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Synthesis of copper benzoate

crazyboy25

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Copper benzoate is used in pyrotechnics to create a blue flame. This is desirable because the blue spectrum is often difficult to achieve and a good blue star is the mark of an accomplished pyro. The formula for copper benzoate blue is 82/18 ammonium perchlorate/copper benzoate.

 

Overview of synthesis:

 

Step 1: synthesis of benzoic acid via hydrolysis of sodium benzoate

Step 2: synthesis of copper carbonate

Step 3: synthesis of copper benzoate

 

 

Step 1: synthesis of benzoic acid via hydrolysis of sodium benzoate

 

NaC6H5CO2+HCl=C6H5COOH+NaCl

 

26.28g sodium benzoate are added to 100ml water in a 250ml beaker. When all the sodium benzoate is dissolved 28.5ml HCl are measured out and slowly added with stirring. a fluffy precipitate forms instantly. The mixture is stirred well then filtered. The precipitate is washed with 200ml ice cold water and recrystallized from hot water/IPA (75/25) yielding 24.8g shiny needle like crystals.

 

http://i179.photobucket.com/albums/w318/crazyboy25/IMG_0107.jpg

 

Step 2: synthesis of copper carbonate

 

62.3g copper sulfate pentahydrate are dissolved in 300ml hot water in a 600ml beaker. 26.45g sodium carbonate are dissolved in 150ml hot water in a 250ml beaker.

 

http://i179.photobucket.com/albums/w318/crazyboy25/IMG_0065.jpg

 

The sodium carbonate solution is slowly added to the copper sulfate with stirring. Carbon dioxide bubbles and a chalky green precipitate form. Once all the sodium carbonate solution is added the mix is stirred until no more carbon dioxide forms.

 

http://i179.photobucket.com/albums/w318/crazyboy25/IMG_0066.jpg

 

The slush is boiled several times with several liters hot water to wash away the contaminants (copper carbonate is insoluble and will settle quickly after boiling.) Then the mixture is filtered and dried yielding about 20g copper carbonate.

 

http://i179.photobucket.com/albums/w318/crazyboy25/IMG_0068.jpg

 

Step 3: synthesis of copper benzoate

 

25g water are placed in a 600ml beaker. To this is added 6g benzoic acid. 3g copper carbonate are weighted out and slowly added to the mixture while stirring and gently heating. When all the copper carbonate is added the heat is increased while stirring until the mixture becomes a paste. This paste is dried to give copper benzoate.

 

http://i179.photobucket.com/albums/w318/crazyboy25/IMG_0069.jpg

 

 

Step 3 was scaled down from a procedure detailed in The Best of AFN lll the original ratios were 50g water 12g benzoic acid and 6g copper carbonate but the formula can be modified to use other carbonates producing other metal benzoates. These may or may not produce as brilliant a color as copper benzoate.

 

http://i179.photobucket.com/albums/w318/crazyboy25/benzoates.jpg

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6 Comments

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Miech

Hmm, thats interesting. Your copper benzoate is of a way darker shade of blue than mine was. I made mine by percipitating sodium benzoate with copper sulfate, maybe thats what causes the difference?

 

Copper benzoate is excellent for whistle mix like bursting charges, as it is not hygroscopic at all. It won't whistle however.

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Swede
Thank you for taking the time to share this... It is appreciated! Nice job!
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andyboy
I was wondering why you go through all those steps, when I made Copper Benzoate I just crashed two solutions of Sodium Benzoate and Copper Sulphate Pentahydrate, is it to get as clean a product as possible? I didn't get any discolouration in the flame from any Sodium remaining in the Copper Benzoate.
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psyco_1322

psyco_1322 66

Posted

I synth'd up some copper benzoate a while back. I went at it with the copper sulfate and potassium benzoate method. Nice and instant, two hot pots of liquid, poured together, instant puddle of copper benzoate and lots of it. I really need to buy a vacuum filteration set up, it would make things a lot nicer. I think I made about 200-300g or so, cant remember.

 

Some one on passfire was saying that what you end up with is a hydrated form and it needs to be dried before its used in compositions. I guess the anhydrous(I use that right?) form is a dark blue. I did notice that when drying the product I ended up with some random spots turning to a dark blue and becoming hard chunks. I really havn't done anything with it since I made it since try making blue whistles. That failed.

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Ventsi

Do you think it would be possible to sub the NaCarb with NaBicarb?

I dont have any and don't plan on buying any soon.

Thanks.

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crazyboy25

crazyboy25 5

Posted

Do you think it would be possible to sub the NaCarb with NaBicarb?

I dont have any and don't plan on buying any soon.

Thanks.

 

 

I think that might work you will have to try it.

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