Spectrophotometry and Absorbance Measurement

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Questions and Answers

What is the primary function of a monochromator in a spectrophotometer?

To amplify the light signal

To measure the irradiance of the emergent light

To select a narrow band of wavelengths (correct)

To contain the sample

What is measured in a single-beam spectrophotometer?

The molar absorptivity of the analyte

The transmittance of the sample

The irradiance of the incident light

The irradiance of the emergent light (correct)

Under what condition does Beer's law fail?

When the concentration of the analyte is very high

When the analyte undergoes a chemical change (correct)

When the molar absorptivity is very low

When the pathlength of the cuvet is very short

What is the purpose of a cuvet in a spectrophotometer?

To contain the sample Signup and view all the answers

Why is glass not suitable for ultraviolet measurements?

Because it absorbs UV radiation Signup and view all the answers

What is the common pathlength of a cuvet in a spectrophotometer?

1.000 cm Signup and view all the answers

What is the purpose of recording a baseline spectrum in a double-beam spectrophotometer?

To account for instrumental errors and noise Signup and view all the answers

Why is it important to choose the wavelength of maximum absorbance?

Because it has the greatest sensitivity Signup and view all the answers

What is the ideal range of absorbance values?

0.4-0.9 Signup and view all the answers

What is the main requirement for a compound to be analyzed by spectrophotometry?

It must absorb light, and this absorption should be distinguishable from that due to other substances in the sample Signup and view all the answers

What is the unit of molar absorptivity?

L/mol·cm Signup and view all the answers

What is the purpose of wiping off fingerprints and dust from cuvets?

To reduce the noise and improve the signal-to-noise ratio Signup and view all the answers

What is the product of frequency times wavelength equal to?

The speed of light Signup and view all the answers

What happens to the energy of a molecule when it absorbs a photon?

It increases Signup and view all the answers

What is the relationship between irradiance and transmittance?

Irradiance is inversely proportional to transmittance Signup and view all the answers

What is the unit of pathlength in Beer's law?

Centimeter Signup and view all the answers

What is the unit of molar absorptivity?

M-1cm-1 Signup and view all the answers

What is the limitation of Beer's law?

It only applies to monochromatic light Signup and view all the answers

Study Notes

Measuring Absorbance

In a double-beam spectrophotometer, light is split alternately between sample and reference cuvets.
A baseline spectrum must be recorded first, which will be subtracted from the sample's absorbance.
The wavelength of maximum absorbance should be chosen because it has the greatest sensitivity and least variation in absorbance versus wavelength.
Optimal absorbance is between 0.4-0.9, as too much or too little light absorbed makes measurements difficult.

Beer's Law in Chemical Analysis

A compound must absorb light, and this absorption should be distinguishable from other substances in the sample.
A compound's molar absorptivity can be calculated from its absorbance and concentration.

Properties of Light

Photons are particles of light with energy (E = hν), where h is Planck's constant (6.626 x 10^-34 J-s).
The frequency (ν) multiplied by the wavelength (λ) equals the speed of light (c, approximately 3.0 x 10^8 m/s).
The energy of a molecule increases when it absorbs a photon.

Absorption of Light

When light is absorbed by the analyte, the irradiance (P) decreases.
Transmittance (T) is the fraction of original light that passes through the sample, while absorbance (A) is the logarithm of the reciprocal of transmittance.
Beer's law states that absorbance is proportional to the concentration of light-absorbing molecules in the sample.
A chromophore is the part of a molecule responsible for light absorption.

Measuring Absorbance (continued)

A spectrophotometer requires a continuous source, a monochromator, and a detector to measure the irradiance of emergent light.
Cuvets are typically made of fused-silica (SiO2) and have a pathlength of 1.000 cm.
For IR spectroscopy, cells are constructed of NaCl or NaBr, and solid samples are ground into a fine powder and dispersed in mineral oil (Nujol).

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Description

Learn about the principles of spectroscopy and absorbance measurement, including the use of double-beam spectrophotometers, recording baseline spectra, and selecting the optimal wavelength for analysis.