Water Quality: Determination of Anionic Detergents - Polypotentiometric Titration Method

6. Procedure

6.1 Sample Preparation

When the LAS content of the sample is >10 mg/L, the sampling volume is 10.00 mL; when it is <10 mg/L, the sampling volume is 20.00 mL. Place the sample in a 50 mL beaker.

6.2 Pretreatment

6.2.1 Colorless wastewater sample: Adjust the pH value of sample (6.1) to 4, then test.

6.2.2 Colored wastewater sample: Add 1 drop of sodium hypochlorite solution (3.5) to every 10 mL of colored wastewater sample (6.1), measure its pH value, adjust the pH value to 2 with sulfuric acid solution (3.3), mix well, and heat on an electric furnace until fuming (70℃). Cool to room temperature, adjust the pH to 4 with sodium hydroxide solution (3.4), then test.

6.3 Potentiometric Titration

6.3.1 Electrode and Instrument Preparation

The PVC-AD electrode needs to be soaked in the activation solution (3.8) for half an hour. If it will not be used for a long time, it should be soaked overnight. The ion meter or pH meter needs to be preheated for half an hour. The activated electrode and calomel electrode are then rinsed in a water cup with a stir bar, recording the potential while stirring. The electrode needs to be rinsed multiple times until the difference in cell potential between two rinses is ±1mV, indicating that the electrode has been properly cleaned. This cell potential is recorded as E_water.

6.3.2 Titration

In the sample cup, place the stir bar and insert the PVC-AD electrode and calomel electrode into the water sample. Titrate while stirring, recording the corresponding cell potential for each volume of CPB (3.10) added. Since the cell potential at the titration endpoint is generally close to E_water, E_water can be used to estimate the endpoint. At the start of the titration, a larger volume can be added, but near the endpoint, the volume should be as small as possible (the standard being a significant change in potential ΔE/ΔV per unit volume increment), generally adding 0.1 mL each time. The potential change should be read when it is ≤1 mV/min. ΔE/ΔV is at its maximum at the endpoint, determined using the second-order differential method. After titration, the electrode needs to be rinsed with water several times until the potential stabilizes. If the electrode will not be used again that day, remove it from the water and replace the electrode cap.

Note: The use of the electrode must comply with the relevant provisions of the electrode product instruction manual.

7. Expression of Analytical Results

7.1 The content of anionic detergent, expressed as LAS (mg/L), is given by the following formula:

Where: c—actual concentration of CPB standard titration solution, mol/L;

V1—sample volume, mL;

V2—volume of CPB standard titration solution (3.10), mL;

0.3444—mass of LAS, expressed in grams, equivalent to 1.00 mL of CPB standard titration solution [c(CPB) = 1.000 mol/L].

8. Precision and Accuracy

The analytical results of a collaborative experiment by six laboratories determining uniformly distributed standard solutions containing 5.000 and 25.00 mg/L of LAS are shown in Table 2.

Table 2 Precision and Accuracy of Potentiometric Titration

Additional Notes:

This standard was proposed by the Standards Division of the Science and Technology Standards Department of the State Environmental Protection Administration.

This standard was drafted by Beijing Institute of Light Industry and Beijing Textile Research Institute.

The main drafters of this standard are He Yaming and Jiang Tian.

The China National Environmental Monitoring Centre is entrusted with interpreting this standard.