Potentiometric Titration of Fulvic Acid: Equivalence Point Calculations and Acidic Functional Groups

Abstract

Gran's functions corrected for buffering and H 2 0 dissociation have been incorporated into the iterative calculation for the two equivalence points and the acid dissociation equilibria. The experimental and mathematical conditions required for accurate, dependable operation of the method have been examined. The resulting equivalence point calculations are based directly on a chemical interpretation of the system, without approximations. The first equivalence point is higher than previously reported, and is displaced upward by KCI. Detailed chemical information has been obtained for some of the acidic functional groups, from plots of KA = (rnH6mA/6mAH) us. mequiv./g. Carboxyl groups at the chelating sites are essentially all ionized for H + concentrations less than 1 x lo-' m.Les fonctions de Gran corrigees pour le tampon et la dissociation de H,O ont ete introduites dans le calcul iteratif des deux points d'kquivalence et d'tquilibre de dissociation acide. Les conditions experimentales et mathematiques requises pour une operation sare et precise de la mbthode ont Cti: examinees. Les calculs du point d'bquivalence qui en resultent, sont fondes directement sur une interpretation chimique du systtme sans approximations. Le premier point d'equivalence est plus &lev& que celui precedemment rapport6 et se trouve dtplad par KCI. Une information chimique detaillee a tte obtenue pour certains des groupes fonctionnels acides, a partir des courbes de KA = mH6mA/6mAH en mequiv./g. Les groupes carboxyles sur les sites de chelation sont essentiellement et entierement ionises pour des concentrations en H+ infkrieures a 1 x lo-' rn.

Effects of polymeric charge variations on the proton‐metal ion equilibria of humic materials

“…The data of Table 1 also indicate that the original assumption of group identicalness within experimental error is valid and that the reported variations ( Gamble 1973) in the differential constant as a function of pI1 are caused by changes in the humic charge density.…”

“…3 can be used to analyze humic titration data, the carboxylic acid end point must first be determined by some analytical technique. Pommer and Breger (1960) and Gamble (1972) have formulated Gran-like functions to allow identification of the equivalence point. These equations are, however, not applicable to this theory of electrostatic interaction.…”

Potentiometric measurement of H+ concentrations for aluminum orthophosphate titration curves

“…humic polyelectrolyte (22,24,25). The final result is the averaged titration curve given in Table 4.…”

“…A number of authors have pointed out or demonstrated this same requirement for various chemical problems (14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27). A concentration probe is evidently required.…”

“…Gamble has observed two categories of random errors in the H+ concentration measurement of acid-base titration curves (22,24,25). These were the random scatter within a run, and the randomly occurring biases found in any collection of replicate runs.…”