Examples of UV-Vis Spectrum Discussion | Relationship Between Absorption Maximum, Concentration, and Electronic Transitions

What Is a UV-Vis Spectrum?

A UV-Vis spectrum is a graph showing how much light at each wavelength is absorbed when a sample is irradiated with ultraviolet or visible light.
Wavelength is plotted on the horizontal axis, while absorbance or transmittance is plotted on the vertical axis.
When a substance absorbs light of a specific wavelength, electrons transition from a lower-energy state to a higher-energy state.

Wavelengths at which strong absorption appears contain information about the electronic state and molecular structure of the substance.
Therefore, UV-Vis spectra are used for concentration determination, confirmation of complexes, evaluation of conjugated systems, monitoring of reaction progress, explanation of the causes of color, and other purposes.

Main Items to Include in the Results

In UV-Vis spectrum results, organize the measured sample, solvent, concentration, cell path length, measurement wavelength range, absorption-maximum wavelength, absorbance, calibration curve, quantitative value, and other information.
The absorption-maximum wavelength and absorbance often form the central part of the discussion.
In quantitative experiments, a calibration curve is prepared from the absorbance and concentration of standard solutions, and the concentration of an unknown sample is determined.

Main Items to Include in the Results

• Sample name
• Analyte
• Solvent
• Sample concentration
• Dilution factor
• Cell path length
• Blank solution
• Measurement wavelength range
• Absorption-maximum wavelength
• Maximum absorbance
• Spectral shape
• Peak width
• Absorbance of standard solutions
• Calibration-curve equation
• Correlation coefficient
• Concentration of the unknown sample
• Comparison with literature values and labeled values
• Sources of error and points for improvement

Reference Experimental Values and Analysis Examples for UV-Vis Spectra

Here, reference experimental values that can be used to discuss absorption-maximum wavelength, absorbance, concentration dependence, calibration curves, and electronic transitions obtained from ultraviolet-visible absorption spectra (UV-Vis) are organized.

UV-Vis measurement is used to investigate which wavelengths of light a sample absorbs.
The absorption-maximum wavelength is related to the electronic structure of the substance and the length of its conjugated system, while absorbance can be used for quantitative analysis within a range where it is proportional to concentration.
However, high concentration, turbidity, solvent, pH, and insufficient blank correction may cause shifts in the absorption spectrum or calibration curve.

Reference Experimental Conditions

Main Items Examined in a UV-Vis Spectrum

Examples of Absorption Spectra of Representative Substances

Reference examples of absorption-maximum wavelengths and absorbance are shown for representative colored substances.

The apparent color of a sample is not the color of the absorbed light, but mainly the color of the light that is transmitted or reflected and reaches the eye.
For example, blue methylene blue strongly absorbs light near the red region, so the remaining blue light is visible.

Example Absorption Spectrum of Potassium Permanganate

A reference example is shown in which the absorbance of an aqueous potassium permanganate solution was measured from 300 to 700 nm.

In this reference example, absorbance is greatest near 525 nm, so the absorption-maximum wavelength of potassium permanganate can be read as 525 nm.

Why Measurement Is Performed at the Absorption-Maximum Wavelength

In quantitative analysis, absorbance is normally measured near the absorption-maximum wavelength.
At the absorption maximum, absorbance is high and sensitivity to concentration changes becomes high.

If measurement is performed at a wavelength away from the absorption maximum, absorbance becomes smaller and the difference in measured values becomes smaller even for the same concentration difference.

Beer-Lambert Law

Absorbance is proportional to concentration and optical path length.

A = εcl

Here, A is absorbance, ε is the molar absorption coefficient, c is concentration, and l is the optical path length.
When a cell with an optical path length of 1 cm is used, doubling the concentration approximately doubles the absorbance within the linear range.

Relationship Between Concentration and Absorbance

A reference example is shown in which potassium permanganate standard solutions were measured at 525 nm.

In this reference example, concentration and absorbance are proportional, and the calibration curve can be expressed as follows.

Absorbance = 31000 × Concentration (mol/L)

Example Calculation of Unknown-Sample Concentration

If the absorbance of unknown sample A at 525 nm is 0.496, the concentration is determined using the calibration curve.

Concentration = Absorbance ÷ 31000

Concentration = 0.496 ÷ 31000 = 1.60×10−5 mol/L

Therefore, the potassium permanganate concentration in unknown sample A is determined to be 1.60×10−5 mol/L.

Example Calculation Including Dilution Factor

If the sample is diluted before measurement, the concentration in the measured solution determined from the calibration curve is multiplied by the dilution factor to obtain the concentration in the original sample.

For samples measured after dilution, forgetting to include the dilution factor in the calculation results in underestimation of the concentration in the original sample.

Deviation From the Calibration Curve at High Concentrations

The Beer-Lambert law holds well in low- to medium-concentration ranges, but the proportional relationship may break down at high concentrations.

If absorbance becomes too high, linearity decreases because of effects such as stray light, intermolecular interactions, and concentration effects.
For quantitation, the sample is diluted so that the absorbance falls within an appropriate range.

Example Absorption Spectrum of Methylene Blue

Aqueous methylene blue solutions show strong absorption in the visible region.
The absorption near 665 nm is related to electronic transitions in a dye having an extended conjugated system.

In dyes such as methylene blue, strong absorption appears in the visible-light region because of electronic transitions originating from the extended conjugated system.

Relationship Between Electronic Transitions and Absorption Wavelength

As the conjugated system becomes longer, the energy required for electronic transition becomes smaller and absorption tends to shift toward longer wavelengths.

Changes in Absorption Maximum With Conjugation Length

As the conjugated double-bond system becomes longer, the absorption-maximum wavelength tends to shift toward longer wavelengths.

When absorption enters the visible-light region, the substance begins to appear colored.
Dyes show strong colors because they absorb visible light through conjugated systems or charge-transfer transitions.

Changes in the Absorption Spectrum With pH

In substances such as acid-base indicators, molecular structure and ionization state change with pH, and the absorption-maximum wavelength may also change.
Reference data using methyl orange as an example are shown below.

If the absorption maximum changes with pH, failing to keep the sample pH constant causes absorbance and color to change even at the same concentration.

Changes in the Absorption Spectrum Caused by the Solvent

The absorption-maximum wavelength and absorbance may change depending on solvent polarity and hydrogen-bonding ability.

When the solvent changes, the ways in which the ground state and excited state are stabilized change, and the absorption-maximum wavelength may shift.

Example Absorption Spectrum of a Dye Mixture

In mixed samples, multiple absorption peaks may be observed overlapping one another.

If multiple absorption maxima are observed, the sample may contain multiple dyes rather than a single component.

Baseline Increase Caused by Turbidity

If a sample is turbid, apparent absorbance may become high because of scattering rather than absorption.

If absorbance remains high on the long-wavelength side where almost no absorption should occur, an elevated baseline caused by turbidity or scattering is suspected.

Example of Blank Correction

If the solvent or reagents have absorption, the blank absorbance is subtracted for correction.

If blank correction is not performed, absorption not originating from the sample is also included in the concentration calculation, potentially causing the concentration to be overestimated.

Example of How to Write the Results

When the UV-Vis spectrum of an aqueous potassium permanganate solution was measured, an absorption maximum was observed near 525 nm.
When the absorbance of the standard solutions was measured at this wavelength, the absorbance increased proportionally as the concentration increased.
Therefore, using 525 nm as the quantitative wavelength was considered to allow highly sensitive measurement of concentration changes.

The calibration curve was “Absorbance = 31000 × Concentration.”
Because the corrected absorbance of unknown sample A was 0.496, the concentration was determined to be 0.496 ÷ 31000 = 1.60×10−5 mol/L.
If the sample was diluted before measurement, the concentration in the measured solution must be multiplied by the dilution factor to obtain the concentration in the original sample.

On the other hand, in high-concentration samples, the measured absorbance was lower than the absorbance expected from the concentration.
This was considered to be because the excessively high absorbance increased the effects of stray light and intermolecular interactions, resulting in deviation from the Beer-Lambert law.
Therefore, quantitation must be performed within the linear range of the calibration curve.

Points for Connecting the Results to the Discussion

In a discussion of UV-Vis spectra, it is important to explain the absorption-maximum wavelength, absorbance, concentration, electronic transitions, and measurement conditions in relation to one another.

• Can the absorption-maximum wavelength be read from the spectrum?
• Can the reason for measurement at the absorption-maximum wavelength be explained in relation to high sensitivity?
• Can it be explained using the Beer-Lambert law that absorbance is proportional to concentration?
• Can the concentration of the unknown sample be determined from the calibration curve?
• If the sample was diluted before measurement, has the dilution factor been correctly taken into account?
• Can deviation from linearity at high concentrations be explained in relation to stray light, intermolecular interactions, and concentration effects?
• Can absorption wavelength be discussed in relation to electronic transitions, conjugated systems, and charge-transfer transitions?
• Can it be explained that pH and solvent may change the absorption-maximum wavelength and absorbance?
• Can it be explained that turbidity and insufficient blank correction affect absorbance?

Example Discussion

In this experiment, the absorption spectrum of an aqueous potassium permanganate solution was measured using UV-Vis spectrophotometry.
As a result, an absorption maximum was observed near 525 nm.
Because permanganate ions strongly absorb in the visible-light region, they appear reddish purple.
Because absorbance is high and sensitivity to concentration changes is high at the absorption-maximum wavelength, 525 nm was used for quantitation.

In measurements of the standard solutions, absorbance was proportional to concentration in the range of 0.5×10−5 to 2.5×10−5 mol/L.
This indicates that the Beer-Lambert law was followed.
Substitution of the absorbance of unknown sample A, 0.496, into the calibration curve gave a concentration of 1.60×10−5 mol/L.

In high-concentration samples, the measured values were lower than the theoretically expected absorbance.
When absorbance becomes too high, the transmitted light becomes extremely weak and the relative effect of stray light becomes large.
In addition, at high concentrations, intermolecular interactions and association may change the absorption characteristics.
Therefore, in UV-Vis quantitation, it is important to perform measurements within a concentration range in which linearity has been confirmed.

Methylene blue showed strong absorption near 665 nm.
This was considered to be because the molecule contains an extended conjugated system and transitions of π electrons occur in the visible-light region.
In general, as the conjugated system becomes longer, the energy required for electronic transition decreases and absorption shifts toward longer wavelengths.
Therefore, dyes with absorption in the visible region appear colored to the naked eye.

Possible sources of error include insufficient blank correction, contamination of the cell, scattering caused by turbidity, excessively high sample concentration, and differences in pH and solvent.
Particularly in turbid samples, scattering rather than absorption raises the baseline and causes the apparent absorbance to increase.
In addition, for substances whose ionization state changes with pH, the absorption-maximum wavelength and absorbance may change even at the same concentration.
Therefore, the blank, cell, pH, solvent, and concentration range must be standardized during measurement.

Summary

In UV-Vis spectra, the electronic state of a substance and the cause of its color can be discussed from the absorption-maximum wavelength, and quantitative analysis can be performed using the proportional relationship between absorbance and concentration.

This reference example dealt with absorption-maximum wavelength, the Beer-Lambert law, calibration curves, unknown-sample concentration, electronic transitions, and the effects of pH, solvent, and turbidity using potassium permanganate, methylene blue, and food dyes as examples.
In a report, it is useful to discuss absorption wavelength, absorbance, concentration, electronic transitions, and measurement errors in relation to one another.

What Is an Absorption Maximum?

An absorption maximum is the wavelength at which absorbance is greatest in a UV-Vis spectrum.
It may also be written as λmax.
The absorption maximum is the wavelength at which a substance absorbs light most strongly and reflects the electronic state and structure of that substance.

The absorption-maximum wavelength changes depending on the conjugated system of the molecule, functional groups, coordination environment of complexes, solvent, pH, and other factors.
Therefore, comparing the position of the absorption maximum with literature values or standard substances makes it possible to confirm the target component and discuss structural changes.

What Is Absorbance?

Absorbance is a value representing how much light a sample absorbs.
When light passes through a sample, components in the sample absorb light and reduce the intensity of transmitted light.
The greater the absorbance, the more strongly the sample absorbed light at that wavelength.

Absorbance depends on the measurement wavelength, sample concentration, cell path length, and molar absorption coefficient.
When the same substance is measured at the same wavelength and with the same cell path length, higher concentration results in greater absorbance.
This relationship is used for quantitative concentration analysis by UV-Vis.

Relationship Between Concentration and Absorbance

The relationship between concentration and absorbance is extremely important in UV-Vis measurement.
When the same substance is measured at the same wavelength, increasing the concentration increases the number of molecules that absorb light, so absorbance becomes greater.
Within the range in which this relationship is linear, the concentration of an unknown sample can be determined using a calibration curve prepared from standard solutions.

A = εcl

Here, A is absorbance, ε is the molar absorption coefficient, c is concentration, and l is the cell path length.
If the cell path length is constant, absorbance is proportional to concentration.
However, at high concentrations, the relationship may deviate from linearity because of intermolecular interactions, scattering, detector limitations, and other factors.

What Is an Electronic Transition?

UV-Vis absorption occurs when electrons in molecules or ions absorb the energy of light and move from a lower-energy state to a higher-energy state.
This change is called an electronic transition.
In organic compounds, transitions related to π electrons and lone pairs are commonly observed.

Representative electronic transitions include π-π* transitions, n-π* transitions, d-d transitions, and charge-transfer transitions.
Which transition occurs depends on molecular structure, functional groups, metal ions, ligands, the length of the conjugated system, and other factors.

Discussion of π-π* Transitions

A π-π* transition occurs when π electrons in double bonds or aromatic rings transition to antibonding π* orbitals.
In conjugated double bonds and aromatic compounds, absorption caused by π-π* transitions may appear in the ultraviolet or visible region.
As the conjugated system becomes longer, the required energy generally becomes smaller and the absorption wavelength shifts toward longer wavelengths.

In dyes and organic materials, the length of the conjugated system is closely related to visible-light absorption.
Molecules with longer conjugated systems more readily absorb visible light, which may cause the substance to appear colored.

Discussion of n-π* Transitions

An n-π* transition is a phenomenon in which an electron transitions from a lone pair on an atom such as oxygen or nitrogen to a π* orbital.
It may be observed in compounds containing carbonyl groups, nitro groups, azo groups, and similar structures.
The absorption intensity may be weaker than that of a π-π* transition.

n-π* transitions may be readily affected by solvent polarity and hydrogen bonding.
Therefore, even for the same compound, changing the measurement solvent may change the absorption wavelength or absorption intensity.

Discussion of d-d Transitions

In transition-metal complexes, d-d transitions in which electrons transition between d orbitals of the metal ion may be observed.
The d orbitals split in energy under the influence of ligands, and light corresponding to the energy difference is absorbed.
If this absorption is in the visible region, the complex appears colored.

The absorption wavelength and intensity of d-d transitions change depending on the type of metal ion, oxidation state, type of ligand, coordination number, and structure of the complex.
If the ligand field is strong, the splitting of the d orbitals changes and the absorption wavelength also changes.

Discussion of Charge-Transfer Transitions

A charge-transfer transition is a transition in which an electron moves from one part of a molecule or complex to another.
In metal complexes, transitions may occur in which electrons move from a ligand to a metal or from a metal to a ligand.
Charge-transfer transitions may be observed as stronger absorption than d-d transitions.

Charge-transfer absorption may cause the color of a complex or strong absorption in the visible region.
The absorption position changes depending on the oxidation state of the metal and the electron-donating or electron-accepting properties of the ligand.

When the Absorption Maximum Shifts Toward Longer Wavelengths

A shift of the absorption maximum toward longer wavelengths is called a long-wavelength shift or red shift.
A long-wavelength shift means that the energy required for the electronic transition has become smaller.
Possible causes include expansion of the conjugated system, increased molecular planarity, solvent effects, complex formation, changes in pH, and substituent effects.

For example, when the conjugated double-bond system becomes longer, the energy difference for a π-π* transition becomes smaller and the absorption shifts toward longer wavelengths.
In dyes and conjugated polymers, this long-wavelength shift is related to changes in color.

When the Absorption Maximum Shifts Toward Shorter Wavelengths

A shift of the absorption maximum toward shorter wavelengths is called a short-wavelength shift or blue shift.
This means that the energy required for the electronic transition has increased.
Possible causes include shortening of the conjugated system, twisting of the molecule, protonation or deprotonation, solvent effects, and changes in the state of association.

If a molecule twists and conjugation becomes weaker, electrons become less delocalized and the transition energy may increase.
As a result, the absorption maximum shifts toward shorter wavelengths.

Discussion When Absorbance Becomes Larger

Causes of increased absorbance include a high sample concentration, a longer cell path length, a large molar absorption coefficient, progression of a color-development reaction, and stronger absorption caused by complex formation.
In quantitative experiments, it is fundamental that higher concentration results in greater absorbance.

However, care is required if the absorbance becomes too high.
If absorbance is excessively high, the transmitted light becomes extremely weak and measurement error from the instrument becomes larger.
In addition, at high concentrations, intermolecular interactions and scattering may cause deviation from the linearity of the calibration curve.

Discussion When Absorbance Becomes Smaller

Causes of decreased absorbance include low sample concentration, insufficient color development, decomposition of the target component, an inappropriate measurement wavelength, loss during sample preparation, and problems with cell orientation or blank correction.
Absorbance also becomes smaller if measurement is performed at a weakly absorbing wavelength rather than at the absorption maximum.

If absorbance is too small, the effect of noise becomes larger and quantitative accuracy decreases.
For low-concentration samples, concentration, optimization of the measurement wavelength, or an increase in the number of integrations may be considered.

Discussion of the Calibration Curve

In UV-Vis quantitation, standard solutions of known concentration are measured and a calibration curve is prepared by plotting the relationship between concentration and absorbance.
If the calibration curve shows linearity, concentration can be determined from absorbance within that range.
The absorbance of the unknown sample must fall within the range of the calibration curve.

The higher the correlation coefficient of the calibration curve, the closer the standard-solution data are to a straight line.
However, not only the correlation coefficient but also the size of the intercept, outliers, blank correction, and accuracy of standard-solution preparation must be checked.

Causes of Absorbance Deviating From the Straight Line

Causes of deviation from a linear relationship between concentration and absorbance include intermolecular interactions at high concentrations, excessively high absorbance, scattering, sample turbidity, incomplete color-development reactions, errors in standard-solution preparation, contamination of the cell, and insufficient blank correction.
Deviation from linearity is particularly likely on the high-concentration side.

If absorbance is too low, noise may also cause deviation from the straight line.
It is important to set the standard-solution concentrations so that absorbance falls within an appropriate range when preparing a calibration curve.

Discussion of Blank Correction

In UV-Vis measurement, a blank measurement is performed to remove absorption other than that of the sample.
The blank includes absorption from the solvent, reagents, cell, and other components other than the target component.
Blank correction makes it possible to determine absorbance originating from the target component more accurately.

If the blank is inappropriate, absorption originating from the solvent or reagents remains and the absorbance may be overestimated or underestimated.
Particularly for low-concentration samples, the relative effect of the blank becomes large.

Effect of the Cell

In UV-Vis measurement, the condition of the cell greatly affects the results.
Dirt, scratches, fingerprints, bubbles, and differences in cell orientation may cause errors in absorbance.
In addition, because absorbance changes if the cell path length differs, the same path length must be used for standard solutions and unknown samples.

Quartz cells are commonly used for measurement in the ultraviolet region, while glass and plastic cells have limitations on the wavelength ranges that can be measured.
It is important to select a cell appropriate for the wavelength being used.

Effects of Sample Turbidity and Scattering

If a sample is turbid, light is weakened not only by absorption but also by scattering.
Because the instrument detects a decrease in transmitted light as absorbance, the presence of scattering may cause the measured absorbance to be higher than the actual absorption.
Suspensions, precipitates, fine particles, and bubbles may cause scattering.

To measure absorbance correctly, treatments such as dissolving the sample thoroughly, filtration, centrifugation, and removing bubbles are necessary.
However, if the measurement target is the suspended particles themselves, the evaluation may include scattering.

Effect of the Solvent

UV-Vis spectra may be affected by the measurement solvent.
The absorption-maximum wavelength and absorbance may change depending on solvent polarity, hydrogen-bonding ability, pH, solubility, and transparency.
n-π* transitions and charge-transfer transitions may be particularly susceptible to solvent effects.

In addition, if the solvent itself absorbs at the measurement wavelength, accurate measurement cannot be performed.
When measuring in the ultraviolet region, attention must also be paid to the cutoff wavelength of the solvent used.

Effect of pH

In dyes and indicators with acid-base properties, complexes, proteins, phenolic compounds, and similar substances, the UV-Vis spectrum may change greatly with pH.
When pH changes, the protonation state and charge state of the molecule change, and the electronic state also changes.
As a result, the absorption-maximum wavelength and absorbance change.

The color of an indicator changes with pH because the protonated and deprotonated forms absorb at different wavelengths.
In a UV-Vis spectrum, this change can be observed as a shift in absorption peaks or a change in intensity.

Discussion of Complex Formation

When a metal ion and ligand form a complex, changes may appear in the UV-Vis spectrum.
This is because complex formation changes the splitting of the d orbitals of the metal ion and charge transfer between the metal and ligand.
As a result, a new absorption peak may appear, the absorption-maximum wavelength may shift, or absorbance may increase.

The color of a complex is closely related to absorption of visible light.
The observed color is often seen as the complementary color of the absorbed color.
UV-Vis spectra can be used to discuss whether a complex has formed and differences in the coordination environment.

Discussion of Color-Development Reactions

In UV-Vis quantitation, the target component may be reacted with a color-developing reagent and its concentration determined from the intensity of the color.
In a color-development reaction, the target component reacts with the reagent to form a compound or complex that absorbs in the visible region.
If color development does not proceed sufficiently, absorbance becomes low and the concentration may be underestimated.

Color-development reactions are affected by reaction time, pH, temperature, reagent concentration, and mixing conditions.
It is important to allow a fixed reaction time before measurement and use the same conditions for all samples.

Monitoring Reaction Progress

UV-Vis spectra can also be used to monitor reaction progress.
If reactants and products absorb at different wavelengths, absorbance or absorption peaks change with time.
If the reactant peak decreases and the product peak increases, this indicates that the reaction is progressing.

In reaction-rate experiments, changes in absorbance at a specific wavelength may be measured over time and analyzed as changes in concentration.
In this case, it is important to perform the measurement within a range where absorbance is proportional to concentration.

Discussion When the Peak Is Broad

Possible causes of a broad peak in a UV-Vis spectrum include overlapping multiple electronic transitions, involvement of molecular vibrational levels, interactions with the solvent, association of the sample, turbidity, and scattering.
Molecules in liquids have various vibrational and rotational states, so their absorption peaks are often not as sharp as those in IR or NMR.

In dyes, polymers, and complexes, intermolecular interactions and association states may broaden the peak or change its shape.
Changes in peak shape provide clues to changes in the state of the sample.

Discussion When Peaks Overlap

In samples containing multiple components, their absorption peaks may overlap.
If peaks overlap, it becomes difficult to accurately read the absorbance of only the target component.
Care is required for mixtures, samples containing impurities, and systems in which multiple complexes coexist.

To avoid overlapping peaks, a wavelength at which the target component absorbs most strongly and other components absorb only weakly must be selected.
In some cases, another analytical method or a separation procedure must also be used.

Comparison With Literature Values and Standard Substances

Absorption-maximum wavelengths and molar absorption coefficients can be compared with literature values and standard substances to evaluate sample identification and the validity of measurement results.
If the major absorption peaks are close to literature values, this supports the possibility that the target component is present.
On the other hand, if peak positions are shifted, the effects of solvent, pH, concentration, complex formation, decomposition, impurities, and other factors are considered.

When comparing with literature values, it is important to check whether the measurement conditions are the same.
Even for the same compound, the UV-Vis spectrum may change if the solvent or pH differs.

Causes of Quantitative Values Being Too High

Causes of UV-Vis quantitative values being higher than the actual value include insufficient blank correction, contamination of the cell, sample turbidity, absorption by interfering components, overlapping peaks, mistakes in calculating the dilution factor, and errors in standard-solution concentration.
Particularly if an interfering component absorbs at the measurement wavelength, its absorption is added to that of the target component and the concentration is overestimated.

Causes of Quantitative Values Being Too Low

Causes of quantitative values being lower than the actual value include loss during sample preparation, incomplete color-development reactions, decomposition of the target component, excessive dilution, incorrect selection of measurement wavelength, and absorbance being so low that it is strongly affected by noise.
If some of the target component remains in the apparatus during transfer of the sample, the measured value also becomes low.

In colorimetric analysis, insufficient color development may result from a reaction time that is too short, inappropriate pH, or insufficient reagent amount, causing absorbance to become low.

When UV-Vis Results Can Be Considered Good

UV-Vis measurements can be considered to have given good results when the absorption maximum is clearly observed, generally agrees with a standard substance or literature value, the calibration curve shows good linearity, and the absorbance of the unknown sample is within the calibration-curve range.
It is also important that the blank be appropriate, that the cell be free of dirt and bubbles, and that the sample not be turbid.

In qualitative analysis, agreement of the absorption-maximum wavelength and spectral shape is important.
In quantitative analysis, it is important that the measurement be performed within a range where absorbance is proportional to concentration, that the calibration curve show good correlation, and that the dilution factor be correctly reflected.

Example Discussion When the Experiment Did Not Go Well

When UV-Vis measurement does not go well, possible causes are considered from results such as an unclear absorption peak, absorbance that is too high or too low, a nonlinear calibration curve, a spectrum that is greatly shifted, large noise, a turbid sample, or an inappropriate blank.
Organizing the causes into sample preparation, measurement wavelength, cell condition, solvent, pH, blank correction, and calibration-curve range makes the discussion easier.

How to Write Points for Improvement

In a discussion of UV-Vis spectra, including not only sources of error but also points for improvement makes the report easier to organize.
Improvements can be organized by dividing them into sample preparation, calibration curves, cell handling, measurement conditions, and analytical methods.

Improvements to Sample Preparation

• Dissolve the sample completely
• Perform filtration or centrifugation when necessary
• Manage the dilution factor accurately
• Keep the color-development reaction time constant
• Keep pH constant
• Avoid decomposition or deterioration of the sample

Improvements to the Calibration Curve

• Prepare standard solutions accurately
• Set an appropriate concentration range
• Adjust the concentration so that absorbance falls within the linear range
• Dilute unknown samples when necessary
• Perform a blank measurement
• Investigate the cause of outliers

Improvements to the Measurement Procedure

• Keep the cell clean
• Wipe fingerprints and dirt from the cell surface
• Remove bubbles
• Keep the orientation of the cell consistent
• Select an appropriate measurement wavelength
• Allow the instrument to stabilize sufficiently

Improvements to Analysis

• Read the absorption-maximum wavelength accurately
• Show the calibration-curve equation and correlation coefficient
• Check whether the unknown sample is within the calibration-curve range
• Correctly apply the dilution factor
• When comparing with literature values, also check the solvent and pH

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of UV-Vis spectra, simply writing that “a peak appeared” or “the concentration was high” results in a superficial discussion.
A good discussion connects the absorption maximum, electronic transitions, the relationship between concentration and absorbance, the validity of the calibration curve, and sources of error.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of UV-Vis spectra.
Adjust the necessary parts according to your own experimental results.

• The absorption-maximum wavelength reflects the electronic state and molecular structure of the sample.
• The observed absorption is considered to originate from electrons absorbing light energy and transitioning to higher energy levels.
• As the conjugated system becomes longer, absorption tends to shift toward longer wavelengths.
• Because absorbance increased as concentration increased, absorbance was considered to depend on concentration.
• Because the calibration curve showed linearity, quantitation using absorbance was possible within this concentration range.
• Possible causes of deviation from the straight line on the high-concentration side include intermolecular interactions, scattering, and detector-response limitations.
• If blank correction is insufficient, absorption originating from the solvent or reagents affects the quantitative value.
• Sample turbidity causes light scattering and may result in overestimation of absorbance.
• Changes in pH may change the protonation state of a molecule and therefore change the absorption-maximum wavelength and absorbance.
• It is necessary to confirm that the absorbance of the unknown sample is within the calibration-curve range before quantitation.

Points to Check When Discussing UV-Vis Spectra

Checking the following points before writing the report makes the discussion easier to write.

• Has the absorption-maximum wavelength been recorded?
• Has the maximum absorbance been recorded?
• Has the measurement wavelength range been stated?
• Have the solvent and blank been clearly stated?
• Has the cell path length been confirmed?
• Has the relationship between absorbance and concentration been explained?
• Have electronic transitions been related to absorption wavelength?
• Have the calibration-curve equation and correlation coefficient been shown?
• Has it been confirmed that the unknown sample is within the calibration-curve range?
• Have possible causes of peak shifts been considered?
• Have the effects of solvent, pH, turbidity, and cell contamination been considered?
• Do the points for improvement correspond to the sources of error?

Summary

UV-Vis spectroscopy is an analytical method that measures how a sample absorbs ultraviolet and visible light and evaluates absorption maxima, absorbance, electronic transitions, and concentration.
The absorption-maximum wavelength is affected by the electronic state of the molecule, conjugated structure, coordination environment of complexes, solvent, pH, and other factors.
Because absorbance is proportional to concentration under the same measurement conditions, it can be used for quantitative analysis with a calibration curve.

Electronic transitions include π-π* transitions, n-π* transitions, d-d transitions, and charge-transfer transitions, and the observed absorption differs depending on the structure and components of the sample.
Expansion of the conjugated system or complex formation may shift the absorption maximum toward longer wavelengths, providing a clue for discussing changes in molecular structure and electronic state.

In a report, rather than simply writing that “absorption was observed” or “the concentration was determined,” organize and discuss the absorption maximum, electronic transitions, relationship between concentration and absorbance, validity of the calibration curve, blank correction, and the effects of solvent, pH, turbidity, and cell condition.
In UV-Vis measurement, standardizing sample preparation and measurement conditions is important for obtaining reliable results.