Titration, surface group

Experimental. Characterizations of a heterogeneous surface by means of surface group titration utilizing visible and ultraviolet chemical indicators to define the titration end point have frequently been employed with white solid catalysts(7-12), (17-20). Aspects of the surface acid group distribution have often correlated with the catalytic activity of the solid(2-9), (21-25). An adaptation of the technique appears to be suitable for studying the interactions between the surface acid groups in mixtures of carbon black and white reference solids. 

Polystyrene Latexes. The polystyrene latexes used were the mono-disperse LS-1102-A, LS-1103-A, and LS-1166-B (Dow Chemical Co.) with average particle diameters of 190, 400, and llOOnm, respectively. The latexes were cleaned by ion exchange with mixed Dcwex 50W-Dowex 1 resin (9). The double-distilled and deionized (DDI) water used had a conductivity of 4x10 ohm- cm-. The surface groups of the ion-exchanged latexes determined by conductometric titration (10) were strong-acid sulfates the surface charge densities were 1.35, 3.00 and 5.95 jiC/cm, respectively. 

Conductometric titration of the surface groups gave a surface charge density of strong acid (-S0 ) of 0.56 C/g and of weak acid (-COO ) of 0.23 C/g. Preceding the conductometric titration the latex was cleaned by serum replacement with doubly distilled water and 5 10 M HCl as described in ref. ( 3). 

Polystyrene latexes have been prepared using persulfate initiator for many years, but only recently have methods been developed to determine the number and loci of the sulfate surface groups. To determine these surface groups, the latex is cleaned to remove the adsorbed emulsifier and solute electrolyte, then the surface sulfate groups in the H+ form are titrated conductometrically with base. The latexes can be cleaned effectively by ion exchange (2-5) or serum replacement (6) dialysis is not effective in removing the adsorbed emulsifier and solute electrolyte (3,5,6). +... 

For serum replacement (6), the latex is confined in a cell with a uniform-pore-size Nuclepore filtration membrane. Distilled, deionized water is pumped through the latex until the conductance of the effluent stream is about the same as that of the distilled, deionized water. This serum replacement removes the adsorbed emulsifier and solute electrolyte quantitatively and allows recovery of the serum in a form suitable for further analysis however, it does not+replace the Na+ and K counterions of the surface groups with Vl ions. To do this, dilute hydrochloric acid (ca. 10 N) is pumped through the latex, followed by distilled, deionized water to remove the excess acid. The latex is then titrated conductometrically to determine the surface charge. 

The latexes were cleaned by ion exchange and serum replacement, and the number and type of surface groups were determined by conductometric titration. The molecular weight distributions of the polymers were determined by gel permeation chromatography. The stability of the latexes to added electrolyte was determined by spectrophotometry. The compositional distribution was determined by dynamic mechanical spectroscopy (Rheovibron) and differential scanning calorimetry, and the sequence distribution by C13 nuclear magnetic resonance. 

Thus obtained complexion quotients, are extrapolated to pHpzc similarly to Davies et al. method. The difference between constants, calculated for different ionic strengths is small and changes few hundredths of the pK unit when the concentration is changing ten fold. Hydroxyl surface group complexion constants, calculated according to this method, differ much from that of obtained from potentiometric titrations of the suspension. 

For carbons with large amounts of surface groups per unit weight the method of direct titration can be... 

The titratable surface charge measurements can be interpreted to give a quantitative assessment of the interactions between the solid and the supporting electrolyte ions. Potentiometric titrations measure the adsorption or release of protons and the model developed by Yates ad. (29) and Davis et al. (17) proposes reactions between oxide surface groups and... 

James, R. 0., Davis, J. A., and Leckie, J. 0., Computer simulation of the conductometric and potentiometric titrations of the surface groups on ionizable latexes. J. Coll. Interface Sci. 65, 331-344 (1978). 

For monofunctlonal surfaces with weak surface groups the first leg of a conductometric titration plot Is also ascending, though less so than the second leg. Hence the number of groups can still be determined. Different types of groups on one surface can be conductometrically distinguished by titration at different pH s If the various proton affinities are sufficiently apart. 

Properties of a° (pH) curves are basic elements in the Interpretation of more complicated systems involving oxides. One of these is the adsorption of hydrolyzable ions (Cd, Al, etc.) or anions that themselves can be titrated (HPO, etc.). In sec. 3.14 some of the relevant applications will be discussed. Another application is that of mixed oxides. The systems include mechanically mixed pure oxides and mixed crystals (such as spinels and ferrites). A number of authors have studied such mixed oxides, thereby reporting the variation of the pH° as a function of the mole fraction of the solid. Sometimes linearity was found, sometimes not. No genered rules can be given. The surface composition is not necessarily identical to that of the bulk, molecules of one oxide may leach and adsorb onto the other and lateral interactions of surface groups of the two constituents affect their pK s and pK s. Mixed oxides are important for a number of technical applications (heterogeneous catalysts with special properties, components of batteries) and also occur in clay minerals, the topic of the following subsection. 

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

The site density number determined from the titration is 1.1-1.2 / nm. This parameter is used to fit the titration and zeta curves of particulates in NaCl by using a non-linear programmation. Four main surface reactions were determined two in relation to the amphoteric dissociation of =A1-0H surface groups (pKi=6.4 and pK2=10) and two with complexation of Cl on =A1-0H2 site and Na" " on =Al-0- site (pK3=3.4 and pK4=3.5). 

---