Dissolved organic carbon (DOC) in water is measured by removing inorganic carbon, oxidizing the remaining organic carbon, and then quantifying the resulting carbon dioxide (CO2).
Steps Involved in Measuring DOC
Here's a breakdown of the process:
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Removal of Inorganic Carbon (IC):
- The water sample is first treated with acid (e.g., hydrochloric acid or phosphoric acid). This converts all inorganic carbon forms (bicarbonate, carbonate) into CO2.
- The acidified sample is then sparged, which means bubbling with a gas (typically nitrogen or purified air) to remove the generated CO2. This ensures that only organic carbon is being measured in the subsequent steps.
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Oxidation of Dissolved Organic Carbon (DOC):
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The remaining carbon in the sample is assumed to be dissolved organic carbon. The DOC is then oxidized to convert it into CO2. Different oxidation methods can be used:
- Combustion: The sample is heated to a high temperature (e.g., 680°C) in the presence of a catalyst, which combusts the organic carbon to CO2.
- UV Persulfate Oxidation: Ultraviolet (UV) light is used in conjunction with a chemical oxidant like persulfate to oxidize the DOC to CO2.
- Ozone Oxidation: Ozone is used as an oxidant to convert the DOC to CO2.
- Advanced Oxidation Processes (AOPs): These processes often combine UV light, ozone, and hydrogen peroxide for powerful oxidation of DOC.
- UV Fluorescence: While not directly oxidizing the DOC to CO2, UV fluorescence can be used to estimate DOC concentration based on the fluorescent properties of certain organic compounds.
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Detection of CO2:
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The CO2 produced from the oxidation of DOC is then measured. Common detection methods include:
- Non-Dispersive Infrared (NDIR) Spectroscopy: This is the most common method. The CO2 absorbs infrared light at a specific wavelength, and the amount of absorption is proportional to the concentration of CO2.
- Conductivity Measurement: The CO2 can be converted to carbonic acid in solution, which changes the conductivity of the solution. This change in conductivity can be correlated to the original DOC concentration.
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Summary of DOC Measurement
| Step | Description | Purpose |
|---|---|---|
| Inorganic Carbon Removal | Acidification and sparging to remove inorganic carbon as CO2. | Ensures only organic carbon is measured. |
| Oxidation | Conversion of DOC to CO2 using combustion, UV persulfate, ozone, or other methods. | Transforms organic carbon into a measurable form (CO2). |
| Detection | Measurement of CO2 using NDIR or conductivity. | Quantifies the amount of CO2 produced, which is proportional to the DOC. |
