Organic matter in water can be measured using several methods, each with its own advantages and limitations. The primary methods include loss on ignition, oxidation techniques, and specific organic carbon analysis.
Methods for Measuring Organic Matter in Water
Here's a breakdown of common methods used to quantify organic matter in water:
1. Loss on Ignition (LOI)
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Principle: This method involves determining the weight loss of a dried water sample upon ignition at a high temperature. The weight loss is attributed to the volatilization and combustion of organic matter.
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Procedure:
- Evaporate the water sample to obtain a dry residue.
- Weigh the dry residue.
- Ignite the residue in a furnace at a controlled temperature (e.g., 550°C).
- Weigh the residue again after ignition.
- The difference in weight represents the organic matter lost during ignition.
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Limitations:
- LOI may overestimate organic matter due to the loss of other volatile compounds, such as carbonates (CO2) and water from clay minerals. A correction factor may be needed for CO2 loss from carbonates.
- It doesn't differentiate between different types of organic matter.
2. Oxidation Techniques
These methods involve using oxidizing agents to break down the organic matter present in the water sample. The amount of oxidizing agent consumed is then used to estimate the organic matter concentration. Common techniques include:
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Potassium Permanganate Oxidation: Measures the amount of potassium permanganate required to oxidize the organic matter. This gives an indication of the oxidizable organic matter content.
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Chemical Oxygen Demand (COD): COD measures the amount of oxygen required to chemically oxidize all organic compounds in water. It uses a strong chemical oxidant (e.g., potassium dichromate) under acidic conditions. COD provides a comprehensive measure of both biodegradable and non-biodegradable organic matter.
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Biochemical Oxygen Demand (BOD): BOD measures the amount of oxygen consumed by microorganisms while decomposing organic matter in water over a specified period (usually 5 days). BOD provides an indication of the biodegradable organic matter.
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Total Organic Carbon (TOC): TOC analysis involves oxidizing the organic carbon to form carbon dioxide (CO2), which is then measured using a non-dispersive infrared (NDIR) detector. It provides a direct measurement of the total organic carbon content in the water sample. TOC analysis is often considered the most accurate and reliable method for measuring organic matter in water.
3. Specific Organic Carbon Analysis
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Principle: This approach identifies and quantifies specific organic compounds present in the water sample, rather than measuring total organic matter.
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Techniques: Gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) are common tools to separate, identify, and quantify various organic compounds.
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Applications: Helpful in identifying pollution sources and understanding the types of organic contaminants present (e.g., pesticides, pharmaceuticals).
Choosing the Right Method
The choice of method depends on the specific application, the type of water sample, and the desired level of accuracy. TOC is often preferred for general organic matter assessment, while COD and BOD are useful for wastewater treatment monitoring. LOI is a simple, inexpensive method suitable for screening purposes. Specific organic carbon analysis using GC-MS or LC-MS is used when detailed information on individual organic compounds is required.
