Total Carbonates Detection in Soil
In use from 2008-07-14
Free carbonates are compounds that coat soil particles. They form under certain conditions such as in dry climates where the pH is above 7. They are also found in some soil profiles that have parent materials made of carbonates (such as limestone).
Carbonate can be detected by measuring the CO2 generated by dissolving the carbonates. By dissolving carbonates in a fixed volume, the pressure increase in samples relative to the blanks will indicate the CO2 generated by carbonate dissolution. The Ideal Gas Law can be used to calculate the moles of CO2 in the headspace, from which the approximate quantity of carbonate can be estimated, and results can be expressed as % of total dry weight using the amount of soil that was added. Short- and long-term increases in pressure can be assumed to reflect calcite and dolomite, respectively.
- Absolute Pressure Gauge (MKS Baratron Type 122A)
- Needle adapter
- Stopwatch or clock
- Data book
- Safety glasses
- 12-mL Labco Exetainers with screw caps and septa
- 10-mL Glass syringe with 24G or 22G needle (for acid addition)
- 5 M hydrochloric acid
Soil samples ideally would be dried and finely ground (< 0.5-mm diameter) to facilitate reaction with acid.
- Weigh approximately 1.0 g (+/- 0.2 g) of dry soil into the vial, recording the exact weight. Seal the vial with the cap and septum.
- Inject 2 mL of the 5-M HCl into the vial and mix it with the soil, holding it in the hood and pointing the top away from you just in case the septum blows out. Wear safety glasses from this step onward. Also inject acid into a couple of vials containing 1.0 g of carbonate-free soil to serve as blanks.
- After 10 minutes, mix the solution again and record the pressure using the transducer. The 10-min period is necessary for full dissolution of calcite.
- Remove the vial from the transducer and briefly open the vial to vent excess pressure wearing safety glasses, and then seal the cap again.
- Measure pressure in the system again after 2-4 hours. Record time and pressure to check for additional dissolution of carbonate. Any pressure increase between 10 min and 2 hours is most likely due to dolomite dissolution.
Evangelou et al. (1984), An automated manometric method for quantitative determination of calcite and dolomite. SSAJ 48: 1236-1239.
Loeppert, R.H. and D.L. Suarez. (1996) Carbonate and gypsum. Pp. 437-475 in: Sparks, D.L. [ed.], Methods of soil analysis, Pt. 3 – Chemical methods. SSSA Book Series No. 5.