[Ineedhelp1993]

Pick an indicator for use in the titration of each acid with a strong base
a) HF
b) HCl
C) HCN

HF, HCN are weak acids and HCl is a strong acid. I figured out the indicators for A and b but for c I don't know how to figure it out.

How am I suppose to know what indicator to use?

From the K_a? then convert it to pK_a?
the pK_a of HCN is 9 but the book says that Alizarin yellow R is the indicator for HCN. How do I figure out what indicator to use?
Thank you in advance.

[Ineedhelp1993]

[Borek]

http://www.titrations.info/titration-end-point-detection

http://www.titrations.info/acid-base-titration-end-point-detection

[Big-Daddy]

I'll summarize some key points. (I still remember one of the first things I did on this forum was have this discussion with Borek!  )

The indicator consists of two forms, the protonated (HA) and deprotonated (A^-). Each has its own colour. When [H⁺]=K_ind, then you will have an exactly even ratio of [HA]/[A^-]. So if you want an exactly even mix of colours to show your end point, then find an indicator so that pH at equivalence point = pK_ind (or as close as possible to pK_ind). Reason suggests that this will only given you usable results if the colour change is very sharp and sudden.

More realistically, you want a range over which the colour changes. From the rearrangement [A^-]/[HA]=K_ind/[H⁺], it's obvious that HA dominates in very acidic conditions and A^- in very alkaline ones. The moment when [A^-]/[HA]=1 is the end-point itself, but we want a range, so let's define it like this. If the lowest pH at which we can notice the colour change is defined as the moment when [A^-]/[HA]=0.1 (so A^- is 1/10th of the colour of HA), and the last moment when we can notice the colour changing is the moment when [A^-]/[HA]=10 (after this [A^-] dominates by too much for us to notice HA), we can define the first one as pH_min and pH_max of our range, so that pH_min=pK_Ind-1 and pH_max=pK_Ind+1. What if you want some factor other than 10? pH_min=pK_Ind-log₁₀(n) and pH_max=pK_Ind+log₁₀(n).

Well, this was good for my revision! Given that you can select any indicator (with any pK_Ind you want), look for one that captures the equivalence point within the range highlightened by pH_min and pH_max. I'm going to assume calculating the pH at the equivalence point is a problem you can handle, and once you know that you just consider which indicator has a range it most neatly fits into.

[Borek]

This is slightly backwards. Better approach is to calculate pH at 99.9% and 100.1% titration and look for an indicator which completely changes color between these two points (so using your symbols pH_99.9% < pH_min and pH_max < pH_100.1%).

[Big-Daddy]

This is slightly backwards. Better approach is to calculate pH at 99.9% and 100.1% titration and look for an indicator which completely changes color between these two points (so using your symbols pH_99.9% < pH_min and pH_max < pH_100.1%).

How does one calculate pH at 99.9% or 100.1% titration, i.e. what do the percentages mean? Number of moles of titrant divided by number of moles of analyte? (By this definition the equivalence point would be at 100% by definition)

And then when we have the two values we want pH_99.9%<pH_min<pH_max<pH_100.1% with an indicator that fits these pH inequalities as you said.

[Borek]

How does one calculate pH at 99.9% or 100.1% titration, i.e. what do the percentages mean? Number of moles of titrant divided by number of moles of analyte? (By this definition the equivalence point would be at 100% by definition)

You are forgetting the stoichiometry, but otherwise you are right.

[Big-Daddy]

How does one calculate pH at 99.9% or 100.1% titration, i.e. what do the percentages mean? Number of moles of titrant divided by number of moles of analyte? (By this definition the equivalence point would be at 100% by definition)

You are forgetting the stoichiometry, but otherwise you are right.

So what is the quantitative full definition then? Maybe moles of OH^- that the titrant could produce if all dissociations were strong, divided by moles of H⁺ the analyte could produce if all dissociations were strong? (I'm thinking of what might happen if the titrant and analyte are mixtures of acids/salts/bases rather than being pure in their own right, to try and find a proper definition of equivalence point)

[Borek]

100% is the equivalence point, period.

[Big-Daddy]

100% is the equivalence point, period.

100% of what? When I defined it as number of moles of titrant/number of moles of analyte, you said this doesn't factor stoichiometry - what did you mean by that? And it should be possible to generalize to a titrant and analyte each having mixture of many acids, salts and bases, so just a single acid/base reaction equation wouldn't explain it ...

[Borek]

Number of moles of titrant divided by number of moles of analyte?

When you titrate sulfuric acid with NaOH, you need to remember they react in 2:1 ratio, that's stoichiometry. Moles per moles would not give you 100% at equivalence point in this case.

100% is the equivalence point, period.

100% of what?

Of titration. This is definition. 100% titration is when you added enough titrant to reach the equivalence point.

[Big-Daddy]

Number of moles of titrant divided by number of moles of analyte?

When you titrate sulfuric acid with NaOH, you need to remember they react in 2:1 ratio, that's stoichiometry. Moles per moles would not give you 100% at equivalence point in this case.

100% is the equivalence point, period.

100% of what?

Of titration. This is definition. 100% titration is when you added enough titrant to reach the equivalence point.

You haven't explained how I calculate it. Let's say I have the starting volumes of analyte and titrant and the total concentrations of each acid/salt/base in each of them. How do I calculate the volume of the titrant solution I need to add to the analyte solution to reach the equivalence point?

The problem is you keep saying "100% of titration" without defining what this means. As I said a single acid/base reaction WON'T explain it so the case of sulphuric acid and NaOH does not give a generalized idea. What if the analyte or titrant are each MIXTURES of acids and/or salts and/or bases.

For example: we have V_analyte of analyte solution and V_titrant of titrant solution. In the analyte solution is Ca1 for H₃PO₄, Ca2 for H₂SO₄, Cs1 for NaHSO₄. In the titrant solution is Cb1 NaOH and Cs2 for NaHCO₃. What is the ratio V_titrant/V_analyte we have to mix to reach the equivalence point?

[Borek]

What is the definition of the equivalence point?

[Big-Daddy]

What is the definition of the equivalence point?

That is what I have been asking you. I was under the impression it is the point when the number of moles (of each component, summed) in the analyte solution = number of moles (of each component, summed) in the titrant solution. But now I am not certain this is true. e.g. if both the titrant and analyte consisted of just one base, would the equivalence point still be a defined value like this, or does it not exist?

I've heard before it is the "stoichiometric equivalent" but I can't get my head around this because I don't understand how it generalizes. For my example: we have V_analyte of analyte solution and V_titrant of titrant solution. In the analyte solution is Ca1 for H3PO4, Ca2 for H2SO4, Cs1 for NaHSO4. In the titrant solution is Cb1 NaOH and Cs2 for NaHCO3. What is the ratio V_titrant/V_analyte, in terms of the various concentrations, we have to mix to reach the equivalence point? If you explain how it is done for this case I would understand how it is generally done.

[Borek]

Equivalence for a mixture is when you add enough titrant for all reactions to complete. But then titration percentage doesn't make much sense, as it would be function of the solution composition, which you don't know beforehand.

Not that it matters much - knowing composition you should be able to find out which substance will he the last one titrated and it should help you find out parameters of the end point. But I refuse to discuss details, as you will get lost pretty soon and we will waste time. You are looking for the most generalized approaches without understanding how to deal with simple problems, which just makes you more and more confused.