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Solution-making strategies & practical advice

Stock up on stock solutions so you can spend your time on the fun stuff! Stock solutions are just where you make a solution of something at a higher concentration than you’ll actually want to use it at (more solute molecules stuffed into that volume) - and then you add however much of it you want to get to your desired final concentration - we call the desired final concentration the “Working concentration” because it’s the one you actually want to be working with when you’re doing the experiment work not just the prep    blog form: http://bit.ly/sciencestocksolutions ;   more on stock solutions: YouTube:    • Making stock solutions - how and why       Stock solutions are huge time-savers because, just like a bartender doesn’t want have to go ferment some whatever you ferment to get gin every time a customer orders a cocktail with gin in it, you don’t want to have to dissolve solid NaCl (table salt) every time you want to make a solution that has NaCl in it. And, just like different gin-containing cocktails have different amounts of gin in them, different solutions I want to make have different amounts of NaCl in them. So I want to keep a stock solution that’s at a higher concentration than any of the solutions need it at - with room to add any extra ingredients. Here’s more about how (and why) to make them).     Molarity is an “absolute” concentration, it tells you how much stuff there is compared to the volume. But we can also talk in terms of “relative concentrations” which tell you [how much stuff there is compared to the volume] compared to [how much stuff you want there to be compared to the volume]. This is how we end up with bottles labeled “10X,” “5X,” etc. The high-concentration version is the “stock solution” because it’s the one you keep in stock. Its concentration is the “stock concentration” and the final concentration you want is the “working concentration” because it’s the one you want to work with when you’re doing your actual experiment (not just the prep part).     10X means the stock is 10 times more concentrated than you want to use it at. So to figure out how much of it to add, you need to reverse the X which means you need to divide. So if you want to make 1L (1000mL) you divide that 1000mL by 10 - you need 100mL of the stock solution. Just dilute that to 1L with water and voila - 1X!    What if you thought a stock was 10X and it was really 50X? You can fix it if you find out soon enough (but you can’t go the other way as I’ve “learned” the hard way a time or too…). So, if we added 100mL of 50X, how much total volume do we really need to have a 1X working concentration?    Why don’t we bring our old friend C₁V₁=C₂V₂ in to help. This equation says that the initial concentration times the initial volume equals the final concentration times the final volume. We can plug in relative concentrations like “10X” and “50X” just as we would absolute concentrations (Molarity-type things). So,    (50X)(100mL) = (1X)(V₂)    now we solve for V₂…    V2 = (50X)(100mL)/1X  V2 = 5000mL = 5L    So now you can just adjust the volume to 4L and hopefully save your experiment!    Most of the time, you know the V₂ you want (desired final volume) and you can use C₁V₁=C₂V₂ for each individual ingredient in your cocktail, using the same V₂ and solving for each ingredient’s V₁ (the volume you have to add). Then you can add up all those V₁s and subtract that sum from V₂ to figure out how much water you’ll need to add if you’re doing experiments on the tiny scale where you’re not working in graduated cylinders & filling to the line.     In addition to these instances where you always want the same working solution, this type of stock solution is great for making a stock “background” for when you have multiple solutions you might want to make that have things in common but also differences. Like a wash buffer and an elution buffer for affinity chromatography (a protein purification technique) - that have the same salt & buffer concentrations but the elution buffer also has an addition competitor molecule to push your protein off the column it’s stuck to (get it to elute). You can start from the same high-X stock to get those “generic” parts and then add the unique parts. Since it’s at a higher x there’s still extra room to add extra stuff whereas if you started at 1x there wouldn’t be enough! http://bit.ly/2Fs8taY    These stock solutions save space and time (you don’t have to make it as frequently) and they often also have another benefit - keeping higher concentration versions of solutions is good because some solutions tend to be more stable at higher concentrations, kinda like extending the expiration date on that bottle. It’s harder for bacteria to grow in them because instead of a solution mimicking body-like conditions, which are what most of the solutions I make do, you have an overload. And since stuff isn’t growing in them they can last longer.   Finished in comments