Nitrates in decent yield from the oxidation of melamine by calcium hypochlorite

A few months ago I read a post on here about how calcium hypochlorite will oxidize melamine to NO2, CO2, and water that seems to have been mostly forgotten. I decided to try and see if it would be possible to use this to make nitric acid in sizeable quantaties, and it is. The reaction seems to take place through a red-colored intermediate which persists for a few minutes before disappearing with bubbling, shown in this image.



For this run, I used 5 grams of melamine powder and 95 grams of 73% calcium hypochlorite meant for pools. The theoretical yield of HNO3 is 15 grams by the idealized reaction C3N6H6 + 12 Ca(ClO)2 -> 3 CO2 + 6 HNO3 + 12 CaCl2. In reality, it is probably less efficient, with some nitrogen only being oxidized to N2 gas which then escapes. The melamine was added to about 500 ml of water in a one-liter beaker with very long stirring. Then, the calcium hypochlorite was poured in as quickly as possible without having the stirring stop. This is necessary because before this, I tried the same thing but with only 250 ml of water and just dumping all the calcium hypochlorite in at once. The foaming was much worse and it overflowed and went everywhere.

Once the hypochlorite is added, the mixture rapidly turns red, as you can see above, before the red color disappears over a few minutes with some bubbling. Once the suspended solids are filtered off, the filtrate has a lavender color which I think is from Mn impurities in the calcium hypochlorite being oxidized to permanganate, which you can see in this image.



At this point, the filtrate appears to have a pH of 11-12, but the test strips are very rapidly bleached so I dont know for sure. I boiled down the liquid, and near the end the lavender color rapidly disappeared. Once it cooled down, which you can see in the image below, there were a few crystals that fell out of solution. The pH was now 6-7, and it no longer rapidly bleached the strip of pH paper.



The solubility of calcium nitrate (~1.2 g/ml @20 C) is high enough compared to calcium chloride (~0.7 g/ml @20C) that by boiling down the solution until stuff starts to crystallize out, most of the calcium chloride impurities can be removed with little loss of nitrate. If you don't do this, when you treat it with sulfuric acid and try to distill off the nitric acid the HCl from the calcium chloride present will react with the nitric acid to produce nitrosyl chloride and chlorine, destroying the nitric acid. Because not all of the chloride can be crystalized out, this will still happen to some extent, but because three moles of HCl are needed per mole of HNO3 this should not completely destroy the yield. Because the liquid was not boiled down enough in that picture, I concentrated down the salts until much more of them had percipitated out, although I did this by eyeball because an error earlier on destroyed both some chloride and nitrate so there would be no way to calculate how much it had to be boiled down by because I didn't know how much was dissolved. I then filtered out the CaCl2 and added an excess H2SO4 with some water to the remaining filtrate and set it up for distillation, which you can see in the image below.



As you can see, there was a substantial amount of NO2 gas generated, confirming that a decent amount of nitrate is in the filtrate. I had a rubber tube which all the gases had to go through which was several meters long and went to right behind a strong fan, which was okay for this scale, but especially for larger scales it might be good to use a proper scrubber. The HNO3 collected in the receiving flask was slightly yellowish, and pretty dilute because the sulfuric acid was diluted in about 3x its own volume in water before it was added to the flask. I continued distilling until the excess H2SO4 became concentrated enough that it started bumping badly.

To figure out the yield, I put the collected acid in a beaker, covered it, and added two US 5 cent coins, which dissolved with the generation of a large amount of NO2 as you can see in the image below. Each coin weighs 5 grams, and is 25% nickel and 75% copper by weight. This dissolves in nitric acid via the reaction Ni + 3 Cu + 16 HNO3 -> Ni(NO3)2 + 3 Cu(NO3)2 + 8 NO2 + 8 H2O. 6.16 grams of metal dissolved, which would imply that there was 24.9 grams of nitric acid in the product, a 166% yield. Obviously this isn't the case, so either a substantial amount of the NO2 is oxidized back to nitric acid by air in the beaker, the nitric acid is contaminated by HCl and/or H2SO4, or both. I added another coin after the first one completely dissolved, and will weight it tomorrow to try and figure out how much was dissolved in total. Still, the large amount of NO2 in the beaker shows that a decent yield of nitric acid is made by this method.



While melamine the chemical is not very OTC, melamine-formaldehyde resins are used to make some brands of kitchenware, sponges, and some types of flooring, so if these can be used this may be a viable route to nitrate production. A few days ago, I put some pieces of a melamine-formaldehyde sponge in a beaker with water and calcium hypochlorite. It did become discolored with some bubbling, but now the color is gone, although there is still some bubbling, but the sponge is still visibly intact. This may mean that it takes a long time for the polymer to be oxidized, or it may mean that it is basically not attacked at all, the initial discoloration was due to unreacted melamine, and the bubbling that is still going on is due to the breakdown of remaining calcium hypochlorite. If this is the case, it may be possible to get melamine from melamine-formaldehyde resin by removing the methylene bridges that link together the amine groups of the melamine in the polymer. Alternatively, although urea doesn't work, salts of cyanuric acid might because they are based on a triazine ring, like melamine. It might be good to further investigate the mechanism of this reaction too. The thread from several years ago linked at the beginning of this post says that sodium hypochlorite does not work, and a paper i read said that when melamine is chlorinated normally, no nitrates are made, and instead some of the hydrogens on the amine groups are replaced with chlorine. Last night, I posted this in the science in general subforum, but in the morning I realized that it was the wrong subforum and that this post was written in a really rambling way, so I fixed it up, deleted the old one, and reposted it here. I don't think that breaks the rules, but I'm pretty new to the forum.

[Edited on 20-6-2020 by garphield]

[Edited on 20-6-2020 by garphield]

Quote: Originally posted by Bromolone

Alternative sources to nitrates are the need of the hour, especially when they're getting rapidly phased out OTC.

Quote: Originally posted by symboom

Nope even that's getting rare you just have to stock up while in season after that it is illegal for vendors to sell outside specified days. [Edited on 20-7-2020 by symboom]