Sciencemadness Discussion Board

CO2 in water

aeacfm - 4-4-2012 at 00:21

I tried to determine CO2 in water according to the method in standard methods " titrimetric method for free Carbon dioxide " page 298... i used the pH electrode to detect the end point @ 8.3
but ............!!!!
every time i proceed in the titration , when pH reached (8 ) suddenly it decreases and with adding huge amounts of the titrant bicarbonte no change occur and i cant reach the end point at ( 8.3)

any help please

woelen - 4-4-2012 at 02:29

If you want help, then please explain in more detail what you have done. I don't have that page 298 you mention in your post. Just write down the steps you have taken. Also tell something about the liquid which you are analysing.

One reason of not reaching the endpoint may be the presence of a buffering agent, which may keep the pH more or less constant over a large range of added acid or base.

weiming1998 - 4-4-2012 at 02:36

1, Why would you ever use a salt and a weak base as a titrant? All titration of acids and acidic substances proceed via a strong base as a titrant, like NaOH. When CO2 reacts with the NaOH, it forms a buffering solution of NaHCO3, which has a pH of around 8.3 (as you said) when all the CO2 has converted to NaHCO3. After that, the NaOH will start reacting with the NaHCO3, forming Na2CO3. When all that has been reacted, the pH will suddenly spike up. It should be noticeable.

2, Titrating with a bicarbonate solution is bound to fail. The H2CO3 doesn't react with the NaHCO3, simply creating a buffering solution that will never reach the endpoint of pH 8.3.

[Edited on 4-4-2012 by weiming1998]

francis - 6-4-2012 at 02:45

Quote: Originally posted by aeacfm  
I tried to determine CO2 in water according to the method in standard methods " titrimetric method for free Carbon dioxide " page 298... i used the pH electrode to detect the end point @ 8.3
but ............!!!!
every time i proceed in the titration , when pH reached (8 ) suddenly it decreases and with adding huge amounts of the titrant bicarbonte no change occur and i cant reach the end point at ( 8.3)

any help please


Refer to 2320B in the Standard Methods: bicarbonate is not the titrant. Determination of CO2 in wastewaters is no longer given by method 4500 but by total alkalinity, aka acid neutralising capacity (2320).

They advise checking the phenolphthalein colour changes at the HCO3- endpoint via a preliminary titration of HCO3- with your standard acid. But it is not the titrant.

You must first make up a sulfuric acid solution of desired concentration, then standardise it against Na2CO3. Then titrate your sample with the standardised acid, using phenolpthalein and bromocresol green for the two endpoints, and/or your properly calibrated pH meter.
Quote: Originally posted by weiming1998  
1, Why would you ever use a salt and a weak base as a titrant? All titration of acids and acidic substances proceed via a strong base as a titrant, like NaOH. When CO2 reacts with the NaOH, it forms a buffering solution of NaHCO3, which has a pH of around 8.3 (as you said) when all the CO2 has converted to NaHCO3. After that, the NaOH will start reacting with the NaHCO3, forming Na2CO3. When all that has been reacted, the pH will suddenly spike up. It should be noticeable[Edited on 4-4-2012 by weiming1998]


In this case, the analyte is the carbonate anion (CO3^2-), so the titration proceeds via addition of strong acid.

But also, not all titrations are acid-base titrations. Take the determination of oxalic acid for example: the titrant is not a strong base, but an oxidant (permanganate), the endpoint of the titration is the point at which the solution turns clear (from purple):
it represents the reduction of permanganate to Mn2+.

There are all sorts of titrations: precipitation titrations, spectrophotometric titrations, EDTA titrations, and many others.

unionised - 6-4-2012 at 03:33

"Refer to 2320B in the Standard Methods:"
How?
You have already been told that at least one of us doesn't have that reference.
If you don't tell us what you actually did then you can't blame us if we don't follow what you say.

francis - 6-4-2012 at 05:57

Quote: Originally posted by unionised  
"Refer to 2320B in the Standard Methods:"
How?
You have already been told that at least one of us doesn't have that reference.
If you don't tell us what you actually did then you can't blame us if we don't follow what you say.


I'm not the original poster. Read the thread again.

aeacfm - 14-4-2012 at 18:49

Quote: Originally posted by woelen  
If you want help, then please explain in more detail what you have done. I don't have that page 298 you mention in your post. Just write down the steps you have taken. Also tell something about the liquid which you are analysing.

One reason of not reaching the endpoint may be the presence of a buffering agent, which may keep the pH more or less constant over a large range of added acid or base.


I am sorry to be late , I was offshore


- the Edition of the standard methods i think is old but I am sure it is page 298 in standard methods of testing water and wast water

- the steps as follows :

1- I collected the samples by a ruber tubing inserting it inside the container " of 500 ml volume " and made it overfilling 5 times the volume of the container and preserved it at temperature lower than the temperature of the collection.

2- I took 50 ml of the sample by siphoning in to a titration jar and quickly titrated it by using " 0.045 N sod. carbonate solution " - freshly prepared - and deyecting the end point at 8.3 as ststed and this is the dilemma to me :mad:

but !!! the book stated that the electrodes must be calibrated by 7and 10 buffers which i didn't do , i used 4 and 7 - i think it is not with that importance :D

thats all what is did

thanks in advance

aeacfm - 14-4-2012 at 18:52

Quote: Originally posted by weiming1998  
1, Why would you ever use a salt and a weak base as a titrant? All titration of acids and acidic substances proceed via a strong base as a titrant, like NaOH. When CO2 reacts with the NaOH, it forms a buffering solution of NaHCO3, which has a pH of around 8.3 (as you said) when all the CO2 has converted to NaHCO3. After that, the NaOH will start reacting with the NaHCO3, forming Na2CO3. When all that has been reacted, the pH will suddenly spike up. It should be noticeable.

2, Titrating with a bicarbonate solution is bound to fail. The H2CO3 doesn't react with the NaHCO3, simply creating a buffering solution that will never reach the endpoint of pH 8.3.

[Edited on 4-4-2012 by weiming1998]


i thought that also , it is that buffering action , but my boss told me it isn't buffering action :(

Pyridinium - 14-4-2012 at 19:20

Quote: Originally posted by weiming1998  
1, Why would you ever use a salt and a weak base as a titrant? All titration of acids and acidic substances proceed via a strong base as a titrant, like NaOH.


The carbonates can be used as primary standards, unlike the NaOH which is much more affected by air CO2 / moisture.

Last time I did a really careful titration I believe used Na2CO3 as the titrant. The endpoint was pretty sharp, but there was a particular dye that would have sharpened it (can't remember offhand which dye it was- have to look it up, but it could have been one of the Eriochromes).