70% alcohol, whether ethanol or isopropyl, is the industry standard for effective surface disinfection. Many people mistakenly believe that a higher concentration, such as 99% alcohol, is more effective, but the opposite is true for microbial killing. The presence of water in the 70% solution is necessary to slow evaporation, which increases the contact time on the surface. This water also acts as a catalyst, aiding the alcohol in penetrating the microbe’s cell wall to fully denature its internal proteins, ultimately leading to cell death.
Selecting the Starting Alcohol and Materials
The preparation process begins with selecting a high-concentration alcohol base, typically either isopropyl alcohol (IPA) or ethyl alcohol (ethanol). Both are effective disinfectants, but IPA is commonly sold as rubbing alcohol, while ethanol is often found in higher grades. Common starting concentrations available to consumers include 91%, 95%, or 99% alcohol by volume. The specific percentage you start with will determine the exact amount of water needed for the final dilution.
It is helpful to understand the difference between pure and denatured alcohol, especially with ethanol. Denatured ethyl alcohol contains bittering additives that make it unsuitable for consumption, but it remains effective for surface disinfection purposes. For accurate dilution, use clean, precise measuring tools, such as graduated cylinders, kitchen measuring cups, or syringes with clear markings. The dilution agent should be distilled or purified water, as tap water often contains minerals that can leave residue on surfaces after the alcohol evaporates.
Calculating the Dilution Ratios
Diluting alcohol requires a simple mathematical principle to ensure the final product is exactly 70% by volume. This process is based on the concept that the amount of pure alcohol in the starting solution must equal the amount of pure alcohol in the final solution after water is added. Though often expressed by the formula C1V1 = C2V2, the underlying principle is that the initial concentration multiplied by its volume must equal the desired final concentration multiplied by the final volume.
If starting with 99% alcohol, the goal is to make a 70% solution. To create a final volume of 100 milliliters (mL), you need 70.7 mL of the 99% alcohol. You then add 29.3 mL of water to reach the 100 mL total volume.
A simpler, practical ratio for diluting 99% alcohol is to mix seven parts of the alcohol with three parts of water. Using a common measuring cup, this translates to combining 700 mL of 99% alcohol with 300 mL of water, yielding one liter of 70% disinfectant. This approximate 7:3 ratio is a quick and effective method for non-laboratory purposes.
When starting with a 91% alcohol solution, the ratio changes because the base material already contains more water. To make 100 mL of 70% solution, you would require 76.9 mL of the 91% alcohol. The remaining volume, 23.1 mL, would be the amount of water to add. A practical, easy-to-remember ratio for 91% alcohol is to mix approximately four parts of the alcohol with one part of water, which results in a concentration near 73%, a highly effective level for disinfection.
Safe Mixing, Handling, and Storage
Once the precise volumes of alcohol and water are calculated, the physical mixing process must be executed safely. Alcohol vapors are flammable and can be irritating to the respiratory system, so the mixing should always occur in a well-ventilated area, preferably near an exhaust fan. It is also important to wear protective eyewear and gloves to prevent accidental splashes and skin irritation from the concentrated alcohol.
The calculated amount of water should be slowly added to the alcohol, and the solution should be gently agitated to ensure thorough mixing. Never mix alcohol near an open flame, spark, or any other ignition source, including pilot lights or operating electrical equipment. Even diluted alcohol remains flammable, and the vapors can travel across a room to find a source of ignition.
The finished 70% solution should be transferred to an airtight container that is clean and non-reactive, such as a sturdy plastic bottle or glass container. The container must be clearly labeled with the contents, including the 70% concentration and the date it was prepared. Proper storage involves keeping the container in a cool, dark location, safely secured away from children, pets, and any source of heat or direct sunlight.