  Whether the new line at ~3.5 keV, recently detected in different samples of
galaxy clusters, Andromeda galaxy and central part of our Galaxy, is due to
Potassium emission lines, is now unclear. By using the latest astrophysical
atomic emission line database AtomDB v. 3.0.2, we show that the most
prospective method to directly check its Potassium origin will be the study of
K XIX emission line complex at ~3.7 keV with future X-ray imaging spectrometers
such as Soft X-ray spectometer on-board Astro-H mission or microcalorimeter
on-board Micro-X sounding rocket experiment. To further reduce the remaining
(factor ~3-5) uncertainty of the 3.7/3.5 keV ratio one should perform more
precise modeling including removal of significant spatial inhomogeneities,
detailed treatment of background components, and further extension of the
modeled energy range.
