Maths Help Please (Hand Sanitiser Recipes)

Fourstar · 23 Mar 2020 at 21:22

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

TrannyBill · 23 Mar 2020 at 22:40

Let's assume you have a litre (1,000mL) to start with. You can easily modify all figures afterwards if you've started with more or less by multiplying all figures by the amount you actually started with then dividing by 1,000.

90% initial concentration means 900mL of alcohol to start with
That 900mL needs to be 75%
(900/75)*100 = 1,200mL gives you the total volume needed for the new dilution
(1,200/100)*2 = 24mL gives you the 2% glycerine needed
1,200 - 1000 - 24 = 176mL of water to add

Hopefully I've made this easy enough to follow for other figures

muon · 23 Mar 2020 at 22:47

I also have 4 litres of 99% IPA. Have it for cleaning electronics and computer parts. Always wondered if this was possible. How did you get glycerol?

TrannyBill · 23 Mar 2020 at 22:49

muon said:
I also have 4 litres of 99% IPA. Have it for cleaning electronics and computer parts. Always wondered if this was possible. How did you get glycerol?

Chemist or vape shop would be easy options

muon · 23 Mar 2020 at 22:51

TrannyBill said:
Chemist or vape shop would be easy options

Any need to actually add water?

TrannyBill · 23 Mar 2020 at 22:57

60 to 95 percent alcohol is the effective range for hand sanitiser, so do as you please. Remembering that water is rather cheap as a solvent.

muon · 23 Mar 2020 at 22:58

TrannyBill said:
60 to 95 percent alcohol is the effective range for hand sanitiser, so do as you please. Remembering that water is rather cheap as a solvent.

Just being lazy. IPA is pretty cheap as well (well normally).

I'll try it out. Have nothing better to do.

Ryan-3 · 24 Mar 2020 at 00:16

"c1v1=c2v2"
Concentration 1 x Volume 1 = Concentration 2 x Volume 2

We would normally do this using a set concentration, but we can simplify to percentage here. So, say you wanted 400ml hand sanitizer containing a 75% concentration of IPA:
(90%)(v1)=(75%)(400ml)
v1 = 333ml IPA.

Glycerol is readily soluble in water, so I'd just suggest making said solution and adding 2-5% v/v of glycerol to it.

TrannyBill · 24 Mar 2020 at 00:24

Ryan-3 said:
"c1v1=c2v2"
Concentration 1 x Volume 1 = Concentration 2 x Volume 2

We would normally do this using a set concentration, but we can simplify to percentage here. So, say you wanted 400ml hand sanitizer containing a 75% concentration of IPA:
(90%)(v1)=(75%)(400ml)
v1 = 270ml IPA.

Glycerol is readily soluble in water, so I'd just suggest making said solution and adding 2-5% v/v of glycerol to it.

I don't know what maths you did there, but it's plain to see that 300 is 75% of 400. So surely you'd need 333mL of 90%

Edit: I see you have corrected while I was replying haha

Fourstar · 24 Mar 2020 at 07:54

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 · 24 Mar 2020 at 07:56

muon said:
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 · 24 Mar 2020 at 07:58

muon said:
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

touch · 24 Mar 2020 at 09:20

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

Fourstar · 24 Mar 2020 at 09:59

touch said:
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.

touch · 24 Mar 2020 at 10:24

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

The solution you have is 90% alcohol and 10% water. If you want to end up with a 75% alcohol concentration, you should mix this existing solution 83% to whatever other water/glycerol.

It's just a generalisation of the calculation given in the first reply: @TrannyBill said that to make 1200ml, you need to take 1000ml of your solution and dilute with 200ml of other fluid*
1000ml is 83% of 1200ml.
Whatever amount you want to make, it needs to 83% of the existing solution in order to end up with 75% alcohol content.

*he actually had 176ml of water and 24ml of glycerol

Fourstar · 24 Mar 2020 at 11:08

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.

jpaul · 24 Mar 2020 at 11:51

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.

aldi checkouts this morning , they were sensibly using sanitizer on gloved hands between each customer ...
personally alcohol on hand makes them horrendously dry/crack ... waiting for the boots low alcohol variant to come back in stock.
https://ecohydra.com/professional/product-information/hand-sanitiser
but a DIY recipe would be good

ElliorR · 24 Mar 2020 at 11:53

You better not be trying to open a speakeasy.

jpaul · 24 Mar 2020 at 12:24

Distillery (or maybe they made up gin from bought spirit) was in the news - making up hand sanitizer as a sideline
... whether hmrc demand it has a spoiler, so it couldn't be drunk. .. glycerin can be food safe - fondant icing

Fourstar · 24 Mar 2020 at 13:55

jpaul said:
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

ElliorR said:
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!