Titration error affecting the PH of the endpoint

[RoboNerd]

Homework Statement

Hi everyone.

1) If I perform a titration with a buret not rinsed in the titrant, then how will the pH of the endpoint be affected?

2) Also, if the volumetric pipet used to measure the analyte is not rinsed with the analyte, how will that affect the pH of the endpoint?

Homework Equations

none.

The Attempt at a Solution

[/B]
I assume that I can analyze the changes in pH of the equivalence points and because endpoints approximate equivalence, the pH of the endpoint should be close.

for question 1: I will have diluted titrant, requiring a greater volume to reach equivalence. if this is a strong acid/strong base titration, then my pH will be 7 as all of the H+ and OH- are neutralized. If this is a titration involving a weak acid/base and a strong titrant [where if the analyte is a base, titrant is an acid and vice versa], then my volume in the flask at the endpoint will be larger due to a larger volume of the titrant added, and will affect the pH accordingly. The question is whether my guess at this is right, and if so, how will the pH be affected?

for question 2: I will have diluted analyte, requiring lesser volume to reach equivalence. if this is a strong-strong titration, then pH will be approximately 7 at the endpoint. However, if this is not the case, then there will be less volume in the flask at the endpoint. I guess this will affect pH accordingly, but the question is how.

Any input is valued. Thanks in advance for your help!

 

[epenguin]

This is what I think. That quantitative chemistry at this level is limited - that if your understanding is sound, there is something you need to know, say some of the common reactions used, but you can master it and score 100% in tests and exams if you try, which is harder to do in other types of question. A fact teachers and coaches are very conscious of. It seems to me that ingenuity gets applied to bring you down. This seems to me close to a trick question, not educative. So I have no hesitation in giving you my answer, but there may be others as you tend to get different opinions on the sort of thing.

I'd say that however you determine it, whether by an indicator or a pH meter, the endpoint pH is it just the pH at the endpoint!. This pH will not be changed by any of these errors!

If instead the question was about how water in burette or pipette would change titration volumes slightly, and then your calculation of the concentration you are trying to determine, then you are answering the along the right lines, though I think you're putting in some inessentials - what does it matter to the answer if it's acid or base, strong or weak? Try to reduce what you are saying to one sentence for each question. If burette or pipette are dry then rinsing should make no difference! However it is a good practice to rinse with the solution you going to use. There may be a bit of moisture you don't see. Then again, the question is letting you think there may be a little water contamination - but suppose the contamination instead is concentrated acid or base? You can't be sure of what previous user or maybe technicians charged with cleaning the stuff have done.

 

[Borek]

I am with epenguin here. pH at the endpoint is what you determine and it doesn't depend on these errors. If anything they can change the theoretical equivalence point (because of dilutions) and the titration result (for the same reason).

Note that rinsing the volumetric glass with the solution is not necessary if the glass is dried, it is only required if you use a wet glass.

 

[RoboNerd]

However, what happens if I have a strong/weak titration [does not matter which]?

At equivalence point, I get my conjugate of my weak acid/base formed, which will then hydrolyze water. With less volume of titrant needed, the resulting volume in the flask [as volumes are additive] will be greater, and so the equilibrium position will be different, affecting the pH. So thus, the pH would be affected, right?

 

[Borek]

Is your question about endpoint, or about equivalence point?

 

[RoboNerd]

It is about endpoint, yes.

 

[RoboNerd]

But the endpoint approximates the equivalence point, no?

 

[RoboNerd]

Never mind. I realized this now.

PH at endpoint is determined by concentration of the excess titrant, and regardless of these errors, the concentration of the excess will remain the same.

Thanks so much for helping me jog my thoughts on this one! I really appreciate it!

 

[RoboNerd]

Wait. No. The pH at the endpoint will remain the same for the buret scenario.

However, with the pipet, I will have less moles of the analyte, so I will need less volume of the titrant. This will mean that I will have a lesser concentration of excess titrant molecules, so the PH will be affected accordingly... no?

 

[epenguin]

You are still making the same confusion. It is something like mixing your petrol with a a bit of lower octane fuel will change the volume you can measure of fuel used to get from A to B, but you are like saying it's going to change the distance!

I suggest that instead thinking about it and discussing with us, you go a a textbook (it doesn't sound like you have during this time) and look at a titration curve (pH against volume added) and see what is meant by endpoint - hopefully in the end you will see the point!

There are hundreds of sources - I just quote the first one I came upon now http://www.creative-chemistry.org.uk/alevel/module4/documents/N-ch4-05.pdf

 

[Borek]

But the endpoint approximates the equivalence point, no?

Yes, but it doesn't matter here. In a typical scenario additional volume diluting the sample (because of using a wet burette/pipette) is tens of μL added to tens of mL, so the impact on the theoretical equivalence point is negligible and much lower than typical difference between end point detections in consecutive titrations.

Try to calculate pH for these solutions:

1. 25 mL of 0.1 M acetic acid + 25 mL of 0.1 M NaOH (theoretic equivalence point for a titration)
2. 25 mL of 0.1 M acetic acid + 25 mL of 0.1 M NaOH + 0.1 mL H₂O (theoretic equivalence point for a titration with accidentally added extra water)

3. 25 mL of 0.1 M acetic acid + 24.95 mL of 0.1 M NaOH (endpoint measured one drop before the equivalence point)
4. 25 mL of 0.1 M acetic acid + 25.05 mL of 0.1 M NaOH (endpoint measured one drop after the equivalence point)

What matters, what doesn't matter?

 

[RoboNerd]

Borek, for your first scenario, I got pH = 8.7279
for your second scenario I got pH = 8.7275
for your third scenario I got pH = 7.4538
for your fourth scenario I got pH = 9.9995

I have no idea what matters and what does not matter, but it seems that the accidentally added water does not have a noticeable impact.

I took a look at a graph of the titration curve, but I have no ideas resulting from my attempts to look at it.

 

[RoboNerd]

I am still confused as to how my endpoint will be affected.

Here's what I wrote for the following questions [for my AP Chem spring break hw]:

1. 
   How will the concentration of unknown change if the volumetric pipet used to measure the analyte is NOT rinsed with the analyte? How will it affect the end point?
   1. The volume of the titrant needed to reach the endpoint will decrease. The pipet will have remaining dH2O and will dilute the analyte’s concentration, so there will be less moles of the analyte in the beaker for the titrant to neutralize. Thus, a lesser volume of the titrant will be required as less moles of the titrant are needed to neutralize the lesser number of analyte moles. M1 * V1 = M2 * V2 where substance 1 is the titrant and substance 2 is the analyte. Because the volume of the analyte and the concentration of the titrant are constant and V1 decreases, M2 decreases. Thus, the experimental concentration of the unknown will be less than its actual value. It will affect the endpoint by requiring a lesser volume of the titrant to reach it. <FIX ABOUT PH OF ENDPOINT>
2. 1. 
      How does the concentration of unknown change if the buret is not rinsed with titrant?
      1. The volume of the titrant needed would increase as left over dH2O in the buret would make the solution more dilute, and thus require a greater volume to provide the set number of moles needed to reach the equivalence point.
         
   
   
   M1 * V1 = M2 * V2, where substance 1 is the titrant and substance 2 is
   
   the analyte. Because V1 increases, M2 increases correspondingly. Thus, the experimental concentration of the unknown is higher than that
   
   of its actual values.
   
   1. How does that affect the endpoint?
      1. Again, for the reasons stated in the previous question’s answer, there will be a greater volume of the titrant needed. Thus, the volume of the titrant needed for the endpoint will increase. The pH of the endpoint will remain the same as its pH is determined by the concentration of the excess titrant and the amount of excess titrant will remain approximately the same as approximately the same number of moles of titrant are added to reach equivalence.

 

[RoboNerd]

As you can see I am still suffering over how to figure the changes in PH of endpoint out.

 

[RoboNerd]

Could you please provide input? Thanks

 

[Borek]

Perhaps you don't understand the difference between the endpoint and the equivalence point.

Can you list what they both mean?

 

[epenguin]

Could you please provide input? Thanks

We have. Have you looked at Figs. 1 and 2 of the link I gave you in #10?

 

[RoboNerd]

Borek: Here's what I know:
Equivalence point: number of moles of acid and base are equal.
Endpoint: a little past equivalence point during a titration when my indicator changes color.

And epenguin, I have looked at figures 1 and 2. However, I fail to see how they would describe changes in endpoint pH with the errors.

 

[Borek]

Endpoint: a little past equivalence point during a titration when my indicator changes color.

No. Endpoint is where the person performing the titration detects the color change. It doesn't have to be after, it doesn't have to be before the equivalence point. It doesn't have to be identical for each titration, it doesn't have to be identical for every chemist involved.

 

[RoboNerd]

Yes, of course. That is what I meant. But my teacher asked me how this affects the pH and I have no idea how to respond to this question, given that the endpoint is not the same for all titrations.

 

[Borek]

my teacher asked me how this affects the pH

IMHO - it doesn't.

Color change depends on the pH, so the observed color change - on average - will occur always at the same pH, no matter what are exact concentrations of titrant and titrated substance. Note: ON AVERAGE. There is no way of answering the question for a single titration, as the final pH (the one at which we state "this is where I declare the endpoint") has no unique value, it is a random variable.

 

[epenguin]

Are

And epenguin, I have looked at figures 1 and 2. However, I fail to see how they would describe changes in endpoint pH with the errors.

Well we have been saying there aren't any such changes.

Here you have volume added on one axis, pH on another. You just seem to be looking at the wrong one everytime! And there is an arrow to show you the equivalence point.

Suppose the error is not the small one we have been fussing about. Suppose it is 100%. The concentration of the added acid or base shown in the figures is half what the experimenter thinks it is.

In the diagram along the horizontal volume axis there are no numbers. But it looks like the equivalence point could be arrived at at 20 ml. Let's say that's what it is with 0.1 M acid or base. But because of the mistake, the concentration of the added solution is 0.05 M - then 40 not 20 ml should be added to reach equivalence. Good titration curve would be described by exactly the same figures, except you would label the fourth notch on the horizontal axis 40 instead of 20. The curve has to remain exactly the same. What is happening to the equivalence point? Nothing. Look.

Edit. Or put it yet another way, the pH equivalence is just the same thing as the pH if you dissolve this corresponding quantity of the corresponding salt, e.g. sodium acetate or disodium oxalate crystals. That is not going to change because of a little or big error in the volume added that you needed to get there.

 

[RoboNerd]

Thank you very much for the help! I just was able to see this reply. It somehow got lost among other threads. This clears things up a lot.

 

[RoboNerd]

I see what you are saying and I whole heartedly agree!