Water Quality: Determination of Anionic Detergents - Principle of Potentiometric Titration

2. Principle

A working cell is constructed using a PVC-AD electrode as the working electrode and a saturated calomel electrode as the reference electrode. Potentiometric titration of anionic detergents in polluted water is performed using CPB as the titrant. Within the Nernst response region of the working electrode, the cell potential and the concentration of the anionic detergent are related as follows:

E = E0 - Klg a_anionic

Where: E—cell potential, mV;

E0—its value is related to the reference electrode used, the junction potential, and the inner surface membrane potential, etc.;

K—the slope of the Nernst equation, i.e., the electrode gradient;

a_anionic—the activity of the anionic detergent ions.

At a constant temperature, E0 and K are constants for the electrode.

Titration reaction:

CPB + LAS → CPB·LAS↓ (white precipitate)

As CPB is added, the concentration of LAS in the water sample continuously decreases, and the cell potential correspondingly increases. Near the equivalence point, there will be a sudden change in [LAS−] in the solution, and the cell potential will also change abruptly. The endpoint is determined using the second-order differential method, and the amount of anionic detergent in the sample is calculated from the CPB consumption (mL) corresponding to the endpoint.

3. Reagents

Unless otherwise specified, all analytical reagents conforming to national or professional standards, deionized water, or water of equivalent purity are used in the analysis.

3.1 Sulfuric acid (H2SO4): p = 1.84 g/mL.

3.2 Sodium hypochlorite alkaline solution (antifomin): C.P., available chlorine content not less than 8.5%.

3.3 Sulfuric acid solution: (1+4).

3.4 Sodium hydroxide solution: 80 g/L. Weigh 4 g of sodium hydroxide (NaOH) and dissolve it in 50 mL of water, then shake well.

3.5 Sodium hypochlorite solution: available chlorine is 10 g/L. Prepared by diluting sodium hypochlorite alkaline solution (3.2). Store at approximately 4°C for half a month.

3.6 LAS Standard Stock Solution: Accurately weigh 220–250 mg of LAS standard (accurate to 0.0001 g) using the subtraction method, dissolve in freshly boiled and cooled water, and dilute to the mark in a 250 mL volumetric flask. Store at approximately 4°C.

3.7 LAS Standard Working Solution: Pipette 10.00 mL of stock solution (3.6) into a 100 mL volumetric flask, dilute to the mark with freshly boiled and cooled water, then take 10.00 mL of this solution into a 100 mL volumetric flask and dilute to the mark. This solution should be prepared on the day of use for CPB standardization.

3.8 PVC-AD Electrode Activation Solution: Measure 50 mL of stock solution (3.6) into a 100 mL volumetric flask, dilute to the mark with freshly boiled and cooled water, and store at approximately 4°C.

3.9 PVC-AD Electrode Internal Filling Solution: Add 3 mg of sodium chloride to 20 mL of stock solution (3.6), dissolve, and transfer to a 50 mL volumetric flask. Dilute to volume and store at approximately 4°C.

3.10 CP6 Standard Titration Solution: Weigh approximately 140 mg of CPB, dissolve in freshly boiled and cooled water, and dilute to volume in a 1000 mL volumetric flask. The titer is approximately 0.12 mg/mL. Standardize using LAS standard working solution (3.7). During standardization, take 10.00 mL of the standard working solution and perform potentiometric titration with the prepared CPB solution.

4. Instruments

General Laboratory Instruments and

4.1 Digital Ion Meter (or pH Meter): Accuracy ±1 mV.

4.2 PVC-AD Electrode: Grade difference ≥ 50 mV.

4.3 Saturated Calomel Electrode.

4.4 2 mL Microburette.

4.5 Microporous removable filter: M50 type.

4.6 0.45µm mixed fiber filter membrane.

4.7 pH precision test paper (0.5～5.0).

Note: Glassware must be cleaned with chromic acid cleaning solution.

5 Sampling and Samples

Clean glass bottles should be used for sample collection and preservation. After water sample collection, acidify to pH=4 with sulfuric acid solution (3.3), and determine whether filtration is necessary based on the cleanliness of the sample. Slow-speed quantitative filter paper or 0.45µm filter membrane (4.6) can be used as the filter. Samples should be analyzed as soon as possible. If preservation is required, the pH should be adjusted to ≤2 and refrigerated at 4℃. It can be stored for three days.