Determination of the Stoichiometry of Palladium Hydride by Data Capture.

Experiment submitted by         Dr John Briggs
                                Dalziel High School
                                Crawford St
                                Motherwell
                                ML1 3AG
                                UK

This investigation is just one of many that show how the analysis power of Datadisc can be harnessed to support the most advanced A-level investigations.

Download pallad.ddw

The experiment

The ability of the platinum metals to act as catalysts in reactions involving hydrogen is thought to be because of the ability of the metals to occlude hydrogen on the surface of the metal. Of these metals, palladium is known to physically absorb hydrogen to form a non-stoichiometric compound with a formula in the range PdH _{0.6 to 0.7} ¹.

The technique described enables the measurement of the stoichiometry of palladium hydride using masses of hydrogen as low as 0.2 mg, an order of magnitude below the range of gravimetric determination possible in a school laboratory.

Apparatus.

• PC running Datadisc Pro for Windows,
• Philip Harris universal interface and "blue box" milliammeter sensor.
• 1cm² zinc foil
• 1cm² palladium foil 0.025 mm thick ( Sigma-Aldrich cat no 26,712 -0)
• 50 cm³ hydrochloric acid 1 mol l ^-1
• connecting leads,
• crocodile clips
• electrode support
• 100cm³ beaker.

Safety Notes

Hydrochloric acid - harmful, irritant.

Wash off with copious quantities of water in the event of skin contact

Eye protection must be worn.

Diagram.

Method

Approximately 1cm² of palladium foil was cut and weighed accurate to ± 0.001g.

The apparatus was set up as shown in the diagram. The electrodes are not inserted until measurements commence.

Datadisc Pro was initiated and measure selected from the menu bar. From the drop down menu, select calibrate and set channel 1 to read in the 500mA range inserting "34" in the sensor number box. Close the dialogue box and select autotime from the measure menu.

Insert the electrodes as shown and click "start" to measure the current in the circuit. Measurement should be continued until hydrogen bubbles freely from the palladium surface. (Approximately 10 minutes)

Results and Discussion.

The graph of current against time shows a high initial current which falls to a steady value over a period of approximately 3 minutes. During this time there is no evolution of gas from the palladium anode. After 3 minutes bubbles of hydrogen form on the anode and are evolved. It was reasoned that reduction of H⁺(aq) from the acid solution to atomic hydrogen would allow hydrogen atoms to diffuse into the palladium lattice and form the hydride. Once the lattice has become saturated with atomic hydrogen, bubbles form on the surface of the palladium. The formation of H₂ gas on the surface of the palladium has a higher activation energy than the formation of hydrogen in lattice interstices and results in a lower current flowing in the circuit. As bubbles of gas are evolved from the anode, there is a distinct increase in the noise level in the measured current. This helps to determine the time required to saturate the palladium lattice with hydrogen. Integration of the current flowing with respect to time will measure the quantity of electricity passed through the external circuit.

Close inspection of the graph of current vs. time suggested that the formation of the hydride was complete after 172 s and inspection of the table of results determined that the charge passed was 19.920 C

Since the mass of Pd used was 0.033 g (3.09 x 10^-4 mol )

Error analysis.

There are 3 possible sources of error in the measurements;

the milliammeter sensor has an accuracy of ± 5mA.

the time measured using the auto time facility for 10 minutes is accurate to ± 4s.

the measured mass of palladium is accurate to ± 0.001g

These errors combine to give an overall accuracy for the experiment of ±10% although this is appears to a pessimistic estimate.

Further observations.

The absorption of hydrogen by the palladium produces a distinct change in the appearance of the metal. The hydride has a darker lustre and appears to be much more brittle than the palladium metal. It appears quite stable at room temperature, but the palladium metal can be reconstituted by heating the palladium hydride foil in a clean test tube for about 30 minutes over a moderate Bunsen flame. An alternative might be to reflux the hydride in hex-1-ene for 30 minutes relying on the ability of the adsorbed hydrogen to reduce the double bond. We were able to partly reconstitute the palladium by simply immersing the hydride in hex-1-ene for a few minutes.

Reference;

1. W. Mueller, J. P. Blackledge and G. G. Libowitz. "Metal Hydrides", Academic Press, London and New York, 1968. p633