DEPTH STUDY PART A: RESEARCH
1. Describe the principle of colourimetry.
Colourimetry is a quantitative technique used for determining the concentration of a
chemical in a solution using its colour and concentration in a solution. Colourimetry only
focuses on one frequency usually in the visible spectrum (measures absorbance and
wavelength between 400 to 700 nm), unlike ultraviolet-visible spectrophotometry. Objects
appear as colours due to
the light that has been
absorbed by that object. If
white light (containing all
wavelengths of the visible
spectrum) was incident on
an object, this object will absorb the complementary colour of its appearance, and thus,
when reflected, will show proportionately higher intensities of the wavelength of light that
the object appears to be. For example, a blue object will absorb its complementary colour
(orange), and thus will reflect the other colours (particularly blue), hence it appears blue to
our eyes. The analysis technique of colorimetry applies this concept. If a blue solution (like
in copper (II) sulfate pentahydrate) has orange light incident on it (when an orange filter is
applied), it will absorb a large percentage of that light, and only very little can pass through
to the detector.
Light travels from the
Tungsten Lamp (which emits
all wavelengths of visible
light), which then passes
through a slit. The stream of
white light passes through a
colour filter (which is the complementary colour of the solution) and into the cuvette. As it
passes through the cuvette, light is absorbed by the solution and the remaining light travels
through and is incident on the photometer. The photons of light are detected and amplified
to create an intensity signal. The colorimeter then works out the absorbance of the sample.
Colourimetry enables the concentration of particular species to be identified using the fact
that the colour intensity is proportional to concentration – the higher the concentration the
more intense the colour. The instrument used for colourimetry is the colourimeter, which
measures the amount of a specific wavelength of light absorbed by the chemical being
analysed.
, The principle of the colorimeter works according to the Beer-Lambert law, which states the
quantitative relationship between absorbance and concentration:
A = εlc
where:
- A: Absorbance - It does not have units since it is a
𝑙𝑜𝑔10 of the ratio between the light entering and light
exiting the solution.
- ε: Molar absorptivity - It is a constant for a particular
chemical. It is a measure of the amount of light that a
solution containing that chemical absorbs per unit of
concentration. The units for molar absorptivity are
𝐿𝑚𝑜𝑙−1 𝑐𝑚−1 .
- l: The path length of the light through the sample -
The length of the cuvette (sample holder), which is measured in cm. Usually, the
length is 1cm.
- c: The concentration of the solution - The units for concentration are 𝑚𝑜𝑙𝐿−1 .
Since ε and l are constants for an investigation, the Beer–Lambert law indicates that there is
a direct relationship between absorbance and concentration:
Aαc
2. Describe alternative methods used to measure concentration.
Titration
Titration is a method used to measure the unknown concentration of a solution
(titrand/analyte) that reacts with a known concentration of another solution
(titrant/reagent). The slow addition of the titrant to a known volume of another solution of
unknown concentration is conducted until the reaction reaches the equivalence point,
which is often indicated by a color change. Titration is the main technique used for
volumetric analysis, which involves determining the concentration of a sample by measuring
the volume of this sample that reacts with a known volume of other substances of known
concentration. The titration process also involves using specialist glassware like volumetric
flasks, volumetric pipette and a burette.
1. Describe the principle of colourimetry.
Colourimetry is a quantitative technique used for determining the concentration of a
chemical in a solution using its colour and concentration in a solution. Colourimetry only
focuses on one frequency usually in the visible spectrum (measures absorbance and
wavelength between 400 to 700 nm), unlike ultraviolet-visible spectrophotometry. Objects
appear as colours due to
the light that has been
absorbed by that object. If
white light (containing all
wavelengths of the visible
spectrum) was incident on
an object, this object will absorb the complementary colour of its appearance, and thus,
when reflected, will show proportionately higher intensities of the wavelength of light that
the object appears to be. For example, a blue object will absorb its complementary colour
(orange), and thus will reflect the other colours (particularly blue), hence it appears blue to
our eyes. The analysis technique of colorimetry applies this concept. If a blue solution (like
in copper (II) sulfate pentahydrate) has orange light incident on it (when an orange filter is
applied), it will absorb a large percentage of that light, and only very little can pass through
to the detector.
Light travels from the
Tungsten Lamp (which emits
all wavelengths of visible
light), which then passes
through a slit. The stream of
white light passes through a
colour filter (which is the complementary colour of the solution) and into the cuvette. As it
passes through the cuvette, light is absorbed by the solution and the remaining light travels
through and is incident on the photometer. The photons of light are detected and amplified
to create an intensity signal. The colorimeter then works out the absorbance of the sample.
Colourimetry enables the concentration of particular species to be identified using the fact
that the colour intensity is proportional to concentration – the higher the concentration the
more intense the colour. The instrument used for colourimetry is the colourimeter, which
measures the amount of a specific wavelength of light absorbed by the chemical being
analysed.
, The principle of the colorimeter works according to the Beer-Lambert law, which states the
quantitative relationship between absorbance and concentration:
A = εlc
where:
- A: Absorbance - It does not have units since it is a
𝑙𝑜𝑔10 of the ratio between the light entering and light
exiting the solution.
- ε: Molar absorptivity - It is a constant for a particular
chemical. It is a measure of the amount of light that a
solution containing that chemical absorbs per unit of
concentration. The units for molar absorptivity are
𝐿𝑚𝑜𝑙−1 𝑐𝑚−1 .
- l: The path length of the light through the sample -
The length of the cuvette (sample holder), which is measured in cm. Usually, the
length is 1cm.
- c: The concentration of the solution - The units for concentration are 𝑚𝑜𝑙𝐿−1 .
Since ε and l are constants for an investigation, the Beer–Lambert law indicates that there is
a direct relationship between absorbance and concentration:
Aαc
2. Describe alternative methods used to measure concentration.
Titration
Titration is a method used to measure the unknown concentration of a solution
(titrand/analyte) that reacts with a known concentration of another solution
(titrant/reagent). The slow addition of the titrant to a known volume of another solution of
unknown concentration is conducted until the reaction reaches the equivalence point,
which is often indicated by a color change. Titration is the main technique used for
volumetric analysis, which involves determining the concentration of a sample by measuring
the volume of this sample that reacts with a known volume of other substances of known
concentration. The titration process also involves using specialist glassware like volumetric
flasks, volumetric pipette and a burette.
