SDA 3-A and SDA 3-C are the same base ethyl alcohol denatured two different ways: SDA 3-A adds five gallons of cyclohexane or methyl alcohol to every 100 gallons of ethanol, while SDA 3-C adds five gallons of isopropyl alcohol. Their authorized use lists are nearly identical, so the decision almost always comes down to one question: can methanol touch your process? If it cannot, choose SDA 3-C. If you are manufacturing reagent alcohol, you are legally required to use SDA 3-A.
The formulas, side by side
| SDA 3-A | SDA 3-C | |
|---|---|---|
| Regulation | 27 CFR 21.35 | 27 CFR 21.37 |
| Denaturant per 100 gal ethanol | 5 gal cyclohexane or methyl alcohol | 5 gal isopropyl alcohol |
| Approximate ethanol content | About 95% of volume | About 95% of volume |
| Contains methanol | Usually yes | No |
| Permit required | TTB industrial alcohol user permit | TTB industrial alcohol user permit |
| Allowed for reagent alcohol | Yes, and it is the only formula permitted | No, expressly prohibited |
Both formulas are published in 27 CFR Part 21, the federal rulebook of denatured alcohol formulas. Verify the current text there before you write a specification, because formulas do get amended.
The methanol question
In practice, most SDA 3-A on the market is denatured with methyl alcohol rather than cyclohexane. Methanol is toxic, it is a regulated substance in a number of consumer product categories, and plenty of downstream customers, quality systems and export markets simply will not accept it. If any of the following apply, SDA 3-C is the safer specification:
- The product is used on skin, near food, or in an enclosed space where vapor exposure matters
- A customer specification, safety data sheet review or export requirement excludes methanol
- Residual denaturant carries through into the finished article rather than evaporating
Where the alcohol is a processing aid that is fully removed or where you are working in a controlled industrial setting, SDA 3-A is usually the lower-cost option and is entirely appropriate. Isopropyl alcohol is not harmless either, but it is far less acutely toxic than methanol and is already present in a great many industrial and cleaning formulations.
The reagent alcohol rule that decides it for labs
There is one place where the choice is not yours to make. Reagent alcohol, the roughly 90/5/5 ethanol, methanol and isopropanol blend that laboratories buy by the case, is a general-use formula defined at 27 CFR 20.117. It must be made with 95 parts by volume of SDA 3-A plus 5 parts by volume of isopropyl alcohol, and it may not contain any other ingredient.
SDA 3-C carries an explicit condition governing use stating that it shall not be used in manufacturing reagent alcohol under that general-use formula. So if reagent alcohol is your end product, SDA 3-A is the mandatory input. Our guide to reagent alcohol covers the resulting blend and its labeling requirements.
What each formula is authorized for
The authorized use lists for SDA 3-A and SDA 3-C overlap almost completely. Both cover a broad industrial range:
- Solvent uses such as cellulose and synthetic resin coatings, plastics and resins, photographic film and emulsions, adhesives and binders, soldering flux, inks, stains and polishes
- Processing uses including pectin, food products, crude drugs, glandular products, vitamins, hormones, antibiotics, vaccines, medicinal chemicals, dyes and rubber latex
- Dehydration of cellulose compounds, sodium hydrosulfite and other products
- Biocides and cleaning, covering disinfectants, insecticides, fungicides, sterilizing and preserving solutions, industrial detergents and cleaning solutions
- Raw material, fuel, fluid and laboratory uses, including laboratory reagents for sale and general laboratory work for your own use
The lists are close enough that authorized use is rarely the deciding factor. Confirm your specific use code against the current regulation and against the scope written on your permit, since your permit is what actually constrains you.
Permits, proof and packaging
Both formulas are specially denatured alcohol, which means both require a TTB Industrial Alcohol User Permit covering the formula and use code before anyone can ship to you. If that is a blocker, the permit-free alternative is completely denatured alcohol such as CDA 12-A, at the cost of a much heavier denaturant load. Our post on TTB permit rules for denatured alcohol lays out which path applies to you.
Proof is a separate decision from formula. 190 proof ethanol sits near the ethanol-water azeotrope of about 95.6% ethanol by weight and suits most water-tolerant processes. 200 proof ethanol is dehydrated to under 0.5% water and is what you specify when residual moisture would ruin a reaction, a coating or a dehydration step. Pristine Alcohol produces SDA 3-A and SDA 3-C from 100% Michigan corn in a 100% stainless steel facility in Marysville, Michigan, in drums, totes and tankers.
Frequently asked questions
Is SDA 3-C just SDA 3-A without the methanol?
Functionally that is the practical difference, but they are separate regulatory formulas. SDA 3-A uses five gallons of cyclohexane or methyl alcohol per 100 gallons; SDA 3-C uses five gallons of isopropyl alcohol. You cannot substitute one for the other on a permit or a specification without the paperwork to match.
Which one is cheaper?
SDA 3-A is typically the lower-cost of the two because methanol is a cheaper denaturant than isopropyl alcohol, though the gap moves with feedstock markets. The cost difference is usually small relative to the ethanol itself.
Can I use SDA 3-C to make reagent alcohol?
No. 27 CFR 21.37 states that SDA 3-C shall not be used in manufacturing the reagent alcohol general-use formula under 27 CFR 20.117. Reagent alcohol must be built on SDA 3-A.
Do both formulas require the same TTB permit?
Both require an industrial alcohol user permit, but the permit lists the specific formulas and use codes you are authorized to receive. If your permit covers SDA 3-A only, you cannot take delivery of SDA 3-C until it is amended.
Does either formula come in 200 proof?
Yes. Both are available on 190 proof or 200 proof base ethanol. Specify 200 proof, under 0.5% water, when your process is moisture-sensitive.
Not sure which formula your process needs? Request a bulk ethanol quote or contact us through the Pristine Alcohol contact page and we will help you match formula, proof and package to your specification and permit.