Measurement of Sodium and Potassium
in Silicate Rocks 

This method for the measurement of sodium and potassium in silicate rocks uses a borate fusion technique.

The sample is fused with a suitable fusion agent such as boric acid (H3BO3) and lithium carbonate (Li2CO3) and the ground product is dissolved in dilute citric acid.The presence of other metals such as aluminium, calcium, iron, magnesium and silicon should not interfere with the measurement of sodium or potassium using this method.   


Fusion Agent
The prefusion mixture is prepared by mixing and grinding together 200g H3BO3, 60g of Li2CO3, 30g of SrCO3 and 10g of Co3O.


Preparation of sample

The sample is prepared by mixing 200mg of the ground rock powder with 4g of fusion agent. The fusion is carried out at 960ºC in a graphite crucible. 


The material obtained is ground to a fine powder. 


Method 1

A 500mg sample of the powdered fusion product is dissolved in 25ml of 10% HNO3. After filtering the precipitated SiO2 using a medium grade filter paper, the solution is diluted to 50ml using distilled water. In some cases a second filtration step may be required prior to dilution. 


Method 2

A 500mg sample of the powdered fusion product is added to 25mls of 2.5% citric acid and heated to 90ºC. 


The fusion product should completely dissolve, apart from graphite powder from the crucible, which is filtered off using a medium grade filter paper. 


The filtrate is transferred to a 50ml volumetric flask and made up to volume. The solutions are then analysed using standards prepared from a similar solution (10% HNO3 + H2O or 2.5% citric acid + H2O).

1 Gavindaraju, K. ‘Rapid Flame Photometric Determination of Sodium and Potassium in Silicate Rocks’, Appl.Spectroscopy, 20 (1966), p.302-304.