BST 121-3 General Chemistry – Bachelor of Biosystems Technology
Exp 4 - Extraction of pH indicators from Plant Materials and
Characterization of their color changes using a pH meter
Introduction
Natural pigments extracted from various plant sources have historically served as valuable pH indicators.
These compounds exhibit a color change over specific pH ranges, enabling the visualization of acidic or
basic solutions . In this experiment, we focus on two commonly available plant materials: red cabbage
(Brassica oleracea) and Asian pigeonwings (Clitoria ternatea) . Our goal is to extract pH indicator dyes
from these plants and evaluate their color change properties across a range of pH values.
Red Cabbage (Brassica oleracea)
Red cabbage contains water-soluble anthocyanin pigments that act as natural pH indicators. These
pigments exhibit distinct color changes based on the pH of the solution.:
Asian Pigeonwings (Clitoria ternatea)
Asian pigeonwings, a flowering vine native to Southeast Asia, also contribute to our investigation. The
flower petals of Clitoria ternatea contain a pigment called clitorin, which serves as a natural pH indicator.
pH Range of Indicators and pKa Importance
Indicators function similarly to weak acids. We can represent a general indicator as HInd, where "Ind"
signifies the remaining molecule after releasing a hydrogen ion.
An equilibrium constant, Kind, describes this indicator's acidity, similar to Ka for a weak acid.
The color change of an indicator reaches its midpoint when the acid and its conjugate base have equal
concentrations. At this point, the Kind expression cancels out
Certainly! In a weak acid-strong base titration, the half-equivalence point is significant. At this stage, when
half of the initial acid has been neutralized, the solution’s pH directly corresponds to the weak acid’s pKa.
Experiment 3
1
, BST 121-3 General Chemistry – Bachelor of Biosystems Technology
This relationship simplifies pH calculations and highlights the buffering capacity during the titration.
The pH Range as a Rule of Thumb
As a rule of thumb, the visible color change of an indicator typically encompasses approximately 1 pH unit
on either side of its pKa value. For instance:
Litmus: pKa = 6.5, pH range: 5.0 - 8.0 (wider range)
Methyl orange: pKa = 3.7, pH range: 3.1 - 4.4
Phenolphthalein: pKa = 9.3, pH range: 8.3 - 10.0
While litmus provides a convenient choice for basic laboratory acid-base detection around neutral pH (pH
7), more precise determinations may require the use of methyl orange or phenolphthalein due to their
narrower and targeted pH ranges.
Experiment 3
2
Exp 4 - Extraction of pH indicators from Plant Materials and
Characterization of their color changes using a pH meter
Introduction
Natural pigments extracted from various plant sources have historically served as valuable pH indicators.
These compounds exhibit a color change over specific pH ranges, enabling the visualization of acidic or
basic solutions . In this experiment, we focus on two commonly available plant materials: red cabbage
(Brassica oleracea) and Asian pigeonwings (Clitoria ternatea) . Our goal is to extract pH indicator dyes
from these plants and evaluate their color change properties across a range of pH values.
Red Cabbage (Brassica oleracea)
Red cabbage contains water-soluble anthocyanin pigments that act as natural pH indicators. These
pigments exhibit distinct color changes based on the pH of the solution.:
Asian Pigeonwings (Clitoria ternatea)
Asian pigeonwings, a flowering vine native to Southeast Asia, also contribute to our investigation. The
flower petals of Clitoria ternatea contain a pigment called clitorin, which serves as a natural pH indicator.
pH Range of Indicators and pKa Importance
Indicators function similarly to weak acids. We can represent a general indicator as HInd, where "Ind"
signifies the remaining molecule after releasing a hydrogen ion.
An equilibrium constant, Kind, describes this indicator's acidity, similar to Ka for a weak acid.
The color change of an indicator reaches its midpoint when the acid and its conjugate base have equal
concentrations. At this point, the Kind expression cancels out
Certainly! In a weak acid-strong base titration, the half-equivalence point is significant. At this stage, when
half of the initial acid has been neutralized, the solution’s pH directly corresponds to the weak acid’s pKa.
Experiment 3
1
, BST 121-3 General Chemistry – Bachelor of Biosystems Technology
This relationship simplifies pH calculations and highlights the buffering capacity during the titration.
The pH Range as a Rule of Thumb
As a rule of thumb, the visible color change of an indicator typically encompasses approximately 1 pH unit
on either side of its pKa value. For instance:
Litmus: pKa = 6.5, pH range: 5.0 - 8.0 (wider range)
Methyl orange: pKa = 3.7, pH range: 3.1 - 4.4
Phenolphthalein: pKa = 9.3, pH range: 8.3 - 10.0
While litmus provides a convenient choice for basic laboratory acid-base detection around neutral pH (pH
7), more precise determinations may require the use of methyl orange or phenolphthalein due to their
narrower and targeted pH ranges.
Experiment 3
2
