BS ISO 21436:2020 pdf download – Pulps — Determination of lignin content — Acid hydrolysis method
9.1.2 Hydrolysis procedure A Hydrolysis procedure A is based on the use of 300 mg of pulp. This procedure is more commonly used in laboratories equipped with an autoclave, and is similar to that described in other methods [7]-[9] in situations in which analysis of carbohydrates is required, in addition to analysis of lignin.
For larger pulp specimens, the acid concentrations for both primary and secondary hydrolysis must remain the same as those used for 300 mg of pulp; thus the volume of solutions must be adjusted accordingly. Add 3,0 ml of 72 % sulfuric acid (6.2) to the test specimen in the beaker. Add the acid gradually in small increments while stirring and macerating the material with a glass rod. To avoid losses, ensure that no material is sticking to the glass rod when it is removed. NOTE Some pulps do not absorb the acid and therefore do not disperse readily. In such cases, after addition of acid, place the beaker under vacuum, in a vacuum desiccator for at least 15 minutes to facilitate wetting and absorption. Place the beaker in a water bath adjusted to 30 ± 1,0 °C for 1 h. Stir occasionally. Add 84,0 ml of water. Mix, cover the beaker with aluminium foil and place it in an autoclave at (120 ± 3) °C for 1 h. Allow the insoluble lignin to settle and for the beaker and contents to cool to approximately 80 °C.
NOTE In some cases, the lignin requires an overnight or a longer period to settle, especially if it is finely dispersed. 9.1.3 Hydrolysis procedure B Hydrolysis procedure B is based on the use of 1,0 g of pulp, and is similar to that described in other methods [10][11] . This procedure is also used in some laboratories.
NOTE Hydrolysis procedure B is not used when carbohydrate analysis is also required. For smaller pulp specimens, the acid concentrations for both primary and secondary hydrolysis must remain the same as those used for 1,0 g of pulp; thus the solution volumes shall be adjusted accordingly. Place the beaker with the test specimen in a water bath (5.2) at 20 ± 1 °C and add gradually 20,0 ml of 72 % sulfuric acid (6.2), maintaining a reaction temperature of 20 ± 1 °C Stir thoroughly with a glass rod for about 1 min.
To avoid losses, ensure that no material is sticking to the glass rod when it is removed. NOTE Some pulps do not absorb the acid and therefore do not disperse readily. In such cases, after addition of acid, place the beaker under vacuum, in a vacuum desiccator for at least 15 min to facilitate wetting and absorption.
Keep the beaker in the bath for 120 ± 10 min with occasional stirring. Transfer the material quantitatively from the beaker into a 1 000 ml Erlenmeyer flask and dilute with water to 750 ml. Boil the solution for 4 h, maintaining a constant volume of solution by frequent addition of hot (80-90 °C) water. Allow the insoluble lignin to settle and for the flask and contents to cool to approximately 80 °C. NOTE In some cases, the lignin requires an overnight or a longer period to settle, especially if it is finely dispersed.
9.2 Filtration
Without stirring the insoluble lignin residue, decant or siphon off the supernatant solution through a pre-weighed filtering crucible (5.1) placed on a 250 ml filtering flask (5.1) (hydrolysis procedure A) or a 1 000 ml filtering flask (5.1) (hydrolysis procedure B). Proceed to filtration procedure A if hydrolysis procedure A is used, or to filtration procedure B if hydrolysis procedure B is used. Filtration procedure A: Transfer the filtrate to a 250 ml volumetric flask. Wash the precipitate with 2 × 30 ml warm water (6.1) and add the washings to the volumetric flask. Allow to cool to room temperature and fill up to the mark with water (6.1). This filtrate will be used for the acid-soluble lignin determination.
NOTE Dilution to precisely 250 ml also allows accurate quantification of carbohydrates in the filtrate, if required. Filtration procedure B: Proceed as in filtration procedure A, but transfer the filtrate to a 1 l volumetric flask, wash the precipitate with 2 × 100 ml warm water (6.1), and add the washings to the volumetric flask. Allow to cool to room temperature and fill up to the mark with water. This filtrate will be used for the acid-soluble lignin determination.