Classification of soil is important for environmental control and construction purposes. Soil classification is based on several factors such as absorption, the ability to shrink or swell, water retention and permeability. Most of these factors independently correlate with the particle size (clay fraction) of the soil. The clay fraction is measured according to for instance ISO 11277 and allows the soil to be categorized within stated classes.

The analysis is carried out in three steps:

  1. Weighing the sample and separating the fractions
  2. Removal of Organic Matter and Carbonates
  3. Determination of the clay fraction

Skalar has developed a concept to automate the second and third step in this process using respectively a SP50 and SP1000 Robotic analyzer. Automation of the third step is of particular interest because of the precise timing and pick up height involved in the procedure.

A typical Organic Matter/Carbonates removal sequence

Steps of Automation
  • Removal of Carbonates
  • Removal of Organic matter
  • Rinsing of the sample

After weighing, the samples are placed in a bottle. The bottles are placed in the SP50 and an amount of water is added. The bottles with sample are heated until the boiling point has been reached. A sample is boiled for 30 min. During heating and boiling every 30 min, 5 ml of hydrogen peroxide is added to remove the organic matter. Sample is cooled down. Same process is started but now with the addition of hydro chloric acid to remove the carbonates. During the complete process water is added in intervals to avoid excessive boiling of the sample.

A typical Clay fraction analysis sequence

Steps of Automation
  • Addition of pyrophosphate
  • Addition of dilution water
  • Sample homogenizing
  • Taking the sample
  • Sample transferring
  • Sample dispensing
  • Sample heating
  • Sample weighing
  • Result calculation

The sample is transferred into a volumetric cylinder of 500/1000 ml. The system, a SP50 or SP1000, automatically adds Sodium pyrophosphate solution and brings the suspension to volume by adding distilled water. During a user-definable time, the sample is homogenized. After a settling period of several hours (up to 8 hours) a fraction of the suspension is taken at a predefined depth from the cylinder and this fraction is transferred to an evaporation dish by the system. After drying the remainder of the sample is weighed and the Clay fraction is calculated via the software.

This procedure can also be performed unattended allowing for overnight operation. In addition, it is also possible to measure different fractions, for example < 16 µm, or to measure directly after the cylinder is brought to volume.

The Skalar Robotic analyzers offer an economical and “walk-away” automation concept for the lengthy clay fraction analysis procedure, which modern advanced laboratories will find greatly beneficial.

Comparison Skalar
Clay fraction Analyzers
Sample capacity 42 bottles (500 ml) 54 bottles (1000 ml)
70 bottles (500 ml)
Fractions Up to 5 Up to 5
Barcode reader Option Option




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