Discussion Boehm titration results - Procedure and Calculation?


by FaNgS
Tags: boehm, calculation, discussion, procedure, results, titration
FaNgS
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#1
Jan13-13, 12:22 PM
P: 92
I'm posting this here because I did not get much help from the Chemistry section. Please read further.

Anyone familiar with the Boehm titration procedure and calculations?

I've done the titrations based on what I was able to gather from literature and the calculations but the results don't make sense to me.

Even if you are unfamiliar with this method here's a quick idea about it: Boehm titration is an acid-base titration method which is used to determine the amount of surface oxygen groups (acidic or basic) present on carbon surfaces (activated carbon, carbon black, graphene, carbon nanotubes, etc). There are several authors that perform the titration in various techniques direct titration, back titration and potentiometric titration. But the most common is a back titration technique, which consists of mixing the initial base with an excess of acid and titrating with base, while in the case of an initial acid, it is mixed with an excess of base and titrated with an acid.

The method uses NaOH, Na2CO3, NaHCO3 and HCl and its main principle is that the number of acidic sites is determined under the assumptions that NaOH neutralizes carboxylic, lactonic and phenolic groups on the carbon surface; that Na2CO3 neutralizes carboxylic and lactonic groups; and that NaHCO3 neutralizes only carboxylic groups. The number of basic sites is calculated from the amount of HCl required in the titration.

I'll skip the procedure I followed, unless someone would like to know what I did, and I will just go straight to the results. I've calculated the amount of surface groups in mmol/g of carbon.

From the NaOH titration I found that there are 1.423 mmol/g of carboxylic, lactonic and phenolic groups.

From Na2CO3 titration there are 1.225 mmol/g of carboxylic and lactonic groups.

From NaHCO3 titration there are 1.575 mmol/g of carboxylic groups only.

So from the results there are 1.575mmol/g of Carboxylic groups, and by subtracting NaHCO3 titration and Na2CO3 titration this shows that there are 0.350 mmol/g of Lactonic groups. And finally the number of phenolic groups is calculated by subtracting the NaOH titration from the Na2CO3 titration to get 0.198mmol/g of phenolic groups.

Wouldn't it make sense that the NaOH titration would result in the highest amount in mmol/g since it neutralizes all 3 groups together? But from I've got, the highest amount was from the NaHCO3 titration 1.575mmol/g (only carboxylic groups neutralized).
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Mandelbroth
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#2
Jan13-13, 01:47 PM
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Quote Quote by FaNgS View Post
I'm posting this here because I did not get much help from the Chemistry section. Please read further.

Anyone familiar with the Boehm titration procedure and calculations?

I've done the titrations based on what I was able to gather from literature and the calculations but the results don't make sense to me.

Even if you are unfamiliar with this method here's a quick idea about it: Boehm titration is an acid-base titration method which is used to determine the amount of surface oxygen groups (acidic or basic) present on carbon surfaces (activated carbon, carbon black, graphene, carbon nanotubes, etc). There are several authors that perform the titration in various techniques direct titration, back titration and potentiometric titration. But the most common is a back titration technique, which consists of mixing the initial base with an excess of acid and titrating with base, while in the case of an initial acid, it is mixed with an excess of base and titrated with an acid.

The method uses NaOH, Na2CO3, NaHCO3 and HCl and its main principle is that the number of acidic sites is determined under the assumptions that NaOH neutralizes carboxylic, lactonic and phenolic groups on the carbon surface; that Na2CO3 neutralizes carboxylic and lactonic groups; and that NaHCO3 neutralizes only carboxylic groups. The number of basic sites is calculated from the amount of HCl required in the titration.

I'll skip the procedure I followed, unless someone would like to know what I did, and I will just go straight to the results. I've calculated the amount of surface groups in mmol/g of carbon.

From the NaOH titration I found that there are 1.423 mmol/g of carboxylic, lactonic and phenolic groups.

From Na2CO3 titration there are 1.225 mmol/g of carboxylic and lactonic groups.

From NaHCO3 titration there are 1.575 mmol/g of carboxylic groups only.

So from the results there are 1.575mmol/g of Carboxylic groups, and by subtracting NaHCO3 titration and Na2CO3 titration this shows that there are 0.350 mmol/g of Lactonic groups. And finally the number of phenolic groups is calculated by subtracting the NaOH titration from the Na2CO3 titration to get 0.198mmol/g of phenolic groups.

Wouldn't it make sense that the NaOH titration would result in the highest amount in mmol/g since it neutralizes all 3 groups together? But from I've got, the highest amount was from the NaHCO3 titration 1.575mmol/g (only carboxylic groups neutralized).
I don't profess myself to be an expert at this, but I think if you subtract the Na2CO3 titration from the NaHCO3 titration, the latter needs to be larger (you shouldn't get negative mmol/g of anything).
FaNgS
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#3
Jan13-13, 02:43 PM
P: 92
I avoided the negative by subtracting the NaHCO3 titration from the Na2CO3 titration.

I've been trying to figure it out, I'm not sure whether it's just my tiredness or do the numbers really don't make sense. You do agree that the results don't make sense, right?

Mandelbroth
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Jan13-13, 02:49 PM
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Discussion Boehm titration results - Procedure and Calculation?


Quote Quote by FaNgS View Post
I avoided the negative by subtracting the NaHCO3 titration from the Na2CO3 titration.

I've been trying to figure it out, I'm not sure whether it's just my tiredness or do the numbers really don't make sense. You do agree that the results don't make sense, right?
If I understand it correctly, yes. Is it possible that you accidentally switched some of the values?

Also, you should describe your full procedure. It may be easier for us to see fallacy if we see its source.
FaNgS
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#5
Jan13-13, 03:14 PM
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Quote Quote by Mandelbroth View Post
If I understand it correctly, yes. Is it possible that you accidentally switched some of the values?

Also, you should describe your full procedure. It may be easier for us to see fallacy if we see its source.
I'll double-check my calculations.

As for my procedure I followed the general Boehm titration procedure as mentioned in this journal article: Standardization of the Boehm titration Part I CO2 expulsion and endpoint determination, S. L. Goertzen et. al, Carbon 48 (2010) 1252-1261.

I'll summarize my procedure here:

1.5g of activated carbon was added to 4 separate flasks each containing 50mL of 0.05M NaOH, Na2CO3, NaHCO3 and HCl (these solutions are called the reaction bases or acid). The flasks were sealed and kept in a water bath at room temperature and shaken at 225 RPM for 24hrs. After 24hrs, the carbon was filtered out.

10mL aliquot of NaOH reaction base was taken and an excess of 20mL of 0.05M HCl was added with 2 drops of Phenolphthalein indicator. The mixture was titrated against 0.05M NaOH and the volume of NaOH required to reach the endpoint was noted.Each titration was repeat twice.

Similar was done for NaHCO3 reaction base, while for Na2CO3 reaction base an excess of 30mL of 0.05M HCl was added (rather than 20mL, due to the diprotic property of the base to ensure complete reaction with the acid). As for the HCl reaction acid, 20mL of 0.05M NaOH was added and it was titrated against 0.05M HCl.

The paper mentions the equation used for calculations but I get negative values based on it. The author uses a ratio of volume of reaction base and acid to account for the monoprotic and diprotic property of the base and a ratio of moles of acid and moles of initial reaction base or acid (in the 50mL) which is not mentioned the reasoning for nor could I understand why.

So I've followed the same equation only neglecting the ratio of volumes and moles and dividing the result by the mass of carbon I've used.

mol carbon functionality = ( [R a or b]*Vr - ( [X]*Vx - [T]*Vt) ) / m

[R a or b] is the concentration of the reaction base or acid initially in the 50mL in units of mol/L
Vr is the volume of reaction base or acid initially which is 50mL or 0.05L
[X] is the concentration of excess acid or base added to the 10mL aliquot in mol/L
Vx is the volume of excess acid or base added in L
[T] is the concentration of the titrant used in mol/L
Vt is the volume of titrant required to reach the end point which is determined by experiment in L
m is the mass of carbon used in g
chemisttree
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#6
Jan13-13, 09:33 PM
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Try standardizing your sodium carbonate and bicarb primary standards vs KHP. How did you prepare your NaOH solutions? From solid pellets of an open bottle? If you made them from solids, did you dry to constant weight?
FaNgS
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Jan13-13, 11:02 PM
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For all the bases I used 99% or higher purity pellets (in the case of NaOH) or powder (NaHCO3 and Na2CO3). I calculated the mass required to achieve 0.05M and dissolved in the corresponding volume of distilled water I need in a volumetric flask.
FaNgS
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Jan16-13, 08:15 AM
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Quote Quote by chemisttree View Post
Try standardizing your sodium carbonate and bicarb primary standards vs KHP. How did you prepare your NaOH solutions? From solid pellets of an open bottle? If you made them from solids, did you dry to constant weight?
I standardized my NaOH solution with KHP and the concentration was 0.047M rather than the intended 0.05M.

Is it acceptable to use the (now standardized) 0.047M NaOH solution to standardize my HCl solution. Then using the HCl solution to standardize the NaHCO3 and Na2CO3 solutions?

A bit of a side track but I want to make solutions of 0.1M Sodium Nitrate (NaNO3) for pH Point of Zero Charge analysis and the powder is all clumped up due to moisture absorption. The MSDS mentions it is a strong oxidizer and this chemical is used in explosives so are there any precautions I should take when drying? And would I need to standardize this solution as well?
zhengerest
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#9
Apr24-13, 09:40 AM
P: 1
any updates, FaNgS? I met with the same problem - NaOH always consumes the least and don't know why.


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