From: fsmith@morgan.ucs.mun.ca (Frank Smith)
Newsgroups: sci.physics.fusion
Subject: Re: Enthalpy
Summary: Flanagan article on heats of absorption of D by Pd
Keywords: palladium deuterium calorimetry 298 K
Date: 14 Sep 92 00:05:57 GMT
Organization: Memorial University of Newfoundland

In article <920908173124.20c01971@FNALD.FNAL.GOV> ames!FNALD.FNAL.GOV!DROEGE writes:
>
>Perhaps someone can point out some obvious error in my thinking. A factor of
two would be nice.
>
>Could someone who has read the Flanagan paper tell me how they made the
>measurement?

Quote from Flanagan, Luo and Clewley, J. Less-Common
Metals,172-174(1991),42-55: 
3.416 g 99.99% pure Pd foil cut into small pieces
was used, mixed with finely divided copper and placed in reaction cell. The
blank cell contained copper with an equal heat capacity to Pd + Cu in the
other. Pd was not activated. Sample exposed to H2 several times, then evacuated
at 445 K for 1 h and overnight at 298 K. Reaction volume was 24.33 cm3; this
was corrected for expansion of Pd by H2. Hydrogen contents were monitored from
pressure changes in the known volumes, using MKS 0-10,000 Torr gauge. D2 was
99.5% deuterium.

Twin-cell differential heat leak calorimeter previously employed for studies of
other metal-hydrogen systems (same authors, same journal,141(1988)103;same
authors+ S. Majorowski, J.Phys. Chem. N.F. 163(1989)81 - I suspect this may be
the German journal of similar name?). The two cells are placed in a massive,
cylindrical Al calorimeter which is within a constant temperature water bath.
Thermopiles, located adjacent to the reaction and blank sides and within the
massive block of the calorimeter, are electrically connected so that a null
signal appears when the cells are thermally equilibrated. When reaction occurs
or when a current passes through the calibration heater, the thermopiles become
unbalanced and a voltage is recorded. The areas of the voltage-time plots are
proportional to the heats evolved; these areas are converted to the heats of
reaction for the actual doses of hydrogen employed, using a heat-area
calibration relationship determined from the electrical calibrations.

The calibration heater is located in the blank cell; electrical calibrations
were carried out frequently and over different pressure and hydrogen content
ranges. Most of the calorimetry was at 298 K. The procedure was to open a valve
separating the dosing and calorimeter volumes allowing a given amount of
hydrogen to be absorbed (or evolved) by (or from) the sample. Before, a steady
baseline indicated thermal equilibration; after, a signal appeared which was
recorded with time until return to the baseline. The area of this deviation
yielded the heat for the amount of hydrogen absorbed or evolved. The measured
heats converted to enthalpies of reaction by dividing by half a mole of H2(D2);
no corrections for expansion work are needed in a twin-cell calorimeter.
Reaction was completed rather quickly (10 min) but 40-60 minutes were necessary
for the two cells to return to the same temperature.

For the alpha-beta "plateau" reaction involving D being taken up, the molar
enthalpy change was -17.27 kJ/mol D at 298 K; for the evolution of D from the
beta phase the molar enthalpy change was +17.23 kJ/mol D. The authors conclude
that hysteresis played no part in the results. The plateau region for PdD
exists up to D/Pd = 0.65 approx. At higher D contents there is a decrease in
the magnitude of the molar enthalpy of absorption from about -17 kJ to -10.83
kJ at D/Pd = 0.754, attained at 1.2 x 10*6 Pascals pressure of D2,
i.e. about 10 atmospheres. The molar
enthalpies of evolution of D from the beta phase are larger in magnitude, being
12.13 kJ at D/Pd = 0.755 but a maximum of 20.89 kJ at D/Pd = 0.650.

NOTE. Tom, I don't know if the above will help you by a factor of two? The
quoted enthalpies are per mole of H atoms (D atoms) not per mole of molecular
hydrogen. I haven't had enough time to look at what you have done in the
context of the above, but will do. There is one other comment from my reading
of the Flanagan paper: it does take a finite time for H or D contents to change
in their experiment - are you expecting too fast a response?

Frank R. Smith