Acid-base indicators

This is my topic for my A2 chemistry coursework and I'm a bit stuck as I haven't been taught this topic before.

I've been titrating various bases into various acids and seeing when the acids change colour (due to the presence of an indicator).
The volume of acid used was 60cm^3.
Here are my results (numbers represent cm^3 of base added in order for indicator to change colour):

Strong acid/strong base 61.5 (Phenolphthalein ) 68 (Methyl orange)
Strong acid/weak base 65 (Phenolphthalein) 36 (Methyl orange)
Weak acid/strong base 62.5 (Phenolphthalein) 10.7 (Methyl orange)
Weak acid/weak base 63.8 (Phenolphthalein) 4.4 (Methyl orange)

I've attached my pH curves so you can see the pH changes. I just want to know if there's anything dodgy with my results so I can do repeats tomorrow (as it's my last chance). The weak base is sodium carbonate by the way.

I think that for strong acid/strong base, something's wrong, shouldn't the methyl orange change at about 61.5 and the phenolphthalein at 68?? Ie: have I got them the wrong way round??

For weak acid/weak base, I thought that none of the indicators would work, my friend is doing the exact same thing as me and she said that there were absolutely no colour changes for her, but both my indicators did did change colour.

I'm just really confused. All i know is that the two indicators change colours at different ranges of pH, but can they only change colour if this pH range happens to be in the 'steep bit' of the graph??
If any of you could just say what you think seems right/wrong then it would really help me.

Strong acid-strong base.jpg32.2KB

strong acid-weak base.jpg31.3KB

weak acid - strong base.jpg31.5KB

weak acid-weak base.jpg30.5KB

Reply 1

charco

Study Forum Helper

the volume added has nothing to do with the pH at the equivalence point - you need to reread your notes

Reply 2

peachylife

Erm... the volume added changes the pH of the solution and so would affect whether the indicator changes colour or not, right?

Reply 3

charco

Study Forum Helper

The point here is that you are only interested in the pH at the equivalence point. Obviously adding an acid or a base to another solution changes its pH but this is not relevant to your question.

There is only one volume added where the number of moles acid equals the no of moles base. You have to choose the correct indicator at this point depending on the type of acid and base.

Weak acid + strong base = basic solution at the equivalence point - use phenolphthalein (or similar alkali range indicator)
Weak base + strong acid = acidc solution at the equivalence point - use methyl orange(or similar acidic range indicator)
Strong acid + strong base - use any indicator
Weak acid + weak base - cannot be titrated accurately

Reply 4

peachylife

charco

There is only one volume added where the number of moles acid equals the no of moles base.

And this would be 60cm^3 right?

I get all the stuff about which indicators are best for which combinations, but I don't know how you can explain which is best by using the results that I got. Ie: are the indicators meant to change colours at around the same volume (the volume when the equivalence point is reached)?

Reply 5

charco

Study Forum Helper

If the molarity of the acids and the base are the same (let's say 0.1 mol dm-3) then equivalence will be reached at exactly the same volume (if the acids are monoprotic) - in your experimental case = 60cm3

The point the expt is making is that at this equivalence point some of the neutralised compounds are outside the sphere of operation of the indicator.

For example: if you are adding strong base to weak acid using methyl orange indicator. The pH at the equivalence point is 9-10, BUT at the region of operation of methyl orange is 3-4. The colour of the methyl orange will change BEFORE the equivalence point i.e. when less base than required has been added. i.e. <60cm3 is recorded.

Example: If you are using phenolphthalein as the indicator of strong acid titrated with weak base, then the equivalence point of the acid/base mixture is about 3-4, BUT the phenolphthalein operational range is 9-10. So adding base to acid produces NO colour change at the equivalence point.
Instead you arrive at the indicator range AFTER 60 cm3 have already been added, i.e >60cm3 is needed before seeing a colour change.

Reply 6

charco

Study Forum Helper

Having re-read your OP I see that your weak base is sodium carbonate, which further complicates matters as it has TWO points of inflexion being dibasic.

Na2CO3 + HCl --> NaHCO3 + NaCl
NaHCO3 + HCl --> NaCl + CO2 + H2O

If you are adding acid to this weak base then you see the two points, the first of which can be indicated by phenolphthalein (as it occurs in the basic region of the pH scale) and the second can be indicated by methyl orange.