Overview
ISO 23470:2018 - Soil quality - Determination of effective cation exchange capacity (CEC) and exchangeable cations using a hexamminecobalt(III)chloride solution - specifies a laboratory method to measure effective CEC and the content of exchangeable cations (Al, Ca, Fe, K, Mg, Mn, Na) in soils. The method uses a hexamminecobalt(III)chloride extractant and is applicable to air‑dry soil samples prepared according to ISO 11464. For calcareous soils a calcite‑saturated hexamminecobalt(III)chloride solution is specified to reduce carbonate dissolution and avoid inflated Ca results.
Key topics and technical requirements
- Principle: Adsorbed soil cations are displaced by hexamminecobalt(III) ions; CEC is calculated from the difference between initial and remaining hexamminecobalt in extract.
- Extraction conditions: Exchange reaction typically performed with shaking for (60 ± 5) min at (20 ± 2) °C (as specified in the standard).
- Analytical determinations:
- Hexamminecobalt can be quantified by total ammonium nitrogen (distillation), direct spectrophotometry, or total cobalt measurement.
- Exchanged cations are determined from the same extract using spectrometric methods (e.g., ICP‑AES).
- Reagents: use analytical‑grade chemicals and deionized water; the standard gives preparation details for hexamminecobalt(III)chloride solution (β ≈ 0.0166 mol/L) and the calcite‑saturated variant for calcareous soils.
- Scope limits & notes:
- Method yields the effective CEC at the soil’s natural pH (recommended for soils with pH ≤ 6.5).
- Calcite‑saturated extract is recommended for soils with pH ≥ 6.5 to minimize carbonate dissolution and erroneous exchangeable Ca.
- Known limitations include potential underestimation in microporous materials (where hexamminecobalt ions cannot reach some exchange sites) and increased Ca from gypsum dissolution.
Practical applications and users
- Who uses it: soil testing laboratories, agronomists, environmental consultancies, soil scientists, land‑use planners and regulatory agencies.
- Uses:
- Assessing soil fertility and nutrient-holding capacity (CEC and base cation status).
- Monitoring soil amendments, liming effects, and remediation projects.
- Research and interlaboratory comparison studies for soil chemical characterization.
- Benefits: one‑step extraction with high affinity extractant, comparable to established CEC methods, and specific procedures to avoid carbonate-related errors in calcareous soils.
Related standards
Keywords: ISO 23470:2018, soil quality, cation exchange capacity, CEC, hexamminecobalt(III)chloride, exchangeable cations, calcite saturated, soil testing, ICP‑AES.