Plaidboy@sh.itjust.works
on 04 Jul 12:08
nextcollapse
If the buffer says 7.5 and it makes up 6 when mixed, either far too little of the buffer was used/there was already something else buffering the solution or the buffer was prepared incorrectly.
By selecting the correct buffer and the correct ratio of salt to acid (such as acetic acid and sodium acetate) an exact pH within a certain range can be reached.
I know this is a meme, but in case someone is interested, we usually do not want to use strong acids/bases to maintain buffers, instead, weaker acids (for eg, acetic acid), weaker bases (nh4oh), or their correponding weak-strong salt pair, or if it is really close to 7, then weak-weak, weakly dissociating salts (likr (nh4)2 co3).
I am probably forgetting the proper names for this, but idea is that waker acids/bases do not dissociate completely. for example, iirc, nh4oh is something close to 9-10, so if you want a basic buffer, then you use nh4oh in bulk to get close to required absolute amount of oh- ion concentration (maybe, because you want some reaction to happen in proper stoichiometric ratios), and for fine tuning, use very low concentration of stronger acid/base (depending on the fact that your target is above or below the value of bulk). stronger acids/bases almost immediately completely ionise. for example, i want to make something like 8.5, then i start with nh40h with 9, and slowly add hcl to reduce ph. with this, you make nh40h + hcl -> nh4cl + h2o. this nh4cl, is now acting as weakely dissociating. this reaction is also reversible. around the equillibrium ph, if you add more h+, then reaction goes forward. but simply by adding more water (or diluting), you can reverse this. with salt like nacl(nacl + h2o -> na+ + cl-) , they will practically never recombine, and you can not use this to your advantage.
the actual salt/acid/base to be used will also depend on solubility of present ions, miscibility, organic or non organic (in this context, organic means carbon related molecules)
stiephelando@discuss.tchncs.de
on 05 Jul 19:23
collapse
threaded - newest
Spent my first 3 years in academia doing exactly this. This is no 7.5 buffer. You’ve got suspension with zcp at around 7 probably.
Gotta use a finer pipette or burette!
(Though ruining a titration also feels bad…)
If the buffer says 7.5 and it makes up 6 when mixed, either far too little of the buffer was used/there was already something else buffering the solution or the buffer was prepared incorrectly.
By selecting the correct buffer and the correct ratio of salt to acid (such as acetic acid and sodium acetate) an exact pH within a certain range can be reached.
The Henderson Hasselback equation is always your friend
I know this is a meme, but in case someone is interested, we usually do not want to use strong acids/bases to maintain buffers, instead, weaker acids (for eg, acetic acid), weaker bases (nh4oh), or their correponding weak-strong salt pair, or if it is really close to 7, then weak-weak, weakly dissociating salts (likr (nh4)2 co3).
I am probably forgetting the proper names for this, but idea is that waker acids/bases do not dissociate completely. for example, iirc, nh4oh is something close to 9-10, so if you want a basic buffer, then you use nh4oh in bulk to get close to required absolute amount of oh- ion concentration (maybe, because you want some reaction to happen in proper stoichiometric ratios), and for fine tuning, use very low concentration of stronger acid/base (depending on the fact that your target is above or below the value of bulk). stronger acids/bases almost immediately completely ionise. for example, i want to make something like 8.5, then i start with nh40h with 9, and slowly add hcl to reduce ph. with this, you make nh40h + hcl -> nh4cl + h2o. this nh4cl, is now acting as weakely dissociating. this reaction is also reversible. around the equillibrium ph, if you add more h+, then reaction goes forward. but simply by adding more water (or diluting), you can reverse this. with salt like nacl(nacl + h2o -> na+ + cl-) , they will practically never recombine, and you can not use this to your advantage.
the actual salt/acid/base to be used will also depend on solubility of present ions, miscibility, organic or non organic (in this context, organic means carbon related molecules)
This happens a lot with hydroponics, too