Determination of total silicon by carbonate fusion

Only evaporation and fusion with alkali carbonate release larger amounts of silica from suspended inorganic material and make it available for spectrophotometric analysis. 

Sodium carbonate solution. Dissolve 10 g of anhydrous sodium carbonate, Na2C03, in pure water and dilute to 100 mL. Store in a polyethylene bottle. 

Sulphuric acid, 0.45 mol/L Dilute 10 mL of the 4.5mol/L sulphuric add (reagent 1 of the silicate determination, see Section 10.2.11) to lOOmL with pure water. Store in a polyethylene bottle. 

Pour 25 mL of sample (using a bulb pipette) into a platinum crudble and evaporate to about 5 mL at a low temperature (about 100 °C) on a sand bath or in an oven. With a plastic pipette add S mL of carbonate solution (see reagents). Evaporate further to dryness and fuse imtil the melt is quiescent and clear. Allow to cool and leach the melt with two 5 mL portions of hot water. Transfer quantitatively into a 100 mL plastic beaker. Add 0.2 mL of the indicator (see reagents) followed by dilute sulphuric add (see reagents) until colourless (about 11 mL). Then add a further 4mL. Transfer into a SOmL plastic graduated cylinder, with a stopper, and dilute with water to 40 mL. Allow to stand for at least 1 h and then use 25 mL for analysis. Proceed as described for the determination of sUicate (see Section 10.2.11). Multiply the resulting sihcate concentrations with a dilution factor Fu = 1.6 to correct for the dilution during the carbonate fusion procedure. 

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