Carboxylic acids potentiometric titration

The method of choice for determining carboxyl groups in lignin is based on potentiometric titration in the presence of an internal standard, /)-hydroxybenzoic acid, using tetra- -butylammonium hydroxide as a titrant (42). The carboxyl contents of different lignins are shown in Table 6. In general, the carboxyl content of lignin increases upon oxidation. 

Naphthenic acid is a collective name for organic acids present in some but not all crude oils. In addition to true naphthenic acids (naphthenic carboxylic acids represented by the formula X-COOH in which X is a cycloparaffin radical), the total acidity of a crude may include various amounts of other organic acids and sometimes mineral acids. Thus the total neutralization number of a stock, which is a measure of its total acidity, includes (but does not necessaiily represent) the level of naphthenic acids present. The neutralization number is the number of milligrams of potassium hydroxide required to neutralize one gram of stock as determined by titration using phenolphthalein as an indicator, or as determined by potentiometric titration. It may be as high as 10 mg KOH/gr. for some crudes. The neutralization number does not usually become important as a corrosion factor, however, unless it is at least 0.5 mg KOH/gm. 

Generally, the results of the measurements indicated that, where dissociation constants were determined by conductometry and also potentiometric titration, they were in agreement with each other further, KHX is low, e.g., about 10 4-10 6moll 1 for aromatic sulphonic acids and 10 13-10 16 moll-1 for carboxylic acids, Xhx2 is high, e.g., 102-104, and KBis low again, e.g., 10 5-10 6 for aliphatic amines and 10 10 for aromatic amines. 

Here we shall confine ourselves to the solvents benzene and 1,2-dichloroethane (class 8). Considering benzene, many investigators have demonstrated since the 1930s the feasibility of titrations in this solvent using both potentiometric and spectrophotometric methods, paying much attention to acid-base indicator reactions under the influence of primary, secondary and tertiary amines. Association of carboxylic acids in benzene was studied at a later stage, mainly on the basis of colligative properties, IR spectroscopy and solvent extraction. ... 

Gur ev, I. A. Gushchina, E. A. Gadashevich, M. Z., Dual-phase potentiometric titration of aromatic carboxylic acids with liquid ion selective electrodes, J. Anal. Chem. USSR 36, 803-810 (Engl.) (1981). 

Less complex techniques have been reported to be useful to study the acidic and alkaline treatment processes of biosorbents and the role of carboxyl and carboxylate groups in metal adsorption. Rakhshaee and coworkers101 used potentiometric titration curves to assess the content of such groups in L. minor biomass treated with NaOH and HC1. The results showed an increase (up to 25%) in the adsorption of Hg(II), Cr(III), Cr(VI), and Cu(II) with NaOH-treated biomass as a consequence of an increase of -COO- groups (0.92-2.42 mmol/g). On the contrary, the -COOH groups increase observed (1.50-2.41 mmol/g) due to the acidic treatment led to a decrease in the metal ions uptake (up to 33%) despite activation by the chloride salts. 

Studebaker (63) studied the potentiometric titration of surface oxides in nonaqueous medium. Using sodium aminoethoxide in ethylene diamine, he found indications for the appearance of two breaks in the titration curves. The first break was attributed to carboxyl groups or groups of similar acidity, the second one to phenols. Only two-thirds of the acidity that was determined by NaOH neutralization could be titrated in nonaqueous medium. 

The basicities of the few 1,3-thiazines studied are located between those of sodium bicarbonate and ammonia. Ethyl 2-methylamino-l,3-thiazine-5 carboxylate (72) has pKa 5.41 (65JOC2290). Russian authors have determined the basicities of some compounds by potentiometric titration with perchloric acid in ethanol. The 2-methyliminothiazine (119), pKa 7.84, is more basic than the isomeric aminothiazine (120), pKa 7.12 (71KGS946). 

Fig. 6 a, b. Acceleration of dissociation of poly(carboxylic acid)s according to the complex formation with various polycations, (a) Potentiometric titration of poly(methacrylic add) (PMAA) and its complexes (b) Amount of protons released from poly(carboxy]ic add) in the formation of polyelectrolyte complexes. Polyanions PMAA and poly(acrylic add) (PAA). Polycations and their low molecular weight analogues ... 

The degree of induction of the dissociation of PMAA in the presence of various polycations is estimated by its apparent dissociation constant (pKJ calculated from the potentiometric titration results as shown in Fig. 6, using the Henderson-Hasselbach equation (see Table 5). In the presence of polycations, pKa and one of the interaction parameters relative to adjacent ionic sites, n, decreases. This confirms the induction of dissociation. Furthermore, the difference in the composition of the polyelectrolyte complex composed of one kind of polycation and different poly(carboxylic acid)s, may be affected by pKa and the conformation of them. 

Fig. 2.14. (A) Potentiometric titration curves of a polymer imprinted with L-PA (PLPA), a polymer imprinted with benzylamine (PBA), a blank non-imprinted polymer (PBL) and acetic acid in MeCN/O.lM NaCl 70/30 (v/v). The NaOH equivalents (x-axis) are calculated based on the theoretical amount of carboxylic acid groups present in the polymer. In (B) is seen the calculated distribution as a function of the degree of ionization (a ). The polymer swelling (ml/ml) in this solvent system was constant in the pH interval 3-12 and was for PLPA 1.32 and for PBL 1.26. From Sellergren and Shea [67].

Since the second term in both the numerator and the denominator are identical, an increase in a s will obviously lead to a decrease in a. In support of this explanation, the non-specific binding increases above pH pp = 6. This is clearly seen in the plot of k versus pHapp for BA on the L-PA MIP (Fig. 5.34) and in the parallel increase in a for the latter (Fig. 5.35). This can also be seen from the plot of the estimated separation factor of BA ( ba) versus pHapp (Fig. 5.34), where a is highest at low pH values (below pATa(BA)) and decreases over a large pH interval. Furthermore, the potentiometric titrations showed that the LPA MIP had a lower average pAfa than a non-imprinted blank polymer (see Chapter 2). This strongly suggests that the carboxylic acid groups of the selective sites are more acidic than those of the non-selective sites. 

Acid-base potentiometric titration of phenol in aqueous solution is precluded because of its high pATa value (9.98), while 4-nitrophenol (7.41) and 2,4,6-trinitrophenol (0.71) can be directly titrated in that solvent. Nonaqueous titrations of phenol are possible however, difficulties are met when nitrophenols are also present in the system. The determination of carboxylic and phenolic groups in humic acids was carried out by acid-base potentiometric titrations in NaCl solutions up to 1 M. Titration data were processed by linear and nonlinear calculation techniques. ... 

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