R-ATCHE-18

Graph

Context

$2HCl + Na_{2}CO_{3} \to H_{2}O+CO_{2}+2NaCl$

$HCl$ is a strong acid
$Na_{2}CO_{3}$ is a weak base.

Since $CO_{2}$ is a product, and we know $CO_{2} + H_{2}O \rightleftharpoons H_{2}CO_{3}$, it follows when equivalence occurs (initial acid has fully react with added base and vice versa), the resulting solution will be acidic.

Hence, we want to use an indicator that will change colour preferably around acidic solutions.

The main characters

Phenolphthalein - for titrations with a basic equivalence point
Methyl red/orange - for titrations with an acidic equivalence point.

When stoichiometrically equivalent amounts of acid and base have been added, this is called the equivalence point, and this is where we want to stop the titration! Because this forms the basis of our calculations, i.e. where we've stopped is where we've assumed stoichiometrically equivalent amounts of acid/base have been added to neutralise.

This yields pH curves.

The endpoint, in comparison, is when a colour change occurs. In general, the equivalence point will never be the same as the endpoint! Our calculations will always be incorrect!

The endpoint is when we stop titration - we stop at the first permanent colour change of indicator!

Hence, we want to choose an indicator that will produce an endpoint closest to the equivalence point, so using an acidic(don't take that literally, i mean turns in acid conditions) indicator for a titration with an acidic equivalence point is better than using a basic indicator.

Reconsider this system: $2HCl + Na_{2}CO_{3} \to H_{2}O+CO_{2}+2NaCl$

Since it is acidic, phenolphthalein is a horrible indicator!!!! It changes colour when its basic, and a basic pH is no where near the actual pH when equivalence is reached! >:(

Instead, we use methyl orange (as it changes around pH 4, which is slightly(by slightly i mean 1000 times more acidic than water) acidic).

Indicator choice

Correct indicator choice means equivalence and end point are close (1 drop, due to how steep the curve is)
Strong acid + strong base, either acidic or basic indicator works (cause once again, pH curve so god damn steeeeeeep)
Strong acid + weak base, eg. $HCl/Na_{2}CO_{3}$, resultant is a weak acid, hence an acidic indicator good, basic indicator bad!
Strong base, weak acid, e.g. $NaOH/CH_{3}COOH$, resultant is a weak base, hence a basic indicator good, acidic indicator bad!
Weak acid, weak base: THIS IS BAD! BUFFER FORMS, SO PH CURVE VERY STEEP, NO POINT TITRATING THIS!
- This is why we sometimes need to do multiple titrations for some things, e.g. say we have $Na_{2}CO_{3}$ as our primary standard, and we want to find the concentration of $CH_{3}COOH$. Then, we would have to do something similar to the following:
  - Standardise $HCl$ using primary standard (weak base to strong acid)
  - Standardise $NaOH$ using $HCl$ (strong acid to strong base)
  - Use the standardised $NaOH$ to titrate $CH_{3}COOH$(finally, we use our resultant i)
Remember from the "main characters"(ah yes naming convention 101) that the best basic indicator is phenolphthalein, the best acidic indicator is methyl orange!

TLDR

You can use 4 sig figs for additional accuracy! e.g. 26.65 vs 26.6

Tricksy

If they ask like "what if x was done incorrectly", it usually means the volume measured is changed due to number of moles of something changes. use that, alongside the $c=\frac{n}{V}$, to determine difference between calculated and real value.

They can use choice of indicator for this as well.