1192 Journal of Applied Microscopy 



The microchemical detection of lithium is satisfactory only when this element 

 is present in considerable amount and in quite simple mixtures. 



At their best the methods are apt to prove unsatisfactory and require not a 

 little care and experience. 



Practically only three reagents are available ; these are : 

 I. Sodium Phosphate. 

 II. Ammonium Carbonate. 

 III. Ammonium Fluoride. 

 Only I and II will be described, since it is not advisable for the beginner to 

 make use of fluorides, because of the great danger of corrosion of the objectives 

 of the microscope. 



/. Sodium Phosphate added to solutions of Lithium salts precipitates Lithium 

 Phosphate. 



SLiaSO^ + 2HNa2P04 = ^^LigPO^ H2O + 2Na2SO^ + H2SO4. 



Method. Allow a drop of a solution of sodium phosphate to flow into a drop 



of a moderately concentrated solution of the substance to be tested. Heat the 



preparation almost to boiling. An exceedingly fine crystalline precipitate is at 



once formed. Examined with a high power, this precipitate is seen to consist of 



strongly refractive lenticular and fusiform bodies either singly or arranged in 



cross-, star-, or dagger-like groups which are very characteristic of lithium (see 



t^y^ Fig. 41). These crystals extinguish when their length 



J. Vi^ xaS '^S lies parallel to the cross-hairs of the polarizing micro- 



C3 ^ ^ scope. Globulites are also formed in abundance, which 



^ /} '^<U ~^_ '"y when examined between crossed nicols show the black 



n i^^ '^A cross of " sphero-crystals." 



ri rx 00 V Reinarks. The reaction should be performed in 



\j \j 60 neutral (or slightly alkaline) solution. If acid, neutral- 



*^^^3* //' V^ ize with sodium carbonate or sodium hydroxide. Since 



' \-p■^'^, --o.oim.^ i^ ^^ reaction an uncombined acid probably results. 



Fig. 41. it is advisable to have a small amount of free alkali 



present, and in practice it is found that faintly alkaline solutions yield the best 



results. An excess of alkali is to be avoided. 



If much sulphuric acid is present it is advisable to heat the material on plati- 

 num until most of the acid has been driven off, after which the residue is dis- 

 solved in water and the solution neutralized. 



Elements of the calcium group can be advantageously removed by treatment 

 with sulphuric acid or ammonium oxalate. 



If much magnesium is present, globulites seem to predominate. 

 In the presence of ammonium there arises the possibility of the formation of 

 a double phosphate LiNH4P04« GHgO : hence if any ammonium salts have been 

 employed in preceding operations, they should be removed by ignition before 

 testing for lithium. 



In testing mixtures, if in doubt as to the nature of the phosphate obtained,^ 

 draw off the supernatant liquor, wash the precipitate, dissolve in dilute acid and 



