January 17, 2022, 09:19:16 PM
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Topic: error propagation of (C4H11N3O3) pH monitoring and further analysis.  (Read 245 times)

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Offline Om799ar

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Hello, I’m a higher-level chemistry student and I conducted an experiment with the purpose to determine the pKa of creatine monohydrate through titration curves.
In the experiment, we did an acid-base titration with 0.1M HCL and 0.1M NaOH. We measured 5g of creatine monohydrate using an analytical balance and dissolved it in the HCL then did the titration. We then determined the neutralization point from color changes due to indicator. We did 3 trials with creatine dissolved in HCl, and 3 without creatine dissolved in HCL (titration of HCL and NaOH only).

What is the average uncertainty of the creatine monohydrate mass? And what is the average uncertainty of the neuralization volume? The systematic uncertainties are 0.001g and 0.028ml respectively

In the analysis, we need to plot dpH/dV vs volume of NaOH to find peaks which are the equivalence and half equivalence points. But why is that? Should it be dpH vs volume? And what is the uncertainty of this graph to add error bars and how find it? The uncertainties of pH and volume are 0.043 and 0.028ml respectively. (note (d) states for derivative or change in)

One last thing, it took 9.6ml NaOH to neutralize the solution without the creatine powder and 9.8 to neutralize it with the creatine powder. Why is this? Please include a detailed explanation for this part if you can

Do you think this experiment is solid? What do you think are its limitations other than the systematic errors mentioned?
I would really appreciate answers because I’m kind of lost in all of this. Thank you!

Offline jeffmoonchop

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Re: uncertainties and analytical limitations of pKa-determining experiment
« Reply #1 on: December 19, 2021, 10:51:47 PM »
Sounds to me that theres minimal difference between HCl and HCl plus creatine, but if the difference is real, why do you think it is? What properties does creatine have that would cause such a result? Have you looked at its structure?

Offline Om799ar

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error propagation of (C4H11N3O3) pH monitoring and further analysis.
« Reply #2 on: December 20, 2021, 07:15:55 AM »
I have conducted an experiment with the purpose to determine the acid dissociation constant of creatine monohydrate through analysis of the pH curves read by a pH electrode. The methodology and analysis of the experiment are as follows (it’s kind of a lot so bear with me):

Creatine monohydrate powder will be dissolved in a 10cm3 solution of 0.1M HCl with a few drops of phenolphthalein indicator and titrated with 0.1M NaOH. A pH probe monitors the change in pH which will be used to determine the neuralization and half neutralization points. With reference to the Henderson Hasselbalch equation, the pH at the half equivalence points is the pKa of the amino acid.

Now that the methodology is clear, this is how I calculated the propagation of error. The table below shows the data collected:

Mass of creatine ± 0.001g   The initial volume of NaOH in burette ± 0.020ml   Neutralization volume of NaOH ±XXXX
5.007   0.000   9.810
5.007   0.000   9.830
5.019   0.000   9.820
Is the uncertainty is the average mass of creatine also 0.001g? since (3*(0.001))/3 = 0.001

The uncertainty of the neutralization volume should be 0.020ml since it is the same as the uncertainty of the initial volume read from the burrete but to calculate the uncertainty value of the average volume:

do I take V(max)-V(min)/(number of trials)=(9.830-9.810)/3 = 0.006 or is it just 0.020 as well?

Alright now digging into the ugly stuff, When we plot the data we have we get a graph that looks similar to the first one attached:

The uncertainty of the pH and NaOH is easy to calculate so added errors bars are easy. The confusing part is when we create the first derivative of this graph. I know it will plot the slopes of this graph and thus the equivalence and half equivalence points will be peaks such as the second graph attached

But how do I add errors bars for this graph? For the horizontal error bars, it's simply the uncertainty of the volume of NaOH. But what about the vertical? my supervisor required me to do so and I attempted to do it but he told me it's incorrect. This is my attempt (where U is absolute uncertainty):
〖2U〗_v/〖Δv〗_max +〖2U〗_(pH )/〖ΔpH〗_max =(2 ×0.28)/6.00+(2×0.043)/1.22=0.159

my rationale is that since the derivative of pH and derivative of Volume is divided then I should add the relative uncertainties of both. If I want to find average uncertainty then I should take (absolute uncertainty) / max-min values.

What do you guys think? I know it's kind of messy but welcome to higher-level advanced chemistry!

Offline Om799ar

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Re: uncertainties and analytical limitations of pKa-determining experiment
« Reply #3 on: December 20, 2021, 07:22:12 AM »
Sounds to me that theres minimal difference between HCl and HCl plus creatine, but if the difference is real, why do you think it is? What properties does creatine have that would cause such a result? Have you looked at its structure?
You're right, it's only around but a 0.2ML difference but that's exactly it, it should be more which is why I'm really focused on uncertainties. I posted a new post with more details do you mind taking a look at it, please?

Offline billnotgatez

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Re: error propagation of (C4H11N3O3) pH monitoring and further analysis.
« Reply #4 on: December 20, 2021, 07:29:08 AM »
I merged your posts for continuity
I apologize if this was not what you wanted

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