When the electric potentials or charge densities are not too high, the PB equation can be simplified to the Debye-Hückel (DH) equation. By expanding the exponential function in the PB equation into a Taylor series, one obtains
:        where       
The parameter κ is referred to as the Debye length, and some representative values for a monovalent salt in water at 25°C with ε ≃ 80 are given in the table on the right. In non-aqueous solutions, Debye length can be substantially larger than the ones given in the table due to smaller dielectric constants. The DH model represents a good approximation, when the surface potentials are sufficiently low with respect to the limiting values
The numerical value refers to a monovalent salt and 25°C. In practice, the DH approximation remains rather accurate up to surface potentials that are comparable to the limiting values given above. The disjoining pressure can be obtained from the PB equation given above, which can also be simplified to the DH case by expanding into Taylor series. The resulting expression is
The substantial advantage of the DH model over the PB model is that the forces can be obtained analytically. Some of the relevant cases will be discussed below.