[dave]

Hi all,
This is my first message here, sorry if what I am going to ask is not very interesting but I am a biologist and my chemistry knowledge is too low to answer this question that I have in mind (here I go  ):
- My project consist in studying the effect of lactate on cells. I measured the concentration of lactate that these cells can produce  over time (rather high after 48 hours) and also noticed that this correlated with a significant drop of the pH media. For my experiments, I basically treat cells with the same concentration of sodium lactate (in fresh media). My question is why does sodium lactate not lower pH of my media whereas lactic acid that cells produce really significantly lowers it ? Is that because of the sodium ?

Thanks for the help 
Cheers,
David

[Fry]

Sodium lactate is a salt, its what you get when you neutralize lactic acid with something like NaOH.

Lactic acid lowers the pH because it generates H+ ions in the solution, making it acidic. Sodium lactate cant lower the pH because it has no acidic protons, if anything it would increase the pH of a strongly acidic solution.

[Borek]

Sodium lactate cant lower the pH because it has no acidic protons, if anything it would increase the pH of a strongly acidic solution.

Remember it is a salt of weak acid - so it behaves as a weak base. It will increase pH of neutral solution as well.

[dave]

Thanks ! So if I understand correctly, it's only because cells produce lactic acid i.e. lactate/ H+ which supply protons whereas sodium lactate i.e. lactate/Na+ does not?
By the way (last one, i swear), why is lactate a weak base?
Thanks !

[Nerro]

Because it is the conjugate base of a weak acid the pKa of lactic acid is above -1.76 which technically makes it a weak acid, which implies that the conjugate base is a weak base. (strong acids also makes incredibily weak conjugate bases which are so weak that they aren't really considered bases anymore.)

If you want to study the effect of lactic acid on a microorganism you might be better off making a buffer of lactic acid and sodium lactate. Look up "Henderson-Hasselbach equation" and "buffer solution"if you're interested.

[Borek]

the pKa of lactic acid is above -1.76 which technically makes it a weak acid

Lactic acid pKa is around 3.9.

There is something wrong with what you posted. pKa = -1.76 is a pretty strong acid.

[Nerro]

It's below -1.74 that you start calling acid "strong acids" in the sense that they completely dissociate in water. I never said lactic acid has a pKa of <-1.74, I said it has a pKa that is above that! It's more acidic than acetic acid (pKa = 4.75) and it's not a strong acid so it must have a pKa between 0 and 4.75. 3.90 sounds about right.

Anyway it doesn't matter here. Lactic acid is a weak acid.