  This paper presents an exhaustive how-to guide on measuring the total
vanadium concentration and state of charge of vanadium electrolytes using
UV-Visible spectroscopy. The study is provided with an open-access database
(https://github.com/AngeAM/SOC_Vanadium_Spectra_2023.git) that supports the
methods and procedures and facilitates access to the calibration data. The
study covers the three types of electrolyte solutions relevant to vanadium
redox flow batteries, namely the anolyte $V^{II}/V^{III}$, the catholyte
$V^{IV}V^V$, and the $V^{III}/V^{IV}$ commercial electrolyte, meant to be
preconditioned to either $V^{III}$ or $V^{IV}$ before battery operation.
Analytical expressions to calculate the concentration of different vanadium
species in the electrolyte solutions are provided based on either empirical
correlations or spectral deconvolution methods. The paper also examines the
limitations of the measurement technique and provides insightful
recommendations for future research. The open-access database provided by the
authors is expected to serve as a valuable repository for scholars and
scientists working in the field of vanadium redox flow batteries.
