Charge determinations of cellulose fibres of different origin -: Comparison between different methods

Eleven different pulps, both chemically and thermomechanically produced and of both softwood and hardwood origin, were acid/base characterised. The measurements were performed as series of potentiometric and conductometric titrations. The potentiometric titration data revealed that all pulp materials were characterised by two buffer regions with an intervening buffer minimum at pHapproximate to6 and, from these measurements, it was possible to evaluate the buffer capacity in the low pH region. The capacity of the second buffer region could not be quantified since drifting readings characterised the data and since the region extended beyond the area of accurate pH determinations. With the conductometric titration data it was not possible to strictly separate the two buffer regions, and it was concluded that the method, as used in the present work, determined the low pH region buffer capacity plus a minor part of the buffer capacity in the upper buffer region. This conclusion was based on a finding that the differences in buffer capacity, as evaluated with the two methods, was related to the relative amount of the weaker acidity at pH>6. From the potentiometric titration data it was also possible to evaluate additional information, namely the initial charge of the pulps and the thermodynamic acidity of the acidic groups on the fibre. In an attempt to determine the origin of the weaker acidity and to determine the practical importance of this weaker acidity, paper sheets were made from one of the softwood pulps at neutral and alkaline pH and these sheets were spectroscopically and physically characterised.