Total acidity determination barium hydroxide method

Total acidity (meq/g) and carboxyl groups (meq/g) were determined by the barium hydroxide method and the calcium acetate exchange method, respectively (X4). Phenolic hydroxyl groups (meq/g) were calculated as the difference between total acidity and carboxyl groups. These determinations were carried out under a nitrogen atmosphere to minimize absorption of carbon dioxide. 

Total acidity determined by the barium hydroxide method (14). 

Although all potentiometric methods of functional group analysis are inherently operational, the total acidity can be determined by the barium hydroxide method, if appropriate precautions are taken to remove all colored material from the reaction mixtures prior to the final titration. Any method that purportedly distinguishes between different classes of functional groups should be used with full awareness of the operational nature of the results that will be obtained. If potentiometric data are augmented with calorimetric andlor spectroscopic data, more accurate estimates of functional group concentrations may possibly be obtained. 

Methods of functional group analysis that are based on pKa values of the functional groups (all potentiometric methods) can only yield operationally defined estimates of the concentration of a particular class of acidic functional groups. For this reason, an increased reliance on spectroscopic methods of functional group analysis is recommended. Only total acidity, as determined by the barium hydroxide method, appears to be a potentially accurate potentiometric method of analysis, if some type of ultrafiltration technique is used whenever normal filtration fails to remove all color from reaction mixtures. 

Several problems are inherent in this method, so the results should be considered as quite operational (Dubach et al., 1964 van Dijk, 1966 Stevenson and Goh, 1972 Holtzclaw and Sposito, 1979 Perdue, 1979 Perdue et al., 1980). First, unlike the barium hydroxide reagent used for total acidity, 0. IM calcium acetate is poorly buffered. The equilibrium pH, which determines the extent to which the acidic functional groups of humic substances will react, is dependent on the amount of humic substances added to the 50 mL of calcium acetate. Perdue et al. (1980) demonstrate that the binding of Ca to the humic substance sample displaces additional protons that do not react if sodium acetate or pyridine is used as the exchange base. It cannot be assumed that those excess protons are derived exclusively from carbbXyr groups. 

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