There have been a number of different baths proposed for the electrodeposition (plating) of (Beta) Lead Dioxide but the bath that has gained general acceptance is the bath containing Lead Nitrate. The Lead Nitrate bath plates well under the widest range of parameters and is easy to make up. There is rather a lot of contradictory results in scientific literature regarding Alpha and/or Beta Lead Dioxide being deposited from acidic Lead Nitrate baths, see JES, 149 (9), 2002 . High plating current density gives Alpha, low plating current density gives Beta. Beta type Lead Dioxide is desired as the outside layer of Anodes.
The plating process can be represented by the following overall equation:
The following hardware will also be needed.
The Plating tank can be a glass beaker as it is good to be able to
see the Anode plating. Use a good tank as
you do NOT want it to break and spill toxic Lead Nitrate all over the
Heater for tank. This can come in the form of an aquarium heater with it's control altered to let it heat to 70°C. A magnetic hot plate stirrer can be used or other system that will heat the solution to 70° approx. Test out your system using water first to see how it is working.
A DC supply capable of putting out a few Volts and the required plating current is needed. Generally lower plating currents give smoother coats of Lead Dioxide and about 20mA/cm^2 is good. This current density will give Beta Lead Dioxide from the Lead Nitrate tank. For some ideas on power supplies see Power supplies.
When deciding what plating current to use you simply calculate the surface area
that you have to plate (cm squared) and multiply it by the current density (in
mA/cm squared) you want to plate at. This will tell you the amount of mA that you need
to put into the Anode. You simply adjust the Voltage on the supply to give you
that current. If using the battery charger supply in the link above, you simply attach the required
number of flying Leads to the Anode. A constant current source is good for this
type of application. The Anode is connected to the positive lead.
The Cathodes for the plating tank can be Copper. Don't use Gouging rods, there are reports that some Gouging rods contain Iron particles. Iron is NOT wanted in the tank. The Cathodes surround the substrate that you are plating and three or four should be OK. The surface area of the Cathode(s) facing the substrate to be plated should be at least half the surface area that you are plating. Alternatively a spiral of Copper wire can be used surrounding the substrate or some Copper strips. Distance between substrate (Anode) and Cathode is not critical and can be of the order of about an inch or two. There should be no sharp corners or indents on the Anode to be plated.
Nitric acid and Lead ion concentration control.
Nitric acid is formed as plating progresses. To stop the concentration of Nitric acid from becoming too high you must have a suitable large tank (if using a one tank system) If using a two tank system with a pump, then the total plating solution volume need not be so large. You must add neutral solution to the plating tank at the appropriate rate.
The pH of water and Nitric acid can be calculated from this page. pH values corresponding to Nitric acid amounts per liter of water are given in the table (as per calculator at link) and they correspond to actual measured values of pH when Nitric acid was added to a Lead Nitrate solution of 340 grams per liter. The molarity of 70% HNO3 is 15.6 and it's density is 1.42 g/cc.
The amount of Lead ion per gram of PbO2 is 0.839 grams. The amount of Lead ion per gram of Lead Nitrate (Pb(NO3)2) is 0.6256 grams. Therefor each gram of
deposited Lead Dioxide will use up 1.341 grams Lead Nitrate. The concentration of Lead Nitrate should not be let fall below 200g/liter. If the tank size is large enough to keep the Nitric acid concentration at reasonable levels then Lead Ion depletion of the plating solution will not be a problem. It is advantageous to have the Lead ion and Nitric acid concentration as constant as possible during plating.
When going for a one or two tank system and adding Lead Compound as plating progresses, the total plating solution volume need not be as large as when using a one tank system with no addition of Lead Compound.
If using a one tank system and adding Lead compound you will need to add the amounts in the table below
Each mole of Electrons (26.8 ampere hours) that flows will produce one mole Nitric acid (63 grams), assuming 100% current efficiency, which is equal to 2.35 grams Nitric acid forming per hour per amp.
Another way to look at this is to say that a half mole of Lead Dioxide is plated onto the Anode per 26.8 ampere hours, (119.6 grams Lead Dioxide per 26.8 ampere hours) which is equal to 4.462 grams Lead Dioxide per amp per hour. You need to add Lead compound to replenish the transfered Lead Ions and depending on the Lead compound you are using the weight added will need to equal the molar amounts of Lead Ion being transfered. The table below gives the relevant molecular weights of the Lead compounds that are used and thus the amounts to add to the tank per amp per hour. This amount of addition will keep the extra acid generated neutralized as well. This is the rate at 100% CE.
|Compound||Grams of compound to add to tank
per amp per hour @100% CE
|Litharge (molecular weight = 223)|| |
|Lead Carbonate (molecular weight = 267)|
|Basic Lead Carbonate (molecular weight = 775)|
The addition of Carbonate or Basic Carbonate will cause problems with foaming, bubbles and floating. A slurry of these compounds may have to be made (by adding some of the compound + some neutral plating solution to a well capped container and shaking) in order stop them floating on the surface of the plating solution. Lead Carbonate comes in a large array of molecular weights so check what you have.
It may be possible to have an excess of Litharge at the bottom of a one tank system (if stirring is not used or perhaps intermittent stirring) which will react fairly slowly with the Nitric as it is produced and help keep the Lead ion concentration constant and the Nitric acid concentration in the wanted range. If the Litharge is too finely divided (small particle size) it may react too easily with the Nitric acid and be inclined to keep the pH of the bath too high (concentration of the Nitric acid will be too low). You cannot use Lead Carbonate or Hydroxide or Basic Lead Carbonate in excess in the one tank system as they will keep the pH too high (they react too readily with the Nitric acid formed) and the Carbonate will also cause bubbles/foaming as it reacts with the acid. See here for some observations regarding addition of Lead compounds to the tank.
From US Patent No. 4,130,467, we have Litharge addition rates of 4 grams per amp per hour in one example and 3.5 grams per hour per amp in another example. The additions were made every two hours. They used a six liter tank in the second example which gives an idea of tank size/deposition rate between each addition of Litharge. It is next to impossible to get a figure for surface area they were plating.
|When using a two tank system with Lead Dioxide deposition being performed in one tank and an excess of neutralizing Lead compound in the other tank (neutral solution), then you must pump the neutral liquid into the plating tank at the appropriate rate to keep Nitric acid at the desired concentration and keep Lead ion replenished.
The amount you need to pump out of the plating tank into the neutral tank (the same volume will be pumped from the neutral tank into the plating tank) is given by the formula below:|
The simple arrargement shown may work as a two tank system with a very simple pump based on an aquarium air pump and a bent tube'. Keep the internal diameter of the tube supplying the air fairly big as it may clog up with crystals of Lead Nitrate. Careful of spray, use covers. The air pump could be turned on and off at a suitable frequency and duty cylce using a timer something like this. If an on/off frequency of below 10 minutes or so is chosen then the pumping can be considered to be the equivalent of continous. Perhaps a branch from the main air tube could be used for agigation of the Lead compound in the neutraliser tank or perhaps a second pump and tube. Some plastic tape wrapped around the substrate where there may be variation of plating solution level will give an abrupt known edge to the plating area. The tank should be set up first using a solution of (say) Ammonium Nitrate at a density of approximately 1.3 grams per cc to ascertain pumping rates/times etc.
There is a useful Microsoft Excell spreadsheet (in a .zip file) here that can be used for calculations regarding the Lead Nitrate plating tank.
Formation of Nitrites
Nitrites are formed at the Cathode as a side reaction and can build up in the tank.
NO3- + 2e + 2H+ ===> NO2- + H2O
One mole Nitrate + two moles acid + 2 moles electrons ===> One mole Nitrite + 1 mole of water Then the reaction below starts to occur at the Anode.
NO2- + H2O - 2e - ===> NO3- + 2H+
One mole Nitrite + one mole water - 2 moles electrons ===> One mole Nitrate + 2 moles acid Since the potential required for the above process (0.80V) is far lower than that required for the formation of PbO2 (1.46V) , current efficiency will drop as the unwanted reaction if favoured. The addition of Hydrogen Peroxide to the tank has been used to counteract this problem by Oxidizing Nitrites back to Nitrates. The build up of Nitrites lowers current efficiency and will give bad coats of Lead Dioxide. Resting the tank for a day or two after it has been neutralized will allow Nitrites to convert back to Nitrates and allow the tank to plate at its best. Bubbling air through the tank will speed up the process. Careful of splashes/spray.
In US Pat. 299464 it is stated that a Sodium Nitrite content of 0.1% will reduce plating CE to 30%. Hydrogen Peroxide (35%) was added slowly over a half hour period to the 1364 liter tank to eliminate Nitrite. Note that they are not making Anode in this patent and the current density is very high on SS that is being plated.
Nitrite testing kits are available for aquarium's and may be useful for detecting Nitrites in the tank.
It may also be possible to test for Nitrites by using a redox indicator like Methylene blue. When Nitrites are present the indicator will be colorless. When eliminated with Hydrogen Peroxide this indicator will turn blue. This indicator may be difficult to see because of the blue colour of Copper Nitrate in the tank as well.
During plating some Copper may be deposited
onto the Cathodes and if the blue colour of the plating solution gets weak, add more Copper Nitrate. There will also be some bubbles on the Anode that are not
welcome but are inevitable. Plating should not be interrupted.
Weigh the Anode substrate before plating so that you will know how much Lead Dioxide was electro-deposited for calculating plating CE etc.
Keep an eye on evaporation from the tank as it can lower the level of
electrolyte in the tank and leave you with a shorter than expected
Anode. A good way to counteract this problem is to wind on a few layers of insulation tape to the substrate with the bottom edge of the tape at where you want an abrupt end to the LD plate. The substrate in them positioned so that the plating solution surface is in the middle of the taped strip. Some solution volume variation can then be tolerated without giving a feathery edge to the top end of the Lead Dioxide plate.
It is a very good practice to test the tank using a small piece of Graphite about one inch or so at about 20mA/cm^2 and see that the plating is good. If the tank will not plate Graphite there is a problem with it. Keep Gouging rods away from the tank.
Ions that are bad news in a plating tank, according to the literature, are Chloride (US 3,463,707), Iron (US 3,463,707), Cobalt, Selenium and Arsenic.