Welp, I was intrigued. Right now my batch is cooling in the fridge. This is my first chemistry experiment, but getting that menthol whiff was a sure sign of success. Step 3: Profit!
I've never done anything with Robotussin or it's components, but I'm gonna do this with an open happy mind. Lemme run down my afternoon:
Got razorblades, 1 gallon ziplocks, 64oz ammonia, 12pk of 1pt (16oz) mason jars, duct tape from Walmart - $15
Got 1qt naphtha from hardware store - $8
Got 2

z Robotussin Nighttime cold/cough/flu (the only syrup I could find with CPM) - $21
Cleaned out 3 mason jars (they had a hint of citrus inside), got out 84oz soda mug. Filled a jar

z naphtha, 4oz water, shook, separated contents in a ziplock, put the "scrubbed" naphtha in a fresh mason. Repeated.
In retrospect, I should only have done this step only once. This produced way more than I needed for 16oz Robotussin.
Dumped an

z bottle into each 16oz jar, added 6-7oz of ammonia, shook, filled the jar the rest of the way with scrubbed naphtha, shook some more. All 32oz fit nicely into a gallon ziplock. No suds or bubbles. Let it hang for a few minutes. I could clearly the see the clear stuff riding the crimson stuff.
Made the small hole at the lowest corner. When the red stuff fully drained into my soda mug, I could clearly see just the remotest amount of residue cling to the bottom, then get washed into a clean mason jar as I collected the clear stuff. When I repeated the step into a fresh ziplock, the lowest 5 millimeters of the bag had red fluid. I used a paperclip to cordon off that section, then manhandled the clear stuff to the other corner. I completely amputated the red corner. I could see nary a drop of red in the clear corner, so I drained it directly into a clean 9x9 pyrex casserole dish. It was about 2mm deep.
By this time it was about 6:30. No hairdryer, so I took the pan outside. The sun had gone behind the trees but it was still warm out. I set a timer for 30 minutes. It didn't seem to have changed; there was a slight naphtha smell, no hint of menthol. I brought the dish in. Filled a skillet with water, turned an electric burner onto simmer (1/6), put the dish over the skillet, started cleaning up my mason jars.
By the way, if anyone knows, is there a proper way to dispose of the ammonia/cough syrup waste? I, erm, flushed it down the toilet. Same with the water I used to scrub the naphtha.
Anyway, it didn't seem like it was evaporating. I increased the heat to 5/6, which is right below the water boiling point on our stove. After awhile I could see bubbles start to form -under- the pyrex. At this point, my old lady comes out of the bedroom screaming about the fumes. I start running around opening more doors and windows and turning the swamp cooler up. I come back and the dish looks empty. I give it a sniff and WHOOF I get a head full of minty goodness. I take it off the hot plate, come back and consult the directions.
At this point I get confused:
Try putting the pan in the fridge and letting it cool to jumpstart crystallization, which does work, but I find that crystallization takes place much quicker when a hair blow dryer is used and the pan is very dry and warm
Warm or cool? I finally went with refrigerator, and began typing this down.
Moment of truth: time to check the fridge!
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Nothing even remotely close to the pictures above. No crystals, no powder. There's definitely a cold fluid stuck to the bottom. When I scrape the pan with a razor blade I can the sounds of something coming off. The razor blade has a goo-like substance on it, something like corn syrup in color and viscosity.
I'll let it sit in the fridge overnight, see what happens in the morning. But to those who have had success, please tell me:
If I don't have a hair dryer, how should I go about the evaporative process?
When the naphtha evaporates and I have menthol, fridge or room temperature? And for how long?
Update! About 4 hours after putting the dish into the fridge I'm pleased to find crystals, yes, like the pictures on page 1! Having scraped the contents into a pile, and chopped them into a fine powder. Much like the consistency of powdered sugar, the powder is fine enough to nestle into my fingerprints. I put it all into a spoon, stirred into a shot of vodka (shoulda chugged faster, blargh!), chased it with some cola.
As I close this, I am T+20 minutes, been typing a bit funny (could be the vodka talkin'), I'll post up on events tomorrow.
Señor Bistéc, thank you for posting this! Not often I decide to go out and play Johnny Chemist.