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Author Topic: INDICATORS QUICK QUESTION  (Read 7916 times)  Share 

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I.09

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INDICATORS QUICK QUESTION
« on: March 19, 2021, 05:20:43 pm »
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Hey Guys,

pH indicators used in titrations are weak acids and bases, but WHY can't they be strong?

Thanks,
Irene

fun_jirachi

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Re: INDICATORS QUICK QUESTION
« Reply #1 on: March 19, 2021, 06:12:28 pm »
+1
If you've looked at buffer solutions, it's a very similar situation to buffers.

If not, here are a few things to consider:
- What happens to a strong acid/base in solution?
- An indicator is usually just a weak acid/base with its conjugate in equilibrium, with the conjugates being of different colours. What would happen if the acid/base was strong instead?
- What happens if a strong acid/base was added to the indicator solution, which is a weak acid/base with its conjugate in equilibrium? Hint: Use LCP

I've asked you questions instead of giving you a straight answer because this gets you thinking about why this works (which will give you a better understanding than if you rote learned an answer a stranger gave you on the internet). This is actually a question that can be asked (a 6 marker in the old syllabus, or something to that effect) - you could be expected to write out Ka or Kb equations, etc.

Hope this helps :)
« Last Edit: March 19, 2021, 08:17:19 pm by fun_jirachi »
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I.09

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Re: INDICATORS QUICK QUESTION
« Reply #2 on: March 19, 2021, 07:38:08 pm »
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I have not learned about buffer solutions yet.
So a strong acid/base will fully dissociate in a solution, so if an indicator was a strong acid/base it will fully dissociate into the solution affecting the analyte's pH. Therefore, an indicator must be a weak acid/base so that it only partially dissociates into the analyte solution. Is that right?

But I am a bit confused about what happens if a strong acid/base was added to the indicator if it was itself a strong acid/base?

Btw, Thank you so much for your reply.

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Re: INDICATORS QUICK QUESTION
« Reply #3 on: March 19, 2021, 08:16:08 pm »
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Yeah, the first part's pretty much correct, sounds like you get it - which is what I was going for, well done! That's one part of why indicators are weak acids/bases.

To be honest, the part you don't quite get relates heavily to buffers (so totally understandable), and was probably as a result of my awful wording (I'll go fix that up). I actually meant what happens if a strong acid/base is added to an indicator (which we now know is a weak acid/base) - sorry! What actually happens with an indicator is that it changes colour when the hydrogen/hydronium concentration changes ever so slightly at a certain point - adding strong acids and bases will alter the equilibrium such that the colour of the solution ends up changing due to the concentration of hydronium changing. This is also why strong acids (and particularly the really strong ones) can't be used as indicators (there's either no detectable colour change, no equilibrium exists, etc.).

In the erroneous case you describe (apologising again), the indicator would just get neutralised, I suppose.
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I.09

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Re: INDICATORS QUICK QUESTION
« Reply #4 on: March 20, 2021, 06:18:58 am »
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Thank you so much for clearing it up. I think I get it now.

But when asked about the importance of titrations, would the following answer be sufficient:

Titrations are important in both the medical area and the food industry. Titrations allow the determination of the concentration of vitamins and minerals in the pharmaceutical preparation of medical drugs, such that the needs of the patient (eg: iron deficiency) are sufficiently met. It also allows food manufacturers to determine the quantity of a reactant in a food sample, such as the amount of salt or sugar which then impacts the colour and taste of the product.

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Re: INDICATORS QUICK QUESTION
« Reply #5 on: March 20, 2021, 10:24:58 am »
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There's no marking criteria for this question, so this answer being sufficient really depends on the question. All I can really say is that everything content-wise is correct and that this answer may be perfect or you may need more information, depending on the question and its value. Your question isn't really directed at content and as such I can't help you as much, I've not seen and I don't remember how a lot of this works anymore - you're best suited to asking your teacher something like this, unfortunately. They'll have seen exams come and go and questions like this pop up. I don't really feel it's okay if I misdirect you and cause you to overprepare and use a verbose answer or be confident in an answer which is in fact not detailed enough.

Hope this makes sense :D
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I.09

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Re: INDICATORS QUICK QUESTION
« Reply #6 on: March 20, 2021, 11:59:46 am »
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ok I understand.
Sorry to keep bothering you. I am really struggling to understand these concepts.

I know that the equivalence point is the point at which the number of moles of the titrant and the analyte is equal and the endpoint is the point at which the pH indicator changes to indicate the end of the titration. This means that if an appropriate indicator is chosen the colour will change (end-point) at the same pH as the equivalence point of the acid-base reaction. But in reality, this doesn't happen, as the endpoint occurs after the equivalence point, I am so confused as to why?

Thanks so much. You don't understand I really appreciate your help.
« Last Edit: March 20, 2021, 12:03:27 pm by I.09 »

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Re: INDICATORS QUICK QUESTION
« Reply #7 on: March 20, 2021, 01:59:36 pm »
+1
You're not bothering me, don't worry about that :D - you don't know what you don't know, and it's totally fine to ask questions!

See the thing is an indicator changes colour when one particular ion dominates the equilibrium - it's a totally separate 'entity' to the neutralisation reaction it's supposed to detect. While the indicator itself does move its equilibrium based on the hydronium concentration going on in the neutralisation reaction, it may not necessarily have the colour change at the pH of neutralisation. We choose appropriate indicators to approximate the equivalence point (it's actually ridiculously hard to detect in most cases) and for simplicity's sake, we just say endpoint ~ equivalence point. In reality, the colour change is never going to occur at the same pH as the equivalence point (usually within an order of magnitude, or \(\pm\)1 pH). It can happen before or after depending on the indicator you choose as well.

I'm not sure if I explained this one quite well enough, so let me know if this doesn't make as much sense.

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I.09

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Re: INDICATORS QUICK QUESTION
« Reply #8 on: March 20, 2021, 02:17:09 pm »
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So in a titration curve obtained from results in physical titration, would the endpoint be equal to the equivalence...


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Re: INDICATORS QUICK QUESTION
« Reply #9 on: March 20, 2021, 02:39:13 pm »
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Maybe, due to the scale of the graph - but this isn't the case most times. If you can display it as something different, do so but only if you can prove it or you know for sure where both are. Usually the endpoint isn't on a lot of titration curves because the pH change is so drastic when the base/acid gets neutralised.
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Re: INDICATORS QUICK QUESTION
« Reply #10 on: March 20, 2021, 11:24:41 pm »
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In terms of primary standards, what is their specific property and why is each property needed?

Thanks

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Re: INDICATORS QUICK QUESTION
« Reply #11 on: March 20, 2021, 11:37:22 pm »
+1
- Low reactivity (especially with components of air); consider what happens if while weighing the standard if it is reactive to air components
- Purity; consider what happens to your standard if it is contaminated
- Cheap and available; you almost won't be able to obtain it otherwise
- High molar mass; this will minimise the effect of a constant deviation on your scale for determining an accurate concentration for your standard
- Minimal or non-hygroscopic; similar to the air one but for humidity. if not you probably have to dry it out using a dehydrator
- Soluble in water; water's literally everywhere, it's just easier to have your standard dissolve in water as opposed to something else (albeit not necessary)

Was there a particular standard you had in mind when asking this question? Try justifying to yourself how it fits each of the above criteria (it should be reasonably easy if you're using a rather common solute).
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Re: INDICATORS QUICK QUESTION
« Reply #12 on: March 20, 2021, 11:48:05 pm »
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Yes Potassium Hydrogen Phthalate KHP

I.09

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Re: INDICATORS QUICK QUESTION
« Reply #13 on: March 21, 2021, 04:27:58 am »
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Are primary standards only weak acids/bases?
Why can't strong acids/bases be used to make primary standard solutions?

And why can't titrations be between weak acid-weak base? Is it due to the short range of the end point?

One last question, how do you know that KHP is a weak monoprotic acid, is it through the following equation:
KHP(s) + H2O (l) <=> K^+ (aq) + HP^-(aq)
HP^-(aq) + H2O (l) <=> P^2−(aq) + H3O^+(aq
in which KHP only donates one H^+

In the same way, how do you know that NaOH is a strong monoprotic base, is it through the following equation:
NaOH(aq) + H2O(l) ⇌ Na^+(aq) + OH^−(aq)
in which NaOH accepts only one H^+......

I am sooooo confused, please help
« Last Edit: March 21, 2021, 10:54:34 am by I.09 »

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Re: INDICATORS QUICK QUESTION
« Reply #14 on: March 21, 2021, 06:08:02 pm »
+1
Are primary standards only weak acids/bases?
Why can't strong acids/bases be used to make primary standard solutions?

Think about reactivity

And why can't titrations be between weak acid-weak base? Is it due to the short range of the end point?

Yes, it's just ridiculously hard to pinpoint. Especially when you usually want to determine the properties of one substance, it's easier to just sub the other one for a stronger acid/base.

One last question, how do you know that KHP is a weak monoprotic acid, is it through the following equation:
KHP(s) + H2O (l) <=> K^+ (aq) + HP^-(aq)
HP^-(aq) + H2O (l) <=> P^2−(aq) + H3O^+(aq
in which KHP only donates one H^+

In the same way, how do you know that NaOH is a strong monoprotic base, is it through the following equation:
NaOH(aq) + H2O(l) ⇌ Na^+(aq) + OH^−(aq)
in which NaOH accepts only one H^+......

Strength is usually denoted using dissociation. Usually for stronger substance you'll see an arrow in one direction to indicate no equilibrium (which is usually close enough to the truth for high school anyway). No of proton donations is usually something seen either by the number of equivalence points when titrating or simply analysing equations where you can see how many they donate.
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