Surface group titration procedure

The potentiometric titration was carried out in order to determine the functional groups present in the biomass surface. During the titration experiments, the C02-free condition was always maintained to avoid the influence of inorganic carbon on the solution pH. Detailed potentiometric titration procedure and estimation method of functional groups are available in the previous reports [4,6]. 

Boehm s procedure [6] was used to titrate carboxyls, lactones, phenols and carbonyls functional surface groups. 

Another key contribution of the Schwarz group was the recognition of the dramatic influence of oxide surfaces on bulk solution pH. In a landmark 1989 paper, Noh and Schwarz [7] demonstrated the method of mass titration, in which successive additions of oxide cause stepwise shifts in solution pH. This procedure is illustrated in Figure 6.7 [7], As indicated in Figure 6.1, the protonation-deprotonation chemistry of the surface hydroxyl groups is coupled to the liquid-phase pH. In mass titration, as the mass (or more appropriately, the surface area) of oxide in solution increases, the solution pH is brought to the PZC of the oxide, at which point no driving force for proton transfer exists... 

Boehm and Johnson Matthey titrations. Both titrations give information on the acidic or basic character. The procedures are described in [13] and [14]. Boehm method classifies the surface acidic sites in three types (called phenolic, lactonic, carboxylic), by growing strength of acidity. However, this arbitrary classification fails to attribute one surface acidic group to one type of acidity. Moreover, this method does not give very accurate results with solids containing few acidic sites. Nevertheless, it appears for our active carbon that "phenolic" sites are a majority. The amount of each type of acidic sites does not change very much with the nitric treatment. 

The density of silanol groups was measured by means of an acid-base titration using a solution of NaOH in a water/methanol mixture (50 50 % w/w), following a procedure described by Sears [20]. It should be noted that only silanol groups on the silica surface are measured with this method. 

The acidic groups have different pK values depending on their location on the carbon surface relative to the location of nonacidic groups that can exert an inductive effect on them [46], Then, by a potentiometric titration method it is assumed that the system under study consists of acidic sites characterized by their acidity constants K. The site population can thus be described by a continuous pK, distribution function f(pKJ [54-57]. The experimental titration data are thus transformed into a proton-binding curve from which the distribution of acidity constants is obtained by using, for example, the splines-based numerical procedure SAIEUS suggested by Jagiello [58]. 

---