It is well known that the exchange interaction between the spins of the rare-earth ions and the crystal field acting on the 4f-electrons are key factors which determine the magnetic behaviour of this system. In this paper we adopt a simplified description of the crystal field, keeping only the two lowest levels associated to the splitting caused by the crystal field on the 4f-state. Using the wave functions of these two levels, which are taken as singlet (ground level) and triplet (excited level), we construct an effective Hamiltonian in matrix notation which includes the crystal field and magnetic interactions and from this Hamiltonian we derive a set of magnetic state equations. Finally, this study is applied to the available experimental data of PrAl₂.