Acid-Base Indicators

[randomuser]

Can someone please explain how to select a suitable acid-base indicator  in an acid-base titration from using the Data Book? I really don't understand.
Also what would you do if there isn't a a suitable indicator which falls in between the given pH ranges of the ones present in the Data Book?

Much appreciated!

[brightsky]

you want to choose an indicator that changes colour close to the equivalence point. remember that that you want the end point to be as close to the equivalence point as possible. the change in colour tells you when to stop dispensing solution from the burette. if it doesn't closely match the equivalence point, then you'll either undershoot or overshoot significantly, rendering your results useless. for a reaction between strong base and strong acid, the equivalence point is at pH = 7. but remember the pH curve nosedives, so an indicator that changes colour anywhere between pH 4 - 10 is good. you can choose either methyl orange or phenolphthalein.

[Limista]

Also what would you do if there isn't a a suitable indicator which falls in between the given pH ranges of the ones present in the Data Book?

Much appreciated!

Not going to happen in VCE Chemistry. The only 'dodgy' situation that comes to mind is if a weak base is titrated with a weak acid, meaning that a back-titration would have to follow through so that pH change is not gradual and can be detected.

[randomuser]

Okay, so how would I determine where the equivalence point is by using a pH curve, or just generally if i get a question, how would I know where the equivalence point occurs?

[Limista]

Point where the gradient of the curve is steepest on the pH curve determines the endpoint, knowing that we can not accurately judge the equivalence point. The endpoint comes just after the equivalence point.

As for generally, you'd get the equivalence point through stoichiometry.

[brightsky]

all you really need to know is this:
- strong base + strong acid: equivalence point at pH = 7
- strong base + weak acid: equivalence point at pH > 7
- weak base + strong acid: equivalence point at pH < 7
- weak base + weak acid: who knows?

i'll explain what happens when you mix strong acid with strong base. the explanation for the others is pretty much the same.

e.g. NaOH(aq) + HCl(aq) --> NaCl(aq) + H2O(l)

when equivalence point si reached, in the vessel you would have: Na+(aq) ions, Cl-(aq) ions, a bit of H3O+(aq), a bit of OH-(aq), and heaps of H2O(l) acting as the solvent.

now Na+(aq) has no acid-base properties so its presence does not effect the pH of the solution. Cl-(aq) can function as a base, but since it is the conjugate base of a strong acid, it is a pathetic base, and so we can safely ignore it. so the only species that will affect the pH are H3O+(aq) and OH-(aq) from the self-ionisation of water. we know that at 25 degrees celsius, Kw = 1.00 * 10^(-14) M^2. so [H3O+] = [OH-] = 1.00 * 10^(-7) M. so we have pH = -log_10[H3O+] = -log_10(10^(-7) = 7.

now remember how we discounted Cl-(aq) because it's the conjugate base of a strong acid? well, when you mix a weak acid and a strong base, the corresponding anion would be the conjugate base of a weak acid, making it a weak base (not a base of negligible strength anymore). so its presence would affect the pH of the solution. in fact, its presence will increase the pH. so at equivalence point, pH > 7.

for weak acid + weak base, it's a bit more complicated, because you aside from the hydronium ions and hydroxide ions produced by the self-ionisation water, you have a weak base AND a weak acid.

[lzxnl]

Do be careful when you say a metal cation has no acid-base properties. Does Fe 3+ (aq) have acid properties? Let's see.
Fe 3+ (aq) is a misnomer. Really, we have Fe(H2O)6 3+ where the iron accepts electron pairs from six water molecules and forms a covalent bond where the oxygen donates both electrons (a coordinate covalent bond).

Now what does this thing do? The iron has a 3+ charge on it. This large charge can drag some electron density from the oxygen, which doesn't like having less electron density, so it in turn pulls on the hydrogen electron density just a bit more. The net result is that the hydrogens are plucked off more readily and so in fact, aqueous "Fe 3+" ions are actually acidic. This holds for many multivalent metal ions like Al 3+ and many of the transition metals with +2 or +3 charges too.

You don't need to worry about this for VCE though; just take note that sometimes ions may have strange acid-base properties.