An instrument has been designed and constructed for the semi-continuous monitoring of sulfuric acid mist. The instrument utilizes the selective condensation method of acid collection with a subsequent conductivity measurement of the aqueous isopropanol-acid solution. The instrument utilizes a heated quartz extraction probe, along with state-of-the-art electronics. The sampling rate can be varied and preset by use of a mass flow controller to assure isokinetic sampling. The instrument was designed to semi-continuously monitor sulfur trioxide-sulfuric acid mist in the tail gases of contact acid plants on an unattended basis.