# Determination of Cellulose Degree of Polymerization in Historical Papers with High Lignin Content
## 3 Results and Discussion
### Determination of Kappa Number and Solubility in CED  
The Kappa number [29] is an important parameter for determining the residual lignin content in a paper made from finished or in-process pulp. It is thus a parameter indicative of the completeness of pulping process for many types of chemical and semichemical pulps, both bleached and semibleached. The Kappa number is based on the reaction of a highly oxidizing chemical, potassium permanganate, with lignin as well as a low content of certain other organic impurities that remain in the pulp at various stages of its processing. The Kappa number provides valuable information to both the manufacturer of the pulp and the papermaking user of the pulp about the properties of the pulp as well as the paper made from it, especially the content of residual lignin present [33]. The Kappa number of the pulp was determined for all samples before and after delignification (Table 2) according to the procedure of Kaminska [12]. The standard applies to many types of chemical, semichemical, unbleached, and semibleached pulps within the Kappa number range from 1 to 100. Above a Kappa number of 100, the accuracy of the test, as well as the relationship between Kappa number and lignin content, may decrease. Therefore, for two paper samples (No. 1 and No. 5), where the determined Kappa values were above 100, the exact values are not given. In Table 2, it is only indicated that these two samples exceed the value of 100. Kappa numbers were the highest in the samples containing the highest percentage of groundwood (samples No. 1 and No. 5) and the lowest in the sample (No. 13) with the lowest content of groundwood pulp (Table 1). The Kappa number decreases with delignification time for all the samples studied, as evident from Table 2. Solubility in CED was tested before and after delignification of the samples for 6 h (and for sample No. 1 also after 3 h). In the cases where the delignified samples did not dissolve in CED, a fresh portion of the reaction mixture was added, and the time of delignification was prolonged for an additional 3 h. In the study by [PERSON] et al. [23], during delignification of different cellulose biomass samples with higher concentration of reagents (0.6 g sodium chlorite and 0.6 mL acetic acid per gram of paper; in our case, the ratio was 0.375 g sodium chlorite and 0.1254 mL acetic acid per gram of paper) and fresh loading of reagents added every 2 h, more than 90% of the lignin was removed after 6 h. In the study, reaction time beyond 6 h appeared to have low impact on further delignification [23]. The differences in delignification of investigated samples can be attributed to different concentrations of the reagent used in our study. As evident from Table 2, all samples with lower Kappa numbers, i.e., less than 23, completely dissolved into CED. According to the standard SCAN-CM 15:88 [17], the method for viscometric determination of the average degree of polymerization can be used for all types of pulps that dissolve in the CED solution. According to the Tappi standard [29], for pulps below 70% total pulp yield, the Klasoni lignin content (in %) is approximately equal to the Kappa number \(\times\)0.13. If the Kappa number is to be used to determine a precise numerical value for the lignin content in a particular pulp, more specific relationships have to be developed for each pulp type, since the relationship varies with wood type and delignification procedure [33; 34]. Since the relationship cited above [29] has not been proven and specific relationships have not been determined for each pulp used in the present study, we can only roughly estimate [33], from the results presented in Table 2 that the lignin content that might still be present in the paper samples should be lower than 3%.