IMID0SULPH0XATE8. 71 



twice its volume, or more, of water, treated with sodium hydroxide 

 solution till it tastes slightly caustic, and then cooled in an ice-l)ox. 

 Tlie trisodium salt begins to separate almost at once after the addition 

 oi the sodium hydroxide, but the water previously added retards the 

 separation and enables the crystals to grow in the cooled liquor suffi- 

 ciently Jarge to be afterwards l)etter freed from their mother-liquor, 

 Strained out, and pressed in calico, the salt becomes almost pure, ;ind 

 can be rendered fully so by recrystallisation. 



Trisodium imidosulphonate is a very stable and easily prepared 

 salt. It forms thin, overlapping, hexagonal plates, which may grow 

 to considerable size,:. but are .seldom if ever to be seen single and per- 

 fect, or with much thickness. The crystals readily effloresce in dry 

 air, iuid have a mildly alkaline taste. The salt is sparingly soluble in 

 very cold water and very soluble in hot water, taking for dissolution 

 at 27^-° about 5'4 parts of water. Its solution readily shows super- 

 saturation. It is alkaline to litmus, and even to phenol-phthaleln, but 

 exercises no action whatever on iodine-solution, and is |)recipitated 

 from its aqueous solution by alcohol without decomposition. It can 

 be repeatedly recrystallised without any loss of alkali. Aqueous solu- 

 tions are quite stable even when continuously boiled. The crystals 

 melt when gently heated, and their water may be ra])idly boiled off 

 without causing any decomposition of the salt. In a vjicuum over 

 sulphuric acid they lose only eleven molecules of the twelve they 

 contain. The salt loses more water when heated, but even at 160° it 

 retains some, ])Ossibly through hydrolysis, so that heated until decom- 

 position begins, it yields always a very little water and ammonia (gas 

 and sublimate). Heated in a vacuum, liowever, it yields even still 

 less, so that atmospheric moisture seems to be active in hydrolysing it 

 when heated in air. 



riie pi'esence of hydrogen in the dried salt might have particular 



