A "recipe" question for chemists

I really don’t know where to post this. While it’s recipe-like in nature, it doesn’t seem like Cafe Society.

I’m trying to make a shampoo for a friend with psoriasis. (Yes, we have her doctor’s clearance.) I want a 1.8% salicylic acid shampoo, from what I read. I know asprin is, if not entirely, then mostly salicylic acid. How many 325 mg (regular adult strength) asprins do I need to disolve into the 12 ounces of shampoo to get 1.8% in the final product? I say 1.8 because that’s what other shampoos contain. I don’t think it would be the end of the world for it to be 1.5%, or even 2.0%, so some sloppiness is tolerable.

Are these percentages generally based on weight or volume? Is there a better source of salicylic acid that would make this all easier?

12 fluid ounces is 355 milliliters.
You want 1.8 grams per hundred mls, so you need (1.8 X 3.55) = 6.39 g aspirin.
6.39g/0.325g = 19.7.
Call it 20 aspirin per 12 ounce bottle of shampoo.

I’m sitting here trying to remember. I spent far too much time dealing with ZPT.

Anyway, according to some of my old lab writeups, it was by weight. That would be easier and more intuitive than volume anyway. Basically, what you need to do is weigh the shampoo (tare the bottle if at all possible, or pour into a different, empty vessel, already tared) and find about 2% of that, then crush the asprin and add it in. Assuming the density of shampoo is about 1 (a somewhat dangerous assumption and why I’m telling you to get the weight of the shampoo), the shampoo would weigh about 350 grams. 2% of that would be 7 grams, so call it 22 asprin tablets to be ground up (use a mortar and pestle) and stirred into the shampoo.

There will be no other easily-available source of salicylic acid, unless you can get a stronger dose of asprin. Note that there is other things in an aspirin tablet to help it hold together, so you might need to strain them out (I’m remembering that one is rather papery–another lab experiment) before adding your powdered aspirin.

To be frank, what’s wrong with Neutrogena T-Gel or Head and Shoulders?

Out of curiosity, could or would a willowbark rinse work?

She’s tried both and doesn’t like them. It’s all my fault, really. I gave her some of my homemade herbal stuff two years ago (everyone got some at Christmastime, because I was poor) and she really, really likes it. She stopped using it when it ran out, got this psoriasis thing on her scalp and her doctor wants her to try a medicated shampoo. So she asked him and me if I could make one. I said I’m willing to try, just needed some help with the “medicated” part!

Salicylic acid is water soluble, right? Would it help to disolve the asprin in some water and strain out anything that doesn’t disolve, then add the liquid to the shampoo?

You know, I had thought of that too. The thing with willowbark is it doesn’t release its salicylic acid until it hits the small intestine, if my Chemical Constituents of Herbs professor was correct. That’s why it can be used by people who have stomach sensitivities to asprin. I don’t remember exactly how or why it happens, nor do I know how to reproduce it outside the body. That is, I know there’s salicylic acid, or it’s precursors, in white willowbark, but I don’t know how to extract that and only that.

Mol weight of:
Aspirin (acetylsalicylic acid): 180.15
Salicylic acid: 138.1

180 mg (1 milliMole) of aspirin will yield 138 mg (1 milliMole) of salicylic acid

The acetyl group in aspirin spontaneously hydrolyzes (combines with water and breaks off) especially at neutral or alkaline pH. This is actually why it’s there. Salicylic acid is the active ingredient in aspirin, but it can cause such irritation of the stomach that a life threatening stomach bleed results (It was known, at least as far back as the Roman empire, that willow bark tea was effective for headaches, but that it was dangerous and potentially lethal. “Salicylic acid” comes from the Latin word “salyx” or willow) It wasn’t a practical medicine until a chemist at the Bayer (Bavarian for “Bavaria”) Pharmaceutical company tacked on that acetyl group, which made it much less irritating to the stomach. As soon as food drink or drug leaves the stomach, the stomach acid is neutrailized by pancreatic solutions containing bicarbonate (to avoid burning the intestines with stomach acid) At this alkaline pH, the acetyl groups slowly break off, creating the clinically effective salicylic acid and a little acetic acid (vinegar). This is also why aspirin smells like vinegar as it goes bad.

Certain salts will greatly speed up the hydrolysis. Sodium carbonate (aka "washing soda is among these) and may be suitable for your formulation. Adding some Sodium Bicarbonate will help you maintain a slightly alkaline pH (since the hydrolysis will give off some acetic acid). Basically --if it were me- I’d mix a bottle of shampoo including 1-3 tsp each of washing soda and baking soda to a slightly alkaline final pH, of say 7.4-7.6, then put it on the shelf, loosely capped for 3-10 days to let the hydrolysis fiish on its own. You can then adjust the pH to whatever is optimal for your shampoo. (pH can affect cleaning, manageability and the effect of conditioners.)

There’s no reason to go to a pH of 8 or more. And above all, remember: this is NOT medical advice. It’s what I’d do, but I’d hope my background would help me spot and correct any problems I might have I stupidly forgotten to warn you against. I don’t know you, your background, your medical condition, your risk factors for baldness, etc… Only your friend can decide if they want to risk putting -gasp!- baking soda and washing soda in their hair shampoo. I take no responsibility.

Aspirin is actually acetylsalicylic acid. It is an ester, a combination of a salicylic acid molecule and an acetic acid molecule. Unless you have assurances from a chemist that your plan will work, that is, unless you have reason to believe that the acetylsalicylic acid molecules will fall apart in the shampoo, then I do not think the shampoo will be efficacious. On the other hand, salicylic acid can be bought in bulk on the net.

Huh. So it can. So what’s the difference between a listing for “Salicylic Acid, Powder, USP, 500g” and “Salicylic Acid, Powder, O.R., 500g” besides price? That is, what do USP and O.R. stand for? Here’s the site I found, but it’s obviously not labeled for the layperson.

As for my credentials, absolutely none when it comes to chemicals. I’ve been an herbalist for 5 years, following a full-time 4 year course of study in Traditional Chinese Medicine and Western Herbalism. Herbal medicine making and beauty product making is my favorite aspect of herbalism, although I do some herbal counseling as well. Rest assured, if I don’t feel it’s safe, I won’t add the chemicals, just make a nice, safe herbal shampoo that will help her.

Are you sure they will ship to a private residence? My experience with most chemical suppliers is that they will only ship to a business or educational institution. Then again, it looks like they’re not selling anything too odd. I mean, this isn’t exactly Aldrich we’re talking about here.

And yes, I’ll admit I forgot about possible complications from the ester. If it didn’t hydrolyze, what effect would that have on the potency?

Not a problem. That is, I don’t know yet if they’d ship to my apartment, but I can always have it shipped to the school I ran and sometimes still work at, which does lab orders all the time.

USP stands for United States Pharmacopeia - it is an indication that the product has been manufactured to meet the standards set by this body. Generally, this means that the product is safe for use as a drug in that there will be no nasty contaminants in it. I don’t know about the O.R., but is is probably another purity grading.

When I was googling to try to find a definition for O.R., I did come across the following, which gives some good info about salicylic acid:


O.R. means ‘organic reagent’; see here. This is often just called ‘reagent grade’; it means it’s pure enough to do research-type organic synthesis with. This is a higher standard than USP or NF, since the human body can tolerate minute amounts of impurities that might make a reaction fail or cause unpredictable results in the lab. (I suspected that OR might be reagent grade when I noticed that OR salicylic acid was substantially more expensive than the USP grade.) Analytical grade is still higher, since even very small amounts of impurities in a sample might lead to inaccurate results; technical grade is a lower standard, intended for industrial chemicals. For your purposes, USP will by all means be adequate; it’s the grade that would be used in a commercial medicated shampoo.

OK, so 7 grams of the USP grade salicylic acid powder, check for pH and adjust accordingly, adding baking soda if it’s too acidic?

And, of course, strand test on my own hair, swatch test on my skin and then wash test on my own head. I’d rather be my own guinea pig!

Though this is a few days old, I’d like to suggest you get some pH strips and test the pH of commercial medicated shampoos before settling on an alkaline formula. Shampoos are normally somewhere around pH 5, which is slightly acidic, and an alkaline solution may be damaging to the hair over time. Bases tend to cause the hair shaft to swell and the cuticle to peel back somewhat; this is one of the reasons hair dye has ammonia. While your shampoo wouldn’t end up anywhere near this strongly alkaline, my suspicion is that an alkaline formula, even slightly, would not be conducive to good hair.

Furthermore, according to The Beauty Bible, salicylic acid requires a moderately acidic formula to be effective. This is in regard to skin care, but I suspect the essential principle is the same - it’s a weak acid that helps break down the bonds holding dead skin cells on the scalp. I don’t know if the medication would be effective if the solution is not somewhat acidic. So I’d test the commercial products to see what pH they are, and shoot for that range.