Determination of Total Organic Carbon (TOC) in Water – Principle of Nondispersive Infrared Absorption Spectrometry

water quality—Determination of TOC by nondispersive

infrared absorption method

This standard adopts the international standard ISO 8245—1987, "Water quality—Determination of total organic carbon (TOC)—Guidelines".

1. Subject Matter and Scope

1.1 This standard specifies the nondispersive infrared absorption method for the determination of total organic carbon in surface water.

1.2 Scope of Determination

This standard applies to the determination of total organic carbon in surface water. The concentration range is 0.5–60 mg/L, and the detection limit is 0.5 mg/L.

1.3 Interferences

Common coexisting ions in surface water exceeding the following concentrations (mg/L) can interfere with the determination. Appropriate pretreatment should be performed to eliminate interference: SO₄²⁻–400; Cl⁻–400; NO₃⁻–100; PO₄³⁻–100; S²⁻–100. When the water sample contains large suspended particles, the measurement results often do not include all particulate organic carbon due to the limitation of the syringe needle aperture.

2. Principle

2.1 Differential Method for Determining Total Organic Carbon

The sample, along with purified air (dried and decarbonized), is introduced into a high-temperature combustion tube (900℃) and a low-temperature reaction tube (160℃), respectively. In the high-temperature combustion tube, the water sample undergoes high-temperature catalytic oxygenation, causing both organic compounds and inorganic carbonates to convert into carbon dioxide. In the low-temperature reaction tube, the water sample is acidified, causing inorganic carbonates to decompose into carbon dioxide. The generated carbon dioxide is then sequentially introduced into a non-dispersive infrared detector. Since carbon dioxide selectively absorbs infrared light of a certain wavelength, within a certain concentration range, the intensity of infrared absorption by carbon dioxide is directly proportional to its concentration. Therefore, the total carbon (TC) and inorganic carbon (IC) of the water sample can be quantitatively determined.

The difference between total carbon and inorganic carbon is the total organic carbon.

2.2 Direct Method for Determining Total Organic Carbon

The water sample is acidified and aerated to decompose inorganic carbonates into carbon dioxide, which is then removed. The purified air is then injected into a high-temperature combustion tube for direct determination of total organic carbon.

3. Reagents

Unless otherwise specified, all reagents are analytical grade, and all water used is carbon dioxide-free distilled water.

3.1 Carbon dioxide-free distilled water: Boil redistilled water in a beaker to evaporate (10% evaporation rate), cool slightly, and store in a lower-mouth bottle with a soda lime tube inserted.

3.2 Potassium hydrogen phthalate (KHC8H4O4): High-purity.

3.3 Anhydrous sodium carbonate (Na2CO3): High-purity.

3.4 Sodium bicarbonate (NaHCO3): High-purity, stored in a desiccator.

3.5 Organic carbon standard stock solution: C = 400 mg/L.

Weigh 0.8500 g of potassium hydrogen phthalate (3.2) (pre-dried at 110–120 °C for 2 h and cooled to room temperature in a desiccator), dissolve in water (3.1), transfer to a 1000 mL volumetric flask, dilute to the mark with water (3.1), mix well, and store for 48 days under low-temperature (4 °C) refrigeration conditions.

3.6 Organic carbon standard solution: c = 80 mg/L. Accurately pipette 10.00 mL of organic carbon standard solution (3.5) into a 50 mL volumetric flask, dilute to the mark with water (3.1), and mix well. This solution should be prepared fresh before use.

3.7 Inorganic carbon standard stock solution: c = 400 mg/L. Weigh 1.400 g of sodium bicarbonate (3.4) (pre-dried in a desiccator) and 1.770 g of anhydrous sodium carbonate (3.3) (pre-dried at 105℃ for 2 h, then cooled to room temperature in a desiccator), dissolve in water (3.1), transfer to a 1000 mL volumetric flask, dilute to the mark with water (3.1), and mix well.

3.8 Inorganic carbon standard solution: c = 80 mg/L.

Accurately pipette 10.00 mL of inorganic carbon standard stock solution (3.7) into a 50 mL volumetric flask, dilute to the mark with water (3.1), and mix well. This solution should be prepared fresh before use.

4. Instruments

General laboratory instruments and:

4.1 Non-dispersive infrared absorption TOC analyzer. Operating conditions:

4.1.1 Ambient temperature: 5–35℃.

4.1.2 Operating voltage: Instrument rated voltage, AC.

4.1.3 Total carbon combustion tube temperature selection: 900℃; Inorganic carbon reaction tube temperature control: 160±5℃.

4.1.4 Carrier gas flow rate: 180mL/min.

4.2 Single-pen recorder: Matched to the instrument. Operating conditions:

4.2.1 Operating voltage: Instrument rated voltage, DC.

4.2.2 Recording paper speed: 2.5mm/min.

4.3 Microsyringe: 50.00µL.

4.4 Stoppered colorimetric tube: 10mL.

5. Sampling and Samples

After water sample collection, it must be stored in a brown glass bottle.

Water samples can be stored for 24 hours at room temperature. If analysis cannot be performed immediately, sulfuric acid can be added to adjust the pH to ≤2, and the sample can be stored at 4℃ for 7 days.