A statistical model of proton binding by humus

A stochastic model for the synthesis and degradation of natural organic matter part II: molecular property distributions

“…pK a distributions are not directly experimentally accessible, although they have been estimated from titration data using numerical methods (Perdue and Lytle 1983;Nederlof et al 1993). In particular, several authors have found that a Gaussian distribution of pK a provides a parsimonious representation of pH titration data; in general, only two Gaussian distributions-one for carboxyl acidity and one for phenolic acidity-can provide a reasonable 'fit' to the data (Perdue et al 1984). Titrating Satilla River FA at ionic strength 0.1, Perdue and Lytle (1983) found the carboxyl acidity could be described by a Gaussian distribution with a mean of 3.62 and a standard deviation of 2.31.…”

“…pH titrations of NOM from different sources tend to be similar in shape, showing some slight negative charge (a = 0.1 to 0.2) even at fairly acidic pH, a featureless increase in a between pH 3 and 6, and a smaller increase between pH 6 and pH 8-9 leading to an 'endpoint' between pH 8 and 9. A second area of increasing negative charge at pH [ 9 is not believed to correspond to the deprotonation of carboxylic acid groups (Perdue et al 1984;Leenheer et al 1995a), and can only be considered when successful QSARs have been developed for phenols and aliphatic amines (see comments above under pK a QSAR. Figure 13a compares pH titration data for terrestrial NOM with calculations based on the simulated soil NOM pK a distribution shown in Fig.…”

Dependence on molecular weight of acid-base properties of humic and fulvic acids

“…From the examination of the parameters of the distributions it can be drawn out that the mean pK A values of the t wo classes of functional groups are consistent with the known properties of carboxyl (mean pK A value = 4.5) and phenolic groups (mean pK A value = 10.0), respectively [3]. In our case it is difficult to assess the significance of σ 1 and σ 2 , due to their high variability, except for the fact that they are indicative of fairly wide ranges of the values in both structural classes.…”

“…Because of the huge variety and number of acidic groups, HS acid-base properties can not be described by discrete values of pK a . It was suggested that the acidity of HS has to be mainly ascribed to the presence of carboxylic groups, with pK a values resembling a Gaussian distribution, and to further weaker acidic moieties, like phenolic groups, some carboxylic acids and alcoholic groups deriving from carbohydrate residues, whose pK a belong to a second Gaussian distribution [3]. Owing to these characteristics of the acidic functionalities in HS, a c o n t i nuous bimodal Gaussian distri bution model, ch a ra cterised by two mean values (pK A,1 and pK A,2 ) and two relevant standard dev i ations (σ 1 and σ 2 ) [4] was adopted fo r describing their acidic properties and by fitting at best the experimental data.…”

Dissolved organic matter contribution to rain water, throughfall and soil solution chemistry

“…The carboxylic functional group is the most abundant acidic group. It is characterized by relatively high dissociation constants (pK a ∼ 4) [7].…”