British chemist whose work was instrumental in the development of colloid chemistry.

Donnan was educated at Queen's College in Belfast, N.Ire., and at the Universities of Leipzig, Berlin, and London. From 1904 to 1913 he taught at the University of Liverpool, and from 1913 to his retirement in 1937 he was professor of chemistry at University College, London.

In 1911 Donnan studied the conditions under which equilibrium is established between two electrolytic solutions separated by a semipermeable membrane—that is, by a membrane through which the solvent and some, but not all, of the dissolved ions can pass. In the absence of such a membrane, the solvent and every species of dissolved ion will diffuse freely from each solution into the other, until the composition of the two solutions becomes the same. The semipermeable membrane, however, prevents the transfer of at least one ionic species, and the preservation of electrical neutrality limits the diffusion of that species' oppositely charged partner. Nevertheless, some movement of mobile ions does occur, and the compositions of the solutions change; as a result, the final distribution of the ionic species is unequal, and there is a measurable difference in the electric potential of the solutions on each side of the membrane. The nature of the equilibrium and the existence of the potential have both become associated with Donnan's name.