One of the most common questions we get asked at Sydney Solvents is: "If I want to disinfect, should I buy the 99% or the 70% isopropyl alcohol?" Almost everyone assumes the higher percentage must be the better germ-killer. In reality, for disinfecting surfaces, 70% isopropyl alcohol is usually more effective than 99%.
Why water makes alcohol a better disinfectant
Alcohol kills bacteria and viruses by denaturing (unfolding) the proteins in their cell walls and dissolving their lipid membranes. That process actually needs a little water to work properly. Water acts as a catalyst and slows evaporation, giving the alcohol more contact time on the surface.
Pure 99% alcohol evaporates so quickly that it can flash off before it has fully penetrated and denatured the microbe. It can also cause the outer protein layer of some bacteria to harden into a shell that protects the organism inside — the opposite of what you want.
So when should you use each one?
- 70% IPA — the sweet spot for disinfecting benches, tools, door handles and skin prep. Slower evaporation, longer kill time, better value.
- 99% / 100% IPA — best where water is the enemy: cleaning electronics, circuit boards, screens, lenses and removing flux or light adhesives with zero residue.
The takeaway
Higher is not always better. Match the concentration to the job: 70% to disinfect, 99% to clean electronics. Both are available from 1L bottles up to 200L drums, and our team is always happy to point you to the right one for your application.