  We present an adapted version of the code HII-CHI-Mistry-UV (P\'erez-Montero
& Amor\'in 2017) to derive chemical abundances from emission lines in the
ultraviolet, for use in narrow line regions (NLR) of Active Galactic Nuclei
(AGN). We evaluate different ultraviolet emission line ratios and how different
assumptions about the models, including the presence of dust grains, the shape
of the incident spectral energy distribution, or the thickness of the gas
envelope around the central source, may affect the final estimates as a
function of the set of emission lines used. We compare our results with other
published recipes for deriving abundances using the same emission lines and
show that deriving the carbon-to-oxygen abundance ratio using CIII] $\lambda$
1909 \r{A} and OIII] $\lambda$ 1665 \r{A} emission lines is a robust indicator
of the metal content in AGN that is nearly independent of the model
assumptions, similar to the case of star-forming regions. Moreover, we show
that a prior determination of C/O allows for a much more precise determination
of the total oxygen abundance using carbon UV lines, as opposed to assuming an
arbitrary relationship between O/H and C/O, which can lead to non-negligible
discrepancies.
