Preparation of the cell
An unglazed clay flowerpot with an upper diameter of 8 cm and height of 7 cm was scrubbed inside and outside with medium coarse sandpaper. It was washed well with water and allowed to stand a night in a dilute sulphuric acid solution.
From a clean HDPE bottle, having about the same diameter as the flowerpot, the bottom was cut off at about 7.5 cm height and at the top a small hole was cut. This will serve as container for the anolyte, and inside the flowerpot comes the catholyte.
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Volume used inside the flowerpot is maximum 100 ml, to allow for some breathing room. The cell is usable for reducing small amounts of [substrate], up to 10 grams. Connections are made directly from the copper wire to the electrodes. The copper wire is rigid and can carry high current without problems (mine is 2.5 mm diameter and can carry a maximum of 20 A). Care must be taken that the copper connection to the anode is outside the hole, as contact with the anolyte will dissolve the copper fast.
The cathode and anode are both rectangular sheets of lead of about 20 cm2 surface area (10 cm x 2 cm). They are first scrubbed and defatted with acetone, then cleaned electrolytically by putting them in the above divided cell, both compartments filled with dilute sulphuric acid, and putting a charge (12 V) through them forwards and backwards, about half an hour each time. The one covered with a brown layer of PbO2 will be used as the cathode.
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[...]Cooling of the substrate seems to be the main problem, as considerable heat is evolved during the reduction. Good stirring of the catholyte is essential.
I. Rough electrodes
When one takes a freshly cast pure lead electrode and rubs the surface with cotton wool and wet sandpaper, a rough matt surface is obtained. We will refer to such treated electrodes as "rough" in the text that follows.
When such a rough electrode is polarised in a divided cell then, at least at higher current densities, in contrast to cadmium an immediate rise to a high value of cathode potential is obtained. This high cathode potential is attained at medium current densities around 10-20 minutes in most cases. With higher current densities (150-300 mA/cm2) this potential begins to decrease rapidly and in all cases of continuous polarisation the cathode potential will first more rapidly, then slower and more evenly decrease, so that no lower limit could be attained.
Cathode potentials of a rough cylindrical lead electrode were measured in a 2N acidic solution with a current density of 300 mA/cm2 (RT):
1.356V - 1 min.
1.304V - 5 min.
1.290V - 10 min.
1.272V - 20 min.
1.251V - 30 min.
1.202V - 60 min.
At lower temperatures (around 10°C) higher peak potentials are obtained, around 2.000V using a current density of 150mA/cm2.
There are clearly slight fluctuations in potential values between various seemingly identical shaped "rough" electrodes. In the author's opinion this can be ascribed to differences in density and its ability of the surface to disintegrate.
II. Pre-treated lead cathodes
When the lead cathode was pre-treated with a layer of PbO2 initially no hydrogen evolution is observed and the measured potential was very low. When 1/4 to 1/2 minute has passed hydrogen evolution starts and the potential rises sharply and one minute later a constant value is attained. This value was slightly lower than what was observed using the rough electrode.
E.g. for similarly shaped electrodes in a vertical apparatus using a current density of 125 mA/cm2, when one minute passed a value of 1.845V was obtained for the pre-treated electrode, while for the rough electrode this was 1.902V .
Also with the pre-treated electrode the potential drops more rapidly over time. When 10 minutes passed the potential dropped 0.047V for the pre-treated electrode vs. 0.015V for the rough one. One hour later, though, the potential of the pre-treated electrode only dropped 0.034V so we can conclude that in this case there also seems to be a stabilisation occurring.
III. Influence of acid concentration
Generally, like is observed with a mercury cathode, it is commonly agreed that the cathode potential rises ceteris paribus with the lowering of the acid concentration. In practice this difference is negligible for lead cathodes.
IV. Potential depression
When using a divided cell (Pt anode, 125mA/cm2, 2N H2SO4, 12°C) and a rough lead cathode one observes the potential lower only slightly over 180 minutes (peaks first at 2.010V and lowers gradually to around 1.980V). The last 150 minutes the drop is only 0.005V .
When the cathode is then removed, rubbed with cotton wool and wet sandpaper, and replaced the observed peak is somewhat lower (around 1.950V) and the potential gradually declines to 1.850V at the 160th minute. Around the 180th minute there is a sudden drop observed to 1.350V . A similar depression is observed when cadmium is used as the cathode, but there the drop occurs much sooner than in the case of lead.
Lead also seems to have the characteristic ability -again contrary to cadmium- to relieve itself from this depression. In some cases the potential rises again with 0.200V half an hour after the depression point.
Pre-treated electrodes seem to be somewhat more labile in this aspect, as the occurrence of a depression seems also to be influenced by other factors, such as the contamination with anolyte in case of long polarisation times.
As a rule will pre-treated electrodes, when the -by a sudden sharp rise in potential- characteristic reduction of PbO2 has ended, first of all even show depressed values, or at least values that are situated between depression and peak potential. What happens hereafter is in many cases very variable.
When anolyte can contaminate the catholyte the following situations can be observed:
1. The cathode will constantly attain a depression value (generally around 1.300-1.350V using above conditions, observed for 50 minutes)
This phenomenon occurs most frequently with those electrodes which during pre-treatment have been strongly oxidised for a long amount of time (20 mA/cm2 10-40 minutes).
2. The cathode reaches a low value, gradually recovers and reaches a high value, wherafter the potential decreases again to its depression value (observed over 120 minutes).
This phenomenon occurs most frequently when the electrode has been electrolytically oxidised for 40 minutes (or more).
3. The cathode reaches a low to medium value, then within minutes rises sharply to its peak value and more or less attains it (generally around 1.900V using above conditions, observed for 50 minutes). It is similar to what is observed with rough electrodes.
This phenomenon occurs most frequently when the electrode has been electrolytically oxidised for a very short time (3-15 minutes).
V. Potential elevation
When one re-polarises an depressed cathode - taking care that anolyte cannot contaminate - one can reach again a high potential value. Such method in practice requires more than just dilute H2SO4. Good results have been obtained using a caffeinated H2SO4 solution. In such a case the climbing to a peak potential goes together with the start of the reduction of caffeine.
Eg. A pre-treated cathode that has attained a depression value of 1.440V was placed in caffeinated 2N H2SO4. Using a current density of 125 mA/cm2 the reduction of the caffeine commenced and proceeded fast, after 14 minutes the optimum for a pre-treated lead cathode was reached. After this treatment the electrode had a peak potential in (pure) 2N H2SO4 of 1.910V
Although this procedure is quite sound, once the author experienced a failure, where after 40 minutes only an increase of 0.003V was recorded. No reduction of caffeine occurred in that case.
VI. Maximum overvoltage
Considering the above it is not surprising that for an evaluation of the heights of the elevation values at the lead electrodes, the attempts, which were implemented in an undivided cell, are just as useful as the ones in a divided cell, if only each time fresh electrolytes were implemented and only the behaviour in the beginning of the attempt is drawn in consideration.
It can be concluded that there are no fixed limits on the 'specific overvoltage' of lead, even when considering the depression values. There are, however, several pointers which can be applied to specific conditions. The limits for peak potential values the author has in his extensive experimental work encountered are: (12°C)
10 mA/cm2 : between 1.760V and 1.868V
100 mA/cm2 : between 1.898V and 1.965V
125 mA/cm2 : between 1.902V and 2.037V
Values for pre-treated cathodes are slightly lower, and the bottom limits depend on duration of electrolytical oxidation.
“I have successfully [operated an] electrolytic reduction in a porous cup (flower pot), using a sheet lead cathode wrapped around a glass bottle standing inside the porous cup, cold water circulated through a two hole stopper in the bottle for cooling the electrode. The porous cup contained a catholyte of denatured alcohol, glacial acetic acid and HCl, in which the [substrate] was partially dissolved and the undissolved portion was kept in suspension by a small nylon propellor blade as is used for model boats on the end of a stainless steel rod about 2mm diameter for a propellor shaft, which was sheathed in plastic tubing and the tip and joint at the little propellor were sealed with RTV clear silicone. This stirrer was chucked into a high-speed overhead stirrer with the propellor as close to the bottom of the porous cup as possible and pitched tangent to the inside wall so as to swirl the agitated catholyte around the cathode in a circular flow.
“The porous cup was set in the center of a much larger glass mixing bowl which contained the plain battery electrolyte anolyte and a sheet lead anode, and this outer bowl was itself set in a slightly shallower pan of cooling water. Everything was thus coaxially arranged with provision for cooling water entering the glass bottle in the center of the catholyte compartment and conducted through a long tube in the two hole stopper downwards to near the bottom inside the bottle, exiting through a short tube in the other hole in the stopper and carried through a flexible hose to the outer cooling bath for the anolyte cooling, a level maintained there by an overflow port at the desired depth, having a flexible hose for conducting the overflow back to the picnic cooler containing a block of ice and the circulating pump.
“The power is provided to the cell by a variac and full-wave bridge rectifier and the current monitored by a good ammeter. Make-up alcohol will have to be added to the catholyte unless some sort of cover is provided for it with provision for reflux of the evaporating alcohol. A thermometer should also be suspended in the catholyte so that the reaction temperature can be monitored and a log sheet should be kept concerning the times for which a certain number of ampere hours have passed.
“The reaction product will be in the catholyte[...]. Unreacted substrate or resinous by-products can be extracted with [solvent] and discarded, the aqueous phase filtered and freebased carefully with NaOH.
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“Lead is attacked by the catholyte and a sludge will be found from the erosion of the cathode. The anode is not attacked at all, just has a chocolate brown protective oxide. The best way of making these is to cut them from sheet lead in an " L " shape with the long leg being bent into a cylindrical form electrode and the short leg of the " L " providing a riser tab for connection of an alligator clip and power cable.
“I didn't do a lot of experimentation with this sort of thing because of unwanted interest of unsavoury characters, but the direction which I intended to go next was using a pool of mercury metal as a cathode, to see if it would perform better. IIRC the yields using the lead cathode were something near 80%.
