Replication of Mills Light Water Calorimetry Experiment - Introduction - 22NOV00

In March of this year, we embarked upon a high fidelity replication of the light water Ni electrolysis experiment that Mills described in considerable detail in his 1996 book (p. 469 ff).  In particular, we are replicating the first experiment described in that section of the book in which two Dewars are used.  One Dewar contains the active electrolysis cell and the other contains a similar thermal mass to provide an appropriate temperature reference for differential calorimetry.

Wishing to remain as faithful as possible to Mills original protocol, we obtained a pair of Model 8600 Dewars from Pope Scientific, the same Dewars that Mills used.

We performed a cooling curve experiment to determine the thermal resistance of the two Dewars and discovered that they were neither very similar to each other nor very close to the ideal thermal performance expected from a perfect vacuum.  To make a very long story short, we obtained a total of 17 Dewars from Pope and found them all to have unsatisfactory thermal performance.  This plot compares the thermal performance of eleven Pope Dewars to that of four Dewars made by Thermos Products Corp.  Plotted is the temperature of 250 cc of H₂O (continuously stirred) contained in the Dewar vs time.  In each test the H₂O temperature was initially 50°C.  As the plot shows, all of the Pope Dewars lose heat significantly faster than the Thermos Products Dewars and there is a very large spread of heat loss rates among the Pope Dewars.  Because the differential calorimetry requires the thermal resistance of the two Dewar to be quite similar, we decided to abandon Pope Scientific and conduct our experiment with Dewars from Thermos Products Corp.

This photo shows the anode-cathode geometry of our cell.  Following Mills, the cathode consists of 24 meters of 0.38 mm diameter Ni wire (Alfa #10253).  The wire Mills used, Alfa #10249 (99% pure), is no longer available from Alfa.  Our wire is 99.8% pure.

Mills did not describe his cathode support so we fabricated this open spool structure from PVDF bar stock.  The Ni wire was wound onto the spool following Mills instructions for handling the Ni cathode (i.e. no skin contact).  After winding we cleaned the cathode as Mills did by immersing it in a 0.57M K₂CO₃/3% H₂O₂ solution for half an hour followed by a distilled water rinse.

The anode (central coil) consists of 10 cm of 1 mm dia Pt wire that was cleaned by mechanical scouring with steel wool followed by overnight immersion in concentrated HNO₃, per Mills.

The anode-cathode separation is 1 cm, as it was in Mills cell.

The anode and cathode leads which exit the cell are covered with special heat-shrink teflon tubing which seals to the wire surface thus completely excluding electrochemical action on the leads.

Also visible in this photo are the three glass-jacketed thermistor probes which monitor the electrolyte temperature and the teflon encapsulated calibration resistor.  Located at different depths, the three probes allow thermal stratification in the electrolyte to be evaluated.

This photo shows how the cell lid, with cathode-anode structure attached fits into the Dewar vessel below.  Note the 1.5" thick Styrofoam insulation attached to the bottom side of the lid.

One departure we made from Mills apparatus (which could not conceivably affect the operation of the cell) is to arrange the cell lid so that all of the electrolysis gases must escape through one port (Mills employed a foam lid for his cell so the gas escaped all around the lid).  Our cell is still "open" as Mills cell was but we are able to measure the flow rate of the gas leaving the cell, whereas he could not.

To achieve this gas-tight construction, we equipped the Dewar flask with a machined flange (clear PVC) that is epoxied in place (Devcon white 2-ton).  The cell lid (clear PVC) then seals to this flange with an O-ring (buna).  All the temperature probes and wires that penetrate the cell lid are also sealed with O-rings (buna), as is the central PVDF rod that supports the cathode-anode structure.  Thus the only exit path for the electrolysis gas is through the small plastic hose barb fitting in the cell lid.

Note the holes in the upper end of the PVDF cathode spool.  They permit the electrolysis gas bubbles to escape to the surface of the electrolyte.

The PVDF spool is attached to a 3/8" dia PVDF rod (with a PVDF screw) which passes through the Styrofoam insulation and attaches to the lid (O-ring sealed).  The white teflon sleeve visible just above the PVDF spool holds the anode lead wire firmly to keep the anode properly located in the center of the spool.

The active cell (left) and the reference cell are supported as shown in this photo.  Each cell has a motor-driven rotating magnet (750 rpm) below it to drive the teflon-coated stir bar inside the Dewar.

In this photo you can see the white elastomer tubing that conveys the electrolysis gas from the active cell to the gas flow rate measurement apparatus (center).

Room temperature is monitored by a thermistor coupled to an air heat sink located in the center of the apparatus.

For the first runs, we will use DC constant current as Mills did.  In subsequent runs we will also try the 36% duty-cycle square wave drive that Mills employed

This photo shows a closeup of the gas flow apparatus.  The gas is delivered to a vertical section of glass tubing which is immersed in a beaker of water.  A scale attached to the outside of the beaker facilitates accurate measurement of the rate at which the meniscus between gas and water inside the glass tube descends as the incoming gas displaces water from the tube.   Using the inside diameter of the glass tubing, the rate of descent can be converted into a volumetric flow rate.  The slight head pressure (an inch or two of water) created by this apparatus does not significantly affect the result.

This measured rate can then be compared with the expected gas flow rate obtained from Faradaic calculations involving the electrolysis current.  The degree to which the measured rate is lower than the expected rate is a measure of the fraction of the electrolysis gas which recombines before leaving the cell.

This photo shows the entire experiment.  The computer data acquisition system is on the left.  It monitors and records three temperatures in the active cell, two temperatures in the reference cell, room temperature, and the voltage, current, and true power reported by the Clarke-Hess 2330 Power Analyzer (blue unit on the shelf).

The electrolysis power supply (constant current) is on the shelf on the right.

The power supply for the stirrer motors is below the shelf on the right.

Questions and comments are welcome.  

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