Alcohol Concentration and Hand Sanitizer Gel Formulation

Here are the answers to your questions:

1. What are the other components in alcohol concentrations like 95%, 96.5%, 99.9% that make them less than 100% pure alcohol?
   The "missing" component in alcohol concentrations that are less than 100% is primarily water. Pure 100% alcohol (Absolute Alcohol) is highly hygroscopic, meaning it readily absorbs moisture from the air, making it difficult to maintain 100% purity outside of controlled laboratory conditions. Therefore, alcohol sold at concentrations like 95%, 96.5%, or 99.9% refers to the percentage of alcohol (such as ethanol or isopropanol) by volume or weight, with the remainder being water.

2. Assuming that when making hand sanitizer gel, it is said to use alcohol at a ratio of 70%. Which grade of alcohol is used? If using 95% alcohol, the proportion of actual alcohol would not reach 70% because other components are included, right?
   When formulating hand sanitizer gel to contain 70% alcohol, this refers to the final concentration of the pure alcohol (Active Ingredient) in the finished gel product.

   • Typically, a higher concentration alcohol grade is used as the starting material, such as 95% or 96.5% alcohol (which is mostly ethanol), or sometimes 99.9% isopropanol, depending on the specific formula.
   • To achieve a final concentration of 70%, the formulation is calculated based on the amount of pure alcohol needed. You must calculate how much of the starting alcohol (e.g., 95%) is required to provide the necessary amount of pure alcohol to make the final gel 70% alcohol by weight or volume.
   • Example: If you want to make 100 grams of gel containing 70 grams of pure alcohol, and you are using 95% alcohol as your starting material, you would need to use slightly more than 70 grams of the 95% alcohol to get 70 grams of pure alcohol (approximately 70 / 0.95 = 73.68 grams of 95% alcohol). The rest of the formula would be filled with water, gelling agents (like Carbomer), glycerin, and other additives.
   • Therefore, using 95% alcohol does not mean the final concentration won't reach 70%. It means you need to use the correct calculated amount of 95% alcohol to ensure that the proportion of pure alcohol in the final gel product is 70% of the total weight or volume.

In summary, the concentration on the alcohol bottle (e.g., 95%) is the proportion of alcohol to water in that bottle, while the desired concentration in hand sanitizer gel (70%) is the proportion of pure alcohol to the total amount of gel, requiring a correct calculation of the starting alcohol amount to achieve the target final concentration.