Abstract: Disclosed is an apparatus for injecting a gas such as CO.sub.2 into a flowing liquid. The liquid is contained in a three-section conduit consisting of converging, bottleneck and diverging sections. The sections are joined end-to-end with different radii adjacent their inner surfaces, i.e., they are joined end-to-end without concordance radii so as to produce a turbulence effect in the liquid flowing through the conduit. An annular chamber is disposed about the bottleneck section and includes a connection for connecting to a source of pressurized gas or the like. The annular chamber is separated from the bottleneck by a wall perforated with micro-holes which allow gas entrainment into the flowing liquid. A plurality of hollow needles which extend varying amounts into the liquid flow are disposed in the wall to provide multiple sites of gas injection in the annular chamber. The arrangement is useful for neutralizing alkaline water by injecting CO.sub.2 gas therein.