GB/T 11275-2007Surface active agents -- Determination of water content (English PDF)

4.1 Principle

The water in the specimen reacts with iodine and sulfur dioxide in the Karl Fischer reagent, which has been pre-calibrated by titration with a known accurate amount of water. The mass fraction of water content is calculated from the amount of reagent used.

4.2 Reagents and materials

WARNING. Inhalation or ingestion of chloroform or dichloromethane is harmful to humans. It may irritate the skin and cause permanent damage. Long-term exposure poses a serious risk of health damage.

4.2.1 Distilled water or water of equivalent purity, meeting the requirements for Grade

II water in GB/T6682.

4.2.2 Anhydrous methanol. analytically pure. If anhydrous methanol is known to interfere with the determination (e.g., the determination of water in aldehydes or ketones), chloroform or dichloromethane may be used, and this shall be noted in the test report.

4.2.3 Karl Fischer reagent uses commercially available pyridine-free Karl Fischer

reagent. Alternatively, pyridine-containing reagents may be prepared according to the method described in Annex C. If pyridine-free reagents are used, this shall be noted in the test report.

4.2.4 Sodium tartrate dihydrate or oxalate dihydrate. analytically pure.

4.3 Instruments and equipment

4.3.1 Trace moisture analyzer, fully automatic or semi-automatic, consisting of the

following parts.

a) Double platinum electrode titration apparatus;

b) Piston burette. 20 mL;

c) Drying tube containing activated silica gel, calcium chloride, or magnesium perchlorate;

d) Titration cell;

e) Magnetic stirring device.

4.3.2 Microsyringe. 50 µL.

4.3.3 Volumetric flask. 100 mL.

4.3.4 Graduated pipette. 20 mL.

4.3.5 Balance. sensitivity is 0.1 mg.

4.4 Determination

4.4.1 Sampling

Test samples of surface active agents shall be prepared and stored in accordance with

GB/T 6372.

4.4.2 Calibration of water equivalent of reagents using the Karl Fischer method

Calibrate the water equivalent of each new bottle of Karl Fischer reagent. Thereafter, calibrate at least once a week.

Add a certain volume (immersing the electrode 2 mm~3 mm) of anhydrous methanol to the titration cell. Titrate with Karl Fischer reagent while stirring until the reading remains stable for 30 s (do not record the volume of Karl Fischer reagent consumed).

Inject 0.04 g (accurate to 0.0001 g) of distilled water or 200 mg~250 mg (accurate to

0.0001 g) of sodium tartrate dihydrate or oxalate dihydrate into the titration cell using a microsyringe. Retitrate until the reading remains stable for 30 s, which is the endpoint.

Record the volume of Karl Fischer reagent used.

Calculate the value of water equivalent T using formula (1) or formula (2).

Where,

T - Water equivalent of Karl Fischer reagent, in milligrams per milliliter (mg/mL);

m1 - Mass of water, in grams (g);

V1 - Volume of Karl Fischer reagent consumed in titration, in milliliters (mL);

m0 - Mass of sodium tartrate or oxalic acid, in milligrams (mg);

w - Water content of sodium tartrate dihydrate (15.66) or oxalic acid dihydrate (28.57), expressed as a mass fraction;

100, 1000 - Unit conversion factors.

4.4.3 Specimen preparation

If the water content (mass fraction) of the specimen is less than 1%, then weigh 5 g~10 g (accurate to 0.0001 g) which is equivalent to a specimen containing 10 mg~50 mg of water.

If the water content (mass fraction) in the specimen is higher than 1%, weigh 1 g~5 g (accurate to 0.0001 g) into a volumetric flask. Add anhydrous methanol and dilute to the mark. Shake vigorously to dissolve the water in the sample. Let stand. Use a pipette to pipette the supernatant equivalent to 10 mg~50 mg of water.

When the water content (mass fraction) of the specimen is higher than 1%, titrate the water in the solvent according to the procedure given in 4.4.4.2.The water content of the solvent shall be deducted from the specimen result.

4.4.4 Determination of water content

4.4.4.1 General rules

Based on the characteristics of the test sample, choose one of the following two methods.

the general method or the dissolution method.

The general method is suitable for samples that are homogeneous and easy to disperse in a titration cell.

The dissolution method is suitable for samples with poor homogeneity and/or those that are difficult to disperse in a titration cell.

Moisture in the atmosphere is the biggest source of interference in Karl Fischer titrations. Special care shall be taken to thoroughly dry the apparatus used and to quickly process the test samples and solvents.

The method of using a moisture meter shall follow the instructions of the instrument manufacturer.

4.4.4.2 General method

4.4.4.2.1 Determination

Add a certain volume (immersing the electrode 2 mm~3 mm) of anhydrous methanol to the titration cell. Titrate with Karl Fischer reagent to the endpoint while stirring (do not record the volume of Karl Fischer reagent consumed). Quickly add the specimen to the titration cell. Stir to dissolve. Titrate to the endpoint. Record the volume of Karl

Fischer reagent consumed.

4.4.4.2.2 Result representation

Water content (mass fraction) is expressed as X, with the value represented as % and calculated according to formula (3).

Where,

T - Water equivalent of Karl Fischer reagent, in milligrams per milliliter (mg/mL);

V2 - Volume of Karl Fischer reagent consumed in titration, in milliliters (mL);

m2 - Mass of the specimen, in grams (g);

10 - Unit conversion factor.

4.4.4.3 Dissolution method

4.4.4.3.1 Determination

Weigh an appropriate amount of the specimen (m1) into a dry flask. Add an appropriate amount of organic solvent (m2) that does not interfere with Karl Fischer's reagent. The amounts of specimen and solvent shall be such that more specimen is dissolved with less solvent. This ensures that the mixture is easily controlled and does not compromise the representativeness of the obtained sample. Record the mass to an accuracy of 0.0001 g. Shake well.

If insoluble salts are still present at this stage, this method is not applicable because.

a) some water may have been removed as water of crystallization by the insoluble salts;

b) the assay procedure shall include accurate determination of the mass of the insoluble salts.

Determine the water content of the solvent (w2) and mixture (wb) according to the steps given in 4.4.4.2.

4.4.4.3.2 Result representation

5.1 Principle

The water in the specimen reacts with iodine produced by ionization at the anode of the titration cell and Karl von Scheres' reagent in the electrolytic cell. The amount of iodine required for the reaction is proportional to the amount of electricity passed through the electrolytic cell. The mass fraction of water content is calculated using the amount of electricity required to reach the titration endpoint. The endpoint is detected by a voltage detection method under current control.

NOTE. If the specimen contains aniline, phenol, impurities that may clog the diaphragm, or impurities that may affect current efficiency, the Karl Fischer coulometric titration method is not suitable for determining the water content. Especially for solid samples, it is necessary to confirm whether they are completely dissolved.

5.2 Reagents and materials

During the analysis, only confirmed analytically pure reagents and Grade II water as specified in GB/T6682 are used. Commercially available anolytes, catholytes, and aqueous solutions of alcohol or propylene carbonate are all acceptable.

5.2.1 Anodic electrolyte

It shall contain iodide ions, sulfur dioxide, pyridine or another base, and organic solvents such as methanol.

For samples containing compounds such as aldehydes or ketones that can react with methanol, use chloroform, 2-methoxyethanol, propylene carbonate, or similar substances instead of methanol.

5.2.2 Cathode electrolyte

It shall contain inorganic salts, organic salts, such as quaternary ammonium salts or amine hydrochloride, and organic solvents such as methanol.

5.2.3 Aqueous solutions of alcohol or propylene carbonate

Add water to 2-methoxyethanol or propylene carbonate so that 1 mL of solution contains approximately 4 mg of water. This solution is then used to add small amounts of water.

5.3 Instruments and equipment

5.3.1 Coulometric titrator. includes titration components, control components, and indicating and recording components. Its structural diagram is shown in Figure 1.

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