Sciencemadness Discussion Board

Purpose of Ammonium Carbonate in Luminol Reaction?

hodges - 13-9-2019 at 13:22

I'm looking at the makeup of glow solutions using luminol. This particular "recipe" came form Elmhurst College's website, but I have seen essentially the same thing with slight variations on several different sites.

4.0 g sodium carbonate
0.2 g luminol
24.0 g sodium bicarbonate (baking soda)
0.5 g ammonium carbonate
0.4 g copper (II) sulfate pentahydrate
1 liter of distilled water

50 mL of 3% hydrogen peroxide diluted to 1 liter

I understand the role of the sodium carbonate and bicarbonate in creating an alkaline solution. I assume both are used instead of just one in order to create an optimal pH that is not too sensitive to exact substance measurements. I understand the role of the copper sulfate as providing catalytic copper ions. I understand the role of the hydrogen peroxide for oxidizing the luminol.

But why ammonium carbonate?

Besides not understanding its purpose, ammonium carbonate is not easy for me to get (I can get the other chemicals easily). I presume I could possibly make ammonium carbonate using dry ice and ammonium hydroxide, but I think it would be tricky. I would need just the right amount of CO2, so as not to get ammonium bicarbonate instead. Presumably I could determine this by monitoring pH. Then I'm not sure how well drying the ammonium carbonate would go either, since I think ammonium carbonate decomposes pretty easily.

Perhaps if I can understand the purpose of the ammonium carbonate, I can figure out something else to substitute.

rockyit98 - 13-9-2019 at 20:47

ammonium carbonate / bicarbonate is used as levitating agent in making cheap stuff .2Kg for a dollar .try local baking shop.

Pumukli - 13-9-2019 at 23:30

Maybe it helps keep the copper in solution. Sort of a complexing agent.

Tsjerk - 13-9-2019 at 23:47

Ammonium ions are part of the pH buffer, but you don't need to isolate ammonium carbonate to use it.

You can try adding a bit of ammonia with an amount of sulfuric acid to get it to pH 9.0 (pH of 1M ammonium carbonate). Sulfate was already present anyway and I don't think the slightly smaller amount of carbonate will matter.

Quote: Originally posted by Pumukli  
Maybe it helps keep the copper in solution. Sort of a complexing agent.

Ah, yes, ofcourse, copper is not soluble at higher pH.

[Edited on 14-9-2019 by Tsjerk]

hodges - 14-9-2019 at 15:50

Ok, so if I am understanding correctly, the ammonium carbonate is part of a complex buffer (along with the sodium carbonate and bicarbonate). This is needed to create a stable pH that is high enough for the luminol to react with the hydrogen peroxide, but not so high as to cause the copper ions to precipitate out as copper hydroxide.

Here is a simpler "recipe" (from United Nuclear luminol experiment page):

To prepare stock solution A, fill a beaker with 100 mL of water. Add 0.18 g of luminol and 3.0 mL of sodium hydroxide solution (1 M).

To prepare stock solution B, fill another beaker with 100 mL of water. Add 1 mL of hydrogen peroxide (3%) and 0.03 g of potassium ferricyanide.

This seems much easier. Presumably the potassium ferricyanide provides catalytic iron, without having to worry about maintaining a low enough pH to prevent precipitation (since the iron is too tightly bound to precipitate out as hydroxide regardless of pH). And I presume sodium ferrocyanide would works the same as sodium ferricyanide, since either valance of iron should be catalytic.