What is the relationship between the color of a solution and the filter used in a colorimeter?

The filter should be the complementary color of the solution. This is because the solution absorbs its complementary color most strongly, providing the maximum change in absorbance for a given change in concentration.

How does a colorimeter differ from a UV-visible spectrophotometer in terms of wavelength selection?

A colorimeter uses colored filters to select a broad range of wavelengths, while a spectrophotometer uses a diffraction grating or prism to select a very narrow, specific wavelength. This makes spectrophotometers more precise and versatile for complex mixtures.

What is the difference between absorbance and transmission in colorimetry?

Transmission is the percentage of light that passes through a sample, while absorbance is the negative logarithm of transmission. Absorbance is used for calibration curves because it has a direct linear relationship with concentration, whereas transmission decreases exponentially.

Why might a calibration curve lose its linearity at very high concentrations?

At high concentrations, the solute particles are close enough to interact with each other, changing their ability to absorb light. Additionally, the light may become so attenuated that the detector cannot accurately measure the small amount of transmitted light.

What is the consequence of failing to 'blank' the colorimeter with pure solvent?

The resulting absorbance readings will be higher than the true value because they will include the light absorbed or scattered by the solvent and the cuvette walls. This introduces a systematic error that can shift the calibration curve away from the origin.

Why is it a mistake to handle the clear sides of a cuvette during an experiment?

Fingerprints leave oils and residues that scatter and absorb light as it passes through the sample. This results in an artificially high absorbance reading that does not accurately reflect the concentration of the complex ion.

Define the Beer-Lambert Law and identify its variables.

The law is expressed as $A = \epsilon cl$, where $A$ is absorbance, $\epsilon$ is the molar extinction coefficient (a constant for the substance), $c$ is concentration, and $l$ is the path length of the light through the sample.

What is a 'calibration curve' in the context of colorimetry?

It is a graph showing the relationship between the concentration of a known set of standards and their measured absorbance. It allows for the determination of an unknown concentration by interpolating its measured absorbance on the line of best fit.

What is meant by the term 'complementary color'?

It is the color that, when combined with another color, produces white light. In colorimetry, it refers to the color of light that a solution absorbs most strongly, which is opposite to the color the solution appears to be.

A cuvette is a small, square-walled tube made of plastic, glass, or quartz designed to hold liquid samples for optical analysis. It has two clear sides for the light beam to pass through and often two frosted sides for handling.

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Colorimetry & Complex Ions

Summary

Colorimetry is a quantitative analytical technique used to determine the concentration of colored transition metal complex ions in solution by measuring their light absorbance. By applying the Beer-Lambert Law and utilizing calibration curves, chemists can precisely correlate the intensity of a solution's color with the molarity of the solute.

1. Definition & Core Concepts

Colorimetry is an instrumental method used to measure the concentration of a colored substance in a solution by detecting the amount of light it absorbs. This technique is particularly effective for transition metal complexes, which exhibit characteristic colors due to d-orbital splitting and electronic transitions.
The complementary color principle is central to this method; a solution appears a certain color because it transmits that color while absorbing its opposite on the color wheel. For example, a solution that appears pale blue is actually absorbing red light most efficiently.
A colorimeter or spectrophotometer is the primary tool used, consisting of a light source, a wavelength selector (filter), a sample holder (cuvette), and a light-sensitive detector.

Schematic diagram of a colorimeter showing light passing through a filter, a sample cuvette, and reaching a detector.

2. Underlying Principles

3. Methods & Techniques

Preparation of Standards: A series of solutions with precisely known concentrations of the complex ion are prepared. These are used to establish a baseline relationship between concentration and light absorption.
Calibration Curve: The absorbance of each standard is measured and plotted against its concentration. This should result in a linear graph passing through the origin, known as a calibration curve.
Sample Analysis: The absorbance of the unknown solution is measured under identical conditions (same filter and cuvette). Its concentration is then determined by locating its absorbance value on the calibration curve and reading the corresponding concentration on the x-axis.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions