Detailed information on chemical testing of soil

N.1 General

N.1.1 Test procedures

(1) The above routine chemical tests are based on traditional testing methods which are within the capability of many geotechnical laboratories. Chemical tests for the presence of other substances should normally be performed by a specialist chemical laboratory.

(2) One hundred grams of dry soil is sufficient for most chemical testing. Usually a much larger sample of dried soil will be required at the outset, but a very small sample of dried soil is required for the specific testing. Thorough mixing of the initial sample, and correct sub-dividing procedures, are essential.

(3) Storage temperature before testing may influence the biological degradation of organic matter. Whenever possible, the sample material for chemical tests should be kept at a temperature of 5 °C to 10 °C.

(4) Most test methods include a calibration routine using "blind" samples and reference samples. Electrochemical methods like pH have well defined calibration schemes with a number of solutions with known pH.

(5) Special requirements may call for deviations from the standard procedures, including specimen preparation. Any procedural deviations should be clearly reported, including the reasons for the deviations.

NOTE Examples of test procedures for the five chemical tests dealt with are given in the documents listed in X.4.2. Equivalent methods also exist in other national standards and in textbooks.

N.1.2. Number of tests

(1) The number of tests specified should take into account the fact that the organic content, carbonate content, sulfate content, pH value and chloride content can vary widely even within a geological stratum. Multiple tests on closely-spaced samples may be necessary to define the local variability.

N.2 Organic content determination

N.2.1 Test procedures

(1) The loss on ignition is normally determined on a representative sample of the soil finer than 2 mm as the mass lost by ignition of a prepared specimen at the specified temperature. The organic content is calculated on the assumption that the organic mass is totally burned by the ignition, and that the mass loss is only due to the ignition of the organic matter.

(2) The loss on ignition generally relates to the organic content of soil containing little or no clay and carbonates. For soil with higher percentage of clay and/or carbonate, factors unrelated to organic content could be responsible for the major proportion of the ignition loss.

(3) A drying temperature lower than the usual (105 ± 5) °C is necessary to avoid oxidation of some organic matter during drying. The examples listed in X.4.2.2 specify a drying temperature of (50 + 2,5) °C, which might not remove all water. Trials may be necessary to establish a suitable drying temperature.

(4) The ignition temperature specified in the examples referred to in (1) is (440 ± 25) °C, but other standards specify temperatures up to 900 ºC. Caution should be exercised when specifying an ignition temperature, taking into account the following:

some clay minerals can begin to disintegrate at temperatures of about 550 °C;
chemically-bound water may vanish at lower test temperatures; for example in some clay minerals, this can start at 200 °C, and gypsum disintegrates at from about 65 °C;
sulfide may oxidise, and carbonates disintegrate, in the range 650 °C to 900 °C. For most purposes, an ignition temperature of 500 °C or 520 °C is appropriate.

(5) The drying and ignition periods should be sufficient to ensure that equilibrium has been achieved. If the period of ignition is less than 3 h, the report should document that constant mass was confirmed by repeated weighing.

NOTE Examples of lest procedures for the determination of organic content are given in the documents listed in X.4.2.2.

N.2.2 Evaluation of test results

(1) The quantity of organic carbon and organic matter can be related to loss on ignition, if the latter is corrected for other expelled constituents.

(2) The organic content can be determined by a direct measurement of the content of organic carbon, by which the errors in the loss on ignition method can be avoided.

N.3 Carbonate content determination

N.3.1 Test procedures

(1) Examples of test procedures for the determination of carbonate content are given in the documents listed in X.4.2.3. For the purpose of this standard, the rapid titration method is the preferred procedure. This method should give results that are accurate enough for soil, provided that care is taken to ensure that the dissolution process is finalised, and that sufficient duplicate tests are performed.

(2) Other examples given in the documents listed in X.4.2.3 determine the carbonate content by measuring the liberated carbon dioxide (CO₂) in a gasometer under controlled temperature and atmospheric pressure.

N.3.2 Evaluation of test results

(1) The carbonate content in percentage carbonate in the sample is expressed as the amount of CO₂. This is formally correct but impractical for design. The results may be given in equivalent calcium carbonate CaCO₃, i.e. the carbonate composition for most soil types. The amount of equivalent CaCO₃ is obtained from the amount of CO₂ by the equation:

CaCO₃ = 2,273 · CO₂

where

CaCO₃ is the CaCO₃ content as percentage of dry weight;

CO₂ is the CO₂ content as percentage of dry weight.