Water Analysis (Leaving Cert Chemistry): Revision Notes

Water Analysis

Water analysis involves several instrumental methods that help identify and quantify various substances in water, ensuring its safety and suitability for different uses.

Key methods include pH metres, atomic absorption spectrometry (AAS), and colourimetry. Each of these methods plays an essential role in detecting pollutants, measuring water quality, and ensuring compliance with environmental standards.

Instrumental Methods of Water Analysis

pH Meters

Principle:

• A pH meter measures the hydrogen ion concentration $\text{H}^+$ in water, indicating how acidic or basic the water is.

• The device uses a glass electrode that generates an electric potential based on the concentration of $\text{H}^+$ ions in the solution. Application:

• pH is crucial for water treatment, environmental monitoring, and ensuring that the water is safe for drinking and industrial use.

• For example, fish in rivers and lakes need a specific pH range to survive.

Atomic Absorption Spectrometry (AAS)

Principle:

• AAS measures the concentration of specific metal ions in water by analysing the light absorbed by the atoms of these elements.

• When a water sample is vaporised, the atoms absorb light at characteristic wavelengths, which correlates with the concentration of the metal ions present. Applications:

• Detection of heavy metals: AAS is used to detect harmful metals like lead ($Pb²⁺$), cadmium ($Cd²⁺$), and mercury ($Hg²⁺$), which can contaminate water sources.

• Environmental monitoring: Used for monitoring river and lake water for metal contamination.

Colourimetry

Principle:

• Colourimetry involves measuring the intensity of colour in a water sample, which correlates to the concentration of a specific compound.

• This is achieved by adding a reagent to the water that reacts with the substance of interest to form a coloured compound. Applications:

• Analysis of chemicals: Colourimetry is commonly used to detect substances like nitrates, phosphates, and chlorine in water.

• Monitoring pollutants: It is used for detecting organic pollutants from sewage, industrial waste, and other sources.

Tests for Anions

Water analysis also involves testing for anions (negatively charged ions) to determine the presence of contaminants:

Biochemical Oxygen Demand (BOD)

• BOD is a measure of the amount of dissolved oxygen required by microorganisms to decompose organic matter in water over a specific period (typically 5 days) at 20°C.
• Significance: High BOD levels indicate a high concentration of organic pollutants (e.g., sewage, silage, milk waste) and can lead to oxygen depletion in aquatic environments, endangering aquatic life.