cen:proj:81759

Earthworks — Chemical tests — Part 6: Determination of pH in soil samples (CEN/TS 17685-6)

Scope

The aim of this standard is to describe a test for the determination of pH within the range pH 2 to pH 12 using a glass electrode in a 1:5 (volume fraction) suspension of soil in either demineralized water (pH in H2O), in 1 mol/l potassium chloride solution (pH in KCl) or in 0,01 mol/l calcium chloride solution (pH in CaCl2). The pH of the suspension obtained is measured using a pH-meter.
This method is applicable to all types of air-dried soil samples, for example as pretreated in accordance with ISO 11464.
To this end, a volume-to-volume shaking ratio is chosen because all test samples can be treated in the same way. If a mass-to-volume ratio were chosen, the weighed amount of test sample would have to be adapted for e.g. soils with a low or high density, to enable the preparation of the suspension. For the purpose of this document, it is sufficiently accurate to measure the required volume of test portion with a measuring spoon.
However, it should be noted that in samples with a high content of charged particles (e.g. organic matter, clay) the suspension effect can modify the potential difference between the electrodes, and thereby have an influence on the recorded pH value. This problem is minimized by gentle stirring of the suspension. For calcareous soils, carbon dioxide can be absorbed by suspension, which makes it difficult to reach an equilibrium value. Other sources of error are associated with soils containing sulfidic minerals or volatile acids.