Sodium salicylate question

[DavidF]

When I made my own salicylate, the product ended up grayish. And boy, was it ever hard to drive off the water! Sodium salicylate is said to be light sensitive. And it is. For some tests I made up some straight sali whistle- no catalyst. So the stuff was white. After half an hour in the sun, the color was noticeably darker on the surface. I put it in the shade right away. I wonder what is happening?

[WSM]

When I made my own salicylate, the product ended up grayish. And boy, was it ever hard to drive off the water! Sodium salicylate is said to be light sensitive. And it is. For some tests I made up some straight sali whistle- no catalyst. So the stuff was white. After half an hour in the sun, the color was noticeably darker on the surface. I put it in the shade right away. I wonder what is happening?

I've noticed that sodium salicylate changes color when exposed to light also, but mine turns a pink color. It still performs as a whistle fuel, even if it changes color.

 

If drying it is difficult, you could try dropping the wet mass into absolute alcohol and see if that doesn't help. Pure alcohol will grab the water and make the salicylate dry faster. Vacuum filtering helps (Google Buchner funnel and see how it's done).

 

Alcohol from the hardware store is typically denatured ethanol and is often 95% ethanol with 5% methanol to make it undrinkable.

 

Good luck with it.

 

WSM

Edited October 7, 2016 by WSM

[a_bab]

David, do you mind sharing how you synth your sally ? Something more elaborate then "I simply mix salicylic acid lye and water" ?

[DavidF]

Thanks WSM. a_bab, I'll admit that my note-taking stinks. My faded post-it note says I used 138 grams of salicylic acid from the drugstore and 84 grams of baking soda. This yielded 160 (theoretical) grams of sodium salicylate, 18 grams of CO2, and 48 grams of water.

 

My memory says I dissolved the baking soda in warm water, and the salicylic acid in methyl alcohol. Several additions were made, allowing the production of gas to stop in between to prevent overflowing.

 

The goal was to produce pyro chems from readily available materials, and to get a better 'feel' for the things we use.

 

I used this salicylate in whistle rockets and strobe rockets. They performed as expected.

[FlaMtnBkr]

"...as expected"

 

That could be good or bad given the context of readily available. Which was it?

 

I don't know that salicylic acid in a pure form is readily available at pharmacies in the US. I think there is pink stuff and creams for pimples but never seen something pure here.

[DavidF]

The rockets performed as I expected they would, had I used 'regular' purchased chemicals. We can buy the salicylic acid in crystalline form in most pharmacies here. Perchlorate, not so much

[WSM]

The rockets performed as I expected they would, had I used 'regular' purchased chemicals. We can buy the salicylic acid in crystalline form in most pharmacies here. Perchlorate, not so much

 

Where do you live, that salicylic acid is available in pharmacies?

 

Long ago, my first purchase of salicylic acid was from a pharmacy, and it was as large pills, coated with a hard red shell. It didn't work out very well. It was sold as a "heavy-handed" (strong) analgesic.

 

WSM

Edited October 19, 2016 by WSM

[FlaMtnBkr]

I believe David is somewhere in Canada.

[WSM]

I believe David is somewhere in Canada.

 

Well, that's just another reason I need to renew my passport.

 

WSM

[NeighborJ]

Salicylic acid is used in the production of some soaps and can be ordered from soap suppliers in 50lb bags. I can't speak for the purity otherwise I'd order a bag but I don't want to get stuck with something I can't use.

David, when you mix the solutions together, do you end up with a precipitate or is it another solution which then needs dehydrated?

[OldMarine]

Hey Neighbor, If it's Pharma grade, it's probably good. They should have an MSDS on their site so you can check.

[NeighborJ]

Soapgoods.com is the cite, they offer 55lb bags of non fda approved salicylic acid for $467. It's cheaper to just buy the stuff already made and avoid the hassle of DIY. But I'm just curious about how it's made. If it forms a precipitate then it could be washed an the impuritys may stay in the solution, or not.

[OldMarine]

Neighbor, You'd do better getting it from Hobby chemical. Cheaper, with time proven quality.

The site you listed had pharma grade stuff but cost per lb plus shipping ruined them.

[DavidF]

Mine came from a place called Xenex Laboratories in BC. I've also seen it on the shelf in drugstores. It's soluble in water and methyl hydrate, and more so at higher temperatures. I will say it sure is 'interesting' playing chemist. I feel that it gives me a better feel for pyro, preparing my own chemicals. Sometimes. Other times, not so much.

As I recall, the salicylic acid solution effervesced with every addition of bicarbonate solution, and the products were all a clear liquid when I was done. I may have heated it to get full solution, but I doubt it. I'm pretty sure I used enough liquid to easily dissolve the product. That part is fuzzy.

That was the easy part. Now, the simple process of evaporating off the water could begin. Yeah, not so fast. The edges of the evaporating solution form dendrites that climb right out of the tray. I used Pyrex baking trays on a food warming station. It seemed like all the slushy crystal growth wanted to happen out of captivity. I had to keep turning the slush back into the tray with a plastic card. By the time I was able to get the water to evaporate off completely, the last liquid got fairly dark. The sodium salicylate ended up being a light beige color, but it worked well.

Never again, if I can help it.

[styropyro]

If you do ever do this again, I'd suggest using lye instead of baking soda as your base. Salicylic acid is a weak acid, and sodium bicarbonate is a weak base, so you'll never get a complete reaction between the two. How much of each is left I don't feel like calculating now, but it could be significant and have a part in why it is so hard to dry! Lye would react fully with the salicylic acid because it is a strong base.

As for why salicylic acid degrades in sunlight, it's molecular structure has enough conjugation in it to make it absorb UV light. Thus the UV radiation in sunlight is likely exciting electrons in its chemical bonds to higher energy states, making them much more susceptible to side reactions that cause degradation. This is along the lines of why UV light is bad for the skin!

[Mumbles]

The equilibrium between salicylic acid and bicarbonate is already toward sodium salicylate based on pKa arguments. The continuous evolution of CO2 makes it essentially quantitative, especially if heated. I would not worry. Sodium hydroxide would make the reaction quicker, but also likely more exothermic, but would eliminate the bubbling. I'd rather have a little sodium bicarbonate than lye in my materials anyway.

[NeighborJ]

Would a simple ph test of the sodium sali solution be a good method of determining if all the lye or bicarb has been reacted?

And the solution may be easier to dehydrate in a vacuum chamber?

[DavidF]

Well, that's a good question NeighborJ. I had some leftover home-made stuff, some store-bought, and a good pH meter. The most informative posts from you guys made me look a little deeper. Also, as we are talking about this, a guy on FW is complaining that his whistle rocket cores have "hair" growing inside them. A respected chemist there suggested that the hair is actually crystals of the acid component of the fuel, be it sodium salicylate or sodium benzoate.

 

Taking the comments from here and there, I expected to find that my home-made product had a higher pH than the store-bought product, which should be on the acid side, if it produced crystals of free acid in the core.

 

Here is what I found. I am sorry that I don't know how to make pics work here with my new computer.

 

http://pyrobin.com/files/IMG_8241.JPG The raw chems.

 

http://pyrobin.com/files/IMG_8247.JPG The pH results.

 

Learning from each other is good.

[NeighborJ]

DavidF, I'm left with more questions then answers. Do you believe your high ph level is the result of an incomplete reaction or is it the result of the photo reactivity side reactions. Also would the store bought stuff be better in some way if the ph was adjusted to 7 or is it better to have the slightly lower ph?

[DavidF]

I could guess, but I was the guy with the question in the first place! The only MSDS I could find online that gave a pH value gave a value of 5-6. I'll let the real chemistry guys take a stab at your question.

 

In a similar case, I made my own perchlorates, using acid base reactions. I found that my potassium perchlorate never quite seemed perfectly dry and didn't mill easily, like the purchased stuff I had previously used. I was adjusting the pH at the end to +7. This left a tiny bit of potassium hydroxide in the perchlorate. As I understand it, the hydroxide would later become potassium carbonate by sucking CO2 out of the air. Anyhow, I asked somebody about it (Mumbles?) and I was told that it would be better to end up on the slightly acid side (just below 7). Since I made that seemingly insignificant change, the texture of my PP is much nicer.

 

While we are on this topic though- some folks complain that sodium salicylate and benzoate are hygroscopic. I've never had an issue, but I don't live in Florida either. I wonder if making potassium salicylate instead would be better than making the sodium salt?

[Mumbles]

I found another MSDS that mentions a pH between 6-8. Sodium Salicylate is a salt of a strong base and a weak acid. It should yield a basic solution, or at least weakly basic. I wont bore you with the math unless you really care, but I came up with a pH of 8.40 for a 10% w/w soln.

[NeighborJ]

Thanks Mumbles, yeah explaining the chemistry math to me would be like trying to teach my dog to do algebra. I'd want to understand but I'd just get that glassy eyed look.

DavidF I've never heard of potassium sali but I'd assume it would be the same process as sodium only with potassium carbonate dihydrate? I'm sure there is a good reason why it is not used and I'd be afraid the test would reveal why, but I'm still curious, maybe you're onto something awesome.

I'm still thinking of the sodium reaction when I ask, do the two solutions need to be a true solution to get a complete reaction or could they be super saturated in a thick suspension to avoid the agonizingly long drying time? Sorry if this is a dumb question.

[WSM]

I wonder if making potassium salicylate instead would be better than making the sodium salt?

 

I have some potassium salicylate (a small sample, maybe 50 g.). It does whistle, but doesn't appear to be a product of commerce.

 

I plan to make a big batch one day to do more experiments with it.

 

WSM

Edited October 26, 2016 by WSM