Effect of Alkali and Alkaline Earth Chlorides on the Anode Temperature of the dc Arc*

Abstract

Temperatures of graphite anodes have been measured for the dc arc in helium and argon by means of a recording thermistor pyrometer when pure chlorides of the alkali and alkaline earth metals are present as samples. Anode temperatures and gap voltages were depressed by all of the samples. The data for helium indicate a correlation of anode temperature to the boiling point of the sample salt and the lowest excitation potential of the cation. Correlation was also observed between the boiling point of the salt and its rate of volatilization. Temperatures in argon are lower and correlation between anode temperatures and either boiling points or excitation potentials was not observed. However, rates of volatilization were related to the boiling points and excitation potentials of the samples. These effects are large enough to account for the poor precision of the dc arc as a spectrochemical source and show the physical-chemical basis for the use of “spectroscopic buffers.”