Weakly acidic groups

Charged and uncharged forms of the ionizable —COOH and —NH3+ weak acid groups exist in solution in protonic equihbrium ... 

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

Diprotic Surface Groups. According to the diprotic model, the surface is represented as an ensemble of identical diprotic weak acid groups, which react according to ... 

In addition, Pauling and Sherman (222) had proposed a ring structure for the anion of aluminium tetrametaphosphate on the basis of a crystal structure determination. The triphosphate NaBP3Oi0 must, then, have the chain structure already proposed by Schwarz (263), for Rudy and Schloesser (250) showed that the acid corresponding to the triphosphate has three strongly and two weakly acidic OH groups with the weakly acidic groups... 

The pH of the humic acid solutions behaved as expected for polymers containing weakly acidic groups. The pH of the 16-mg/mL stock, as detected by a combination glass-reference electrode, changed slowly and did not stabilize. A typical value for the pH after 30 min of stirring was 4.5. When the humic solution was supplemented with 0.5 M KC1 to aid the equilibrium of protons with titratable groups, the pH came to a stable value after a few minutes. A pH of 3.9 was then recorded. If the humic acid solution was progressively diluted to 20 /zg/mL in 0.5 M KC1, the equilibrium pH rose to 6.4. The pH of the humic acid diluted to 2.0 /zg/mL in the synthetic hard water was stable and did not differ detectably from the pH of the water before addition of the humate. 

The eluent is rather acidic in most protein separations. With silica-based packings, the reason for the selection of this condition is related to the surface silanols which are weakly acidic groups. Above pH 3.5-4, these groups are dissociated and ionic interactions will occur with basic sample compounds. These interactions can be reduced simply using acidic mobile phases that prevent ionization of the silanols. Another reason is the stability of the silica which is higher in acidic media72). 

Thus, for weakly acidic groups, as found in amino-acids ... 

K. F. Bonhoeffer, L. Miller and U. Schindewolf (20) performed similar measurements i.a. at cation exchange membranes with weakly acid groups and strongly acid groups and K+, Na+, Li+ and H+ ions. 

To determine the effect of different polymerization conditions on the polymer endgroups produced, polymerizations were carried out using the standard bicarbonate buffer as well as other variations. Table V (13,16) shows that the use of the persulfate-bicarbonate combination with and without emulsifier gave latexes of final pH 7-8 with only sulfate groups. The addition of 10 5 silver ion gave a latex of pH 8.5, but with weak-acid groups, presumably because of oxidation of the sulfate groups. 

The omission of the bicarbonate buffer gave latexes of lower pH with weak-acid groups. The formation of weak-acid groups in these latexes is not merely an effect of pH during the polymerization potassium dihydrogenphosphate, sodium borate, and sodium... 

Assignment of the weakly acidic group (pK 10.4) of amicetin to the amide linkage between cytosine and p-aminobenzoic acid was confirmed163 by the synthesis of JV-benzoyl-1 -methylcytosine (XLIa) by benzoylation of 1-methylcytosine. This cytosine derivative (XLIa), upon titration,... 

NMR-spectrum of its tetramethyl ester, and its conversion, by total decarboxylation with zinc or copper, into apoadifoline, a methyl derivative of 1,6-diazaindene (XXXVIII). The destruction of ring A in adifoline by oxidation clearly indicates the presence of an oxygenated substituent, possibly a hydroxyl group. Such a substituent would account for the second, weakly acidic group in adifoline it is also consistent with the production of two /3-carboline derivatives, one of which is phenolic, in... 

Katz S, Beatson RP, Scallan AM (1984) The determination of strong and weak acidic groups in sulfite pulps. Sven Papperstidn 87 R48-R53... 

Conversion of keto and quinone groups to hydroxyl and phenol groups by sodium borohydride reduction is a possible approach toward decreasing hydrogen bonding by conversion of basic functional groups to weak-acid groups that become the predominant characteristic after reduction. Sodium borohydride reduction is performed under conditions shown in reaction (5) ... 

There is no potentiometric method that can be used to determine the phenolic hydroxyl content of humic substances. Consequently, phenolic groups are often simply assumed to be the difference between total acidity and carboxyl content. This author prefers the use of weakly acidic groups to represent these functional groups, as recommended by Stevenson (1982). While some of the weakly acidic groups are probably phenols, weaker carboxyl groups, alcoholic groups in carbohydrate entities, and enols may be of some importance (see Fig. 5). In an earlier section of the chapter, it was pointed out that the phenolic hydroxyl content of humic substances is probably limited by the amount of aromatic carbon in the sample. Before all the weakly acidic groups are assumed to be phenols, the structural implications of such an assumption should be examined. 

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