Complex coacervating systems form liquid-liquid phase separated solutions with unique viscosity, wetting, and adhesive properties. To utilize these characteristics, it is important to experimentally determine the phase behavior of specific polymer systems. Currently, separate solutions of varying concentrations must be prepared to investigate coacervate phase diagrams, which is both time- and material-intensive. We present a microfluidic device which enables the investigation of the salt-polymer phase diagram in a single measurement. We propose a gradient generating design to produce a range of salt and polyelectrolyte concentrations. This device allows for the imaging of mixtures of salt and polyelectrolytes at various concentrations to observe the conditions under which coacervation occurs. Thorough mixing is needed for the accurate characterization of output concentrations from the gradient generator to map the location of points on the phase diagram. Channel walls are patterned with a staggered herringbone mixer (SHM) to promote mixing. The mixing of a fluorescein isothiocyanate-dextran solution with a diffusivity representative of that of the polymer solutions on the device will be quantified to experimentally validate the expected steady-state gradient. We use poly(acrylic acid) (PAA) and poly(allylamine) (PAH) as a model polyelectrolyte system to validate the device by comparing on-device results to off-device phase diagrams from literature.