Acidic functional groups, dissociation

Electrostatic discrete functional group models. The development of charge on the surface of the humic macromolecule decreases the tendency to dissociate protons from the acid functional groups. To overcome this problem an electrostatic correction factor is introduced into the acid dissociation and complexation constants. This is similar to the approach adopted for the SCMs for inorganic surfaces. 

Oiganic colloids (mainly humic substances) also possess large specific surfaces and are generally negatively charged due to the acidic functional groups that tend to dissociate or polarize. Examples of such groups include carboxyls (R-COOH), quinones (-Ar=0), phenols (-Ar-OH), and enols (R-CH=CH-OH). (See Section 5.4.2 for a more complete discussion of humic substances.)... 

The effects of solution acidity and basicity on luminescence spectra result from the dissociation of acidic functional groups or protonation of basic functional groups associated with the aromatic portions of fluorescing and phosphorescing molecules. Protonation or dissociation can alter the natures and rates of non-radiative processes competing with luminescence and, thereby, affect the quantum yields of emission. For example, the antimalarial mefloquine fluoresces very weakly but phosphoresces well in neutral aqueous media. However, at pH < 1, its protonated form fluoresces intensely, and its phosphorescence is very weak. 

Paddison, Eikerling and coworkers performed quantum chemical calculations and ab initio MD simulations of crystal hydrates [89-91], The studies reveal the strong influence of the acid functional group on proton mobility at low humidity they show how chemical modifications of side chain entities can make the dissociation of the acid more complete. 

If a is defined as the degree of dissociation of the acidic functional groups, then ... 

Organic Matter Complexation ModeL Soil humus contains a variety of acidic functional groups, some of which have high specificity for Al complexation or chelation (see section 4.3b). Al-organic complex formation requires that the acidic group, RH , dissociate ... 

These organic compounds possess acidic functional groups, such as carboxylic or phenolic, that dissociate to form anions ... 

The influences of pH on luminescence spectra are derived from the dissociation of acidic functional groups or the protonation of basic functional groups, associated with the aromatic portions of fluorescing molecules. 

Different equilibrium dissociation constant and equilibrium ionisation constant equations may then be derived with reference to the kinetic scheme (Scheme 8.16) on the basis of stasis, such as Equation (8.93), which dehnes an equilibrium dissociation constant equilibrium ionisation constant for acidic functional group 1, or 8.95 that defines the equilibrium ionisation constant K di for basic functional group 2 ... 

The NOM sample, which was concentrated as described in Appendix 1, was titrated using a Metrohm automatic titrator. The titrator was operated in dynamic titration mode. The samples were acidified from ambient pH to pH 2.8 with 0.1 M HNO. s and subsequently alkalised with 0.1 M NaOH to pH 10. It was assumed that at pH 2.8 all acidic functional groups will be saturated, whereas at pH 10 all carboxylic and half of the phenolic groups were dissociated. The limitations of these assumptions were discussed in Chapter 2. 

The titration vessel was purged with nitrogen to eliminate CO 2. From the volume and molarity of added base and the mass of titrated DOC, the content of acidic functional groups can be calculated. Carboxylic acid content was calculated from the amount of base added until the end-point was reached. Phenolic acid content was calculated as twice the difference in titrant required to change the pH of the titrate from 8 to 10, since it was assumed that at pH 10 only half the phenolic groups were dissociated. A solution of a concentration of 20 mgL" as DOC NOM were titrated. The error due to the salt content of NOM is likely to be high. 

Rather, the setting reaction is entirely an addition polymerization process, generally photo-initiated, and involving free radicals. Once the material is set, it is able to absorb trace amounts of moisture, and this allows the acid-functional groups to manifest their acidity by dissociating after which they attack the basic glass to form a small amount of salt matrix [1 ]. The effect of this moisture-initiated reaction is discussed in later sections of this chapter. 

---