ASTM D7503
Scope
1.1 This test method describes the procedures for measuring the soluble and bound cations as well as the cation exchange capacity (CEC) of fine-grained
inorganic soils. Clay minerals in fine-grained soils carry a negative surface charge that is balanced by bound cations near the mineral surface. These bound cations can be exchanged by other
cations in the pore water, which are referred to as soluble cations. The cation exchange capacity is a measure of the negative surface charge on the mineral surface. The CEC generally is
satisfied by calcium (Ca), sodium (Na), magnesium (Mg), and potassium (K), although other cations may be present depending on the environment in which the soil exists. This test method was
developed from concepts described previously in Lavkulich (1981) (1) and Rhoades (1982) (2). In
soils with appreciable gypsum or calcite, dissolution of these minerals will release Ca in solution that may affect the measurement.
1.2 In this test method, the soluble salts from the mineral surface are washed off with de-ionized water and then the concentration of soluble salts within
the extract is measured. The bound cations of the clay are measured by using a solution containing an index ion that forces the existing cations in the bound layer into solution. The total
concentrations of bound and soluble cations in this solution are measured. The CEC is measured by displacing the index ion with another salt solution and measuring the amount of the displaced
index ion.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this
standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 All observed and calculated values shall conform to the guide for significant digits and rounding established in Practice D6026. The procedures in Practice D6026 that are used to specify how data are collected, recorded, and calculated are regarded as
the industry standard. In addition, they are representative of the significant digits that should generally be retained. The procedures do not consider material variation, purpose for obtaining
the data, special purpose studies, or any considerations for the objectives of the user. Increasing or reducing the significant digits of reported data to be commensurate with these
considerations is common practice. Consideration of the significant digits to be used in analysis methods for engineering design is beyond the scope of this standard.
1.5 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in
this standard. Reporting of test results in units other than SI shall not be regarded as nonconformance with this test method.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on
Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Keywords
bentonite; bound cations; cation exchange capacity (CEC); cations; clay; exchange complex; soluble cations;
ICS Code
ICS Number Code 23.060.10 (Globe valves)
DOI: 10.1520/D7503-18