PURPOSE: To prevent the occurrence of unexpected accident when handling a strong acid solution for gelation by adding a finely powdered silicate compound to the strong acid solution.

CONSTITUTION: To a strong acid solution, e.g., conc. solfuric acid, conc. nitric acid, etc., a finely powdered silicic acid or insoluble silicate compound, including synthetic silicates and silica gel, is added, whereupon the fluidity of the strong acid solution is lost to become gelled. Thus, the flow of the strongly acid liquid can be prevented and also the treatment of the strongly acid liquid can be easily performed even when an accident is encountered during the transportation, etc. Also, the strong acid solution can be used little by little when being used, and also the silicic acid content can be separated by coagulation. Thus, no trouble occurs in reactions of the strong acid solution.