Overview
ISO 10693:1995 - "Soil quality - Determination of carbonate content - Volumetric method" specifies a standardized volumetric procedure to determine the total carbonate content of air‑dried soil samples. The method decomposes carbonates with hydrochloric acid, measures the CO2 evolved with a Scheibler apparatus, and expresses results as an equivalent concentration of calcium carbonate (CaCO3). It is applicable to all types of air‑dried soils and is intended for routine soil testing and quality assessment.
Key topics and technical requirements
- Principle: Carbonates/bicarbonates in the soil react with HCl to release CO2; the CO2 volume is measured and compared with CaCO3 standards.
- Scope: Applicable to all air‑dried soil samples pretreated per ISO 11464. Results are reported on an oven‑dried basis (water content determined per ISO 11465).
- Reagents: Analytical‑grade water (ISO 3696 Grade 2), hydrochloric acid c = 4 mol/L (prepared by diluting 340 mL concentrated HCl to 1 L), and CaCO3 powder as standard.
- Apparatus: Scheibler volumetric apparatus (U‑shaped calibration tube, storage vessel), reaction vessels (≈150 mL), small plastic acid cup (~10 mL), stop‑cocks, tongs, and analytical balance (accuracy 0.1 mg). Typical Scheibler units use 100 mL calibration tubes.
- Procedure highlights: weigh representative test portion, add ~20 mL water to vessel, place 7 mL HCl in plastic cup, connect to Scheibler unit, initiate reaction by tipping acid onto soil, shake (typically 5 min up to 1 h) and record stabilized CO2 volume. Include blanks and CaCO3 standards in each series. Maintain measurements in a temperature‑controlled room to avoid pressure/temperature corrections.
- Interferences: Other gases (e.g., H2S from sulfides) can inflate CO2 readings; if sulfides are present, mercury(II) chloride may be added to HCl to precipitate sulfide as HgS.
- Reporting: Results as g/kg CaCO3 (or specified carbonate form), plus sample ID, water content, details of deviations, blanks, and standards.
Applications and users
ISO 10693 is used in:
- Agricultural and soil fertility testing to quantify liming requirements and soil buffering capacity.
- Environmental and geotechnical investigations where carbonate content affects soil behavior (e.g., carbonate‑rich loess, calcareous soils).
- Laboratories performing routine soil chemical analysis, agronomists, environmental consultants, and regulatory agencies monitoring soil quality.
Keywords: ISO 10693, soil quality, carbonate content, volumetric method, Scheibler apparatus, CaCO3, soil testing.
Related standards
- ISO 11464 - Soil quality: pretreatment of samples for physico‑chemical analyses
- ISO 11465 - Determination of dry matter and water content (gravimetric)
- ISO 3696 - Water for analytical laboratory use (specification)
- ISO 5725 - Precision of test methods (repeatability and reproducibility)
