Soap and water is a preferred choice for hand hygiene and reducing microbial and viral contamination. However this isn’t always convenient or available when out walking or shopping for necessities, so the next best thing is a hand sanitizer formulation. The nice commercial ones are in short supply, but it is relatively easy to blend your own if you can access the ingredients. Blending chemicals safely is another chemical engineering specialty. Here’s a recipe for small volumes for home use, with some discussion on the physical chemistry basis for the various components.

The formulation I use is one recommended by the World Health Organization (WHO) and documented in their publication. This publication is meant for larger volumes of 10 Litres, which is a bit much for individual or family use. So I’ve re-calculated the recipe to make up a volume of 250 mL (about 1 cup), which many people could do using kitchen utensils. Following is each ingredient, volume, and comments about its purpose.

Instructions: Add each ingredient to a measuring cup, to achieve a final volume of 250 mL, or 1 cup. Beware: the alcohol is flammable, and this blending should be done in a well-ventilated space away from any open flame or other ignition sources. Also avoid inhaling alcohol vapours as they can have negative health effects.

Isopropyl Alcohol (188 mL, about 3/4 cup, 99.8% purity). Also known as isopropanol, 2-propanol or propan-2-ol. Sometimes labelled “rubbing alcohol” although this may also be something else so check the label carefully. You can usually find this in the pharmacy sections of stores, but make sure it is almost pure (close to 100%), not the 70% stuff. This is the main active ingredient for sanitizing hands. It acts by disrupting the lipid (fatty) membrane surrounding bacteria and some viruses, including coronaviruses. Once the membrane layer is disrupted, the bacterial or viral contents can leak out or be otherwise damaged, which renders the pathogen inactive. Note that later we dilute this down to 75%, because alcohols have the best disinfecting power if they are in the 70 to 80% concentration range. Water plays a role in the membrane disruption action, so using pure alcohol is not so good (and using vodka, which is 40% alcohol, is not good either).

Glycerol (3.6 mL, about 3/4 teaspoon, 98% purity). Also known as glycerin. Also usually available in the pharmacy section, and is a thick or viscous liquid. This acts as a “humectant”, something that helps to protect your skin from the drying effects of the alcohol. It serves no sanitizing function.

Hydrogen Peroxide (10 mL, about 2 teaspoons, 3% purity). Likewise usually found in the pharmacy section, often with other first aid materials. Hydrogen peroxide (H2O2) is a well-known antimicrobial compound. In this WHO formulation however, it is not there to sanitize your hands. Its purpose is actually to sanitize the other ingredients and containers, in case they carry bacterial spores.

Water (enough added to the other ingredients to raise the volume to 250 mL or 1 cup). You’ll need around 1/4 cup to get the total volume up to 1 cup. WHO recommends sterile distilled water, which you can often get in grocery stores or pharmacies. Alternatively, boil some tap water for a few minutes and then let it cool down to room temperature before using it. Don’t add hot water, or you’ll vapourize off some of the alcohol.

With these instructions, you should end up with 250 mL of a sanitizer containing 75% isopropyl alcohol, 1.45% glycerol, 0.125% hydrogen peroxide, and water. Note that this sanitizer is still flammable, and should not be used in the near vicinity of ignition sources (open flames, sparks, smoking, etc.). Alcohol fires are almost invisible.

Other Ingredients? WHO suggests that you could add a bit of colouring if you want it to look different from water. Probably some food colouring would be OK, but I wouldn’t add very much or it might stain your hands. Other ingredients like essential oils or fragrances are not recommended, as they can be irritating or allergenic for some people.

To use, pour a few millilitres into your palm and spread thoroughly around your hands, fingertips, and between fingers. You’ll find that this formulation is not as thick as many commercial brands. It is more water-like in viscosity. If your hands are dirty, this sanitizer may not work so well, and soap and water is a better option.

The WHO document gives another formulation based on ethanol (ethyl alcohol), but it is difficult for the average person to find and purchase 80% (160 proof) ethanol because of the regulatory issues. Methanol (methyl alcohol) is not recommended because of its more significant human toxicity issues. Isopropyl and ethyl alcohols are also toxic, but at higher intake levels than methanol, and so are relatively safer. Minimizing inhalation of the vapours is recommended however for all alcohols and other solvents.