Water Hardness, Pollution, Cleaners, and Sampling

Water Hardness

Hard water is water that contains appreciable amounts of dissolved calcium and magnesium compounds. These compounds form insoluble precipitates with soap. Ion exchangers are used to remove the hardness.

Temporary Hardness

Temporary hardness is caused by the presence of calcium and magnesium bicarbonates. It can be eliminated by heating the water to precipitate the calcium carbonate:

Ca(HCO₃)₂ → CaCO₃↓ + CO₂ + H₂O

Permanent Hardness

Permanent hardness refers to the remaining calcium and magnesium salts (not bicarbonates). It requires the addition of chemicals for removal, for example:

Na₂CO₃ + CaCl₂ → CaCO₃↓ + 2NaCl

Water Pollution

Thermal Pollution

Heat, in general, is not considered a contaminant, but its effects on water can be detrimental. The primary source of thermal pollution is industrial processes that use water as a coolant and then discharge it at a higher temperature. The effects include:

• Reduction of dissolved oxygen in water, as warmer water holds less oxygen.
• Increased rate of chemical reactions (for every 10°C increase, the reaction rate approximately doubles).
• Disruption of aquatic life due to false temperature readings.
• Exceeding thermal tolerance limits for some aquatic species.

Industries are required to cool the water or reuse it for processes that require hot water.

Cleaners

Cleaners are composed of surfactants, structuring agents, and miscellaneous additives.

• Surfactant: Its purpose is to lower the surface tension of water, allowing it to penetrate tissues better. It is a bipolar substance, with one end dissolving in water and the other in organic dirt. The most commonly used surfactant is sodium dodecylbenzene.
• Structuring Agent: Its primary mission is to sequester calcium and magnesium, effectively softening the water. Polyphosphates are used to provide a basic pH (alkalinity), which favors washing. The most commonly used is sodium tripolyphosphate (P₃O₁₀Na₅).
• Miscellaneous Additives: These include brighteners (which absorb ultraviolet light and emit bluish-white light) and anti-redeposition agents, which keep dirt suspended in the water so it can be removed during rinsing.

Troubleshooting Detergent Issues

Some detergents are not biodegradable. Surfactants with branched chains, unlike linear chains, do not degrade easily in water. Nitrogen (N) and Phosphorus (P) are key elements contributing to eutrophication in water bodies. Solutions include finding structuring agents that are not phosphorus-based or using surfactants that do not require structuring compounds.

Types of Water Samples

• Single Sample: Collected at a specific location and time.
• Composite Sample: Consists of several single samples. These can be:
  • Time-composite: Samples taken from the same location at different times.
  • Space/Integrated-composite: Samples taken at the same time but from different locations.

Types of Water Sampling

• Simple Random Sampling: Random samples are taken independently.
• Stratified Sampling: The area is divided into sections, and simple random sampling is performed within each section.
• Systematic Sampling: Sampling is performed at regular time intervals.
• Stratified Systematic Sampling: A combination of stratified and systematic sampling methods.