The effect of potassium addition on the radial distribution of temperature and electron number density in a U-shaped direct current (dc) argon plasma operating at different arc currents has been studied by optical emission spectroscopic techniques and the power interruption method. Spatially resolved electron number densities (n _e) have been determined from measured radial profiles of Balmer-H_β spectral line. The obtained electron number densities have been used for thermodynamic temperature (T _LTE) evaluation with the assumption that the arc plasma is in a state of local thermodynamic equilibrium. The excitation temperatures (T _exc) have been determined from the absolute integral emissivity of the argon line at 430.01 nm. For heavy particle temperature (T _h) evaluations we have used a power interruption method. The obtained results have shown that an addition of KCl decreases both electron number density and temperature of the arc column. The magnitude of such an influence on plasma parameters increases with an increase in the KCl concentration and decreases with an increase in the arc current.