  We present experimental studies of atom-ion collisions using buffer-gas
cooled, trapped ytterbium (Yb$^+$) ions immersed in potassium (K) vapor. The
range of the collisional temperature is on the order of several hundred kelvin
(thermal regime). We have determined various collisional rate coefficients of
the Yb$^+$ ion per K-atom number density. We find the upper bounds of
charge-exchange rate coefficients $\kappa_{\rm ce}$ to be
$(12.7\pm1.6)\times10^{-14}$ cm$^3$s$^{-1}$ for K-$^{171}$Yb$^+$ and
$(5.3\pm0.7)\times10^{-14}$ cm$^3$s$^{-1}$ for K-$^{172}$Yb$^+$. For both
isotopes, the spin-destruction rate coefficient $\kappa_{\rm sd}$ has an upper
bound at $(1.46\pm0.77)\times10^{-9}$ cm$^3$s$^{-1}$. The spin-exchange rate
coefficient $\kappa_{\rm se}$ is measured to be $(1.64\pm0.51)\times10^{-9}$
cm$^3$s$^{-1}$. The relatively low charge-exchange rate reported here
demonstrates the advantage of using K atoms to sympathetically cool Yb$^+$
ions, and the relatively high spin-exchange rate may benefit research work in
quantum metrology and quantum information processing on an atom-ion platform
using K atoms and Yb$^+$ ions.
