70% vs 91% vs 99% Isopropyl Alcohol Compared

The number on the bottle tells you how much of the liquid is isopropyl alcohol and how much is water. Below are the specification limits from our own certificates for each strength, which strength fits which job, how to dilute 99% correctly (the usual recipe gets it slightly wrong), and what the percentage does not change.

What is the difference between 70%, 91% and 99% isopropyl alcohol?

Isopropyl alcohol (CAS 67-63-0) is the same molecule in every bottle; the strengths differ in how much water is mixed in. That water is the slower-evaporating part of the mixture, which is why a 70% film stays wet longer than a 99% one, and why the water left behind matters on electronics and optics. The table compares the four strengths we sell, using the limits on our own published sample Certificates of Analysis and the flash points on our Safety Data Sheets.

Which strength should you buy?

Two things in that table surprise most buyers. First, the percentages are not all on the same scale: our 70% and 91% state concentration by volume, while our 99% and 99.9% state it by weight. By weight, a 70% by-volume solution is only about 62–63% isopropyl alcohol; the worked calculation is in Is Rubbing Alcohol the Same as Isopropyl Alcohol?. Second, the 99% grades leave the least behind: the residue limits are lower than for 70%, and there is almost no water to dry off.

Why do isopropyl alcohol percentages stop at 70%, 91% and 99%?

70% is the rubbing alcohol strength. The USP–NF monograph for Isopropyl Rubbing Alcohol says it contains NLT 68.0% and NMT 72.0% of isopropyl alcohol by volume, with the remainder consisting of water, with or without suitable stabilizers, perfume oils, and color additives. Our 70% USP Grade has the same 68.0–72.0% range, with water as the only other ingredient.

91% sits just above a wall in the chemistry. Isopropyl alcohol and water form an azeotrope at 87.7% isopropyl alcohol by weight, boiling at 80.37°C (PubChem), which is roughly 90% by volume. Ordinary distillation of a water mixture cannot go past that point, so a strength in the low 90s is the natural "wet" grade, and getting to 99% takes a different separation step. The full story is in why isopropyl alcohol stops at 91% and how isopropyl alcohol is made.

99% and 99.9% are the dry grades. If you see a 90% bottle, it is a by-volume strength between the two; check the label for "by volume" or "by weight" before comparing it with anything else.

What percent isopropyl alcohol is best for cleaning?

Pick the strength by how much water the job can tolerate, then by the specification you need.

What is 91% isopropyl alcohol used for?

Mostly general cleaning: degreasing, glass, tools and adhesive residue, where you want it to dry faster than 70% and a trace of water is acceptable. On bare circuit boards, connectors and optics, 99% leaves less behind, and it is also the better starting point if you want to mix your own 70% or 91%.

Can I use isopropyl alcohol to disinfect?

We sell isopropyl alcohol as a chemical and solvent and make no germ-killing, disinfecting or antiseptic claims for any strength. Our isopropyl alcohol is sold as a chemical, not as an over-the-counter drug, so it is not for use on skin. If disinfecting is your goal, use a product whose label carries an EPA registration number and follow that label.

How do you dilute 99% isopropyl alcohol to 70% or 91%?

Measure the strong alcohol, then add water up to the final volume. Do not add a calculated volume of water: isopropyl alcohol and water shrink when they mix, so the two volumes do not add up.

Our own 70% shows the effect. Its formulation record lists its composition as 68.3 volumes of 99.9% isopropyl alcohol and 31.7 volumes of deionized water, and the reference result on the certificate is 70.2% by volume. Without shrinkage, those volumes would give only about 68.2%. Run the usual recipe the other way (707 mL of 99% plus 293 mL of water) and the batch comes out slightly stronger than 70%, because it ends up a little under 1,000 mL.

1. Measure the isopropyl alcohol in a graduated cylinder and pour it into a container marked at the final volume.
2. Add deionized or distilled water to just below the mark (tap water leaves mineral residue; see distilled vs deionized water).
3. Mix, let it stand at room temperature, then top up to the mark and mix again.
4. Close and label it with the strength and the date.

For other strengths and volumes, use our isopropyl alcohol dilution calculator or the step-by-step guide to making 70% isopropyl alcohol from 99%.

Is 70% isopropyl alcohol less flammable than 99%?

Not in any way that changes how you handle it. Our Safety Data Sheets give every strength the same classification, H225 Highly flammable liquid and vapor, with closed-cup flash points of 18–22°C for 70%, 14–18°C for 91% and 12°C for 99% and 99.9%. All of those are at or below room temperature, so every strength gives off vapor that can ignite at room temperature. Our 70% still ships as a Class 3 flammable liquid (UN1219).

• Storage: keep away from heat, hot surfaces, sparks and open flames, keep the container tightly closed, and store it in a well-ventilated place.
• Transfers: ground and bond the container and receiving equipment.
• Protection: chemical safety goggles or a face shield and chemical-resistant gloves, per section 8 of the SDS, with adequate ventilation.
• Exposure limits: OSHA PEL 400 ppm (8-hour average); NIOSH REL 400 ppm averaged over a shift, with a 500 ppm short-term limit; ACGIH TLV 200 ppm, with a 400 ppm short-term limit.

Read the SDS for the strength you buy: 70%, 91%, 99%.

USP, Technical or ACS: which grade do you need?

Grade and strength are separate choices. The grade names the specification the product is tested against: our 70% and 91% are USP Grade, our 99% is Technical Grade, and our 99.9% is ACS Reagent Grade. Choose USP where your process calls for the pharmacopeial specification, ACS where a lab method calls for a reagent, and Technical for cleaning and industrial use. More on grade names in understanding chemical grades, and on the other "rubbing alcohol", 1-propanol, in n-propyl alcohol vs isopropyl alcohol.

All four come in quarts through 275- and 330-gallon IBC totes; see every isopropyl alcohol size and grade, or what a 55-gallon drum weighs and costs to ship. A certificate for a specific lot is available on request. For other solvents, see Acetone Technical Grade and Methanol Technical Grade, and our aerospace cleaning application guides.