AS NZS 1301.421:2020 – Method of test for pulp and paperMethod 421: Paper, board and pulps -Determination of pH of aqueous extracts-Cold extraction
AS NZS 1301.421:2020 – Method of test for pulp and paperMethod 421: Paper, board and pulps -Determination of pH of aqueous extracts-Cold extraction.
Weigh 2,0 g ± 0.1 g of air-dry sample (6.2) in a 250 ml flask (5.1).
NOTE Sincc the amount of sample is not critical, there Is no need to determine the dry matter content to adjust
for minor differences in moisture content.
Add 100 ml of water (5.1) to the flask (6.1) containing the sample pieces. Cheek that all pieces are soaked. Seal the flask with its ground-glass stopper and leave it to stand for I h at a temperature between 20 C and 25 °C. Shake the flask at least once during this time.
Filter the extract through a coarse, Iritted glass filter into a small beaker (6.1). Immediately add 2 ml of potassium chloride solution (5.3) and continue with the measurement.
8.3 Determination of pit
Operate the pH-meter in accordance with the manufacturer’s instruction. Wash the electrodes with water (5.1); allow the water to drain from the electrodes, hut do not wipe them. Calibrate the pll•meter (5.2), at a temperature between 20 °C and 25 °C, with two different buffer solutions (5.2) having pH- values such that the pH of the extract is between the pH-values of the buffer solutions. The first buffer solution shall be chosen so that the pH-value of the buffer solution is In the same region as the electric zero point of the pH-meter (usually = 7). The reading for the second buffer solution should agree with its correct value to within 0.1 pH-unit.
If the pH-meter fails to show the correct pH-value for the second buffer solution, consult the manufacturer’s manual. A deviation of, or exceeding, 0.2 pH-units, rounded to the nearest 0.1 pH unit, ndlcates a faulty electrode. In addition, a slow but continuous increase or decrease in the reading indicates a faulty electrode.
After calibration, rinse the electrodes several times with water (5.1) and once In a small quantity of the extract. Check that the temperature of the extract is between 20 C and 25 °C. Immerse the electrodes in the extract. Record the pH when there is no measurable drift, within 30 s.
Before measuring the next sample, rinse the electrodes carefully with water (5.1) to remove any traces of sample or buffer solution.
At the end of a series of measurements, check the electrodes with the same buffer solutions. The results for bath solutions should agree with their correct value to within 0.1 pH unIt. Otherwise repeat the procedure In 8.3.
Calculate the mean of the duplicate determinations.
Report the p11-value to the nearest 0,1 pH-unit. The individual results should not differ by more than 0.2 pH-unit; if they do. repeat the determination with two additional extracts, and report the mean and the range of all measurements.
NOTE See Annex B for precision.
10 Test report
The test report shall include the following information:
a) a reference to this document, I.e. ISO 6588-1;
b) the date and place of testing;
All the reagents used shall he of recognized reagent grade. The buffer solutions shall he renewed at least once a month. The anhydrous salts In A.1 and A.2 shall be dried at 120 °C.
A.1 Buffer solution pH 4,0: Potassium hydrogen phthalate, 0,05 mol/l solution
Dissolve 10,21 g of potassium hydrogen phthalate (K11C8H404) in water (5.1) in a 1 litre volumetric flask and dilute to the mark.
The pll•value of this solution Is 4.00 at 20 °C and 4.01 at 25 °C.
A.2 Buffer solution pH 6,9: Potassium dihydrogen phosphate and disodium hydrogen phosphate solution
Dissolve 3,39 g of potassium dihydrogen phosphate (KH2PO4) and 3.54 g disodlum hydrogen phosphate (Na2HPO4) in water (5.1) in a 1 litre volumetric flask and dilute to the mark.
The pH-value of this solution is 6.87 at 20 °C and 6.86 at 25 C.
A.3 Buffer solution pH 9,2: Disodium tetraborate solution
Dissolve 3.80 g of disodlum tetraborate decahydrate (Na2B4Oi.10H20) In water (5.1) In a 1 litre volumetric flask and dilute to the mark.