19A1 Carbonates – rapid titration

In this titration procedure, soil is treated with dilute HCl and residual acid is titrated with standard 0.5 M sodium hydroxide (NaOH). Results are referred to as ‘CaCO₃ equivalent’ since the reaction is not selective for calcite; other carbonates including dolomite will be included to some extent.

Table 19.1. Summary details of method codes, method titles, technologies and notes for soil carbonate tests described in this chapter^†.

Code	Technology	Test method	Notes
19A1	Chemical reaction with dilute HCl; residual acid titrated with standard 0.5 M NaOH.	Carbonates – rapid titration.	Semi-quantatitive for carbonates.
19B1	Increase in pressure generated by the CO₂ released after reaction with HCl at room temperature.	Carbonates by pressure change – manometric.	Semi-quantatitive for carbonates plus indication of dolomite from reaction rate plots.
19B2		Carbonates by pressure change – transducer.	Semi-quantatitive for carbonates.
19C1	Chemical reaction with dilute HCl and subsequent observations.	Spot field test for the presence of soil carbonates with HCl	Qualitative for presence or absence.
19D1		Field test for soil carbonates with HCl and effervescence class assessment.	Qualitative for presence, absence, plus amount.

^†See also Method 6E1 for carbonate by loss-on-ignition.

Reagents

1 M Hydrochloric Acid

Dilute 100 mL of 10 M HCl, mix and make volume to 1.0 L with deionised water.

0.5 M Sodium Hydroxide

Dissolve 20.0 g sodium hydroxide (NaOH pellets) in deionised water and make to 1.0 L. Stand-ardise against potassium hydrogen phthalate (KHC₈H₄O₄) in a similar manner to that described in Method 4D1. Special precautions to exclude CO₂ prior to standardisation are unnecessary.

Phenolphthalein Indicator (0.1%)

Dissolve 100 mg phenolphthalein (C₂₀H₁₆O₄) in 100 mL ethanol (C₂H₅OH).

Procedure

Weigh 5.0 g air-dry soil (<2 mm) into a 250 mL wide-mouth plastic extracting bottle. Include two blanks (no soil) plus either a reference sample or 0.5 g CaCO₃ powder. Use 2.5 g of air-dry soil (<2 mm) if the soil is known to contain >30% carbonate.

Add 100 mL 1 M HCl, cover in a manner that permits release of any CO₂, and swirl occasionally for 1 h at ≈25°C. Allow to stand overnight, cap securely, then mechanically shake for 2 h.

Let the suspension settle then filter or centrifuge. Take 10 mL supernatant into a 100 mL Erlenmeyer flask and add 25 mL deionised water. Add 2–3 drops Phenolphthalein Indicator and titrate with standard 0.5 M NaOH.

Calculation

where:

a = mL standard 0.5 M NaOH used for blank

b = mL standard 0.5 M NaOH used for sample

S = weight (g) of air-dry soil

M = molarity of standard NaOH (usually 0.5 M)

50.04 = 50.04 × 10^–3 × 10 × 100% (where 50.04 is the equivalent wt of CaCO₃)

Report CaCO₃ equivalent (%) on an oven-dry basis. Use the air-dry moisture to oven-dry moisture ratio to for the oven-dry conversion. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.