Making 70% ethanol solutions (the right way)

We use 70% ethanol (EtOH) as a disinfectant in the lab, so we often have to dilute ethanol and fill our squirt/spray bottles. In order to do it the right way, we need to understand what that 70% means. It’s 70% volume/volume (v/v) meaning ethanol volume divided by total volume. And here’s where problems can arise. You might think you could mix 70mL pure EtOH with 30 mL water to get 100mL of 70% EtOH. But you can’t! This is a case where basic math fails us and 70+30 does not equal 100! Instead it equals less than 100!!

blog form: https://bit.ly/ethanol_solution

When you mix EtOH and water, the EtOH kinda sneaks into some gaps in the water network and interacts with the water, so the volumes are “non-additive.”  Therefore, the correct way to make 100ml of 70% EtOH from 100% EtOH is to take 70 mL EtOH and dilute it to a final volume of 100ml (which will require slightly more than 30ml water). You can do this in a graduated cylinder or by eyeballing using pre-marked lines on a bottle if you don’t need to be precise. Speaking of precision, if you’re just using the ethanol solution to clean don’t worry if you made it the wrong way (measured out the water). It should still be fine for your purposes! This is just for future reference and for when you really care about the actual %.

Speaking of practical, real-life usage, usually, instead of starting with pure ethanol we start with ethanol that has a bit of water in it. This is much cheaper. How do you know which you’re working with? The proof is in the proof!

Proof can be reported different ways in different places, but in the US it is twice the % v/v. So pure ethanol is 200-proof. Anything lower than that has some water in it already. Which, if your plan for it is to add more water to it, makes sense to use!

A common proof you will see in the lab is 190-proof. Divide that by 2 and you get 95. So this is 95% (v/v) ethanol.

Now say you want to dilute that to 70%. We often use 70% ethanol to disinfect things. Ethanol is able to denature (unfold and incapacitate) proteins and disrupt lipid membranes. This makes it good at killing microbes. But if you use pure ethanol, it can’t penetrate well, evaporates too quickly, and makes a sort of “crust” of precipitated protein gunk that protects what’s underneath.

So you want an ethanol/water solution instead. You don’t want to go too low in though or else there’s not enough ethanol to do the job. The sweet spot is ~70% ethanol (140-proof if we go back to that terminology). There’s some wiggle room which makes things easier in the lab not having to worry about measuring out exactly. more on how alcohols disinfect here: https://bit.ly/handsanitizerss

To figure out how much you need to make 100mL, you can use our handy-dandy

c1v1=c2v2 (initial concentration times initial volume equals final concentration times final volume - much more on this here: http://bit.ly/c1v1equalsc2v2; YouTube: C1V1 = C2V2 (aka M1V1=M2V2) is so use... )

(95%)(v1) = (70%)(100ml)
v1= 73.4mL

If you wanted 500mL?

Either just multiply that by 5
73.4*5= 368.4 ml

Or do the whole c1v1=c2v2 again with a different v2

(95%)(v1) = (70%)(500ml)
v1= 368.4 ml

There’s also a shortcut if you don’t need to make exactly 100ml. Take 70mL of the starting solution and dilute it to a final volume of the starting solution percentage. So, for example, if you start with 95% EtOH, dilute that to 95ml for a 70% solution.

To see why that works…

(95%)(70mL) = (70%)v2
v2= 95 mL

Using the final percentage as your starting volume lets those 2 cancel out, leaving you with your final volume being your starting percentage!

If you want to scale up, just multiply. So, for example, if you know you want ~500ml, multiply 70 by 5 to get 350 ml and dilute that to 95x5= 475 ml total.

To make your life even easier, once you calculate it out, you can draw lines on the side of your spray bottle corresponding to the ethanol fill line and the total volume fill line (just pour in those volumes of water to make the marks). Next time, just follow the lines!

Also, beware that ethanol can dissolve your sharpie ink so use solvent-resistant lab markers and/or cover your markings and labelings with packaging tape or the sealing film for a qPCR plate or crystal screen.

Ethanol is also flammable so make sure you mark the bottle as flammable - and write out the whole chemical name (ethanol not just EtOH).

more (hopefully) helpful random practical lab tips & tricks: https://bit.ly/lab_tricks_page     
 
more about all sorts of things:  #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0 or search blog: http://thebumblingbiochemist.com