  We study the vapour-liquid phase behaviour of an ionic fluid confined in a
random porous matrix formed by uncharged hard sphere particles. The ionic fluid
is modelled as an equimolar binary mixture of oppositely charged equisized hard
spheres, the so-called restricted primitive model (RPM). Considering the
matrix-fluid system as a partly-quenched model, we develop a theoretical
approach which combines the method of collective variables with the extension
of the scaled-particle theory (SPT) for a hard-sphere fluid confined in a
disordered hard-sphere matrix. The approach allows us to formulate the
perturbation theory using the SPT for the description of the thermodynamics of
the reference system. The phase diagrams of the RPM in matrices of different
porosities and for different size ratios of matrix and fluid particles are
calculated in the random-phase approximation and also when the effects of
higher-order correlations between ions are taken into account. Both
approximations correctly reproduce the basic effects of porous media on the
vapour-liquid phase diagram, i.e., with a decrease of porosity the critical
point shifts toward lower fluid densities and lower temperatures and the
coexistence region is getting narrower. For the fixed matrix porosity, both the
critical temperature and the critical density increase with an increase of size
of matrix particles and tend to the critical values of the bulk RPM.
