up to 0.4 dex, and are those with the highest differences in the adopted physical conditions and in the correction applied for the unobserved ions. Again, this illustrates the uncertainties introduced by the choices one has to make in order to calculate chemical abundances.

The \[\] lines are usually the brightest iron lines in photoionized nebulae. Therefore, total iron abundances are generally calculated using $\mbox{Fe}^{++}$ abundances and ICFs derived from photoionization models. However the iron abundances computed in this way differ from the ones obtained by adding up the ionic abundances of Fe$^{+}$, Fe$^{++}$, and Fe$^{+3}$ in some regions and low-excitation PNe. @Rodriguez_05 studied the uncertainties involved in the calculations and proposed that the iron abundances can be constrained using two ICFs, one derived from photoionization models: $$\label{eq:1}
\frac{\mbox{Fe}}{\mbox{O}}=0.9\left(\frac{\mbox{O}^{+}}{\mbox{O}^{++}}\right)^{0.08}\frac{\mbox{Fe}^{++}}{\mbox{O}^{+}},$$ and the other computed from observational data: $$\label{eq:2}
\frac{\mbox{Fe}}{\mbox{O}}=1.1\left(\frac{\mbox{O}^{+}}{\mbox{O}^{++}}\right)^{0.58}\frac{\mbox{Fe}^{++}}{\mbox{O}^{+}}.$$

For low ionization objects, with $\log(\mbox{O}^+/\mbox{O}^{++})\geq-0.1$, equation (\[eq:2\]) should be replaced by $$\label{eq:3}
\frac{\rm {Fe}}{\rm {O}}=\frac{\mbox{Fe}^+ + \mbox{Fe}^{++}}{\mbox{O}^+},$$ since $\mbox{Fe}^{++}$ and $\mbox{O}^+$ will dominate the total abundances of Fe and O. This scheme provides three values for the iron abundance, each of them corresponding to one of the three possible explanations suggested by @Rodriguez_05 for the iron abundance discrepancy. The Fe/O values 