4. Spectrophotometry

What is spectrophotometry?

A quantitative analytical technique that measures how much light a chemical substance absorbs or transmits at specific wavelengths.

According to Beer-Lambert's law, absorbance (A) is directly proportional to which two factors?

Concentration (c) and path length (l) through which light travels.

What is the formula that expresses the Beer-Lambert's law?

A=€lc, where A is absorbance, € is molar absorptivity, l is path length, and c is concentration.

What does the 'A' represent in spectrophotometry?

Absorbance, which is a unitless measure of light absorbed.

What is molar absorptivity?

A constant (€) indicating how strongly a substance absorbs light at a specific wavelength.

What are the limitations of spectrophotometry?

It is used for dilute solutions; higher concentrations can cause deviations due to electrostatic interactions.

What are the typical components of a spectrophotometer?

A light source, monochromator, cuvette, detector, and display/readout system.

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

To isolate a narrow band of wavelengths from the light source.

What are cuvettes used for in spectrophotometry?

Cuvettes are transparent containers that hold the liquid sample.

Why is a stable light source important in spectrophotometry?

To ensure consistent light intensity for accurate measurements.

How do absorbance and color perception relate in solutions?

A solution's color corresponds to the wavelength transmitted; for example, a blue solution absorbs all but blue wavelengths.

How is absorbance often expressed in spectrophotometry?

As A (3 decimal places) or %T (percent transmittance).

What happens to absorbance as the concentration of a solution increases?

The amount of light absorbed increases, and the percentage of light transmitted decreases.

What is the principle behind reflectance spectrophotometry?

It measures the intensity of light reflected from a sample across various wavelengths.

What are common applications of reflectance spectrophotometry?

Evaluating tissue for diseases, measuring paints/colors, and food freshness.

What is the primary difference between absorbance spectrophotometry and reflectance spectrophotometry?

Absorbance measures transmitted light while reflectance measures light reflected/scattered off the sample.

What does nephelometry measure?

Light that has been scattered, particularly in solutions.

What is the key difference between nephelometry and turbidimetry?

Nephelometry measures scattered light, while turbidimetry measures the loss of intensity of transmitted light.

What factors can interfere with the accuracy of a nephelometer?

Microbial contamination, specimen turbidity, and lipemia.

What is the purpose of a standard curve in spectrophotometry?

To plot absorbance or %T against known concentrations for determining unknown samples.

What is the equation used to determine unknown results from spectrophotometry?

Using the line of regression from the standard curve to find corresponding concentrations from measured absorbance.

What does quality control (QC) ensure in a spectrophotometer?

It ensures wavelength accuracy, stray light correction, and linearity of results.

What types of light sources are commonly used in spectrophotometry?

Tungsten lamps for visible light and deuterium lamps for ultraviolet light.

What is the Nernst equation used for in potentiometry?

To relate the measured potential to the concentration (activity) of the analyte in a solution.

What role does an electrode play in potentiometric measurements?

It measures the electrical potential difference to determine solute concentration.

What is chromatography?

A technique used to separate, identify, and purify components in a mixture based on their distribution between stationary and mobile phases.

How are the components of a chromatographic system classified?

According to the physical state of the carrier phase: gas or liquid phase.

What is the purpose of mass spectrometry (MS) in analytical chemistry?

It identifies and weighs chemicals by measuring their mass-to-charge ratio.

What distinguishes gas chromatography from liquid chromatography?

Gas chromatography uses an inert gas as a mobile phase, while liquid chromatography uses liquid solvents.

What is the significance of retention time in chromatography?

It indicates when a substance exits the chromatography column, aiding in identification.

What applications are common for gas chromatography?

Toxicology, blood alcohol detection, and therapeutic drug monitoring.

What advantage does mass spectrometry provide over chromatography alone?

It allows for the identification of compounds based on their mass instead of just their retention time.

What safety precautions should be taken in a spectrophotometry lab?

Proper use of protective equipment and monitoring of chemical exposures.

How is data from spectrophotometric measurements validated?

By ensuring the standard curve has a high coefficient of determination (R² >= 0.95) for accuracy.

What does the term 'electrochemical cell' refer to?

A device that generates electricity from spontaneous redox reactions or drives non-spontaneous reactions using electricity.

What is the function of an anode in an electrochemical cell?

It is the electrode where oxidation occurs (electron loss).

What is an indicator electrode?

An electrode sensitive to the concentration of a specific analyte, used in potentiometric measurements.

What is the purpose of calibration in spectrophotometry?

To ensure the accuracy of measurements and correct any instrument deviations.

What is the basic principle behind electrochemistry?

The study of the relationship between electrical potential difference and chemical change.

What is the advantage of using a fibrous probe in reflectance spectrophotometry?

It allows non-invasive measurements of surfaces or tissues.

What is the significance of cuvette maintenance in spectrophotometry?

Ensuring cuvettes are clean, scratch-free, and appropriate for the measurement’s wavelength.

What does absorbance indicate about a solution?

It shows the concentration of absorbing substances based on light absorption properties.

Why is it important to zero the spectrophotometer before measuring samples?

To eliminate background interference and ensure accurate absorbance readings.

What is the optimal temperature range for gas chromatography?

Typically above 100°C to vaporize samples for analysis.