One or more quartz tubes (1) are utilized in apparatus for the concentration of an acid. Each quartz tube is heated from the outside, and the acid is fed in at the upper part of each tube. Concentrated acid is drained off from the lower part of each tube into a vessel (3) and the vapors driven off from the acid during the concentrating rise inside each tube, and are led off from the upper part thereof. Inside each tube, the acid first passes through a rectification zone (A) in the upper part of the tube, and then passes through a liquid film zone and a reservoir zone.