Maths Help Please (Hand Sanitiser Recipes)

[Fourstar]

I've got some 90% Isopropyl alcohol / 10% water solution I'm wanting to dilute down to 75/25 to use as hand sanitiser. I can't work out the formula I need that will tell me the required amount of water I need to add to a given quantity of 90/10 to make 75/25.

One step further. I'm also going to try adding 2% Glycerol to make it less harsh. What formula would I need to end up with
75% IPA / 23% water / 2% Glycerol from 90/10?

Any help appreciated

 

[Fourstar]

Thanks for the explanations folks.

For anyone else looking for something to keep their sanitiser in I found the "Original Source Mint & Tea Tree Shower 50ml" travel sized shower gel bottles work well and don't leak. They are common in the travel section in supermarkets.

This version. I have not tried the design which tapers in towards the top.

 

[Fourstar]

How did you get glycerol?

I ordered it online. As mentioned above chemists and vape shops are another option.

If you don't need much you can also get it in some supermarkets in the home baking section.

Here is the type Tesco stocks.
https://www.tesco.com/groceries/en-GB/products/254914123

Glycerin and Glycerol are different names for exactly the same thing.

Glycerine (from a google search) is glycerin/ol but only has to be 95% pure. Shouldn't make any difference for our purposes.

It's not actually needed for hand sanitiser to function. It acts as a humectant moisturising the the skin to prevent irritation and dryness.

Example hand sanitiser formulations published by the WHO suggest 1.45% so I figure 2% is a good amount to shoot for.

 

[Fourstar]

Any need to actually add water?

Somewhat counter-intuitively more isn't better and water is important to the operation of hand sanitiser.

Isopropyl alcohol, particularly in solutions between 60% and 90% alcohol with 10 – 40% purified water, is rapidly antimicrobial against bacteria, fungi, and viruses. Once alcohol concentrations drop below 50%, usefulness for disinfection drops sharply. Notably, higher concentrations of alcohol don’t generate more desirable bactericidal, virucidal, or fungicidal properties.

The presence of water is a crucial factor in destroying or inhibiting the growth of pathogenic microorganisms with isopropyl alcohol. Water acts as a catalyst and plays a key role in denaturing the proteins of vegetative cell membranes. 70% IPA solutions penetrate the cell wall more completely which permeates the entire cell, coagulates all proteins, and therefore the microorganism dies. Extra water content slows evaporation, therefore increasing surface contact time and enhancing effectiveness. Isopropyl alcohol concentrations over 91% coagulate proteins instantly. Consequently, a protective layer is created which protects other proteins from further coagulation.

https://blog.gotopac.com/2017/05/15...than-99-isopropanol-and-what-is-ipa-used-for/

The WHO in their guide to small scale Hand Sanitiser production recommends 75% IPA or 80% ethanol.
https://www.who.int/gpsc/5may/Guide_to_Local_Production.pdf

 

[Fourstar]

You'd need an 83% mix to get the right concentration because 90% of 83% is 75%

Where did 83 come from? You're the first person to mention that number.

 

[Fourstar]

I see. You're expressing things in another way.

If I mix 83ml of 90% IPA with 17ml of water I will end up with 100ml of 75% IPA.

 

[Fourstar]

aren't you better off going for the lower 60% concenteation, that way when you rub it on hands, more chance of good coverage before it evaporates
... or, you can just use more product if your ipa is cheap.

With home brew concoctions I'm inclined to give a safety margin. A bit strong has no adverse effects but a bit weak if targetting 60% could have a dramatic effect on performance.

The World Health Organisation also specify 75% IPA in their instructions for local production of hand sanitiser.
https://www.who.int/gpsc/5may/Guide_to_Local_Production.pdf

You better not be trying to open a speakeasy.

IPA might share its name with a rather tasty tipple but you certainly wouldn't want to drink it!

 

[Fourstar]

WHO recipe has hydrogen peroxide in theirs, is that rest necessary considering 75% of the content is alcohol.

This is probably more relevant to larger scale production, storage and distribution where conditions are less sanitary. No DIY recipes I came across include it. The guide says it is not an active ingredient for sanitising.

Hydrogen peroxide: used to inactivate contaminating bacterial spores in the solution and is not an active substance for hand antisepsis.