The Reversal of the Sign of the Charge of Collodion Membranes by Trivalent

The reversal of the sign of charge of collodion membranes (treated previously with a protein) by hydrogen ions has been discussed in a preceding paper 1 and in this paper the influence of trivalent cations on the reversal will be considered. We shall omit the discussion of the action of tetravalent cations (e.g. ThC14) since their soiutions have so high a concentration of hydrogen ions that this alone suffices to bring about a reversal in the sign of charge. We will prove first that collodion membranes which have previously been treated with a protein give the reversal in the sign of charge in the presence of trivalent cations, while collodion membranes not so treated do not show the reversal. When a solution of CaCI~ of not too high a concentration (e.g. below ax/8) is separated from pure water by a collodion membrane, which is negatively electrified, the solution shows no attraction for water, while it attracts water powerfully when the membrane is charged positively. The Ca ion acts as ff it repelled positively charged water and as if it attracted negatively charged water. To find out whether or not trivalent cations reverse the sign of charge of the membrane we have to add a low concentration of a "neutral" salt with trivalent cation to the 'solution of CaCI~. Weak solutions of CeCI~ and LaC18 satisfy this condition. The solutions of CaC12 are for this purpose made up in •/1,024 solutions of CeCla or LaC13 instead of in distilled water; and the distilled water surrounding the collodion bag is also replaced by