pH testing with colorimetric test kit

Today we analysed mudstone, dolerite, basalt and limestone based soils collected from soils excursion exercise yesterday.

pH testing was performed with the use of a colorimetric kit, a quick, portable and reasonably cheap method of approximating soil chemistry. A sample of soil no bigger than the size of a 20 cent coin was placed on a white tile, removing large bits of debris and organic matter. Indicator solution is added and mixed until a thick flat paste is formed. White barium sulphate powder is then lightly sprinkled on the surface and after a minute the change of colour to the edges of the powder applied is compared to a colour chart. This chart is accurate to the nearest 0.5 and is an approximation only. More accurate readings can be obtained using an electronic meter. Probes are useless. 

Only major OH&S issue is to prevent inhalation of the barium sulphate powder. Environmental issues that would need to be considered include disposal of chemically treated soils - would need to do further research on potential impacts of different methods. 

Results :  dolerite pH 6.0, mudstone pH 5.0, limestone pH 6.5, basalt pH 6.0

These results were reasonable, but probably not accurate due to the presence of other matter in the samples. Dolerite and basalt are igneous rock which is high in nutrients and generally more alkaline with typical range of pH 6.2 - 6.5.  Mudstone is a sedimentary rock which is low in nutrients and more acidic with a typical range of 5.0 - 6.0

The use of the colorimetric kit requires practise, especially to reliably match colour changes. A thin layer of barium sulphate scattered evenly and left alone is essential. Interpreting results and comparing colour changes seems to require some experience. 

Following this practical in the classroom I performed further pH testing on soil at home (photos above show the steps). I analysed various soils sites and consistently had readings of between 6.0 - 6.5