Lily P.

asked • 11/09/20

pH in a titration curve

Hi!

In my titration curve for HCl being titrated with NaOH, the initial pH-value was plotted as 2.1 according to a pH-meter. When I found the equivalence point and calculated the concentration of HCl, it was 0.065 M. I calculated how much the initial pH-value should be from that by -lg(0.065) which is around 1.2. That is way off from the original pH-value I got.


Why did this happen? Is it because the risk of me calibrating the pH-meter incorrectly? If that is the case, will the x-value of the equilibrium point also be wrong?? For the latter, I feel like the answer would be no, because what is wrong is that if the pH-meter was calibrated wrong, then all of the values would be 0.9 pH higher than what they should be, so the curve would only differ in the y-direction, but would be exactly the same in the x-direction. Is that correct?


Thanks in advance!


PS: For those who contracted me/ I contacted them in order to setup lessons, I am sorry I could not answer. It required me to put my financial information before I could type anything else, and I saw that you had to choose which state you live in, but I do not live in the U.S. I live in England, so I wasn't sure what to do by then. However, I figured out the solution to my problems, so thanks anyway

1 Expert Answer

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J.R. S. answered • 11/09/20

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There could be several reasons why your calculated pH and the actual pH read on the pH meter were so different.


It could be that the NaOH concentration was incorrect? Was it standardized against a known pH standard? Are you calculations for the concentration of HCl correct? When you ask about the x-value being wrong, what is the x axis? Is it mls of NaOH used? If so, then the x-value will not be wrong.


PS: Thank you for explaining about the lack of response to those who contacted you. I was one of them. No problem and I understand completely. Good luck.


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Lily P.

Thank you for your reply! The NAOH was standardized, it was just the concentration of HCl that was unknown. And yes the x-axis is mls of NaOH used and the y-axis is pH. So if my pH-meter is incorrectly calibrated, it will only affect the curve shifting in the y direction but not in the x? (Unless there are other error sources causing shifts in x)
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11/10/20

J.R. S.

tutor
Yes, it will only affect the x-axis. But, I'm still not confident that the NaOH was "standardized" and what I mean by that is against some other known standard, such as KHP (potassium hydrogen phthalate). How can you be sure the [NaOH] was what you think it is? Did you check the pH of the NaOH with the same pH meter that you used for the HCl?
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11/10/20

Lily P.

Thanks a lot! You mean only affect the y-axis, in your first sentence? I see your point regarding the NaOH. Actually, no, we did not check it's pH, and we were only told that it was standardized. This is actually a good way to improve the same lab if done again, measuring the pH of NaOH and comparing it with the stated concentration. But if the pH-value of NaOH is not what is was supposed to be if it was standard, by then it will affect the x-axis and at which x we will get equilibrium right? But if not, it just means that the y-axis is incorrect but the x-axis should be the same despite if the pH-meter was correct or not?
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11/10/20

J.R. S.

tutor
Yes, I meant only affect the y-axis, not x-axis. Sorry. If the [NaOH] isn't what is stated on the bottle (or what was told to you), it won't affect the actual titration curve, but it will affect THE CALCULATED pH of the HCl. Everything will look the same (same mls, same pH, etc) but since you use volume NaOH x molarity NaOH to find moles HCl and then [HCl] and then pH of HCl, all these calculations will be in error relative to the actual values. See my point?
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11/10/20

Lily P.

Yes I do! Thank so you much, I just have one last question: Let us say that the concentration of NaOH is calculated as the same as stated and that the error is actually from the pH-meter not calibrated correctly. If the values from the titration graph were: (0, 2.1), (5, 2.2) ,(7, 14), for example. This means that the pH-meter showed the acid having pH =2.1. Then, from the equivalence point, we took the volume added NaOH and calculated the concentration of HCl and later it's pH which was 1.2. That is a difference of 0.9 between the plotted value and calculated value. Does that mean all pH-values in the graph should be subtracted with 0.9 to get the correct value? For example then where x= 7 the pH would would be 13.1?
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11/10/20

J.R. S.

tutor
Not sure I understand your question, but recall that pH is a log function so it won't be linear. You should be able to calculate all of these data by knowing the volume of HCl that you titrated. It must have been sizable if it took 5 ml of NaOH to raise the pH of HCl from 2.1 to 2.2. Either that or the [NaOH] was very dilute. It seems strange that the pH would change so little especially since a buffer is NOT being formed. Sorry I can't be more concrete in my answer but not having all the data makes it difficult for me to see exactly what's happening.
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11/10/20

Lily P.

I understand, I will try to rephrase I think. I just wonder how, if the pH-meter was calibrated wrongly, it will affect the titration curve in comparison with if the pH-meter was calibrated correctly? For example we found out that if the error was with the concentration of NaOH then the titration graph would not change at all, but the only change will be with the calculations since we are using 0.1 M NaOH in a calculation that is based on NaOH having a higher/lower concentration. But what will the changes be if the error was with the pH-meter? (PS: Thanks a lot for all the help u are giving me, u r the best!)
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11/10/20

Lily P.

OOh I just realised, what if it was because we added distilled water to the analyte (HCl) because we wanted the pH-meter/electrode to touch the liquid surface. The distilled water does not affect the result of titration in terms of mL added NaOH, but it does only affect the pH, because we have a more diluted solution of HCl?
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11/10/20

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