[waqaszeb]

Hello,

First post here in the forums. We did a 2-part titration experiment in the lab in order to determine the % nicotine content in a sample of tobacco. The first titration was standardization of the perchloric acid, and the second titration was between perchloric acid and an obtained nicotine sample.

For Part A, we are asked to determine the concentration of perchloric acid. The part which confuses me here is that we standardize 0.025M (approximately) perchloric acid solution because? I think its because the 0.025M concentration is an approximation. Why standardize when we already know the concentration? Or maybe the 0.025M approximation is not sufficient. That's just my guess. We standardized by adding 0.1281 grams of potassium acid phthalate and 25 mL of glacial acetic acid to a clean flask. This solution is then titration with the 0.025M (appx) solution. I needed 24.02 mL of perchloric acid to get to equivalence point. I am asked to calculate the concentration of perchloric acid.

I dont understand why we add potassium acid phthalate. Is the perchloric acid reacting with the solvent (glacial acetic acid) or potassium acid phthalate? Thesa are all acid. Perchloric and KHP are both amphiprotic from what I understand.

I'll explain the difficulty with Part B once I tackle this part I guess to keep things flowing smoothly. Thanks!

[Borek]

0.025M is only approximate, you need much higher accuracy than that, which is why you do the standarization. You will see why you did it once you will calculate perchloric acid concentration.

I am not sure why you had to do titration in glacial acetic acid and not in water (perhaps you did the final titration in the glacial acetic acid too?), but the idea is obvious - KHP is the titrated base, glacial acetic acid is just a solvent. Note that even if the glacial acetic acid reacted with the KHP neutralizing the base, perchloric acid is still strong enough to protonate resulting acetate - and the number of moles of acid required doesn't change.

[waqaszeb]

So based on the reaction between KHP and perchloric acid (1:1 ratio), I got:

0.128 grams of KHP used/204.22 grams KHP/mole= 6.272 x 10^-4 moles of KHP,
which means 6.272 x 10^-4 moles of perchloric acid reaction. Total volume of the solution is (25ml glacial acetic acid + 24.02 mL perchloric acid)

Concentration of perchloric acid = 6.272 x 10^-4 moles/(24.02+25)= 1.279 x 10^-5

Does that look right?

And we used glacial acetic acid throughout the whole procedure because its a non aqueous titration which enhances nicotine's otherwise weak basic character.

[waqaszeb]

Sorry, I divided that by 49.02 mLs. It should be divided by 0.04902 which gives 0.0127 M

[Borek]

Concentration of perchloric acid = 6.272 x 10^-4 moles/(24.02+25)= 1.279 x 10^-5

You were right up to now. You are not interested in the concentration of produced salt, but in concentration of acid in the solution used for titration.

And we used glacial acetic acid throughout the whole procedure because its a non aqueous titration which enhances nicotine's otherwise weak basic character.

OK, that's not uncommon approach.

[waqaszeb]

Ok, thanks a lot. Now for part B we have to calculate % nicotine. This titration was done with the approximate 0.025M perchloric acid. In the lab manual it says that perchloric acid will be standardized and then used to determine the nicotine in the tobacco sample. Now, we did three trials for each part so I have three standardized concentrations for perchloric acid. Do I take the average of those three readings (from Part A) and use that number as perchloric acid concentration to determine the percent nicotine?
I needed very little amount of perchloric acid to titrate nicotine.

Also, we added about 100 mL of 9:1 toluene:chloroform mixture to the nicotine sample. Why do we need to how much of this mixture was added to calculate percent nicotine..I thought that was just a solvent..

[waqaszeb]

I think we just need that to know the total volume of the solution in order to calculate the percentage of nicotine. in terms of reaction with nicotine, i believe the mixture serves as an inert solvent. not too sure though

[Borek]

Ok, thanks a lot. Now for part B we have to calculate % nicotine. This titration was done with the approximate 0.025M perchloric acid. In the lab manual it says that perchloric acid will be standardized and then used to determine the nicotine in the tobacco sample. Now, we did three trials for each part so I have three standardized concentrations for perchloric acid. Do I take the average of those three readings (from Part A) and use that number as perchloric acid concentration to determine the percent nicotine?

Yes.

Also, we added about 100 mL of 9:1 toluene:chloroform mixture to the nicotine sample. Why do we need to how much of this mixture was added to calculate percent nicotine..I thought that was just a solvent..

You need to elaborate on the procedure. Yes, that's just a solvent, but have you titrated everything in one step, or have you split the extract to titrate smaller samples?

[mark reda]

Why did you add 25ml to the 24.02. If you dont add it you will get 0.024... something which is closer to the Molarity of what you want, 0.025M?

So based on the reaction between KHP and perchloric acid (1:1 ratio), I got:

0.128 grams of KHP used/204.22 grams KHP/mole= 6.272 x 10^-4 moles of KHP,
which means 6.272 x 10^-4 moles of perchloric acid reaction. Total volume of the solution is (25ml glacial acetic acid + 24.02 mL perchloric acid)

Concentration of perchloric acid = 6.272 x 10^-4 moles/(24.02+25)= 1.279 x 10^-5

Does that look right?

And we used glacial acetic acid throughout the whole procedure because its a non aqueous titration which enhances nicotine's otherwise weak basic character.