UV-VIS Spectrophotometry Review

Comprehensive flashcards covering UV-Vis Spectrophotometry principles, chemical structures (chromophores and auxochromes), the Beer-Lambert Law, shifts in spectra due to pH, and analysis limitations.

What is the wavelength ($\lambda$) range for UV-Vis radiation as defined in the lecture?

$$200 - 800\,nm$$

How does energy change in relation to wavelength in the electromagnetic spectrum?

Shorter wavelengths correspond to higher energy levels.

Which types of electrons are triggered by the energy levels associated with UV-Vis radiation?

Valence electrons, specifically $\pi$-electrons (from double bonds) and $n$-electrons (from lone electron pairs).

What is a 'conjugated system'?

A system of interlinked unsaturated bonds (such as alternating saturated and unsaturated bonds) that allow the free movement of delocalised electrons.

Define 'Auxochromes'.

Functional groups with lone electron pairs attached to a conjugated system that contribute to the absorbance of light and the delocalisation of electrons.

What is a 'Chromophore'?

The structural part of a molecule that absorbs UV-Vis radiation, consisting of the conjugated system and potentially any attached auxochromes.

What happens to absorbance as a chromophore becomes more extensive?

Less energy is required for excitation, and absorbance shifts to a higher $\lambda$ (lower energy).

At what wavelength does $\beta$-CAROTENE absorb, and what color does it appear?

It absorbs at $\lambda = 497\,nm$ (blue-green light) and appears red-orange.

What are the two forms of the Beer-Lambert Law equation presented in the notes?

$A = A_{1cm}^{1\%} \times c \times l$ and $A = \varepsilon \times c \times l$

Define the symbols $A_{1cm}^{1\%}$ and $\varepsilon$.

$A_{1cm}^{1\%}$ is the Specific absorbance coefficient (conc. in $g/100\,cm^{3}$); $\varepsilon$ is the Molar absorbance coefficient (conc. in $mol\,dm^{-3}$).

What is the standard path length ($l$) for cuvettes used in spectrophotometers?

$$1\,cm$$

Under what condition is linearity lost for the Beer-Lambert Law?

Linearity is lost at concentrations greater than $0.01\,M$. It is only valid for dilute solutions where $A < 1.5$.

What is the recommended 'Beer Lambert Range' for absorbance readings to ensure linearity?

Between $0.3$ and $0.8$.

Define $\lambda_{max}$.

The wavelength which produces the highest absorbance value; it is commonly used for quantitative analysis.

What is a 'Bathochromic shift' (red shift)?

An increase in $\lambda_{max}$, often seen in basic solutions (e.g., phenol moving to phenoxide).

What is a 'Hyperchromic shift'?

An increase in the absorbance coefficient ($A_{1cm}^{1\%}$ or $\varepsilon$).

What is a 'Hypsochromic shift' (blue shift)?

A decrease in $\lambda_{max}$, often seen in acidic solutions (e.g., aniline moving to anilinium).

What is a 'Hypochromic shift'?

A decrease in the absorbance coefficient ($A_{1cm}^{1\%}$ or $\varepsilon$).

Why is 'Lack of specificity' considered the biggest limitation of UV-Vis spectrophotometry?

All molecules with similar chromophores, such as impurities or degradants like salicylic acid in aspirin samples, will be accounted for, potentially leading to over-estimation of drug content.

According to the typical procedure, how is an unknown concentration determined after measuring 3-5 dilute samples?

By plotting a calibration curve of concentration ($c$) vs absorbance ($A$) and determining the unknown sample concentration from that standard curve.