  Future and on-going infrared and radio observatories such as JWST, METIS or
ALMA will increase the amount of rest-frame IR spectroscopic data for galaxies
by several orders of magnitude. While studies of the chemical composition of
the ISM based on optical observations have been widely spread over decades for
SFG and, more recently, for AGN, similar studies need to be performed using IR
data. This regime can be especially useful in the case of AGN given that it is
less affected by temperature and dust extinction, traces higher ionic species
and can also provide robust estimations of the chemical abundance ratio N/O. We
present a new tool based on a bayesian-like methodology to estimate chemical
abundances from IR emission lines in AGN. We use a sample of 58 AGN with IR
spectroscopic data retrieved from the literature to probe the validity of our
method. The estimations of the chemical abundances based on IR lines in our
sample are later compared with the corresponding abundances derived from the
optical emission lines in the same objects. HII-CHI-Mistry-IR takes advantage
of photoionization models, characterized by the chemical abundance ratios O/H
and N/O and the ionization parameter $U$, to compare their predicted
emission-line fluxes with a set of observed values. Instead of matching single
emission lines, the code uses some specific emission-line ratios sensitive to
the above free parameters. We report mainly solar and also subsolar abundances
for O/H in the nuclear region for our sample of AGN, whereas N/O clusters
around solar values. We find a discrepancy between the chemical abundances
derived from IR and optical emission lines, being the latter higher than the
former. This discrepancy, also reported by previous studies of the composition
of the ISM in AGN from IR observations, is independent from the gas density or
the incident radiation field to the gas.
