  Analytical equation of state for pure alkali metals (lithium, sodium,
potassium, rubidium and cesium) in equilibrium gas phase is proposed. This
equation has a simple form, generalizes the equation of state for a perfect gas
and is universal for all alkali elements. It correctly reproduces the
experimental data for the equilibrium gas phase over a wide range of pressures
(up to $\sim 10^5$~Pa) and temperatures (up to $\sim 3 \cdot 10^3$~K). On the
basis of this equation of state, expressions for thermal and caloric
coefficients as well as for \textit{other physical characteristics} are
obtained. The results of this study confirm feasibility of the principle of
corresponding states in relation to the group of alkali elements.
