Calcium acetate exchange 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. 

FIGURE 7. The calcium acetate exchange method for carboxyl groups. 

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. 

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. 

Xing-Chu and Yu Sheng [79] have described a spectrophotometric method for the determination of exchangeable calcium in soil. In this method, a portion of an aqueous extract of the soil is treated with ammoniacal ammonium acetate and an aqueous solution of chlorophosphonazo-mA. The solution is evaluated spectrophotometrically at 630 nm. Recoveries of calcium are 99% and relative standard deviations of between 0.9% at the 11 mequiv/lOOg of soil level and 3.1% at the 2 mequiv/100 g soil level are obtained. 

This method makes use of commercially available, thin-layer plates coated with a cation-exchange resin in the sodium form the sodium ions can readily be exchanged for other cations by immersing the plate in an appropriate salt solution. Cupric, calcium, and lanthanum acetates have been used to introduce cations that complex readily. The plate is then developed with water as the irrigant. Compounds that form complexes with the cations are retained and have low values those that complex weakly are... 

---