Tala Alrashid
Student number :1006265391
Lab Section 0101
CHM110
Lab partner: Brea Maloney
Experiment 4:
Spectrophotometric Determination of the Dissociation Constant of
an Indicator
TA: Rodica Pecheanu
Date of lab: November 4
th
, 2019
Date submitted: November 18
th
, 2019
Introduction
A bromophenol blue indicator consists of a weak acid (or base) that indicates the pH of a
solution by exhibiting different colours (1). Its acid form, Hln, is shown by a yellow colour and
its basic form, ln

, is blue. According to Le Chatelier’s principle, the colour of the solution varies
on the concentration of hydronium ions (or the pH), meaning that the reaction will shift
depending on the concentration of products or rea ctants (2). For example, the equilibrium of
bromophenol blue will shift to the left and produce Hln if the solution is acidic, thus resulting in
a yellow colour. The equilibrium will shift to the right and produce ln

if the solution is basic,
thus resulting in a blue colour. This equilibrium can be represented by the following equation:
Hln (yellow)
+ H
2
O = ln

(blue) + H
3
O
+
The following equation represents the relationship between the pH and
the relative amounts of Hln and ln

, which is also known as the acid or base
dissociation constant.
Kln = [H
3
O
+
][ln

]/[Hln]
The previous equation can be further manipulated to obtain this
equation concerning the strength of the acid or base:
pK
ln
= pH + log[Hln]/ [ln

]
Purpose
The purpose of experiment 4 is to find the dissociation constant, Kln and pK
ln
value of the
bromophenol blue through spectrophotometer means. This technique refers to the Beer Lambert
law which stated that the quantity of a particular wavelength absorbed by a substance is directly
proportional to the concentration of the substance. Therefore, this experiment will use a
spectrophotometer to measure the concentration of Hln or ln

in a solution by determining the
amount/wavelength of light that was absorbed. The following equations will aid in determining
the desired numerical values
[HIn]/[In

] = (AA
ln

)/(A
Hln
 A)
pK
ln
= pH +log (A
Hln
 A)
Page 2 of 10
Experimental method
(refer to
CHM110 course manual book from page 7072)
1
Transfer 5mL of the ≈0.02% bromophenol blue solution in a 100mL flask. Add 0.05M
KHP, a pH 4.0 buffer solution to the flask to bring the total volume to exactly 100mL.
Mix the solution well.
2
Measure 50mL of the solution into two flasks labelled as A and B.
3
Measure the pH of both solutions using a pH meter, they should have the same pH. It is
important to note that the pH node must be cleaned with distilled water before usage.
For solution A:
1
Label 3 clean and dry vials A1, A2 and A3. Add KHP and 0.20M HCl according to the
table for each sample. Then measure the pH of the solution using the pH meter and the
absorbance of solution from 440 to 590nm.