The discrimination of the fast second-order, pseudo-first-order and fast pseudo-first-order reaction models on the basis of the second-order reaction model justifies the use of the pseudo-first-order model in the prediction of the multiplicity regions. The use of the fast second-order reaction model leads to a certain pitfall regarding uniqueness and multiplicity. Finally, the use of the fast pseudo-first-order reaction model is justifiable not only because of the fair agreement between the multiplicity regions predicted by this model and the second-order model but also because of the analyticity of the uniqueness and multiplicity regions obtained by this model.