Comparison of the titration and ion adsorption methods for surface charge measurement in oxisols

The pHdependent charge properties of the A and B horizons of two representative oxisols from the Cemdo Region, Brazil, were studied by a comparison of the potentiometric tifration method of net charge measorement and direct measurement of anion nnd cation exchange capacity from K' nnd Cladsorption. Most soil samples bore net negative charge at their native pH, but variability in the umgnitude of this charge was attributed to the effect of Al-blocked exchange sites or contributions from strongly acidic organic fwct i o d groups, since these soils had similar minemlogies. The soils also bore significant pHdependent positive charge at their natural acidic pH valueq which was more evident in the B horizons than the surface horizons. Estimates of net surface charge by potentie metric tifration nnd ion adsorption did not agree, especially as the soil pH was adjusted away from the point of zero chnrge. This lack of agreement is attributed to dissolution resdions of minerals and organic matter at high and low pH, which consume acid or base and overestimate surface chnrge. T HE TWO MOST COMMON methods of studying the charge characteristics of amphoteric surfaces are indirect (potentiometric titration) and direct measurement of anion and cation retention as a function of pH (Parks and de Bruyn, 1962; Schofield, 1949). Potentiometric titration involves the measurement of the net adsorption of H+ or OHat different pH and at different ionic strengths. Because of the characteristics of amphoteric surfaces, there is a pH at which the different curves intersect, i.e., a point at which adsorption of protons is independent of ionic strength. This pH is termed the point of zero salt effect (PZSE), (Parfitt, 1980; Parker et al., 1979). The second method, ion adsorption, involves the measurement of retention of an indifferent ion (adsorbed onto the surfaces by electrostatic attraction), such as Na+, K+, NOj, and Clas a function of pH at a fixed ionic strength (Schofield, 1949). This method provides an estimate of the cation exchange capacity (CEC) and anion exchange capacity (AEC), as well as the net charge (AEC minus CEC) at each pH, and is considered t o 'measure point of zero net charge (PZNC). In general, the A horizons of soils tend to have lower PZNC than their corresponding B horizons. onomy, Cornell Univ., Ithaca, . NY 14853. Contribution from lkp. Of the % p. of Agronomy, Cornell Un~v. Supported in part by the Tropsoils Project. Received 27 May 1988. *Corresponding author. Published in Soil Sci. Soc. Am. J. 53:1040-1045 (1989). This is attributed to the greater negative charge in the upper horizons because of the higher organic matter content (Van Raij and Peech, 1972). Direct comparison of PZNC values from different soils cannot generally be done because of the dependence of the measurement on the ionic strength of the solution in which the AEC and CEC were measured. For example, Gallez et al. (1976) and Morais et al. (1976) showed a decrease in PZNC values of highly weathered soils with increasing ionic strength. The PZNC and PZSE values in soils are very low compared to those of pure oxides, presumably because soils generally contain organic matter, permanently charged materials, and SiOgroups, all of which increase the negative charge of the soil, lowering apparent points of zero charge (Parfitt, 1980). Recent investigations of charge development on A1 oxide as a function of pH have demonstrated that dissolution can introduce a large error into the estimates of surface charge by the potentiometric titration method (Schulthess and Sparks, 1987). A similar error is likely to arise in soils dominated by oxide mineralogy, although the degree to which this occurs is probably sensitive to the crystallinity of the A1 and Fe oxides present. Indeed, Parker et al. (1979) have discussed the pitfalls inherent in measuring net charge in soils by potentiometric titration. The present investigation was undertaken to compare the potentiometric titration and ion adsorption methods for determining pH-dependent surface charge of Oxisols. MATERIALS AND METHODS Soil Characteristics For this study, samples of the A and Bo horizons of two Oxisols from the Cerrado Region, Brazil, were collected by J. Macedo from two representative soil profiles (Macedo and Bryant, 1987). Both soils are classified as Typic Acrustox in the Soil Taxonomy system, and as Dark Red Latosol and Red Yellow Latosol in the Brazilian Classification scheme (Macedo and Bryant, 1987). The samples studied correspond to the A and Bol horizons of the Dark Red Latosol (site A) and the Ap and Bol horizons of the Red Yellow Latosol (site H) in Macedo and Bryant (1987). In this study the soil samples will be referred to as Dark Red Acrustox (DRA and DRB for the A and Bo horizons, respectively) and Red Yellow Acrustox (RYA and RYB). Some of the chemical and physical characteristics of the soil samples are listed in Table 1. Soil pH was measured in a 1:10 soil/solution ration (in order to maintain the same ratio as that used for the ion adsorption measurements) usMARCANO-MARTINEZ & MCBRIDE: COMPAR~SON OF TITRATION AND ION ADSORPTION METHODS IN OXlSOLS 104 1 ing a combination glass-AgC1 electrode. Organic C was determined on a Model 240C Perkin-Elmer elemental analyzer (Perkin-Elmer Corp., Nowalk, CT). Free Fe oxides were extracted by the CBD method (0.3 M sodium citrate, 1 M sodium bicarbonate, and dithionite) of Mehra and Jackson (1960). Organically bound Fe and Al were extracted with a 0.1 M solution of sodium pyrophosphate using a soil/solution ratio of 1: 100 (USDA-SCS, 1984). Both elements were determined by atomic absorption spectrophotometry. All four soil samples belong to the clay textural classification (Macedo and Bryant, 1987). The mineralogical compositions were qualitatively determined for this study by xray diffraction (XRD) on a Norelco x-ray spectrophotometer (Phillips Electronic Instruments, Mount Vernon, NY) (Macedo and Bryant, 1987) and by infrared spectroscopy (IR) using a Perkin-Elmer Model 28 1 infrared spectrophotometer, revealing the same mineralogy for the A and Bo horizons within the same soil profile. The clay fraction, separated by procedures detailed by Macedo and Bryant (1987) consisted mostly of kaolinite and gibbsite in all four samples, based on the intensities of XRD and 1R peaks. The Dark Red Acrustox also contained goethite and hematite, while the only Fe oxide in the Red Yellow Acrustox was goethite. The latter soil sample contained relatively more kaolinite, gibbsite, and goethite. The sand and silt fractions contained, in addition to the previous minerals, quartz and traces of maghemite. Surface areas of the soil samples, measured by a three point Brunauer-Emmitt-Teller (B.E.T.) isotherm for N2 adsorption at 196 "C on a Quantasorb sorptometer (Quantachrome, Syosset, NY), are reported in Table 1. For this study, the soil samples were air-dried and screened through a 0.7-mm sieve prior to use; the results are expressed on an oven-dry weight basis. Charge Characterization Ion Adsorption Method An estimate of the CEC and AEC as a function of pH was determined by measuring the amount of K and C1retained by the soils at different pH using a modification of Schofield's method (Schofield, 1949). Based upon the known mineralogy of these soils, it was assumed that the clays contained essentially no sites capable of specifically absorbing K*, and therefore that KC1 could be treated as an indifferent electrolyte. Triplicate 2-g samples of soil were weighed in centrifuge tubes and washed with 0.1 M KC1 to minimize soluble Al; after discarding the supernatants, 20 mL o f the same solution were then added and the pH adjusted with KOH or HCI to give a pH range between 2 and 8. The samples were equilibrated at room temperature (24 + 2 "C) by shaking intermittently on a reciprocal shaker for 12 h. Then the samples were centrifuged, the supernatants disTable I. Chemical and physical characteristics of the soils. Soil oH (H,O) oH (0.01 M KCI) Omnic Ct C B D Fe