What does '1:1.5 in 50% alcohol' mean? That field has a whole curve in it

About a 6 minute read.

Nine hours ago:

"So I was looking through the good scents company's website for Benzyl Alcohol and Benzyl Acetate and for 'soluble in' it says 'ethyl alcohol, 1:1.5 in 50% alcohol', I understand ethyl alcohol is another name for ethanol (which I have). but the '1:1.5 in 50% alcohol', how should I interpret that when trying to dilute these materials for testing. Also, is there any reason they specifically say ethyl alcohol instead of alcohol like they typically do?"

Two replies:

"TGSC is just weird sometimes. I would just dilute them in ethanol per normal." (berael)

"If you make 50% water ethanol solution you can get 1 part to dissolve in 1.5 parts of the water/ethanol solution you just made" (actual_ask164)

The second is correct. And the database can answer both of his questions more fully.

Question one: the notation

One part material dissolves in 1.5 parts of "50% alcohol", and "50% alcohol" means a half-and-half mixture of ethanol and water, not neat ethanol.

It is pharmacopoeial notation, by volume.

Why nobody could simply tell him: the notation is rare in this database. Scanning the 23,941 records with a solubility field, only 11 use the 1:x in y% alcohol form.

The field carries at least five notations, and the common ones are different:

Notation	Records
A value in mg/L (e.g. 4.29E+04 mg/L @ 25 C)	21,041
A percentage (e.g. alcohol, 12%)	10,503
Mentions y% alcohol	36
x vol. of y% alcohol	18
1:x in y% alcohol	11

He landed on eleven records out of forty-two thousand.

Question two: why "ethyl alcohol"

This one has numbers. Scanning the solubility field across the library:

Term	Records
alcohol	11,424
ethanol	2,941
ethyl alcohol	1,453

All three are in use and there is no rule.

Worse is the double meaning inside one field. Benzyl alcohol's solubility field opens like this:

ethyl alcohol, 1:1.5 in 50% alcohol; ethyl alcohol, 1:8-9 in 30% alcohol; most organic solvents; water, 1:25 in water

The word alcohol appears twice in one sentence meaning two different things. The first is the name of a solvent; the second is "a 50% aqueous ethanol".

He found this confusing because the field is confusing, not because he was reading it wrong.

(Some other quirks of that field are in Reading a material datasheet, though that article does not cover notation.)

The half he did not ask about

He mentioned benzyl acetate in the same question. That record's solubility field reads:

water-alcohol mixtures: 30% 1:200, 35% 1:120 40% 1:70 50% 1:20 60% 1:5

That is not a number. It is a curve.

Alcohol strength	Parts of solvent needed
30%	200
35%	120
40%	70
50%	20
60%	5

From 30% to 60%, the solvent needed drops from 200 parts to 5. Forty-fold.

Over a span of 30 percentage points.

Those five points behave

The standard cosolvency model is log-linear: log solubility against the volume fraction of cosolvent is a straight line.

Fitting the five points above (S = 1 / parts needed):

log₁₀(S) = -3.950 + 5.361 × f

R² = 0.9968.

Alcohol strength	Datasheet	Model
30%	1:200	1:220
35%	1:120	1:119
40%	1:70	1:64
50%	1:20	1:19
60%	1:5	1:5.4

That slope has a name: solubilization power (σ), here 5.36. In a form worth remembering: every 10 percentage points of ethanol multiplies solubility by 3.44.

And the slope can be guessed from logP

That is exactly what Millard, Alvarez-Núñez and Yalkowsky did in Int J Pharm in 2002. For a large number of organic compounds they determined the solubilization power σ of each cosolvent (propylene glycol, ethanol, PEG 400, glycerin) from the slope of log-solubility against cosolvent volume fraction.

What they found: a nearly linear relationship between solubilization power and solute hydrophobicity (log Kow). Their closing line is direct:

"Thus, knowing or calculating a compound's partition coefficient is all that is needed to predict solubilization."

I happen to have two comparable materials:

	Formula	logP	Slope σ	Per 10 points
benzyl alcohol	C7H8O	1.10	3.77	x2.38
benzyl acetate	C9H10O2	2.00	5.36	x3.44

The one with the higher logP has the steeper slope. The direction matches the paper.

(Benzyl alcohol has only two data points, 1:1.5 at 50% and 1:8-9 at 30%, so that 3.77 is a line through two points rather than a regression. Two points have no R².)

Which means you can guess the direction without a table. The higher a material's logP, the more sensitive it is to alcohol strength; for low-logP materials, 190 proof against 200 proof makes little difference.

A limit worth flagging

An R² of 0.9968 is too pretty, so it is worth saying why.

Those five points sit between 30% and 60%, the middle of the range. Machatha and Yalkowsky proposed a bilinear model in J Pharm Sci in 2006 for exactly this reason: the log-linear model goes wrong at the extremes. Their model uses two parameters, σA and σB, for the initial and terminal slopes of the solubility profile, and they report it fitting more accurately than either the log-linear or a general parabolic model.

So the line above is trustworthy between 30 and 60%, and extrapolating it to 95% or to neat ethanol is not.

And 95% or above is exactly what most people make dilutions in.

What you can actually do

• Read 1:x in y% alcohol as "one part material in x parts of a y% aqueous ethanol". The y% is ethanol in water by volume, not neat ethanol.
• The word alcohol can mean two things inside one field. Check whether a percentage follows it.
• Anything beginning water-alcohol mixtures: is a whole curve. Do not read one point.
• High-logP materials are sensitive to alcohol strength; low-logP ones are not. That is the basis for deciding whether higher proof is worth it. (190 proof against 200 proof works another angle on this.)
• Do not extrapolate 30-60% figures to 95%. Log-linear goes wrong at the ends.
• The field has five notations, and 57% of materials do not have it at all. Not finding a figure does not mean it will not dissolve.

What this doesn't establish

• I ran no solubility experiments. This is database fields plus two papers.
• I did not trace the provenance of those five data points. The database does not say who measured them, at what temperature, by what method. An R² of 0.9968 may well reflect that the original data was itself interpolated from a model rather than five independent measurements. That is the biggest uncertainty here.
• Whether "1 in 1.5" is by volume or by weight is not stated in the field. I read it as volume, following pharmacopoeial convention.
• Benzyl alcohol's σ is a line through two points. Two points always give R² = 1, so I did not compute it.
• I did not fit σ against logP. Two materials cannot fit Millard's relationship; I only checked the direction.
• The logP values are estimates. Benzyl alcohol 1.10 and benzyl acetate 2.00, both marked (est).
• The notation counts of 11 and 36 come from regular expressions. Slightly different formats are missed, so these are undercounts.
• The 11,424 for alcohol includes the 1,453 for ethyl alcohol (the former being a substring of the latter). The three figures are not mutually exclusive.
• Millard 2002 and Machatha 2006 both studied drugs, not fragrance materials. What I cite is the model and its shape, not any conclusion about aroma chemicals.