Practical guide to phosphate buffers

Confused about which phosphate’s which? Count the counterions! There will be one per “base” because each of those cations is basically taking a proton’s place!
Dihydrogen phosphate (H₂PO₄⁻) is monobasic
and
Monohydrogen phosphate (HPO₄²⁻) is dibasic
You can remember this by reasoning through it…
Starting by recognizing that a phosphate ion in its fully deprotonated form (PO₄³⁻) has 3 oxygen atoms “up for grabs” by protons (we don’t worry about that 4th one that’s drawn double bonded because it’s super duper duper unlikely to protonate).
If something takes a proton we say it acts as a base. So this is tribasic.
If one of the O’s takes a proton, you get monohydrogen phosphate, which we can call monoprotic. Now you just have 2 open O’s up for grabbing, so it’s dibasic.
If one of those takes a proton, we get something that’s triprotic. And this is basically its fully acidic form, so we just call it phosphoric acid. Sometimes it’s called orthophosphoric acid, with “ortho” meaning “true” - and, similarly, a phosphate ion, .PO₄³⁻, is sometimes called orthophosphate.
So we have the 4 possible forms, but we really only have to worry about the “middle two’ (dihydrogen phosphate (H₂PO₄⁻) and monohydrogen phosphate (HPO₄²⁻)) in our bodies and biochemical buffers because the extremes are too extreme to be found at physiological (bodily) pH. How do I know? By looking at the pKas (2.14 for phosphoric acid, 6.86 for dihydrogen phosphate, and 12.4 for monohydrogen phosphate). Much more on pKa here: bit.ly/phacidbase
but the gist it’s the pH at which half of an acid will be protonated, half deprotonated. Each pH unit above the pKa you are (where there are fewer protons around), you get 10-fold less protonated, and each unit below (where there are more protons available to take) gives you 10-fold more protonated. We’re typically way above phosphoric acid’s pKa, so it will almost all be deprotonated. And we’re way below orthophosphate ion’s, so it will almost all be protonated. This leaves dihydrogen phosphate and monohydrogen phosphate, which are both present at appreciable levels at physiological pH. So much so that you have enough of each to sop up added acid or base, keeping pH stable - good buffer for our cells and cell-mimicking mixtures in the lab.
When we deal with these in the lab, we’re typically dealing with them in salt form, where these negatively-charged ions (anions) are neutralized by hanging with a positively-charged ion (cation). In the lab, these are most commonly sodium or potassium. Each of those has a +1 charge (i.e. Na⁺ or K⁺), and thus can serve as a sort of placeholder for a proton. So the number of these you have is going to be equal to the number of basic sites - so you get:
* monobasic salts: KH₂PO₄ & NaH₂PO₄
* dibasic salts: K₂HPO₄ & Na₂HPO₄
But beware that the one-to-one thing only applies if you have monovalent (singly-charged) cations. If you have a cation with a higher charge, this won’t hold true because you’ll need fewer to neutralize it (eg 1 Mg²⁺ with dibasic to give you MgPO₄²⁻).
Note: A word of caution about phosphate is that it can form salts when we don’t want it to - it can chelate some metals in solutions.
You can use the Henderson-Hasselbalch equation (pH = pKa + log[A⁻]/[HA]) to figure out how much of each to mix to get a buffer of the desired pH. And you can use the same formula to figure out what proportion of each form you’ll have in a solution of a given pH. Here’s much more on it: bit.ly/phbuffers
But, I usually go from 1M solutions of each, and mix those. You can find tables like this: cshprotocols.c...
Beware the dibasic stock solution likes to crash (precipitate) - if that happens, don’t use it (yet). Instead, heat* and bath sonication can help you salvage it. It’s good to have 2 bottles of it just in case, so you can use one while redissolving the other. *Just avoid evaporating out water, increasing the concentration.
More: blog: bit.ly/phosphateintuition ;KZbin: • Phosphate forms (names...
         
more about all sorts of things: #365DaysOfScience All (with topics listed) 👉 bit.ly/2OllAB0 or search blog: thebumblingbioc...

[@MarianaAguilarMorales]

Nice video! Do you happen to have a video related to the Hofmeister series and its effect on protein stability? Thanks and keep up the great work :)

[@thebumblingbiochemist]

Not really, sorry - but this touches on it bit.ly/ionicstrengthsalting

[@shepardsinsequence]

Do you add any sodium azide to your PBS?

[@thebumblingbiochemist]

I do not

[@derpderp2590]

How do you pipette from the 1st stop to the 2nd stop for very small volumes into PCR tubes with nothing inside it? With the tip touching the walls sometimes it may still form bubbles if it's still near the liquid at the base, or that the liquid somehow gets stuck inside the tips and trying to get that liquid out by whatever methods made it worse....

[@thebumblingbiochemist]

Hope these can help: blog: bit.ly/pipettingsmall ; KZbin: kzbin.info/www/bejne/in-VZXedaNx2hLM Good luck!

[@derpderp2590]

@@thebumblingbiochemist i think i may have found a solution (havent tested it yet). Pushing from the 1st stop to the 2nd stop while pulling out/dragging the tip upwards on the sides of the tubes may work. Cos another issue i am facing is when i pull out after fully dispensing to the 2nd stop, through capillary action, some liquid gets automatically sucked back in (even though i did not release the plunger at all). I do watch that video before posting these questions. I did try the "if there is still liquid in the tip, go back in, pull up until liquid reaches the trapped liquid and use it to drag that liquid back down" but it made it worse, as the residual liquid simply went above the new liquid and gets stuck even further lolz