Apparatus for mixing and/or testing small volumes of fluids

An apparatus and method for measuring a range of small volumes of fluids to a high degree of precision, said apparatus including: a) a single reaction chamber (20); b) a piston (30) operable within said chamber to selectively and precisely vary the internal volume of said chamber; c) a first inlet (41) to said chamber in communication with one or more fluid sources of which a portion of a first fluid of said one or more fluids is adapted to be drawn into said chamber; d) at least one further inlet (43) including a second inlet to said chamber in communication with one or more fluid sources of which a portion of a second fluid of said one or more fluids is adapted to be drawn into said chamber; and e) a sealable outlet (45), wherein said piston is adapted to progressive draw said portion of a second fluid into said chamber until either accurare metering of a volume of said first or second portion of fluid is achived or a reaction involving said first and second portions in said chamber (such as a titration reaction) is completed.