Carboxylic acid group titration

The theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

The values of 9 thus calculated for the polyanions (3) having various charge densities are listed in Table 1. It should be noted that all the carboxylic acid groups in the polyions are assumed to be almost fully dissociated in the pH region where the spectroscopic pH titration of the Me residue was performed. 

Titratable Acidity. The possibility of carboxylic acid group generation from excessive oxidation of corn starch was monitored by titratable acidity (TA). A 0.01 N NaOH solution was used to titrate a dilute aqueous starch suspension (20 mL of a 5% w/v sample) for the presence of acidic functional groups, using phenolphthalein as the indicator dye. An unreacted starch sample was also titrated to yield a sample blank value. TA values were expressed as mL of base required to reach the colorimetric phenolphthalein and end-point. 

Fig. 7.7.3. Conductometric titration curve of a refiner mechanical pulp made from sulfonated spruce chips (3.47g). End point A corresponds to titration of the sulfonic acid groups and end point B coresponds to titration of the sulfonic and carboxylic acid groups. (Katz et al. 1984)...

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. 

Numerous tertiary amines that also contain carboxylic acid groups form remarkably stable chelates with many metal ions. Ethylenediamine tetra-acetic acid (EDTA) can be used for determination of 40 elements by direct titration using metal-ion indicators for endpoint detection. Direct titration procedures are limited to metal ions that react rapidly with EDTA. Back titration procedures are useful for the analysis of cations that form very stable EDTA complexes and for which a satisfactory indicator is not available. EDTA is also used for determining water hardness the total concentration of calcium and magnesium expressed in terms of the calcium carbonate equivalent. 

The acid value is obtained by refluxing the product with xylene to convert carboxylic acid groups (formed by exposure of the product to air) into anhydride groups, followed by refluxing with anhydrous methanolic KOH. The cooled mixture is then titrated with 2-propanolic HC1 using phenolphthalein as the indicator. The acid value shown in Table II is the actual titration value. However, since methanol converts the anhydride to the half ester, the true acid value, shown in parentheses, is double the titration value. 

At low pH the solubility of these compounds decreases due to the poor dissociation of the carboxylic acid groups. Carboxylic groups can be titrated to calculate the charge per mg DOC. Due to the wide range of compounds present, no clear equivalence point can be determined, but a plot of Acharge as a function of ApH was found to be similar for all NOM fractions (Newcombe and Drikas... 

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