[Vannasouk]

My group and I have been doing titrations with milk over four weeks to find how the pH of the milk changes with time.
All the titrations are done, but we're having difficulty with the calculations.
We want to find the pH.

A titration was performed with 20ml full cream milk.
5 drops phenothalien was added to the milk.
A burette was filled with 0.1mole NaOH (sodium hydroxide).
It took 5.8ml of the 0.1mole NaOH to turn the milk pink (the end point).

The milk was still fresh, being unopened until the titration began.

Does anyone know how to figure out what the pH of this milk is, assuming that the acid is Lactic acid?

C6H12O6 - formula for lactic acid. Lactic acid weight is 90 (it's 90 right?)

--------------------------------------------------------------------
Reading the forum rules it says I should try doing the working myself. I assure you I have.

I would post what I got but it is so totally wrong. Well... It's confusing you know? I really have no idea, that's why I'm asking.

Can someone put me on the right track?

[enahs]

Assuming it is lactic acid, write a balanced equation of lactic acid with sodium hydroxide.

However many mols of NaOH where required to to neutralize the acid = number of moles of acid (after accounting for stoichiometry, which you will get from your balanced equation).


Though, I am not sure how accurate your results will be, using a color changing indicator in a non opaque solutions.

[Borek]

This is insane, you don't check pH with titration. And that's not for the first time when such question surfaces.

[Vannasouk]

This is insane, you don't check pH with titration. And that's not for the first time when such question surfaces.

Huh? Really?! So there is absolutely no way to figure it out using the information gained from a titration. Not even with some kind of tricky formulae? I thought that was the whole point of a titration. Or is that just a misconception? (not said in a hostile way, honestly don’t know)

Anyway, here's a balanced equation for the reaction between NaOH and Lactic acid

C6H5COOH + NaOH = C6H5COONa + H2O
There's the equation, and it's balanced.

So the number of moles of the NaOH used, should be equal to the number of moles contained in the acid.

Moles = Molarity * Liters no?
The 5.8mL of NaOH has to converted into liters
5.8/1000 = 0.0058L

Moles of NaOH = 0.1M (the molarity of the NaOH used) * 0.0058L

Number of moles in NaOH used = 0.00058
               = 5.8 *10 ^-4
Therefore number of moles within Lactic acid in milk = 5.8 * 10 ^-4

so… now what?

Hoe do I use this information to find the pH?

Sorry, if it is really can’t be done, I won’t bother you guys again.

[Borek]

This is insane, you don't check pH with titration. And that's not for the first time when such question surfaces.

Huh? Really?! So there is absolutely no way to figure it out using the information gained from a titration. Not even with some kind of tricky formulae? I thought that was the whole point of a titration. Or is that just a misconception? (not said in a hostile way, honestly don’t know)

Titration is used to determine amount of acid. pH can be calculated form the known amount of acid, assuming you know what acids are present, what their dissociation constant are. So, in theory it looks like may be able to find pH - but in practice errors are so huge, that this approach is of no use.

Anyway, here's a balanced equation for the reaction between NaOH and Lactic acid

C6H5COOH + NaOH = C6H5COONa + H2O
There's the equation, and it's balanced.

It is balanced, but it is not a lactic acid.

http://en.wikipedia.org/wiki/Lactic_acid

Looks to me more like benzoic.

So the number of moles of the NaOH used, should be equal to the number of moles contained in the acid.

Moles = Molarity * Liters no?
The 5.8mL of NaOH has to converted into liters
5.8/1000 = 0.0058L

Moles of NaOH = 0.1M (the molarity of the NaOH used) * 0.0058L

Number of moles in NaOH used = 0.00058
               = 5.8 *10 ^-4
Therefore number of moles within Lactic acid in milk = 5.8 * 10 ^-4

so… now what?

You know number of moles, you know volume - can you calculate concentration?

Hoe do I use this information to find the pH?

Do you know how to calculate concentration of H+ in weak acid solution using ICE table?

Sorry, if it is really can’t be done, I won’t bother you guys again.

It is not your fault. Even if it was your idea to check pH by titration, you should be advised to choose other method. Titration is not for pH measurement, it is for determination of amount of substance. These two things (pH and acid concentration) are related, but there are many factors that don't allow to use them interchangeably.