387 replies to this topic

[sasman]

.htm[/url]

My guess:

In a lead-acid battery durring "forming" of the plates the anodic reaction is:

PbO + H2O => PbO2 + 2(H+) + 2(e-)

It takes 2 moles of electons to oxidise on mole of litharge to one mole of lead dioxide.  This suggests it takes at least *4* moles of electrons to oxidise solid lead to lead dioxide.  I am unsure how the lead gets on the anode in the first place, that would be an "anodic reduction" reaction, can that even happen?  If lead nitrate was present unionised something like this might happen requiring only two moles of electrons:

Pb(NO3)2 + 2(H2O) => PbO2 + 2(NO3-) + 4(H+) + 2(e-)

No idea how practical that is, seems to me we'd be dealing with Pb++ ions.  Obviously the lead ends up being Pb++++ which is oxidation, so there is at least two moles of electrons envolved, if not more.  Personally my guess is four or six Faradays for one mole of lead dioxide which says you got very close to 100% or something that deserves a nobel prize and would be the salvation of mankind. 

I am probably wrong though, electrochemistry is not something I am good at.

I have just found this equation for the anode reaction?.. i still cant work out how many moles of electrons are used in this equation .. my gues is 2 moles lead? 2 of water = 4 moles?.. which is more or less the one you have explained above..

Pb2+(Ely) + 2 H2O(Ely) -> PbO2(PbO2) + 4 H+(Ely) + 2 e-

[alany]

What's "Ely" mean?

BTW, my dyes arrived today (at 7:40 am, bugger it!).

http://nexus.cable.n...hylene-blue.jpg

Sure looks like there is some perchlorate in my NaCl cell.

I tried using the Indigo Carmine test for chlorate but found it hard to judge the discolouration, the electrolyte scaresly causes any change to the reagent boiling in a test tube, however a small crystal of pure chlorate bleaches out the solution very quickly.

I'll do some calibrated tests with various dilutions of potassium chlorate I think.

[raketpeter]

What's "Ely" mean?

BTW, my dyes arrived today (at 7:40 am, bugger it!).

http://nexus.cable.n...hylene-blue.jpg

Sure looks like there is some perchlorate in my NaCl cell.

I tried using the Indigo Carmine test for chlorate but found it hard to judge the discolouration, the electrolyte scaresly causes any change to the reagent boiling in a test tube, however a small crystal of pure chlorate bleaches out the solution very quickly.

I'll do some calibrated tests with various dilutions of potassium chlorate I think.

Really?? I find the indigo carmine test to be highly reproducible.
When you add the HCl solution to the boiling solution under test and it causes bubbles to form (with a boiling sound) - then you konw significant amounts of chlorate are still present. (The gas evolved is Cl2O - toxic.)

Only when the above shows no apparent reaction is the indigo carmine relevant. I find that the detection limit for chlorate is very low! (Don't have the numbers here.)

Two things are important, though.
1, Add lots of HCl. (1ml HCl to 1ml test solution is what I normally do.)
2, remember to bring the mix to a vigorous boil before you conclude "no decolouration".

(I reduce remaining chlorate with Na2S2O4, what do you use?)
Peter

[alany]

I boil the dye and the acid first, then squirt in the sample, the same volume as the reagent. My initial problem was my acid wasn't strong enough, but 300 g/l HCl seems to be fine. I use 1 part dye to 5 parts acid and 6 parts solution under test. I use a small boiling chip in the tube to make it easier to boil.

To destroy the chlorate I lower the pH to near 1 with HCl and boil which tends to destroy most of it by itself, I clean up the rest by adding sodium metabisulfite until the indigo carmine test is negative, then I add a little more to be sure.

I've written up the latest run but I haven't unhidden the article yet, I will once I add the tests on the product. It is currently drying ready for weighing.

[raketpeter]

I boil the dye and the acid first, then squirt in the sample, the same volume as the reagent.  My initial problem was my acid wasn't strong enough, but 300 g/l HCl seems to be fine.  I use 1 part dye to 5 parts acid and 6 parts solution under test.  I use a small boiling chip in the tube to make it easier to boil.

To destroy the chlorate I lower the pH to near 1 with HCl and boil which tends to destroy most of it by itself, I clean up the rest by adding sodium metabisulfite until the indigo carmine test is negative, then I add a little more to be sure.

I've written up the latest run but I haven't unhidden the article yet, I will once I add the tests on the product.  It is currently drying ready for weighing.

OK - your procedure is clearly different from mine. I looked at your (very interesting web page) and have a couple of questions:

1) Are you using copper wire is the cathode?? - Surely you are not going to make ammonium perch. with sodium perch. from a copper cathode??

I use Ti - no *chemical* corrosion, but there are problems with hydrogen embrittlement which gives rise to bending of the Ti and a small ammount of "physical" erosion of the Ti.

2) It seems you are running at VERY high amperage for such a small cell (and
anode) - also 75 deg C seems very high (I run at 25-35 deg C). What's the cell voltage under your cell conditions for the chlorate step and for the perch. step?

I run a 1900 mL batch size cell for about eight days at 3.3 A.
This, by the way, corresponds to over 95% current efficiency (but I cheat and start with the chlorate which makes a HUGE difference ).

Peter

[alany]

1) Are you using copper wire is the cathode?? - Surely you are not going to make ammonium perch. with sodium perch. from a copper cathode??

I use Ti - no *chemical* corrosion, but there are problems with hydrogen embrittlement which gives rise to bending of the Ti and a small ammount of "physical" erosion of the Ti.

No, I was using graphite, but I am now using Titanium. The old graphite cathode has a brass support pin which is threaded into the graphite. That worried me, but it was fairly well sealed from the cell.

2) It seems you are running at VERY high amperage for such a small cell (and
anode) - also 75 deg C seems very high (I run at 25-35 deg C). What's the cell voltage under your cell conditions for the chlorate step and for the perch. step?

Yes I am cooking the cell. The high temperatures aren't all that bad, but the high current density isn't great for the chlorate step. Shouldn't make any difference for the perchlorate step though. The watts are simpler than brains approach works just fine, you can quite safely run 1 A per 100 ml or so and have passive thermal control work, as long as the cell is fairly "tall".

The power supply meter says 6.5-7 Volts for the chlorate step, but the wiring resistance and electrodes themselves drop hundreds of mV at the high currents I run. I didn't bother to record the voltage for the perchlorate step last time, as I wasn't really sure when it transitioned. I just set the voltage high enough to push the current through the cell without much thought to the exact EMF.

I run a 1900 mL batch size cell for about eight days at 3.3 A.
This, by the way, corresponds to over 95% current efficiency  (but I cheat and start with the chlorate which makes a HUGE difference ).

Having completed a perchlorate run now I've restarted the cell on the reprocessed liquor as an experiment in long term sodium recycling. I am not sure if the Pt/Ir anode will survive this run, it may need to be cleaned and rebuilt. I'll eventually split the chlorate and perchlorate runs to save wear on the anode.

I have a friend here in AU encouraging me to experiment with Lead Dioxide so we can compare notes. That will probably be where I go next. I have some ideas on forming Litharge or Lead into anodes rather than plating it onto substrates that I want to try out. I also want to try Manganese Dioxide.

I've also been working on ion selective electrodes for cell instrumentation. A chloride electrode is pretty easy, but perchlorate is not something I've got to work yet. I hope to eventually build a probe and computer system that measures pH, temperature, Cl- and ClO4- in real time for detailed experiments.

[Old1953]

I wonder how well diamond would work? Diamond coated tools are becoming common, but I haven't heard of anyone trying it for this type of purpose as yet.

Thin diamond films will conduct electricity just as graphite will, but its much more strongly bound to the cubic lattice.

http://www.cvddiamond.com/faq.html

[sasman]

Raketpeter what voltage do you run your 3.3amp cell?..Does your perc perform well with making colours?...how many times do you have to recrystalize?

Alany mentions his first batch was not as good as his commercial perc(obviously he can purify the product more)..Also what size PT anode are you using and what method of control are you using for current..

I am intrested in what that Black coating is on alany's Pt anode very wierd?.. i have made small test batch's using my Pt Anode mesh and had no black coating..

.I have nearly made all my Lead Dioxide anodes...

My first Graphite anodes were 13 mm( 1.5mm PbO2 coating) diameter 15 cm long but these are made to use a current of about 10 amps each (Surface area approx 49 cm x 200 milliamps)..

I made these small electrodes because from what i have read passing large currents through a big anode tend to over heat the connections..etc..

So i thought make small electrodes and share the current between them, so each electrode only has to take a small current..
From my experiments these GSLD anodes can easily take 10 amps so i scaled up my next batch and Now the ones i will be using are much bigger ..

They are 34mm Diameter( 1mm PbO2) 15 cm long can take 25 amps (128 cm Surface area x 200 milliamps)..
I will post some pictures later .. very rare to see PbO2 anodes on the Net?..i have read lots but no photos ..

The actual coating on the rods is superb.. very smooth no pits or pinholes..much easier than i thought to make them..
The secret in making them is do it properly..I have read lots of people on the Rec.pyro ..were there PbO2 anode peel or dont last long..Also use Graphite not Carbon!! i tried Carbon gouging rods..Rubbish... dont bodge.. get Graphite...

The end result is well worth it...Rock hard Black crystaline PbO2 coating Sounds like ceramic when you tap it..

I am running in each electrode at about 25 amps in a Brine solution to test each one.. for a week or two .this also serves to leech out any chemicals that may have been absorbed by the PbO2 coating ..
The best test for the Anode is when the Chloride concentration gets really low even a Pt electrode dont like low levels of Chloride..

The only problem i have is when adding PbO to lower my Ph of my plating tank .. even though i have added lots of excess PbO and have left it stirring overnight at 60 centigrade.. it never reaches PH 7..?.. baffled me..
Also when PbO is added the colour of the solution is not the same pale blue as it was to start.. this worried me until i added a small amount of Nitric acid.. the colour then changed Back from a greenish blue to a nice pale blue...

To be continued....

[alany]

I suspect the black coating is Platinum, I may have accidently plated the cathode with it.

None of my acids touch the material I filtered out of the solution. It is very likely Platinum black. If I had some concentrated nitric acid I could make aqua regia and try that on it to make Chloroplatinic acid and plate some more Titanium. Americian Eagles are pretty cheap, if you have the acids making your own plating bath is pretty easy. I've seen Palladium and Ruthenium on eBay fairly cheap too, they should be OK for anodes. Ruthenium is kinda nasty to handle though, its tetraoxide is toxic and its oxide formation is fantastically exothermic.

After the current run is complete I will try the cathode as an anode with a fresh piece of Ti as the cathode. If the cell doesn't polarise and become useless within hours then there is little else the coating could be but Pt.

I think I need to build myself a Sodium Chlorate reactor that I can just run continously, probably with MMO electrodes in a 20 litre drum with a draining cock at the bottom to tap off crops and a thistle tube airlock for additions. I need a large crop of Sodium Chlorate so I can do parallel experiments on Perchlorate anodes starting with measured amounts of pure Chlorate.

My one lesson from liquor recycling so far: make sure you don't put too much (ideally any) potassium back into the cell. Kinda obvious really, but excess potassium ions going back into the cell means lots of chlorate crystalised out as soon as it forms any kind of concentration. You need to either under-estimate the Potassium needed to get out a reasonable crop of Perchlorate or carefully titrate the addition until it just no longer precipitates more Perchlorate.

Edit:

This is what happened when I ran the Ti rods I have as an anode:

http://nexus.cable.n...c-corrosion.jpg

It did not polarise, rather it just eroded with an increasing current over time, this was at just over 4 Volts. I expected the opposite. Oddly though, when the power was turned off and then back on it took a while for the current to rise again. No significant amount of oxygen was liberated and fluffly slighty orange crud formed on the top of the cell. I am thinking the Vanadium and Aluminium in the alloy is preventing a stable anodic oxide layer from forming.

Edited by alany, 24 March 2005 - 01:52 PM.

[gilbert pinkston]

my experience is if you keep adding NaCl to a properly run Chlorate cell you can get a continuous crop of NaClO3 a Pt cell should make it very easy because of the high evaporation as opposed to a graphite cell
here is some early results from a graphite cell http://eseephoto.com...9294_189045.jpg
this anode cathode arrangment has run totally submerged (nothing above fluid level) for three weeks and shows very little erosion http://eseephoto.com...9294_189046.jpg when using graphite and common salt there is no need to filter ... what carbon doesn't settle out comes out easily with a few vacuum filter washes
add the cleaned product continuously to the NaClO4 cell and NaClO4 will drop out http://eseephoto.com...9294_189044.jpg

[Pretty green flames]

Just a quick question

Does Sodium chlorate have a different shape of crystals than sodium chloride.


Just so i'll know what to look for

[gilbert pinkston]

depends on conditions NaClO3 is very similar to NaCl....above on close inspection looks like a bunch of little squares closely packed........when condensed from a saturated solution quickly in a freezer NaClO3 looks very similar to salt.......here is the result of concentrating the final vacuum rinse solution of solid NaClO3 and allowing to cool slowly http://eseephoto.com...9294_189047.jpg square flat about 3/4 inch on a side 1/8 to 1/4 high

[alany]

Yes, they are thin plates rather than cubes. Perchlorate forms needles but you need to let the solution cool slowly to grow them up large enough to see clearly especially the potassium versions with their fairly low solubility.

Gilbert: those links are all 403 for me?

Chemical tests are the best way to ensure you've seperated Chloride, Chlorate and Perchlorate. Get yourself some Silver Nitrate, Methylene Blue and Indigo Carmine, they aren't very expensive and *tiny* amounts are all you need for hundreds of tests.

[gilbert pinkston]

sorry about 403 (i think)......they don't work for me either.....i just right click, properties, copy, paste i know it's uncool but it's all i got try again they are colour pix

[alany]

Why is it so green, iron contamination?

Where did you get the electrodes from? Is that support structure also immersed in the electrolyte? I am amazed you don't suffer worse corrosion, what temperature and current density do you run?