] + [In - ]
Let represent the fraction of the total indicator amount that is In - . We can then
write this fraction as
[In ]
[totalindicaotr]
This equation can be rearranged to give [In - ] = [total indicator].
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Acid Ionization Constant
If is the fraction that is In - , then the remaining fraction that is HIn is 1 - We
can then write its fraction as
[HIn]
1
[totalindicaotr]
This equation rearranges to give [HIn] = (1 - )[total indicator]. Substituting these values
for [In - ] and [HIn] into the equation for K a gives
[totalindicator]
[H
(1
)[totalindicator]
Ka
3
Cancellation of the [total indicator] terms yields
[H
(1
)
K a
3
O
]
O
]
We measure the [H 3 O + ] with a pH meter. We only need the value for to solve for K a .
We will use visible spectrophotometry to determine .
At very high pH, the ionization equilibrium shifts far to the right and we can
assume that the entire indicator is in its In - form. At high pH, [total indicator] = [In - ] pure .
Absorbance is directly proportional to the concentration of In - , therefore, we can
substitute A pure for [In - ] pure . At pH values in the transition range, [In - ] is a fraction () of
[In - ] pure , or [In - ] = [In - ] pure . Absorbance values (A) in this range measure [In - ]. This can
be represented in equation form as
[In ] A
[In ]
pure
A pure
You will prepare five solutions of bromocresol purple at different pH values and measure
their absobances. Solution 1 will have a value one pH unit above the transition range.
Solutions 2-5 will have pH values that span the center of the transition range. You
should work out these pH values and record them in your notebook prior to class. To
see the results for the indicator dye phenolphthalein, read the sample experiment
problem.
3D Molecules
Go to the web site and click on the Bromocresol Purple molecule. Use the
radio buttons to show different views and properties of each molecule. Answer the
questions based on the manipulation of the molecules.
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Acid Ionization Constant
Experimental Methods and Materials
Safety considerations
Wear suitable protective clothing, gloves, and eye/face protection!
You should read the online MSDS for:
Ammonium Chloride Phosphoric Acid Sodium Hydroxide
Bromocresol Purple
Sodium Citrate
Preparation of solutions
Label 5 clean, dry stoppered Erlenmeyer flasks. Obtain 50 mL of the stock
buffer in a beaker. The buffer contains sodium citrate, phosphoric acid, and ammonium
chloride dissolved in water. Measure the pH. Add 0.1 M NaOH to adjust its pH to the
lowest value you selected in the transition range. Micropipette 5.0000 mL of this buffer
solution into the flask labeled Solution 5. Adjust the remaining buffer solution in the
beaker to the next lowest pH value with NaOH. Micropipette 5.0000 mL into the flask
labeled Solution 4. Continue to prepare the next three solutions in a similar manner.
Make sure your record the exact pH of each solution. With a micropipette, add 130 L
of bromocresol purple solution to each flask.
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Acid Ionization Constant
Calibrating the Vernier Spectrophotometer
Click on Logger Pro from the Programs list on the computer. Click on
Experiment Calibrate Spectrometer on the menu bar. Allow the spectrometer to
warm up for 3 minutes. Place a cuvette ¾ full with distilled water in the cuvette holder.
Click Finish Calibration. Once the OK button is ready, click it.
Collecting data for
Fill a cuvette ¾ full with Solution 1. Place in the cuvette holder. Click on the
Configure Spectrometer Data Collection icon on the right-hand side of the menu
bar. Click Abs. vs. Concentration under Set Collection Mode. Click the OK button to
close the dialog box. Check that the wavelength for absorbance shown along the y-axis
is a close match to max for bromocresol purple. If not, recalibrate. This absorbance is
A pure .
Click on the Collect button at the top right of the menu bar. Record the
absorbance shown in red inside the box below the data table. Hit the Stop button.
Insert Solution 2 into the cuvetter holder. Hit Collect. Record the absorbance value.
Hit Stop. Repeat absorbance data collection for Solutions 3-5.
Laboratory Report
The following should be part of your final report:
Determine for Solutions 2-5.
Determine [H 3 O + ] from the pH for Solutions 2-5.
Calculate K a for Solutions 2-5. Calculate an average K a . Calculate the standard
deviation for K a . Both the average and standard deviation may be done in Excel.
Compare your K a to the accepted literature value. Explain any significant error.
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