GB/T 2678.2-2021Paper, board and pulps - Determination of water-soluble chlorides (English PDF)

3.1 water-soluble chlorides

The chloride ion content in aqueous extracts of paper, board and pulp specimens measured under specified conditions.

4.1 Principle

The specimen is extracted with boiling water for 1h. Then in a solution containing chloride ions, drop into mercuric nitrate standard titration solution. At this time, mercury ions immediately react with chloride ions to form insoluble mercuric dichloride. Add excess ethanol to the titrant to reduce its solubility. When all the chloride ions in the solution become mercuric dichloride, a slight excess of mercury ions immediately forms purple mercury compounds with diphenylcarbazone added to the solution. Use the concentration and volume of the standard solution of mercury nitrate consumed to calculate the chloride ion concentration in the specimen extract.

4.2 Reagents

Unless otherwise specified, only analytically pure reagents are used.

4.2.1 Water, GB/T 6682, grade two.

4.2.2 Hydrogen peroxide (H2O2) solution, 30% (mass fraction).

4.2.3 Ethanol (CH3CH2OH) solution, 95% (volume fraction).

4.2.4 Nitric acid (HNO3) solution [c(HNO3)=1.0mol/L]. Carefully measure 90mL

[c(HNO3)=14mol/L] of concentrated nitric acid (HNO3 content is about 65%). Add to 500mL of water (4.2.1). Then dilute to 1L.

4.2.5 Sodium hydroxide (NaOH) solution, [c(NaOH)=0.50mol/L].

4.2.6 Sodium chloride standard solution [c(NaCl)=0.01mol/L]. Accurately weigh

0.5846g of the benchmark sodium chloride that has been calcined at 500 degrees C ~ 600 degrees C for 2h and dissolve in water (4.2.1). Transfer to a 1000mL volumetric flask. Use water (4.2.1) to dilute to the scale. Certified standard solutions can also be purchased.

4.2.7 Mercury nitrate standard solution {c[Hg(HNO3)2]=0.01mol/L}. See Annex B for

preparation and calibration.

4.2.8 Nitric acid (HNO3) solution (volume fraction 1:1). Carefully measure 90mL of

concentrated nitric acid c(HNO3)=14mol/L (HNO3 content is about 65%)]. Add into 500mL of water (4.2.1). Then dilute to 1L.

4.2.9 Diphenylcarbazone (C13H12ON4) indicator [c(C13H12ON4)=10g/L]. Weigh 0.25g

of diphenylcarbazone and dissolve it in 25mL of 95% ethanol. Store in a brown bottle.

This solution is valid for one week.

4.3 Instruments and equipment

4.3.1 Analytical balance: the resolution is 0.0001g.

4.3.2 pH meter: the precision is 0.01 units.

4.3.3 Constant temperature water bath.

4.3.4 1mL microburette: the minimum division is 0.01mL.

4.3.5 150mL, 250mL, 500mL conical flasks.

4.3.6 Wild-mouth bottle with ground glass stopper.

4.3.7 Reflux condenser.

4.3.8 Buchner funnel.

4.3.9 Other routine laboratory glassware.

4.4 Specimen collection and preparation

If it is to evaluate a batch of pulp, the specimen shall be collected according to GB/T 740-2003. If the test is used to evaluate a batch of paper and board, the specimen shall be taken in accordance with GB/T 450. If evaluating other types of samples, it shall be ensured that the samples taken are representative.

Due to the low chloride content in some samples, contamination of the samples shall be avoided during sampling. Clean gloves shall always be worn during specimen preparation. Tear or cut the sample into 5mm x 5mm specimens.

It shall be ensured that the laboratory is free of dust and fumes of chlorine-containing substances such as hydrochloric acid or chlorinated solvents. If the paper mill uses chlorine or chlorine dioxide as a bleaching agent, special attention shall be paid to specimen collection and preparation in the factory's in-house laboratory. Please do it in the on-site laboratory.

Wrap the sample in aluminum foil or plastic bag after specimen preparation until analysis. Store in the wild-mouth bottle with ground glass stopper.

The determination shall be carried out immediately after specimen preparation.

4.5 Test steps

4.5.1 Do two experiments in parallel. Extract each specimen in duplicate. Carry out a

blank test of the reagents in full accordance with the method for the test specimen.

4.5.2 Accurately weigh 5.0g of the air-dried specimen, accurate to 0.001g. At the same

time, take another specimen of the same mass to determine the absolute dry mass m of Extract the sample with boiling water for 1h. Filter the extract. Use hydrogen peroxide heating to reduce the disturbances that may be caused by carbohydrates. Add nitric acid to dissolve and acidify the test solution. Add acetone. Then use potentiometric titration method to titrate. With the addition of silver nitrate standard solution titrant, the potential of the indicator electrode changes accordingly. Near the end point of the titration, the concentration of the measured ion undergoes a sudden change, causing a sudden jump in the electrode potential. The end point of the titration can be determined from the jump in the electrode potential. From this, the content of chloride ions can be calculated from the volume of the consumed silver nitrate standard solution.

5.2 Reagents

Unless otherwise specified, only analytically pure reagents are used.

5.2.1 Water, GB/T 6682, grade two.

5.2.2 Nitric acid solution (1+1). Use water (5.2.1) to dilute 500mL of

[c(HNO3)=14mol/L] concentrated nitric acid (HNO3 content is about 65%) to 1L.

5.2.3 Acetone (CH3COCH3), chloride free.

5.2.4 Silver nitrate standard solution [c(AgNO3)=20mmol/L]. Accurately weigh 3.397g

of dried silver nitrate. Use water (5.2.1) to make it completely dissolved. Transfer it into a 1000mL volumetric flask. Use water (5.2.1) to dilute it to the scale. This solution shall be stored in the dark.

5.2.5 Sodium hydroxide solution [c(NaOH)=0.1mol/L]. Weigh 4g of sodium hydroxide.

Use water (5.2.1) to dissolve it. Transfer into a 1000mL volumetric flask. Then use water (5.2.1) to dilute it to the scale.

5.2.6 Hydrogen peroxide solution, 30% (mass fraction).

5.3 Instruments and equipment

5.3.1 Potentiometer or other measuring instrument: The measured DC voltage is

0mV~300mV. The accuracy is not less than 2mV. Use a silver electrode (silver ion selective electrode) as the indicator electrode, and a glass electrode as the reference electrode. Use an electrokinetic potentiometer with a motor-driven microburette and graph recording.

5.3.2 Glass microsyringe: 0.100mL, can read to 0.001mL.

5.3.3 500mL conical flask.

5.3.4 Reflux condenser.

5.3.5 Constant temperature water bath.

5.3.6 Analytical balance with a resolution of 0.0001g.

5.3.7 Magnetic stirrer.

5.3.8 Other routine laboratory glassware.

5.4 Specimen collection and preparation

Proceed as described in 4.4.

5.5 Test steps

5.5.1 Do two experiments in parallel. Each specimen is extracted in duplicate. Carry

out a blank test according to the procedure for testing specimens.

5.5.2 Weigh air-dried specimens. Weigh 20g for high-purity electrical paper. For general

paper, weigh 4g, accurate to 0.0001g. At the same time, take another specimen of the same mass. Determine the absolute dry mass m of the specimen according to the provisions of GB/T 462. Put the specimen into the 500mL Erlenmeyer flask (5.3.3).

Add 300mL of freshly boiled water (5.2.1) to high-purity electrical paper. For general paper, add 100mL of water (5.2.1). Install the reflux condenser (5.3.4). Extract in boiling water for 60min+/-5min.

5.5.3 When the extraction time is up, take it out and let the extract cool to room

temperature. Pour out or use a glass filter to filter. For high-purity electrical paper, pipette 150mL of filtrate into a 250mL beaker. For general paper, pipette 50mL of filtrate. Then add 10 drops of sodium hydroxide solution (5.2.5) and 10 drops of hydrogen peroxide solution (5.2.6). Put them on an electric hot plate to heat, oxidize and decolorize. Wait until the solution evaporates to about 5mL. After the test solution is cooled to room temperature, add 1mL of nitric acid solution (1+1) (5.2.2).

5.5.4 Use 10mL of acetone (5.2.3) to wash a 50mL beaker three times. Transfer the

above test solution (5.5.3) to this beaker. Immerse the electrode of the potentiometer (5.3.1) in the test solution. Use a magnetic stirrer to stir continuously at a constant speed.

Read out the potential value on the potentiometer. Use the glass microsyringe (5.3.2) to add 0.01mL of silver nitrate standard solution (5.2.4) each time for potentiometric titration. After each addition of silver nitrate standard solution, read the potential value once. Potential starts to change slowly. With the increase of the amount of silver nitrate standard solution added, the potential change increases. Titrate until the potential value changes slowly again.

NOTE: If an automatic titrator is used, the rate of adding titrant is set to 0.1mL/min~0.2mL/min.

5.6 Result calculation

The water-soluble chloride content of the specimen shall be calculated according to formula (2):

2°C) water (6.2.1). Dissociate the specimen until the specimen is completely dispersed.

Then soak the specimen for 1h. Gently stir while soaking to ensure complete extraction of chlorides.

After the stirring stops, immediately draw a part of the suspension with a syringe (6.3.3).

Filter it with the tea filter (6.3.4) to prepare the specimen solution.

6.5.1.3 Thermal extraction method

Weigh the air-dried specimen. Weigh 20g for high-purity electrical paper. For general paper, weigh 4g, accurate to 0.0001g. At the same time, take another specimen of the same mass to determine the absolute dry mass m of the specimen according to the provisions of GB/T 462. Put the specimen into the 500mL Erlenmeyer flask (5.3.3).

Add 300mL of freshly boiled water (6.2.1) to the high-purity electrical paper. For general paper, add 100mL of water (6.2.1). Install the reflux condenser (5.3.4). Extract in boiling water for 60min+/-5min.

Take it out when the draw is up to the time. Let the extract cool to room temperature.

Pour out or filter through a glass filter. For high-purity electrical paper, pipette 150mL of filtrate into a 250mL beaker. For general paper, pipette 50mL of filtrate. Use a syringe (6.3.3) to draw part of the solution. Filter it to prepare the specimen solution.

6.5.2 Drawing of working curve of chloride ion

6.5.2.1 Use chloride dilute solution (6.2.3) to prepare concentration as c(Cl-)=1mg/L, c(Cl-)=3mg/L, c(Cl-)=5mg/L, c(Cl-)=8mg/L, c(Cl-)=10mg/L series working solution.

The concentration of the working solution can be adjusted appropriately according to the chloride ion concentration in the test solution.

6.5.2.2 Set the test conditions according to the instrument manual. Measure the working

solution in turn. Draw a working curve according to the measurement results and the chloride ion concentration of the calibration solution.

6.5.3 Specimen and blank test

Set the test conditions according to the instrument manual. Determine the specimen and blank solution, respectively. Calculate the chloride ion concentration in the solution according to the working curve.

6.6 Result calculation

The water-soluble chloride content of the specimen shall be calculated according to formula (3):

Where, X - The water-soluble chloride content of the specimen, in milligrams per kilogram (mg/kg);

c1 - The chloride ion concentration in the specimen solution after filtration, in milligrams per liter (mg/L);

c0 - The chloride ion concentration in the blank solution, in milligrams per liter (mg/L);

V - The volume of water used, the specified volume is 250mL;

m0 - The mass of the weighed specimen, in grams (g);

w - The absolute dry matter content of the sample, %.

The average of the two determination values is used as the measurement result. The result is rounded to 0.1mg/kg.

7.1 Nitric acid (HNO3) solution [c(HNO3)=1mol/L]. Carefully measure 90mL

[c(HNO3)=14mol/L] of concentrated nitric acid (HNO3 content is about 65%). Add into 500mL of water (6.2.1). Then dilute to 1L.

7.2 Carefully clean glassware and other instruments that come into contact with the

specimen or extract. All glassware shall be soaked in nitric acid solution (7.1) for 5min~10min. Rinse thoroughly with boiling water. Tweezers and scissors used for sample preparation shall be washed with boiling water.

7.3 During the preservation, collection and preparation of the specimen, the pollution

of the environment and the operation process shall be avoided.

7.4 Calculate with a sampling amount of 5g. The detection limit of mercury nitrate

method is 10mg/kg. The detection limit of silver nitrate potentiometric titration is 5.0mg/kg. The detection limit of ion chromatography is 20mg/kg. The detection limit of the method can be reduced by appropriately increasing the sampling amount, concentration and so on. In this case, the detection limit can be evaluated according to the actual test conditions.