Walk into any chemist or hardware store looking for isopropyl alcohol and you'll find products ranging from 70% to 99.9% purity. For newcomers, this raises an obvious question: which one should you buy? The answer, as with most things in chemistry, depends on what you're trying to accomplish. This article breaks down the science behind different IPA concentrations and provides clear guidance on which to use for various applications.
The Science of IPA Concentrations
Before diving into specific uses, it's worth understanding what these percentages actually mean and why they matter.
What the Numbers Represent
When we say "70% isopropyl alcohol," we mean the solution contains 70% IPA and 30% water by volume. Similarly, "99% IPA" contains only 1% water. That remaining water content (or lack thereof) dramatically affects how the product behaves.
The Water Paradox
Intuitively, you might assume that higher alcohol concentration means more cleaning or disinfecting power. After all, you're getting more of the "active ingredient." However, for disinfection specifically, this assumption is wrong.
Research dating back decades has consistently shown that 70% IPA outperforms higher concentrations at killing bacteria. The reason lies in how alcohol kills microorganisms: it denatures (destroys) their proteins. Water plays a crucial role in this process.
When concentrated alcohol contacts bacteria, proteins on the cell surface coagulate almost instantly, forming a protective barrier. Water in 70% IPA slows this process, allowing the alcohol to penetrate the cell fully before proteins coagulate. The result is complete cell death rather than protected, surviving bacteria.
Detailed Comparison
Let's examine how the two most common concentrations—70% and 99%—compare across different factors:
Disinfection Effectiveness
- 70% IPA: Optimal for killing bacteria, many viruses, and fungi. The gold standard in healthcare settings.
- 99% IPA: Less effective as a disinfectant. The rapid evaporation and protein coagulation issues reduce antimicrobial action.
Winner for disinfection: 70% IPA
Evaporation Rate
- 70% IPA: Moderate evaporation. Takes several seconds to a minute to dry depending on amount applied.
- 99% IPA: Very rapid evaporation. Dries almost instantly, often within seconds.
Winner for fast drying: 99% IPA
Residue
- 70% IPA: May leave minor water marks or residue, especially on electronics or glass.
- 99% IPA: Evaporates virtually residue-free, leaving surfaces clean and dry.
Winner for no residue: 99% IPA
Skin Safety
- 70% IPA: Gentler on skin due to water content. Still causes drying with repeated use.
- 99% IPA: More aggressive. Can cause significant skin drying and irritation with even brief exposure.
Winner for skin contact: 70% IPA
Flammability
- 70% IPA: Still flammable, but water content provides some safety margin.
- 99% IPA: Highly flammable. Requires extra caution around heat sources.
Winner for safety: 70% IPA
Price
- 70% IPA: Generally cheaper and more widely available.
- 99% IPA: Usually more expensive, especially in smaller quantities.
Winner for value: 70% IPA
- Choose 70% for: Disinfection, first aid, skin prep, surface sanitising, general cleaning
- Choose 99% for: Electronics, 3D printing, thermal paste removal, precision cleaning, laboratory work
Use Case Recommendations
Healthcare and First Aid
Use 70% IPA. This is the standard in hospitals, clinics, and first aid applications worldwide. It effectively kills pathogens on skin surfaces and equipment while being relatively gentle on skin. You'll find 70% IPA in pre-injection swabs, hand sanitisers, and surface disinfectants in medical settings.
Surface Disinfection
Use 70% IPA. For killing germs on countertops, doorknobs, and other high-touch surfaces, 70% provides superior disinfection. Allow the surface to remain wet for 30-60 seconds for best results before wiping dry.
Electronics Cleaning
Use 99% IPA. The minimal water content is critical when cleaning electronics. Water can damage circuits, corrode contacts, and cause short circuits. The rapid evaporation of 99% IPA means moisture doesn't linger on sensitive components. Use it for:
- Circuit boards
- Computer components
- Electrical contacts
- Precision instruments
Thermal Paste Removal
Use 99% IPA. When replacing thermal paste on CPUs or GPUs, you need a solvent that dissolves the old paste completely and evaporates without leaving any residue. The 30% water in 70% IPA can cause problems here—both by leaving water on sensitive components and by not dissolving some thermal compounds as effectively.
3D Printing (Resin)
Use 99% IPA (or at minimum 91%). Uncured resin needs to be washed off prints before final curing. High-concentration IPA dissolves resin effectively, and the rapid evaporation means prints are ready for curing quickly. The water in lower concentrations can cause cloudiness on finished prints.
Removing Adhesive Residue
Either works, but 99% is slightly better. Both concentrations dissolve most adhesives effectively. 99% evaporates faster and leaves no water spots, which matters on surfaces like glass or painted items. For general sticker residue removal, use whatever you have on hand.
Glass and Mirror Cleaning
99% IPA or diluted 70%. For streak-free glass, 99% IPA is ideal because it evaporates completely. If using 70%, you may need to wipe with a dry cloth to prevent water spots. Some people dilute either concentration with additional distilled water for economical glass cleaning.
Ink and Marker Removal
Either works. Both 70% and 99% IPA dissolve many types of ink effectively. Test in an inconspicuous area first, as IPA can damage some surfaces.
What About 91% IPA?
91% isopropyl alcohol represents a middle ground and deserves mention. It's commonly available at pharmacies and offers a compromise between disinfection ability and quick evaporation. Consider 91% IPA when:
- 99% isn't readily available
- You need faster evaporation than 70% but aren't working with extremely sensitive electronics
- You want a single product for multiple uses
However, for most applications, either 70% or 99% is the better choice depending on whether you prioritise disinfection or evaporation.
Diluting 99% IPA
If you have 99% IPA but need a lower concentration for disinfection, you can dilute it with distilled water. To create approximately 70% IPA from 99% IPA:
- Mix 7 parts 99% IPA with 3 parts distilled water
- Example: 700ml of 99% IPA + 300ml distilled water = approximately 1 litre of 70% IPA
Use distilled water rather than tap water to avoid introducing minerals that could leave residue or interfere with certain applications.
Common Misconceptions
"Higher Concentration Is Always Better"
False. As discussed, 70% IPA is actually more effective for disinfection. Choose concentration based on application, not the assumption that more is better.
"99% IPA Is Medical Grade"
Not necessarily. "Medical grade" or "pharmaceutical grade" refers to purity and quality control standards, not concentration. You can have medical-grade 70% IPA and industrial-grade 99% IPA, or vice versa.
"70% IPA Is Watered Down"
The water content in 70% IPA is intentional and serves a specific purpose. It's not a cost-cutting measure—it's what makes 70% IPA effective at its primary job: killing microorganisms.
Conclusion
The choice between 70% and 99% isopropyl alcohol isn't about quality—it's about matching the tool to the task. For disinfection, first aid, and surface sanitising, 70% IPA is the scientifically proven choice. For electronics, precision cleaning, and applications where moisture is the enemy, 99% IPA is the way to go.
Many households and workshops benefit from having both concentrations available. A small bottle of 70% IPA in the first aid kit and a larger container of 99% in the workshop covers most needs. Understanding why each works differently empowers you to make the right choice every time.