Bedlasky
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Potentiometric titrations issues
Hi.
I did few potentiometric titrations and have some issues with potential stabilization.
Firstly when I titrated glycine with 0,1M HClO4 in glacial acetic acid (and the same titration with dicyclohexylamine, this time in chlorbenzene +
glacial acetic acid solvent). Each addition of HClO4 was 0,5 ml, around end point additions were 0,1 ml. I waited for potential to be more or less
constant. But potential still slowly rised and never stopped. So I wrote always first relatively stable potential to the table. Especially difficult
was to find potential around end point. Titration curve and curves of the first and the second derivations looks nice, results are good (glycine
slightly over 100%, but this is probably due to the moisture, I didn't dry it before titration). But I don't understand why potential still rised when
it should remain the same.
I also tried titration of nitrate solution with ferrous sulfate in conc. sulfuric acid media. Here was potential really unstable after each addition.
The first attempt was really disaster - titration curve looked well, but curves of the first and the second derivations were really messed up. The
second attempt was successful, because I wrote in to the table first potential which showed up on the display. Any ideas why the potential was so
unstable and goes up and down like crazy?
I did several other potentiometric titrations without issues - titration of nitrating mixture with methanolic cyclohexylamine, titration of potassium
dichromate with ferrous sulfate, titration of sulfanilic acid with sodium nitrite. All these titration didn't have problems with reaching stable
potential (reaction of sulfanilic acid with sodium nitrite have slow reaction rate, so it took long time in comparison with the other two reactions).
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Swinfi2
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Do you have a humid environment? The KF titrator I use at work is always messing up from humidity (and poor mixing as the drops above the bulk fluid
gain moisture then fall in)
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Bedlasky
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Here in CZ is slightly less humid environment than in UK, but practically similar. Good point, titrations in acetic acid media are sensitive to water,
so rising potential was probably due to moisture.
But titration in sulfuric acid media isn't that much sensitive to water. Water content can be up to 25% and results are still relevant. But nitrate is
titrated by aqueous FeSO4, so with each addition rises water content little bit, maybe it have something to do with it?
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maldi-tof
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Try to add some acetic anhydride. It will "capture" some water to form more acetic acid, and maybe your titration can be more stable.
At least, in our methods of analysis, all of our titrations with HClO4, always states to add acetic anhydride and cyclohexanol (if i am correct...).
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Bedlasky
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maldi-tof: Thanks for idea. My titrations were just one-time experiment, I want some data for article. But if I'll next time do this titration, I'll
try addition of acetic anhydride.
Cyclohexanol? Wouldn't OH group have some undesirable effect?
What substances do you titrate with HClO4? I do (in job) amine value of N,N-dimethylcyclohexylamine and N,N-dimethylbenzylamine in acetic
acid/chlorobenzene solvent, without acetic anhydride.
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maldi-tof
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For example, assay of sodium fluoride and sodium citrates are tested against HClO4.
I can't remember now if it is cyclohexanol, I'll check it later and make sure what is the other reagent that we use for this titration. What I know
for sure, is that we use an indicator, violet crystal (or methyl violet in English, CAS number is 548-62-9.
EDIT: I copy the USP monograph for NaF (it was dioxane and not cyclohexanol, I don't why I said that...)
ASSAY
•Procedure
Sample solution:To 80.0 mg add a mixture of 5 mL of acetic anhydride and 20 mL of glacial acetic acid, and heat to dissolve. Allow to cool.
[Note—The heating step to dissolve the sodium fluoride in the acetic acid mixture is critical. It is recommended to heat for a minimum of 30 min and
use sonication, if necessary, to make sure dissolution is complete prior to the titration step.]
Analysis: Add 20 mL of dioxane to the Sample solution. Add 1 drop of crystal violet TS, and titrate with 0.1 N perchloric acid VS to a green endpoint.
Perform a blank determination, and make any necessary correction (see Titrimetry 〈541〉). Each mL of 0.1 N perchloric acid is equivalent to 4.199
mg of NaF.
Acceptance criteria:98.0%–102.0% on the dried basis
[Edited on 9-6-2021 by maldi-tof]
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