E.C. Tubb 
Fifty Days to Doom 

The galaxy is at war, a war between the oxygen-breathing Terran Federation of worlds and the Ginzoes, chlorine-breathing aliens. Neither side dare attack the others’ habitable worlds for fear of reprisals: the war has become a matter of spaceships firing on each other as they emerge from hyperspace.An Earth ship is captured in battle by the Ginzoes, and its surviving crew learn that the aliens have gained possession of a newly-developed catalyst. If dropped into the oceans of oxygen planets the catalyst can liberate the chlorine from the sea and convert them to chlorine worlds suitable for the Ginzoes. The catalyst will be used unless the Terran Federation declares peace within fifty days. Fifty days to save Mankind . . . or fifty days to doom!