# Determination of Cellulose Degree of Polymerization in Historical Papers with High Lignin Content
## 4 Conclusions  
In the present study, the use of viscometry to determine the degree of polymerization of groundwood-containing pulps was evaluated. A significant number of samples were examined; however, the same trend was observed for all samples studied. It was demonstrated that paper samples can be dissolved in cupriethylenediamine (CED) when lignin content calculated from the Kappa number is lower than 3%; consequently, the average DP\({}_{\rm v}\) of cellulose can be determined with viscometry. The degradation of cellulose by delignification is more pronounced in the absence of lignin; therefore, complete removal of lignin is not required.  
As viscometry may provide data subject to systematic errors under certain conditions, such as delignification, is it necessary to compare the results of average DP\({}_{\rm v}\) determinations with those obtained using SEC. The results of the viscometric determinations of average DP\({}_{\rm v}\) are in good correlation with the M\({}_{\rm w}\) determined by SEC both before and after delignification.  
The process of delignification of paper samples was followed using SEC with UV-VIS detector. From the SEC chromatograms of the paper samples with lignin content reduced from 15 to 2.4%, it was evident that the lignin content did not interfere with the determinations of cellulose M\({}_{\rm w}\) after the preparation and analysis of cellulose tricarbanilates (CTCs) using our SEC setup. Under delignification conditions as presented in the study (0.375 g sodium chlorite and 0.125 mL acetic acid per gram of paper), no significant depolymerization of cellulose was observed in the paper samples which is confirmed by the results of the SEC analyses and monosaccharide determination in the filtrates of the reaction mixtures.  
The study suggests that the method of delignification of paper samples by Kamniska [12] can be used prior to viscometric determination of average \(\mathrm{DP_{v}}\) in CED in paper degradation studies of historical papers containing groundwood, when relative results would be sufficient. Therefore, a robust, inexpensive, and efficient classical method of determination of average \(\mathrm{DP_{v}}\) by viscometry can be applied to the historical groundwood-containing samples, and the instrumental technique SEC is not required.  
With the investigated set of historical paper samples and the limitations associated with their accessibility for additional analysis, it is planned to expand the existing historical sample set and conduct further research to obtain absolute results.  
Conceptualization, J.M.; methodology, J.M. and I.K.C.; validation, J.M., I.K. and I.K.C.; formal analysis, J.M., I.K. and I.K.C.; investigation, J.M. and I.K.C.; resources, J.M. and I.K.C.; data curation, J.M. and I.K.C.; writing--original draft preparation, J.M.; writing--review and editing, I.K.C. and J.M.; visualization, J.M.; supervision, J.M. and I.K.C.; funding acquisition, J.M. and I.K.C. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded by the Slovenian Research Agency, research core funding No. P1-0153. The APC was funded by the Slovenian Research Agency.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available because sharing and archiving research data in a publicly accessible repository has not been established yet.  
The authors acknowledge the financial support from the Slovenian Research Agency (research core funding No. P1-0153) and the contribution of paper samples from European Community, Sixth Framework, Energy, Environment and Sustainable Development Programme, Contract No. SSPI-006584 (PaperTreat). [PERSON] is gratefully acknowledged for sharing literature, paper samples, and the basic idea to perform the study. [PERSON], [PERSON], and [PERSON] are gratefully acknowledged for technical assistance. We gratefully thank [PERSON] for language editing.  
The authors declare no conflict of interest.