Nature of certain Solutions. 97 



tested are to placed in a small porcelain basin, a little alcohol 

 added, and then a few drops of test solution. When the 

 quantity of sulphuric acid is something more than a trace, an 

 immediate black precipitate is formed. If the quantity is 

 something less no immediate result is visible ; but as the 

 alcohol evaporates, it leaves behind a film of dark crystals 

 which under a lens are easily recognizable. In this way a 

 quantity of sulphuric acid amounting only to '000015 gramme, 

 about -^ of a milligram, can be detected with certainty, pro- 

 vided the test solution is not used in too great excess. To 

 detect so small a quantity, however, requires some familiarity 

 with the reaction. 



With larger quantities of free sulphuric acid, the charac- 

 teristic violet-black crystalline precipitate falls immediately. 

 In dealing with quantities too small to cause precipitation, it 

 is advantageous to gently warm the basin beforehand ; in 

 this way a crystallization is obtained within two or three 

 minutes, and the danger of secondary reactions, such as 

 might be caused by the presence of oxidizing acids, is 

 diminished *. 



The conclusions derived from the foregoing investigations 

 may be briefly summed up as follows : — 



1. The solution of iodoquinia affords the means of detecting 

 free sulphuric acid even in traces in presence of combined 

 sulphuric acid. 



2. The salts of heavy metallic protoxides do not owe their 

 acid reaction to dissociation. The solutions of their sulph- 

 ates contain no free sulphuric acid. To this there is one 

 exception. Solutions of ferrous sulphate always contained 

 free sulphuric acid. 



3. Sesquisulphates, as far as examined, are always disso- 

 ciated in solution. 



4. Alums are always dissociated in solution with a single 

 notable exception. Chrome alum exists as such in solution. 



5. Acid salts are dissociated in solution. The dissociation 

 is very great and may perhaps be complete. 



* Very faint traces of free sulphuric acid are detected as follows. If 

 the test liquid dries up without showing indications, 1 or 2 cub. centini. 

 of alcohol are poured into the basin and slowly washed round. As the 

 yellow film dissolves it may leave behind it a violet-black film of hera- 

 pathite, which, being much less soluble, resists the action of the alcohol 

 longer. This is a test of very great delicacy and the treatment with 

 alcohol should never be omitted. 



Phil. Mag. S. 5. Vol. 36. No. 218. July 1893. H 



