In order to test for the existence of a pool of unreacted CO2 dissolved in seawater, we devised a "circulating pH cell" for deployment on the ROV swing-arm.  A SeaBird CTD pump was used to circulate the seawater within a small loop through a cell containing a SeaBird SBE-18 pH sensor.  By use of a series of hydraulically actuated valves, we could continuously re-cycle the seawater within the cell (mixing it thoroughly on time-scales of 3-5 seconds) or open the valves and flush the cell with ambient seawater.  When closed, the trapped volume of the system is ~500mL.  The pumping rate is 1.2L/min in the flushing mode and nearly 2.0L/min in the re-circulating mode.  A sampling wand with a tygon tube attached to the inlet valve allows the ROV pilots to sample seawater anywhere within reach of the ROV manipulator.  In this way, we can sample both undisturbed ambient seawater or CO2 enriched seawater from any point beyond the end of the flume.  A small inverted chamber on the end of the swing-arm also allows us to contain small volumes of liquid CO2 at depths where its density is less than that of seawater.

The figure shows the pH trace from the cell during one of the OACE experiments.  The trough was filled with liquid CO2 and the thruster on the flume was set at a slow speed in order to generate a slow current across the liquid interface.  Points A and F show the background seawater pH as water is drawn from a point well above the flume.  Points B and D show the slightly lowered pH when seawater from just beyond the lip of the flume is drawn through the pH cell.  Because of turbulence at this point causing variable mixing of the CO2 enriched seawater just above the liquid CO2 pool in the trough with ambient seawater, a stable baseline in not achieved.  However, when the valves are closed and the enriched seawater is recirculated inside the pH cell allowing the disssolved CO2 to fully react and reach equilibrium, a substantially lower pH is observed conclusively demonstrating the plume when sampled near the end of the flume had not achieved chemical equilibrium.

Shortly after this experiment was conducted, we realized that by adding a small (1mL) hydraulically driven syringe and a mylar reservoir, we could make acid injections into the cell and study the kinetics of the CO2 hydration reaction.  Indeed, if one used a well calibrated electrode and acid, one could do an alkalinity titration at depth at in situ temperatures and pressures.  At the moment of injection, very low pH seawater is produced at the tip of the injector.  At this low pH, the carbonic acid-bicarbonate-carbonate equilibrium is forced way to the left and dissolved CO2 is produced consuming most of the acid.  As this dissolved CO2 is mixed throughout the cell, it encounters much higher pH and slowly begins reacting with seawater, forming carbonic acid which dissociates and lowers the pH of the cell.  Over time, equilibrium of the carbonate system is re-established, and one can follow the course of this reaction by monitoring the variation of pH with time.