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The stable end of the solubility-curve GH, the triple-point H 
indicates the coexistence: 4 Na,O. Al,O,.10H,O + NaOH. H,O + 
solution. 
The equilibriums along HK. 
As last curve of the solubility-isotherm the curve HK is joined 
to GH indicating the solutions coexisting with the monohydrate of 
sodium-hydroxide. As the triple-point AH is situated near: 0,1 °/, 
Al,O,, so very closely on the Na,O-axis, the curve HK is very 
short. Therefore only the end points Mand XK have been determined. 
SUMMARY. 
1. At 30° two stable aluminates arise in the system Na,O— 
Al,O,—H,O: viz. 4 Na,0. 3 Al,O,. [6 H,O and 4 Na,O. Al,O,. 10 HO: 
Both of those form strongly incongruently saturated solutions, in 
other words they are decomposed by water and by diluted NaOH 
solutions. In fig. Il we may see below what concentration-limit 
the NaOH-solution will cause this decomposition. 
2. According to the preparation aluminiumhydroxide may be 
obtained in different forms. Under special circumstances it arises as 
a erystellized hydrate of the composition: Al,O,.3H,0. 
3. The gelatinous hydroxide must be considered as a metastable 
phase of variable composition. It absorbs variable quantities of alkali. 
4. Most probably there is a continuous transition between these 
gelatinous hydroxides and the crystallized hydrate. 
5. Aluminiumoxide is metastable at 30° as regards the hydrate. 
6. The swelling that the dessiccated particles of the hydroxide 
and oxide display, is very much dependent on the alkalinity of the 
solution. 
Delft, Inorganic and physical-chemical Laboratory 
April 1920. of the Technical University. 
