  Knowing the bulk elemental abundances of an exoplanetary atmosphere is not an
easy task, but it is crucial to understand the formation history of planets.
The purpose of this work is to show that the observability of TiO features at
optical wavelengths in the transmission spectra of hot Jupiter atmospheres is
sensitive to the bulk chemical properties of the atmosphere. For that, we run a
grid of chemical models which include TiO formation and destruction, for the
ultra-hot Jupiter WASP-19b and an ultra-hot version of HD~209458b. We take into
account non-equilibrium chemistry and changes in the temperature and pressure
structure of these atmospheres caused by different C/O ratios. We calculate
synthetic transmission spectra for these models, and study the relative
strengths of TiO and \ce{H2O} features quantitatively. To compare with
observations, we use a model independent metric for molecular abundances,
$\Delta Z_{\rm TiO-H_2O}/H_{\rm eq}$ that has been previously used in
observational studies of exoplanetary atmospheres. We find that with this
metric we can differentiate between different chemical models and place
constraints on the atmosphere's bulk carbon and oxygen abundance. From chemical
considerations we expected that the TiO abundance would depend on the bulk
nitrogen, however found that any change to N/H did not result in changes to the
resulting TiO. We apply our method to a set of known exoplanets that have been
observed in the relevant optical wavelengths and find good agreement between
low resolution observations and our model for WASP-121b, marginally good
agreement with WASP-79b, WASP-76b, and WASP-19b, and poorer agreement with HD
209458b. Our method can be particularly helpful for indirect studies of the
bulk abundances of carbon and oxygen.
