# Regarding Kw, titrations, and pH

So I have need of performing a titration upon a fruit juice to determine acid concentration. I will be using 1M NaOH as my base. Seeing as the solution will be coloured, it would be difficult to use an indicator such as phenolpthalein. Thus, I would like to use a pH probe.

My Question:

While titrating, should I be aiming to reach a pH of 7 or should I be aiming to reach the ionic product constant of water (Kw) at the temperature that I am performing the titration at? (e.g.6.63 at 50 degreees celsius)

When using a pH probe, instead of aiming to reach the equivalence point of the titration, you should be titrating beyond it to create a complete titration curve. With an indicator, the end point is clearly indicated by a color change; with a pH probe, however, the end point is indicated by a steep change in pH in a plot of the data, thus requiring the collection of data beyond the end point.

While in theory the pH at the equivalence point of the titration can be calculated (and, for the limiting case of a strong acid and strong base, is exactly the $K_w$ that corresponds to the temperature of your solution), calculating the pH at the equivalence point and aiming to reach that pH is (i) bad lab practice, since if you're able to calculate the pH at the equivalence point, you're presumably trying to experimentally verify theoretical calculations, and (ii) not applicable to your current experiment, because the organic acids present in fruit juice are weak acids, whose conjugate bases are weak bases that will raise the pH at the equivalence point (and render the pH impossible to calculate for your purposes).

You shouldn't use 1M NaOH. It is way too strong and solution will turn basic with just a couple of drops. You should aim to add around 10-20 mL of base to fully titrate your sample. 10 mM solution is probably a good place to start. And you don't want to waste a barrel of juice on this experiment.

You should titrate to roughly pH = 10, but your main goal is to see full titration curve. Juice has all sorts of acids - strong onces, weak once. So don't expect your curve to look like a textbook example. It will have multiple phases.