  An analysis of reductive perturbation method (RPM) is presented to show that
why the solitary structures of nonlinear ion acoustic waves (IAWs) cannot be
obtained in magnetized electron ion plasma by employing this technique. In RPM,
the nonlinear Korteweg-de Vries (KdV) equation is derived using stretched
coordinates in the reference frame of the wave phase speed, considering the
dispersion to be a higher-order effect which balances the nonlinearity to
produce a solitary structure. The maximum amplitude $\mid \Phi_m \mid$ of the
nonlinear solitary wave turns out to be larger than one which contradicts the
small amplitude approximation. In the presence of negative ions, the maximum
amplitude satisfies the condition $\mid\Phi_m\mid <1$. To elaborate these
points, the results have been applied to an experimental plasma consisting of
positive ions of xenon $(Xe^{+})$ and negative ions of fluorene $(F^{-})$ along
with electrons. The amplitude and width of the solitary structures depend upon
the ratio of the electron to positive ion density ($\frac{n_{e0}}{n_{i0}}$).
Since the nonlinear coefficient turns out to be negative, rarefied (dip)
solitons are formed in the magnetized $Xe^{+}-F^{-}-e$ plasma.
