features.

The paper is organized as follows. In Section 2 we describe our sample of Galactic PNe, whereas in Section 3 we present the calculation of electron temperatures and densities for all the objects. Section 4 contains an extensive discussion of the iron abundance and depletion factor calculations. Section 5 details the calculation of C/O abundance ratios from collisionally excited lines (CELs) and recombination lines (RLs), and the comparison between these two estimates. The compilation of infrared dust features observed in the sample is presented in Section 6. In Section 7 we discuss our results and study the correlation between iron abundances, C/O values, and dust features in our sample of PNe. Our conclusions are presented in Section 8.

The Sample {#sec:sample}
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Our sample consists of 56 Galactic PNe with published spectra of relatively high spectral resolution and deep enough to detect the lines we need to calculate physical conditions and iron abundances. The sample includes 28 Galactic PNe studied in @DelgadoInglada_09, but we exclude nebulae with electron densities above 25000 [$\rm{cm}^{-3}$]{} since it is difficult to derive reliable physical conditions and chemical abundances in these objects [@DelgadoInglada_09]. Besides, we set an upper limit on the $I(\mbox{\ion{He}{2}~}\lambda4686)/I(\mbox{H}\beta)$ intensity ratio 