an engineering geological characterisation of tropical clays - GBV

79
The CS-225 is a micro-processor analysis system for determination of carbon and sulphur
contents through infrared absorption. Iron and tungsten filings acting as catalysts were added
to the soil specimen which was then heated in a high frequency induction oven. The
combustion/ heating gases were dried and then directed to the infrared sulphur cell for the
measurement of the sulphur dioxide content. The gases were further passed through a
catalysing oven where carbon monoxide (CO) and sulphur dioxide (SO2) were converted into
carbon dioxide (CO2) and sulphur trioxide SO3, respectively. The SO3 was collected in a
sulphur-trap, thereby permitting measurement of the total carbon content in terms of CO2 in
the infrared carbon cell. The results of the analysis were calculated by the microprocessor
system with the aid of calibration factors and after applying relevant weight compesations.
The organic carbon content was determined by first pretreating the soil specimen with
concentrated hydrochloric acid to remove the carbonate part. The excess acid was evaporated
by heating on a hot plate and finally removed by passing over potassium iodide and antimony
metal.
The sulphur content of both the red soils and black clays was found to be negligible. The
results are therefore not reported.
6.6.2 Results
Results of carbon content determinations of soils performed in this study are given in Table
6.6. The black clays exhibit 0,50-1,48% total carbon (mean: 1,17%) and 0,22-1,19% organic
carbon (mean: 0,93%); and therefore 0,04-0,74% inorganic carbon (mean: 0,24%). The red
soils have 0,45-2,47% total carbon (mean: 1,12%), 0,35-1,90 organic carbon (mean: 0,85%)
and 0,10-0,57% inorganic carbon (mean: 0,27%). The total carbon content in both types of
soil is generally low (less than 2,5%) so that difficulties were encountered in trying to
separate and determine the organic and inorganic components during laboratory analysis. In
this study, therefore, the total carbon and organic carbon contents were determined; and their
numerical difference adopted as the inorganic carbon content, i.e.
Total carbon (%) = organic carbon (%) + inorganic carbon (%), or
Ctotal (%) = Corganic (%) + Cdiff. (%),
where the numerical difference, Cdiff., corresponds to the inorganic carbon component.
The organic carbon component serves to represent the organic matter content of the soils. The
inorganic carbon component represents the carbonate content occurring in the soils in the
form of calcite, dolomite, ankerite, siderite and/or other inorganic carbonates. The Scheibler
method for determination of carbonate content of soils could not be applied in the present
analysis due to the generally low carbonate content (less than 5%) of the black clays and red
soils. This analysis method depends on atmospheric conditions of temperature and pressure,
and is therefore only applicable to soils with carbonate contents of over 5%.