The electrokinetic behaviour of iron‐ and aluminium‐hydroxides and ‐oxides

Measurements have been made of the electrophoretic velocities of ferric and aluminium‐oxides and ‐hydroxides in suspensions of variable pH, in order to establish the isoelectric point of these minerals. The results are: Goethite, isoelectric at pH 3.2; “limonite” at pH 3.5; lepidocrocite at pH 5.4; haematite at pH 2.1; böhmite and diaspore always negative and no isoelectric point; hydrargyllite at pH 4.8–5.2; corundum at pH 2.2. It is evident that the isoelectric points of these minerals are very different from those of freshly precipitated systems (α‐Fe2O3: 8.3; Al(OH)3: 8.2). The findings are discussed on the basis of the crystal structure of the minerals.