SAC Nitrogen in Fertiliser

[JackMichaelson]

So, my percentage obtained was an overestimation, however I'm struggling to find out reasons why there could be an overestimation apart from the reason that not enough HCl was added to reach the endpoint of the titration with NaOH. Be great if someone could help!

[luffy]

So, my percentage obtained was an overestimation, however I'm struggling to find out reasons why there could be an overestimation apart from the reason that not enough HCl was added to reach the endpoint of the titration with NaOH. Be great if someone could help!

O.o Which school do you go to? Glenny?

One of the errors I put for that Prac was that we may not have pipetted exactly 20mL of the two aliquots, which would adjust the results. Also, I added that NaOH has a chemical property to mix with the air, which would adjust the recorded concentration at the beginning of the Prac. I'm not sure if I was correct :S

Also, I don't think your "not enough HCl" is a valid reason, because you would then not see a titre at all and the colour change would not be detected. Thus, the experiment, itself, would not have been completed.

[funkyducky]

colour change/end point does not coincide exactly with the equivalence point; it usually occurs slightly after the equivalence point (ie. when the solution is more acidic in this case)
"not enough hcl" would result in a lower titre volume, therefore lower n(HCl), therefore underestimation of NaOH.
It is much more likely that you added too much HCl, for the reason I stated above, and also because sometimes the human eye cannot detect the slightest colour change, so you  might add a few extra drops before you notice the colour change, thus going over the end point and resulting in an overestimation.

[JackMichaelson]

So, my percentage obtained was an overestimation, however I'm struggling to find out reasons why there could be an overestimation apart from the reason that not enough HCl was added to reach the endpoint of the titration with NaOH. Be great if someone could help!

O.o Which school do you go to? Glenny?

One of the errors I put for that Prac was that we may not have pipetted exactly 20mL of the two aliquots, which would adjust the results. Also, I added that NaOH has a chemical property to mix with the air, which would adjust the recorded concentration at the beginning of the Prac. I'm not sure if I was correct :S

Also, I don't think your "not enough HCl" is a valid reason, because you would then not see a titre at all and the colour change would not be detected. Thus, the experiment, itself, would not have been completed.

When you say "we may not have pipetted 20ml", you didn't explain whether more or less than 20ml was added. I'm not sure where your going with the NaOH thing, car to elaborate more?
What do you mean that I would not see a titre at all and colour change would not be detected? I'm talking about back titrations where if not enough HCl was added, then there would be less mols of NaOH in excess from the titration, which would mean that there would be more NaOH reacted according to the equation n(reacted)=n(initial)-n(excess). Is it me that is wrong? cause I don't understand anything of what you said. =[

[JackMichaelson]

colour change/end point does not coincide exactly with the equivalence point; it usually occurs slightly after the equivalence point (ie. when the solution is more acidic in this case)
"not enough hcl" would result in a lower titre volume, therefore lower n(HCl), therefore underestimation of NaOH.
It is much more likely that you added too much HCl, for the reason I stated above, and also because sometimes the human eye cannot detect the slightest colour change, so you  might add a few extra drops before you notice the colour change, thus going over the end point and resulting in an overestimation.

I thought n(reacted)=n(initial)-n(excess). So if a lesser volume of HCl was added to NaOH to read the endpoint, then this would mean that there would be a less amount of NaOH in excess. which would mean more n(NaOH) reacted which ould lead to an overestimation. Am i just wrong? 

[vea]

colour change/end point does not coincide exactly with the equivalence point; it usually occurs slightly after the equivalence point (ie. when the solution is more acidic in this case)
"not enough hcl" would result in a lower titre volume, therefore lower n(HCl), therefore underestimation of NaOH.
It is much more likely that you added too much HCl, for the reason I stated above, and also because sometimes the human eye cannot detect the slightest colour change, so you  might add a few extra drops before you notice the colour change, thus going over the end point and resulting in an overestimation.

I thought n(reacted)=n(initial)-n(excess). So if a lesser volume of HCl was added to NaOH to read the endpoint, then this would mean that there would be a less amount of NaOH in excess. which would mean more n(NaOH) reacted which ould lead to an overestimation. Am i just wrong? 

You are right but you won't have a lesser volume of HCl added to NaOH unless you stop the titration before the colour even changes.

[funkyducky]

Sorry, my bad, I didn't realise this was a back titration that you were talking about.

In that case, an overestimation could occur because the end point occurred before the equivalence point, which would be "not adding enough HCl".

EDIT: it is possible that the indicator you used changed colour (the end point) before the equivalence point of the reaction - you could see a colour change even if the solution isn't neutralised. Eg. methyl orange changes colour between pH 3.1 and 4.4. Which indicator did you use?

Another possible error is there being other soluble acidic ions in the fertiliser sample that reacted with the NaOH, so the excess amount would be less. Those other ions would be included in your calculation of the amount of NH4+, so you'd get an overestimation.

[JackMichaelson]

colour change/end point does not coincide exactly with the equivalence point; it usually occurs slightly after the equivalence point (ie. when the solution is more acidic in this case)
"not enough hcl" would result in a lower titre volume, therefore lower n(HCl), therefore underestimation of NaOH.
It is much more likely that you added too much HCl, for the reason I stated above, and also because sometimes the human eye cannot detect the slightest colour change, so you  might add a few extra drops before you notice the colour change, thus going over the end point and resulting in an overestimation.

I thought n(reacted)=n(initial)-n(excess). So if a lesser volume of HCl was added to NaOH to read the endpoint, then this would mean that there would be a less amount of NaOH in excess. which would mean more n(NaOH) reacted which ould lead to an overestimation. Am i just wrong? 

You are right but you won't have a lesser volume of HCl added to NaOH unless you stop the titration before the colour even changes.

yeah, that's why I am having trouble finding reasons for overestimation when performing back titrations.

[JackMichaelson]

Sorry, my bad, I didn't realise this was a back titration that you were talking about.

In that case, an underestimation could occur because the end point occurred before the equivalence point, which would be "not adding enough HCl".

I think we concluded that an *overestimation could occur?

[funkyducky]

yeah, overestimation sorry! I hope I'm not confusing you *goes back and edits post*

[VCEMan94]

how would you minimise those errors funkydunky?