The self-consistent electric field acting in a plasma retards the most mobile charged particles, which usually leads to a Boltzmann distribution of electrons. If negative ions cross the discharge volume several times during their lifetime in the volume processes, these particles also obey the Boltzmann distribution. It is demonstrated that this condition is usually satisfied when the characteristic time of electron attachment is small as compared to the time of ambipolar diffusion of the negative ions (ion diffusion at an electron temperature). In the opposite case, the profiles of electrons and negative ions are similar.