This method has been adapted to enable measurement of lithium over a wide range of concentrations (0.10 to >8.0%), in lithium containing materials.
The sample is decomposed using a mixture of nitric, hydrofluoric* and perchloric acids. The effect of interfering ions (principally ferric, chromic and aluminium) is eliminated by buffering the sample solution within the pH range 1 to 4, using a citric acid-ammonium citrate buffer.
All solutions are prepared using deionised distilled water.
A solution containing 1M of citric acid and 1M ammonium citrate was prepared, having a pH of 3.7. The buffer solution was boiled for 30 minutes and stored refrigerated in order to prolong its shelf life and inhibit microbial growth.
Ranging in concentration from 0.5 to 50.0ppm, or appropriate to sample concentration, were prepared from a standard stock solution, using appropriate aliquots to a 250ml volumetric flask to which 25ml of the citric acid-ammonium citrate buffer has been added.
Preparation of sample
A sample containing approximately 10 to 20mg of lithium oxide is decomposed in a platinum dish by treating it with a mixture of 10ml of concentrated nitric acid, 10ml of 48% hydrofluoric acid* and 5ml of 62% perchloric acid. The solution is evaporated at low temperature on a hot plate until fuming, within a well ventilated fume hood. The platinum dish is removed from heat source and allowed to cool.
A 5ml aliquot of hydrofluoric acid* is added to the residue and allowed to stand for approximately 30 minutes and then evaporated to fuming, near dryness. The addition of hydrofluoric acid and subsequent evaporation is repeated twice more or until the silica appears to be volatilised. After fourth evaporation, the sample is allowed to fume to near dryness, but not baked.
Sufficient perchloric acid is left in the residue to allow dissolution with the addition of water.
After the dish has cooled, 30 to 40ml of deionised water is slowly added and the mixture is gently warmed until the residue has dissolved. The solution is allowed to cool and quantitatively transferred to a 250ml volumetric flask containing 25ml of 1M citric acid-ammonium citrate buffer and diluted to volume. The prepared solutions were measured under the same conditions as the standard solutions.
*Hydrofluoric acid is extremely corrosive and is a contact poison. It should be handled with extreme care and used only by personnel trained in appropriate safety procedures.
1 Kassner, J.L., et al, ‘Flame Spectrophotometric Determination of Lithium in Lithium Minerals’, Anal. Chem, 32 (1960), p.1151-1153.