Nitric acid reacts differently according to it's dilution. Pure nitric acid doesn't dissolve copper! The nitric acid must be "cured"to include nitrous acid which is formed very slowly at first in contact with copper. As the acid becomes red with NO, the action increases. Dilution with pure water (rain or distilled) ; that is adding nitric acid to water and never the opposite to reach a 1:1 ratio, and then curing the acid by adding a small sliver of copper or by exposing the acid to sun light will activate the corrosive action. 1:1 is still considered very strong acid. If heated, all but gold and the heavy pgm's will dissolve. If boiled, a golden powder will precipitate with the gold that is the basic oxide of palladium. This oxide of palladium will dissolve in diluted HNO3 or in water, but not in concentrated HNO3. So now some of you must have surmised that the yellow gold that seemed to disappear when washed with water was actually palladium.
Nitric and Palladium
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About 4 months back, I tried to dissolve about 10 gms Pd in 50 ml HNO3, but the reaction did'nt started, I tried again with 1 gm Pd in 50 ml acid, the reaction started and in about 1 hour , 1 gm was completely dissolved, to this I added 2 more gms, It also dissolved in 1 hour, I put 5 gms, again it dissolved completely, to my surprise, this way I manage to dissolve almost 28 gms Pd in just 50 ml acid.
And now again I am trying again but unable to dissolve even 10 gms in 50 ml acid. Yes 4 months back it was summer time in Canada with lot of sunlight but not now.
I will appreciate if someone can explain what all is happening in above process.
And now again I am trying again but unable to dissolve even 10 gms in 50 ml acid. Yes 4 months back it was summer time in Canada with lot of sunlight but not now.
I will appreciate if someone can explain what all is happening in above process.
What state is this palladium in?
palladium oxide is acid resistant.
palladium oxide is acid resistant.
Palladium is in pure metal grain form. 4 months back and now, i bought it from same supplier.
I think, 4 months back,in summer time, very heavy sunlight accelerated this reaction, which is not happening now. But I dont know the exact reason. remaining all conditions are same, same is the conc. of Nitric acid.
I think, 4 months back,in summer time, very heavy sunlight accelerated this reaction, which is not happening now. But I dont know the exact reason. remaining all conditions are same, same is the conc. of Nitric acid.
I am not sure what concentration of nitric you are using.
Nitric when dilute will normally dissolve metals easier than a concentrated solution.
I think there are several reasons for this, too strong it can oxidize a metal into a passivated state, forming an oxide coating hard to dissolve or to remove for further acid attack of metal, and also NOx gases in the dilute nitric acid are actually needed to do most of the work of dissolving metals, dilute nitric not only will have more of these gases of decomposed NOx (than more pure fuming nitric acid would), water helps to keep the NOx gases into solution to do more work, and water can act as a solvent for the metal nitrate solutions as the metal dissolves, as far as the sunlight is involved, can only think of a couple of explanations sunlight will decompose HNO3 (forming more of these NOx gases in solution, and also provide some heat which also can help the process.
Nitric when dilute will normally dissolve metals easier than a concentrated solution.
I think there are several reasons for this, too strong it can oxidize a metal into a passivated state, forming an oxide coating hard to dissolve or to remove for further acid attack of metal, and also NOx gases in the dilute nitric acid are actually needed to do most of the work of dissolving metals, dilute nitric not only will have more of these gases of decomposed NOx (than more pure fuming nitric acid would), water helps to keep the NOx gases into solution to do more work, and water can act as a solvent for the metal nitrate solutions as the metal dissolves, as far as the sunlight is involved, can only think of a couple of explanations sunlight will decompose HNO3 (forming more of these NOx gases in solution, and also provide some heat which also can help the process.
butcher said:
yes you are right, now I have seen that yes Pd grains become passivated after some time. Earlier also and now also I am using 68% nitric acid.
My aim is to get Palladium nitrate with minimum free acid. What i did earlier was, I , took 50 ml acid and started putting in Pd grains, increment of 5 gms, spread over 24 hours time. after about 16 hours, i could see very thick paste of Pd Nitrate along with lots of undissolved Pd grains. To this I added about 40 ml more DI water, and in almost next 1 hours every thing was dissolved , so now I had 28 gms Pd dissolved in just 50 ml Nitric.. This process emitted very little fumes.
Now also I am repeating the exactly the same, but now this process emit large amount of fumes and after some time metal becomes passivated.
Now also I can get 28 gms of Pd Nitrate, but it will be in about 100 ml of acid, and amount of free acid will be too high. Now if I boil off this free acid, I can see formation of extremely fine black color soot at the bottom of solution.
Please suggest what to do to make Pd Nitrate with minimum free acid
This time I also tried to remove oxide layer from grains, by first taking out undissolved grains washing them with water several time, drying them completely , then immersing them in HCl for 5 min, again water washing them and drying them completely. On putting them in old Nitrate solution, reaction starts vigorously and very soon they become passivated again.
I have rarely worked with Palladium, and am more familiar with the reactions of silver.
Lets look at dissolving silver.
Too concentrated of an acid will actually slow the process.
Even in more dilute acid, there can be a delayed reaction.
What I believe happens is the silver first passivates with an oxide layer as the silver is oxidized by the nitric, at the same time some of the nitric in solution begins to form NOx gases, It is these NOx gases in solution, from the decomposition of nitric that help attack and are what combine with silver to make silver nitrate as the nitric is reduced (by oxidation of the silver) to nitrates, after this delayed reaction, the reaction becomes more violent, somewhat I believe from the heat produced, but also from the decomposition of the nitric acid itself, silver oxide layers are dissolved faster by the production of more of these gases, and there is so much of these gases in solutions oxidizing and themselves reducing with the silver the silver no longer is in a state of passivisation but is quickly being dissolved into the solution with the help of the water.
Note here also water can help to keep quite a bit of these gases in solution NO2 in this water can convert back to HNO3 instead of leaving as a red brown cloud of wasted nitric acid and gas that had the possibility to do more work, Note sometimes I add a bit of H2O2 with the water, this can help to provide oxygen in solution, so that NO gas can form NO2 in solution with the water (the idea is help the nitric go further keeping these gases in solution and working on the silver.
After most of the nitric is consumed in this violent reaction, the reaction will slow to a crawl, although there is still quite a bit of free nitric acid in solution, this may be because the heat of the reaction is not as strong, or for some other reason it slows to a crawl, heating the solution at this point will dissolve more silver into solution the heat helps the reaction proceed faster, heat makes the acid in this silver salt solution stronger, and heating can drive off water as the reaction continues helping to concentrate the nitric left, I find I can put an awful lot more silver into solution in this latter process using heat.
Most of the time we recommend diluting your 70% HNO3 with equal volumes of water, nitric is expensive for me and hard to obtain easily, to make my nitric go further many times I will dilute mine quite a bit more (to help keep gases in solution), this may take me a little longer to dissolve the silver, but time is cheaper than nitric for me, I will also add a little H2O2. and use heat after the initial reaction to dissolve the silver, with this process I can completely react all of the nitric acid as long as I have some silver left in the end, so many times I try to limit the amount of nitric I use compared to the amount of silver involved and add small portions of dilute nitric towards the end of the process, to put some more of that excess silver into solution.
I do not know how well I explained this, but diluting your nitric and using heat at the end of the process to consume the nitric acid (leaving some silver at the end) will consume the nitric acid, when done right you can load the remaining water solution with quite a bit of dissolved silver nitrate salts.
Lets look at dissolving silver.
Too concentrated of an acid will actually slow the process.
Even in more dilute acid, there can be a delayed reaction.
What I believe happens is the silver first passivates with an oxide layer as the silver is oxidized by the nitric, at the same time some of the nitric in solution begins to form NOx gases, It is these NOx gases in solution, from the decomposition of nitric that help attack and are what combine with silver to make silver nitrate as the nitric is reduced (by oxidation of the silver) to nitrates, after this delayed reaction, the reaction becomes more violent, somewhat I believe from the heat produced, but also from the decomposition of the nitric acid itself, silver oxide layers are dissolved faster by the production of more of these gases, and there is so much of these gases in solutions oxidizing and themselves reducing with the silver the silver no longer is in a state of passivisation but is quickly being dissolved into the solution with the help of the water.
Note here also water can help to keep quite a bit of these gases in solution NO2 in this water can convert back to HNO3 instead of leaving as a red brown cloud of wasted nitric acid and gas that had the possibility to do more work, Note sometimes I add a bit of H2O2 with the water, this can help to provide oxygen in solution, so that NO gas can form NO2 in solution with the water (the idea is help the nitric go further keeping these gases in solution and working on the silver.
After most of the nitric is consumed in this violent reaction, the reaction will slow to a crawl, although there is still quite a bit of free nitric acid in solution, this may be because the heat of the reaction is not as strong, or for some other reason it slows to a crawl, heating the solution at this point will dissolve more silver into solution the heat helps the reaction proceed faster, heat makes the acid in this silver salt solution stronger, and heating can drive off water as the reaction continues helping to concentrate the nitric left, I find I can put an awful lot more silver into solution in this latter process using heat.
Most of the time we recommend diluting your 70% HNO3 with equal volumes of water, nitric is expensive for me and hard to obtain easily, to make my nitric go further many times I will dilute mine quite a bit more (to help keep gases in solution), this may take me a little longer to dissolve the silver, but time is cheaper than nitric for me, I will also add a little H2O2. and use heat after the initial reaction to dissolve the silver, with this process I can completely react all of the nitric acid as long as I have some silver left in the end, so many times I try to limit the amount of nitric I use compared to the amount of silver involved and add small portions of dilute nitric towards the end of the process, to put some more of that excess silver into solution.
I do not know how well I explained this, but diluting your nitric and using heat at the end of the process to consume the nitric acid (leaving some silver at the end) will consume the nitric acid, when done right you can load the remaining water solution with quite a bit of dissolved silver nitrate salts.
This time I also tried to remove oxide layer from grains, by first taking out undissolved grains washing them with water several time, drying them completely , then immersing them in HCl for 5 min, again water washing them and drying them completely. On putting them in old Nitrate solution, reaction starts vigorously and very soon they become passivated again.
I am not sure but by removing and drying you probably could oxidize the Pd oxide layer further (possibly from pdO to PdO2), I do not know.
HCl will dissolve the oxide, but once this oxide is dissolved HCl will not attack the remaining Pd metal worth a darn, and your introducing some chloride to your nitric when you put this back into you nitric solution (not too much of a problem here, but I see no reason to do this,I am thinking it could just complicate things further.
Maybe someone who has worked more with palladium will chime in here.
I am not sure but by removing and drying you probably could oxidize the Pd oxide layer further (possibly from pdO to PdO2), I do not know.
HCl will dissolve the oxide, but once this oxide is dissolved HCl will not attack the remaining Pd metal worth a darn, and your introducing some chloride to your nitric when you put this back into you nitric solution (not too much of a problem here, but I see no reason to do this,I am thinking it could just complicate things further.
Maybe someone who has worked more with palladium will chime in here.
Yes you are 100% correct in both of your replies. I am also thinking , exactly the same.
The only thing amazing me is that how come I managed to dissolve so much of Pd in just a small amount of Nitric.
And now on repeating of same, now I am not getting the same results, This make me belive that earlier MAYBE UV rays from strong sunlight helped.
The only thing amazing me is that how come I managed to dissolve so much of Pd in just a small amount of Nitric.
And now on repeating of same, now I am not getting the same results, This make me belive that earlier MAYBE UV rays from strong sunlight helped.
Yes it can reduce some of the nitric to NOx gas needed for the reaction.
niteliteone
Well-known member
What is the temperature of the acid solution now compared to 4 months ago :?:
Sun light does have UV rays that will decompose the nitric acid, but of more importance is that it will also "Heat" the acid solution. This heat will determine how fast the reaction will happen and also how much metal can be dissolved with that amount of acid.
Are you using cold acid with this experiment or are you heating it :?:
Sun light does have UV rays that will decompose the nitric acid, but of more importance is that it will also "Heat" the acid solution. This heat will determine how fast the reaction will happen and also how much metal can be dissolved with that amount of acid.
Are you using cold acid with this experiment or are you heating it :?:
niteliteone said:
4 months ago everything was carried out at room temp of about 28 - 32 C absolutely no external heating was involved.
now I have tried at room temp of 11 - 13 C, also used external heating till 85 C, but did not achieve results like previous one
Chemical reactions can be exothermic or endothermic.
Exothermic reaction, evolution of heat. Example heat produced by the chemical reaction of nitric acid dissolving silver.
In exothermic chemical reactions chemical energy is usually converted to heat energy but can also form other kinds of energy changes, liberate light without heat, or electrical energy without light or heat.
http://en.wikipedia.org/wiki/Exothermic
Endothermic reaction absorption of heat. Example reaction of the cold pack where water and ammonium nitrate come into contact pulling heat from the surrounding area.
In endothermic reactions heat energy, light energy or electrical energy is converted into a chemical energy.
http://en.wikipedia.org/wiki/Endothermic
Dissolving a metal in nitric acid produces heat as energy by the chemical reaction.
Chemical changes do not always involve energy changes, and some energy transformations do not involve chemical changes. Example heat energy may be converted into electrical or mechanical energy without chemical changes.
Exothermic reaction, evolution of heat. Example heat produced by the chemical reaction of nitric acid dissolving silver.
In exothermic chemical reactions chemical energy is usually converted to heat energy but can also form other kinds of energy changes, liberate light without heat, or electrical energy without light or heat.
http://en.wikipedia.org/wiki/Exothermic
Endothermic reaction absorption of heat. Example reaction of the cold pack where water and ammonium nitrate come into contact pulling heat from the surrounding area.
In endothermic reactions heat energy, light energy or electrical energy is converted into a chemical energy.
http://en.wikipedia.org/wiki/Endothermic
Dissolving a metal in nitric acid produces heat as energy by the chemical reaction.
Chemical changes do not always involve energy changes, and some energy transformations do not involve chemical changes. Example heat energy may be converted into electrical or mechanical energy without chemical changes.
niteliteone
Well-known member
Well it was worth a shot.rcarbon said:
Where I live it is +/- 100*F through-out the summer and down in the 30's over the winters. This has a big effect on working with chemicals year round, when working with ambient temperature chemicals.
Over time I have learned to only work with heated solutions when dissolving metals, as it takes the guess work out of how things will react this time compared to the last.
freechemist
In Remembrance
Some of my practical lab-experiences dissolving palladium:
1.) General: Constant stirring is a must, also heating is often necessary.
2.) Digestion in AR: Palladium, to dissolve in AR is a dog, especially as Pd-black, also suspending Pd in HCl and adding HNO3 in small increments. Without heat not much happens. At 70-80oC digestion appears to run smoothly, but can, without any evident reason, come to a stillstand. Further heating and stirring, especially when adding a small portion of HNO3, often may lead to a sudden boil-over, mostly resulting in a big mess. Dissolution is always accompanied by escape of copious amounts of NOx.
3.) Digestion in nitric acid: Needs a lot excessive acid; at least about 2.15 ml 65%HNO3 are used to dissolve 1 g metallic Pd. Calculation based on the assumption, that 1 mole of HNO3 is used to oxidize 1 gram-atom Pd to Pd(II), and 2 moles HNO3 are needed, to hold it in solution as palladium nitrate, Pd(NO3)2. Heat helps to accelerate the reaction. While digesting Pd in HNO3 only small quantities of brown NOx escape, like rcarbon, apparently a very good observer, has observed it, too. If the digestion reaction does not start, addition of a few crystals of solid NaNO2 (sodium nitrite) is helpful, as well as adding about 1 ml of 1% KI-solution for each gram of Pd.
4.) Digestion in sulfuric acid: A lot of excessive 62% sulfuric acid (a mixture of equal volumes of 96% H2SO4 and water) is used as the "base-solvent", while oxidizing with about 1.1 moles of HNO3 (as 65% HNO3) per gram atom of Pd. The resulting solution has to be strongly acidic, and all Pd should be dissolved formally as palladium hydrogensulfate, Pd(HSO4)2. The digestion reaction delivers only small quantities of brown NOx, which cannot be driven out quantitatively, even by prolonged, strong heating. An intermittent passivation of metallic Pd has never been observed in this digestion reaction. Difficulties starting the reaction can be overcome, as in #3 above.
1.) General: Constant stirring is a must, also heating is often necessary.
2.) Digestion in AR: Palladium, to dissolve in AR is a dog, especially as Pd-black, also suspending Pd in HCl and adding HNO3 in small increments. Without heat not much happens. At 70-80oC digestion appears to run smoothly, but can, without any evident reason, come to a stillstand. Further heating and stirring, especially when adding a small portion of HNO3, often may lead to a sudden boil-over, mostly resulting in a big mess. Dissolution is always accompanied by escape of copious amounts of NOx.
3.) Digestion in nitric acid: Needs a lot excessive acid; at least about 2.15 ml 65%HNO3 are used to dissolve 1 g metallic Pd. Calculation based on the assumption, that 1 mole of HNO3 is used to oxidize 1 gram-atom Pd to Pd(II), and 2 moles HNO3 are needed, to hold it in solution as palladium nitrate, Pd(NO3)2. Heat helps to accelerate the reaction. While digesting Pd in HNO3 only small quantities of brown NOx escape, like rcarbon, apparently a very good observer, has observed it, too. If the digestion reaction does not start, addition of a few crystals of solid NaNO2 (sodium nitrite) is helpful, as well as adding about 1 ml of 1% KI-solution for each gram of Pd.
4.) Digestion in sulfuric acid: A lot of excessive 62% sulfuric acid (a mixture of equal volumes of 96% H2SO4 and water) is used as the "base-solvent", while oxidizing with about 1.1 moles of HNO3 (as 65% HNO3) per gram atom of Pd. The resulting solution has to be strongly acidic, and all Pd should be dissolved formally as palladium hydrogensulfate, Pd(HSO4)2. The digestion reaction delivers only small quantities of brown NOx, which cannot be driven out quantitatively, even by prolonged, strong heating. An intermittent passivation of metallic Pd has never been observed in this digestion reaction. Difficulties starting the reaction can be overcome, as in #3 above.
solar_plasma
Well-known member
Just to get it right:
NaNO2 or KI...
or did you mean NaNO2 and KI?
..very valuable info, thanks!!
freechemist
In Remembrance
It's "or". Both do their job to the operators satisfaction.
