GB/T 6283-2008Chemical products -- Determination of water Karl-Fischer method (general method) (English PDF)

5.1 Water

Tested water shall meet the specification of Grade-III water stipulated in GB/T 6682.

5.2 Methanol

Analytically pure. If the mass fraction of water in reagent is greater than 0.05%, add 50g of 5A molecular sieve into 500mL of methanol; plug the bottle stopper; place overnight; absorb the supernatant to use.

5.4 Iodine

Analytically pure.

5.5 Pyridine

Analytically pure. If the mass fraction of water in reagent is greater than 0.05%, add 50g of 5A molecular sieve into 500mL of pyridine; plug the bottle stopper; place overnight, absorb the supernatant to use.

5.6 Glacial acetic acid

Analytical pure.

5.9 Anhydrous sodium sulfite

Chemically pure.

5.10 Sulfur dioxide

The sulfur dioxide both loaded with steel bottle and generated through sulfuric acid decomposing the sodium sulfite shall go through the dehydrating and drying treatment.

The sulfur dioxide generation unit can refer to Appendix A.

5.11 Sample solvent

The mixture containing 4 volumes of methanol and 1 volume of pyridine; or the mixture containing 4 volumes of ethylene glycol monomethylether and 1 volume of pyridine (especially applicable to the carbonyl-containing compound). In special cases, other solvent can also be used, such as glacial acetic acid, pyridine, or mixture containing 1 volume of methanol and 3 volumes of chloroform.

5.12 Desiccant

5.14 Sodium tartrate (Na2C4H4O6 - 2H2O)

Analytically pure.

5.15 Water-methanol standard solution (10g water/L)

Use micro burette or pipette to inject 1mL of pure water into fully dried 100mL volumetric flask containing about 50mL of methanol;

5.16 Water-methanol standard solution (2g water/L)

Use micro burette or pipette to inject 1mL of pure water into fully dried 500mL volumetric flask containing about 100mL of methanol;

5.17 Silicone grease

Used for lubricating frost glass joints.

6.1 Direct titration (visual method or coulometric method)

6.1.1 The apparatus in this method can refer to Appendix C, the apparatus of this Standard consists of the following parts.

6.1.1.1 Automatic burette. 25mL, fine tip, scale division of 0.05mL, connect the protection tube filled with desiccant, and prevent the atmospheric moisture into the tube.

6.1.1.5 Reagent bottle with Karl - Fischer reagent, capacity 3L, brown bottle, filling

tube inserting into automatic burette through the ground plug.

6.1.1.6 Bi-join rubber ball, connect with the drying bottle filled with desiccant; so that transfer the dry air to the reagent bottle, and fill the burette with the reagent.

6.1.1.7 End-point electricity measuring device, see Figure C.3 in Appendix C (visual

method can be omitted).

6.1.2 Medical syringe, with appropriate volume, which has been corrected.

6.1.3 Small glass tube (called sample tube), one end closed, the other end sealed by rubber plug; used for weighing specimen and adding materials into the titration vessel;

e.g.. weigh sodium tartrate (about 0.250g) for calibration of Karl - Fischer reagent, or weigh solid specimen.

6.2 Coulometric back-titration method

6.2.1 The apparatus in this method can refer to Appendix D; the apparatus of this Standard consists of the following parts.

6.2.1.5 Electromagnetic stirrer, fixed in the base with height adjustable; use

outsourcing glass or Teflon soft steel rods to stir, the rotation speed is 150r/min~300r/min.

6.2.1.6 End-point electricity measuring device, see Figure D.3 in Appendix D.

6.2.2 Medical syringe, with appropriate volume, which has been corrected.

7.1 End-point determination principle

The color of iodine in the Karl - Fischer reagent vanished as encountered water in the to-be-tested specimen; the first excessive drop of reagent shall show color.

7.2 Operating procedures

7.2.1 Calibration of Karl - Fischer reagent

7.2.2 Determination

Discharge the residual liquid in the titration vessel thoroughly through the discharge nozzle; use syringe, through the rubber plug, to inject 25mL (the stipulated volume of the to-be-tested specimen) of methanol or other solvent; start the electromagnetic stirrer; in order to react with slight amount of water in the methanol, add Karl - Fischer reagent with automatic burette, till the solution turns to brown.

7.3 Result expression

7.3.1 The titer of Karl - Fischer reagent T is expressed in mg/mL; and calculated by Formula (1) or (2).

7.3.2 Water content of specimen

The water content of specimen X is expressed by mass fraction; and calculated by Formula (3) or (4).

8.1 End-point determination principle

Ensure there is a potential difference between two platinum electrodes; when the solution contains water, the cathodic polarization reverses the current flow; then titrate the end-point through cathodic depolarization accompanied by abruptly increasing current (shown by suitable electrical device).

8.2 Operating procedures

8.2.1 Calibration of Karl - Fischer reagent

the volume of consumed Karl - Fischer reagent (V3).

8.2.2 Determination

Discharge the residual liquid in the titration vessel thoroughly through the discharge nozzle; use syringe, through the rubber plug, to inject 25mL (the stipulated volume of the to-be-tested specimen) of methanol or other solvent; start the electromagnetic stirrer; in order to react with slight amount of water in the methanol, add Karl - Fischer reagent stipulated in 8.2.1 till the ammeter pointer become skew abruptly, and maintain for at least 1min.

8.3.2 Water content of specimen

The water content of specimen X is expressed by mass fraction; and calculated by Formula (7) or (8).

9.1 End-point determination principle

Add excessive Karl - Fischer reagent, back-titrate with water-methanol standard solution.

9.2 Operating procedures

9.2.1 Calibration of Karl - Fischer reagent

9.2.1.1 Assemble the apparatus as per the Appendix D; lubricate the joint with silicone grease; use one automatic burette to add excessive Karl - Fischer reagent to the titration vessel; so that the electrodes are immersed; start the electromagnetic stirrer; connect with end-point electricity measuring device; then use the second automatic burette to titrate the standard water-methanol solution till the ammeter pointer suddenly is back to zero.

9.2.2 Corresponding values between Karl - Fischer reagent and standard water-

methanol solution

9.2.3 Determination

Discharge the residual liquid in the titration vessel thoroughly through the discharge nozzle; use syringe, through the rubber plug, to inject 25mL (the stipulated volume of the to-be-tested specimen) of methanol or other solvent; start the electromagnetic stirrer; in order to react with slight amount of water in the methanol, add excessive (about 2mL) Karl - Fischer reagent; then titrate water-methanol standard solution till the ammeter pointer is suddenly back to zero.

9.3 Result expression

9.3.1 The titer of Karl - Fischer reagent T is expressed in mg/mL; and calculated by Formula (9) or (10).