available we use instead \[\] $\lambda7155$ and $I(\lambda8616)/I(\lambda7155) \sim 1$, as found by @Rodriguez_96 in several regions. The values of Fe$^{+}$/Fe$^{++}$ for these eleven PNe go from 0.06 (M 3-7) to 0.80 (NGC 6565 and NGC 6741). Only five of the sample PNe would require a value for the Fe$^{+}$ abundance in the ionization correction scheme described below, in § \[sec:tot\_ab\]. The two of them where it is available show that its contribution is not likely to be critical, since it only increases the value of Fe/H by 0.08 dex (in NGC 40) and 0.17 dex (in IC 418). Some of the calculated values of Fe$^{+}$/H$^{+}$ are also used in the calculations described in § \[withoutICF\].

The Fe$^{++}$ ionic abundances are derived by solving the equations of statistical equilibrium [@Osterbrock_06] for a 34-level model atom, using the collision strengths of @Zhang_96 and the transition probabilities of @Quinet_96. We used PyNeb [@Luridiana_12] to compare the results obtained with these atomic data with those implied by the recent calculations of @Bautista_10. In the three nebulae with the largest numbers of \[\] lines (Cn 1-5, M 2-4, and M 2-6), we find differences lower than 5% in the final Fe$^{++}$ abundances, but the 