Carboxyl group determination direct titrations

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. 

Direct titrimetric method is a classic method for DE determination (National Research Council, 1972). In this method, pectin is first washed by acidic alcohol to convert the free carboxyl groups to the protonated form and is then washed with alcohol and dried. The washed sample is then dissolved in water and titrated with standard base and acid. The uronic content and DE are calculated from the neutralization and saponification equivalents of the pectic carboxyl groups. The existence of acetyl groups in some pectin samples leads to an overestimate of DE in the titration method. In this case, the copper binding method is an alternative approach (Keijbets and Pilnik, 1974). In the copper binding method, the amounts of copper ions that bind to the pectin molecules before and after saponification are measured by the complexometry, the ratio of which gives the DE. 

For simple amino acids, and Ki, are generally so small that their determination by direct neutralization is impossible. Addition of formaldehyde removes the amine functional group, however, and leaves the carboxylic acid available for titration with a standard base. For example, with glycine,... 

The structural formula reveals that this polymer contains two different types of carboxyl group which have different dissociation constants. While the first dissociation step is characterized by a pK value of 3.4, the pK value of the second step is about 7.4. Both pK values were determined via titration of the prepolymer with sodium hydroxide solution. The exchange capacity of the finished stationary phase is directly proportional to its polymer content. It may be calculated in advance, since, owing to the chemical composition, the concentration of the exchange groups in the prepolymer is known. 

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. ... 

If the macromolecule contains a well-defined number of groups which can be determined by chemical means, it is obvious that such a determination may be used to obtain the molecular weight of the substance. For instance, if each molecule carries a carboxyl group at the end, the number of these groups per unit of volume may be measured by direct or by conductometric titration. Obviously, this method determines the total number of molecules, i.e., according to equation (1), the quantity 2iCi I Mi, The average value obtained in this way is, therefore... 

Carboxyl and hydroxyl groups can be estimated by standard titration methods. For the estimation of carboxyl groups, the sample is directly titrated with a standard alcoholic potassium hydroxide solution. To determine hydroxyl number, the sample is refluxed for 1 h with acetylating mixture (pyridine/acetic anhydride) and the excess acetic anhydride titrated back. The overall average functionality (fj (carboxyl and hydroxyl) is the sum of average carboxyl functionality (c ), main-chain hydroxyl functionality (h ) and branching-chain hydroxyl functionality (bj. 

Native polysaccharides with acid groups other than the uronic type are not very common except for the sulfate esters. Total acidity may be estimated by direct titration, but erroneous results are obtained if the polysaccharide is alkali-labile as is the case with many oxidized polysaccharides. Addition of calcium acetate (7, 8) or sodium bromide 8, 9) to the polysaccharide solution increases the accuracy of the titration. Other methods for the estimation of carboxyl and other acidic groups involve determination of the amount of methylene blue absorbed, or determination of the amount of silver salt formed by exchange from a solution which contains silver in combination with a very weak acid. The sulfate content of polysaccharide sulfates, such as agar, is obtained by ordinary sulfate analysis of the completely hydrolyzed or ashed polysaccharide. 

The carboxyl group may be determined by direct titration according to the method suggested in the classification reactions in Chapter IX, Exp. 14. The saponification of estei-s, likewise, is illustrated in laboratory experiment No. 17. 

Carboxylic and sulfonic acid groups import acidity to organic compounds. Most carboxylic acids and sulfuric acids are readily dissolved in water, and their titration with a base is straightforward. If solubility in water is not sufficient, the acid can be dissolved in ethanol and titrated with aqueous base. Aliphatic amines and many saturated cyclic amines can be titrated directly with a solution of a strong acid. Esters are determined by saponification with a measured quantity of standard base. The excess base is titrated with standard acid. 

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