Double Titration

Wolgwang

May 28, 2021 05:11

Why does phenolphthalein not show color change?

Why it shows color change only at the equivalence point?

Ashish Ahuja

Every indicator has a certain range in which it shows change in color; commonly called "color transition range". You can look up the pH values for this range on wikipedia for whichever indicator you're interested in.

That's the reason you don't blindly pick indicators; you theoretically calculate at what pH you want to see a color change and then pick a suitable indicator.

@Wolgwang if you are doing this for double titration, you would split it up into two reactions; the overall reaction is not useful, since Phph shows color change after the neutralization of NaOH and conversion of Na2CO3 to NaHCO3, and methyl orange after complete neutralization of everything.

Wolgwang

May 28, 2021 05:29

@AshishAhuja CTR of Hph is (8.3-10) so why it won't show a color change when ph of the base is (8.3-10)?

Ashish Ahuja

Yes it will. The equivalence point does not lie exactly in CTR as you notice, but it doesn't introduce too much of an error because the titration curve is very sharp (nearly vertical) around the equivalence point, so the error in measurement will only be a drop or two. Phph is an easy to use and cheap indicator, and it does the job here. But for titrations where we don't get a sharp curve, like a weak acid + weak base, we will need more accurate indicators.

Nowadays if you had to perform a titration for some real-world application I believe you'd just use a pH meter.

Wolgwang

May 28, 2021 05:45

Why different reaction takes place in the different indicator?
For example when $\ce{NaOH,Na2CO3}$ are analyte and $\ce{HCl}$ is titrant.
When Hph is used, the reactions that takes place are $$\ce{NaOH + HCl->NaCl +H2O\\ Na_2CO_3 + HCl-> NaHCO3 + NaCl}$$
and when MeOH is used, the reactions that takes place are $$\ce{NaOH + HCl->NaCl +H2O\\ Na_2CO_3 + 2HCl-> 2NaCl + H_2O + CO_2}$$

Why indicators affect the reactions?

Ashish Ahuja

@Wolgwang sorry, I just noticed that the plot you have sent is a weak base + strong acid; the equivalence point comes around pH = 3 so Phph is not a suitable indicator at all.

@Wolgwang do you understand how titrations work (simple titration)?

Wolgwang

Yes

Ashish Ahuja

Ok, so in the case NaOH Na2CO3 and NaHCO3 is present in the beaker (or flask or whatever) as the solution whose concn we have to determine

(concn of each of the components)

HCl is in the burette

Wolgwang

Yes

Ashish Ahuja

First we add a few drops of Phph to the beaker

then we add hcl drop by drop till we see a color change

the rxns which occur are

first NaOH largely gets neutralized since it is the strongest base; NaOH + HCl -> NaCl + H2O

Wolgwang

May 28, 2021 05:52

@AshishAhuja When it is added, it won't show pink color? Because ph of contents in the beaker > 10?

Ashish Ahuja

yes

Wolgwang

@AshishAhuja Yes

Ashish Ahuja

as it becomes acidic it turns colorless, that is the "color change"

note that here we are adding hcl drop by drop

so the most basic reacts first.

Wolgwang

But Na2CO3 is basic.

Ashish Ahuja

yes, Na2CO3 then starts reacting after NaOH has been neutralized.

Wolgwang

May 28, 2021 05:56

Ok

Ashish Ahuja

Then, when all the Na2CO3 has been converted to NaHCO3; Na2CO3 + HCl -> NaHCO3 + NaCl

now the solution is very weakly basic, and PhPh changes color

Wolgwang

Now Hph changes color to colorless?

Ashish Ahuja

yes

Now we know the total amount of HCl required to neutralize NaOH, Na2CO3

Wolgwang

Yes

Ashish Ahuja

Next, we add methyl orange

Wolgwang

May 28, 2021 05:58

wait

> Hph changes color when NaOH is completely neutralised and Na2CO3 is 50% neutralised.

Ashish Ahuja

yes, that's one way of writing the same thing, although it is somewhat confusing. Na2CO3 fully neutralized converts to NaCl, halfway through it is NaHCO3

Wolgwang

Oh Ok

Ashish Ahuja

2 mins ago, by Ashish Ahuja

Next, we add methyl orange

Wolgwang

@AshishAhuja In the same apparatus?

Ashish Ahuja

yes, in the beaker

Wolgwang

May 28, 2021 06:02

After adding Hph, MeOH is added in the same beaker right?

Ashish Ahuja

Note that the NaHCO3 present in the beaker now is partially from the initial solution, partially from the reacted Na2CO3

@Wolgwang yes, after the Phph color change; at the Phph color change we get one data point - amount of HCl required to neutralize NaOH and partially neutralize Na2CO3

Wolgwang

Ok

Ashish Ahuja

Now we keep adding HCl drop by drop till methyl orange changes color

NaHCO3 + HCl -> NaCl + H2CO3

when MeOH changes color, we know the amount of HCl required to neutralize the NaHCO3 present in the beaker (from the initial soln and reacted Na2CO3)

this is the second data point

Wolgwang

So now there is no NaHCO3 in the beaker?

Ashish Ahuja

@Wolgwang yes, everything has been neutralized to NaCl.

From these two data points and whatever data is given in the question you can calculate the composition of the initial soln.

Wolgwang

May 28, 2021 06:08

@AshishAhuja Will it change something if no NaHCO3 was present in the beaker in the very initial solution?

Ashish Ahuja

uhh the calculation will change, because in the second neutralization (with MeOH) now you only need to consider the NaHCO3 which came from Na2CO3

Wolgwang

@AshishAhuja Why n factor of Na2CO3 is 1 when Hph was used?

Ashish Ahuja

@Wolgwang what is the reaction which occurs when only phph is used?

Wolgwang

16 mins ago, by Ashish Ahuja

Then, when all the Na2CO3 has been converted to NaHCO3; Na2CO3 + HCl -> NaHCO3 + NaCl

Ashish Ahuja

For an acid-base reaction the n-factor will be number of H+ accepted/donated (vaguely, I do not remember what it is exactly defined as). Then you can clearly see that the n-factor of Na2CO3 is 1

Wolgwang

May 28, 2021 06:15

And what is n factor of NaHCO3?

Ashish Ahuja

Although I would recommend just sticking with moles for such stuff

@Wolgwang what do you think it should be?

Safdar Faisal

@wolgwang, you might wanna stop using n-factor.. it will become a very complicated thing later on..

Wolgwang

@AshishAhuja 1?

Ashish Ahuja

yes

Wolgwang

@AshishAhuja Does this make any sense to you?

Ashish Ahuja

May 28, 2021 06:19

no, this just seems like random nonsense.

Wolgwang

@AshishAhuja Thanks a lot :-)

@SafdarFaisal I want to do that but my chem teacher had given 177 questions from Neeraj Kumar's book as homework and most of the question uses n-factor :-(

Ashish Ahuja

@Wolgwang if you want to use n-factor (even though you could convert any questions which used n-factors to moles), just keep in mind what is actually happening. The two steps in the titration; I don't see why you would ever need the n-factor of Na2CO3 with MeOH as in the image above.

Safdar Faisal

@Wolgwang during the exam, they aren;t gonna ask you what is the n-factor of X.. moles is way more reliable for slving question and it's simpler..

Ashish Ahuja

@SafdarFaisal yeah but plenty of books have questions which ask about n-factors and such, which is causing the issue I guess.

JEE Chemistry Club

Participants