190 proof or 200 proof — the argument is about the flattest part of the curve

Someone on r/DIYfragrance bought the wrong thing:

"Oops, bought some SDA 40B from Lab Alley and when I got it, I realized it was 200 proof. Is the 200 proof still useful for perfumery or did I mess up? Are there any key differences in its function? I tried to search this subreddit but couldn't find a lot of info, but I read 200 proof can evaporate to 190; should I just let it do that somehow?"

The replies came quickly and agreed:

"There's no difference. Yes, the 200 will naturally become ~191 over time as it absorbs water vapor right out of the air. It doesn't really matter. Mostly we don't buy 200 proof simply because it's usually more expensive, and there's no difference from 190 proof." (beraelen)

"All alcohol will naturally want to absorb water from the atmosphere up until about 5%. You don't have to lift a finger and you won't even notice it. So there's no harm, you just paid a tad bit more than you needed to." (CapnLazerz)

"I use both and there's literally no difference in performance." (zenmaster_B)

And there is a small conflict in the thread worth including:

"If I see one more post about alcohol i think I'm gonna go crazy." (AdministrativePool2)

"🤷 just didn't see much info on this question specifically, sorry dude" (the poster)

"I've been scouring the forums relentlessly for the last 6 weeks."

That last line is why this round covers this question. Searching the same subreddit for "190 proof" finds it asked at least five times across two years: 190 proof vs 200 proof, Why 190 proof and not 200 proof ETOH?, Best Ethanol 200 Proof, Is 190 proof better, or 200 proof?, and a Perfume Solvent Base Formula post that lays out three options. Every time somebody answers correctly, and every time the answer stays inside a comment thread.

The short version

• The replies are right, and the reason is checkable. 190 proof (95% by volume) sits near the ethanol-water azeotrope. Azeotropic mixtures cannot be separated by ordinary distillation, so reaching 200 proof requires another process step, which is why it costs more.
• "It absorbs water on its own" is also right. Ethanol is hygroscopic, and 200 proof drifts back towards the azeotropic composition once opened.
• But "no difference" is worth quantifying. Using the only seven materials in the database with multiple ethanol-strength measurements: benzyl benzoate needs 1:2 at 90% and 1:1 at 95%, a twofold difference.
• The same material needs 1:1000 at 45%. Benzyl acetate moves fortyfold between 30% and 60%.
• So the forum is arguing about the flattest part of the curve. There is almost nothing between 190 and 200. The steep part is between 30% and 60% — the range you pass through turning a concentrate into a finished product — and nobody is asking about it.
• One database answer in passing: ethanol is not odourless. Its strength field reads medium and its odour field reads alcoholic, ethereal, medicinal.

Why 190 proof is the standard

proof is the US alcohol unit, twice the percentage by volume:

Proof	Ethanol (by volume)	The rest
190	95%	about 5% water
200	100%	anhydrous (absolute)

Ethanol and water form an azeotrope. At one atmosphere the ethanol-water azeotrope sits around 95.6% by mass, roughly 97% by volume, and azeotropic means the vapour and liquid have the same composition at that point, so further distillation does not purify it.

Reviewing the separation of ethanol from aqueous solutions in Ultrasonics Sonochemistry in 2022, Naidu and colleagues state the mechanism plainly:

"Conventional distillation is featured by low energy efficiency and inability to separate azeotropic mixtures, and thus novel alternatives, such as ultrasonic separation have been explored to advance the separation technology."

That paper is about bioethanol downstream separation and has nothing to do with perfumery, but it establishes the same point: getting past the azeotrope requires something other than distillation — molecular sieves, entrainers, membranes.

That is why 200 proof costs more, and it is the full version of "you just paid a tad bit more".

"It absorbs water on its own"

Ethanol is hygroscopic. Anhydrous ethanol exposed to air takes up water vapour and moves towards the azeotropic composition.

CapnLazerz's "up until about 5%" points the right way. The equilibrium depends on ambient humidity and temperature, so it is not a fixed 5%, but under ordinary indoor conditions a repeatedly opened bottle of 200 proof settles somewhere in the 190s.

And the poster's question — should I make it evaporate to 190 — has the answer: do nothing. Deliberately leaving it open loses ethanol at the same time (its vapour pressure is 44.6 mmHg at 20 °C, and it goes fast). Let it come on its own, or do not think about it at all.

Quantifying "no difference"

Article #49 built a dataset: of 42,225 database records, only 7 leave two or more solubility measurements at different ethanol strengths.

Applied to this question:

The high-strength end — where this argument lives:

Material	Strength change	Solution needed	Fold change
Benzyl benzoate	90% → 95%	1:2 → 1:1	2×
Vanillin	70% → 95%	1:3 → 1:2	1.5×

The low-strength end:

Material	Strength change	Solution needed	Fold change
Benzyl benzoate	45% → 95%	1:1000 → 1:1	1000×
Benzyl acetate	30% → 60%	1:200 → 1:5	40×
Cinnamaldehyde	30% → 60%	1:25 → 1:5	5×

190 and 200 are five points apart, in the flattest stretch of the curve. The database has no material with a measurement between 95% and 100% at all, presumably because nobody felt that range needed measuring.

The forum consensus is right, and the degree to which it is right can be measured: those five points are nearly horizontal.

The question actually worth asking

What strength is your finished product?

An EDP at a 20% fragrance load, with the other 80% all 95% ethanol, gives a finished product around 76% ethanol. Add water — many formulas do, to adjust evaporation or reduce sting — and it can drop below 70%.

Look at the table again. That is exactly where the curve starts to steepen.

Benzyl benzoate needs 1000 parts of solution at 45%, and it is a diluent in many formulas, used in quantity. Benzyl acetate appears in 317 of 954 published formulas at a median dose of 5.50%. Both live in that steep range.

This series wrote in article #37 that what heat dissolves comes back out on cooling, and added in article #49 that adding water does it too. This is the third face of the same thing: your finished product's strength matters far more than your stock bottle's, and the forum discussion is entirely about the latter.

Ethanol itself is not odourless

While here, the other recurring question. The same subreddit has a thread called "The bite of ethanol" with 26 replies, and another asking whether perfumer's alcohol is better than Everclear's strong alcohol smell.

The database is blunt:

Field	Value
Odour strength	Medium (rank 2)
Odour	Alcoholic, ethereal, medicinal
Odour description	At 100%: strong alcoholic, ethereal, medical
Substantivity	< 1 hour at 100%

Strength rank 2, with "medicinal" in the odour field. Ethanol is not background; it is a material with a smell that happens to leave within the hour.

And the substantivity field reads < 1 hour. This series has handled several > lower bounds — Tonalide's > 400 hours, sotolon's > 400 hours. This is the first upper bound. Same rule: nobody has reported the actual value, only that it is under an hour.

Its vapour pressure is 44.6 mmHg at 20 °C. Against materials this series has covered: hexanal 10.888, DPG 0.0319, TEC 0.000175. Ethanol is four times faster than the fastest fragrance material here, and 250,000 times faster than TEC.

So the opening bite is real, and it is gone within an hour. Which is part of why maceration gets recommended so often, though maceration changes more than just ethanol leaving.

What happens on skin

In 2025, Hadjiefstathiou and colleagues published a measurement device in Talanta for perfume release in the air above a surface.

They used a perfume of eight fragrance molecules in ethanol, measuring headspace evaporation by solid phase microextraction (SPME) with gas chromatography, and compared four surfaces: chemically inert glass, the Strat-M® model, a perfume test strip, and skin on volunteers' forearms. Temperature control, time effects, system dimensions, volume and seal integrity and SPME parameters were all investigated.

The paper reports the device demonstrating originality, effectiveness and repeatability both in vitro and in vivo, with promising results towards a better understanding of the evaporation of fragrance molecules and its link with the physicochemical properties of skin.

This paper did not measure 190 versus 200 proof, or the effect of ethanol strength. I cite it for one thing: how a perfume evaporates on skin is still, as of 2025, at the stage of establishing measurement methods. Part of why forum arguments about alcohol are hard to settle is that the question itself has only just acquired tools in the literature.

The answer for the poster

• You did not mess up. 200 proof is entirely usable.
• Do not make it evaporate. It absorbs water on its own, and leaving it open deliberately loses ethanol at the same time.
• What you overpaid for is the azeotrope. 190 proof is where distillation gets cheaply; 200 proof takes another step.
• Those five points are nearly horizontal on the solubility curve. If you want something to worry about, worry about your finished product's ethanol strength, which is where the curve is steep.
• Ethanol has a smell (medium strength, alcoholic/ethereal/medicinal), and it leaves in under an hour.

Why this keeps getting asked

This section is not for the poster. It is for the person who said one more alcohol post would drive them crazy.

Searching that subreddit for "190 proof" finds this question asked at least five times between 2024 and 2026, plus at least six threads on adjacent questions — where to buy, whether Everclear works, what SDA 40B is, the bite of ethanol, DPG or ethanol.

Every time, someone gave the correct answer, and every time that answer stayed inside a comment thread.

The poster says he searched for six weeks. He was not failing to look. The answers were not in a findable form.

What this doesn't establish

• I measured nothing. This piece is database queries, seven pre-existing solubility records, and two papers.
• The azeotropic composition figure (about 95.6% by mass) is a textbook value, not mine. Naidu 2022 establishes the mechanism — ordinary distillation cannot separate azeotropes — and gives no composition figure.
• The "up to about 5%" equilibrium depends on humidity and temperature. I did not look up that quantitative relationship; the reply's 5% is treated as an order of magnitude.
• The database has no solubility measurement between 95% and 100%. Calling that stretch nearly horizontal extrapolates from the twofold change between 90% and 95%; it is not directly measured.
• The seven materials are everything the database holds, not a sample. That dataset's limits are listed in article #49.
• Hadjiefstathiou 2025 did not measure ethanol strength. I cite it to show that skin evaporation measurement is still developing, not for any conclusion about proof.
• Regulation and purchasing eligibility are entirely absent here. SDA 40B, denaturant types and what can be bought where are a separate set of problems.