[Kudo89]

Hi....
I am a new member of this forum....
I have some problems about determining CaO content in limestone using EDTA...
This is the summary of procedure :
First, I burn limestone to release CO2 from limestone, leaving CaO (and some amount of MgO, Fe, Al and any other component).
Then, I dilute some of sample in sucrose solution. Before the sample is titrated with EDTA I added some buffer pH 10, KCN as masking agent and EBT indicator.
Can I determine the exact amount of CaO with this procedure?
As far as I read, in pH 10 EDTA will remain in half ionized form so it will reacts with both of Ca and Mg...
...
I have already searched in internet and it makes me more confused 
I found that in preparation of EDTA solution, some of them mention about adding some of NaOH in EDTA solution. Is it necessary to add some NaOH? For what reason?
I also found that there is no exact composition of buffer. I found about 2 - 3 composition of ammonia and ammonium chloride using for make buffer. Anybody know how to determine the exact amount of ammonia and ammonium chloride using for make a buffer pH 10?
About EBT indicator, I read that EBT indicator is make a red wine complex with Mg2+ which will release after all Ca form a complex with EDTA. But, when I standardized my EDTA solution using calcium carbonate, I don't any of Mg in both of EDTA and calcium carbonate solution and it still give me a red wine complex. How can it happen?
.
Thank you so much for your time....
If I made some mistake forgive me please...

Regards
Kudo89

[Borek]

Can I determine the exact amount of CaO with this procedure?

No. Mg will interfere.

As far as I read, in pH 10 EDTA will remain in half ionized form so it will reacts with both of Ca and Mg...

It is not EDTA that is a problem. If you don't want to titrate Mg you need much higher pH to precipiate Mg(OH)₂.

I found that in preparation of EDTA solution, some of them mention about adding some of NaOH in EDTA solution. Is it necessary to add some NaOH? For what reason?

Probably to make it easier to dissolve EDTA. Besides, EDTA is sold sometimes as an acid, sometimes as a sodium salt, depending on which one you use, adding NaOH may make sense, or not.

I also found that there is no exact composition of buffer. I found about 2 - 3 composition of ammonia and ammonium chloride using for make buffer. Anybody know how to determine the exact amount of ammonia and ammonium chloride using for make a buffer pH 10?

You should use Henderson-Hasselbalch equation, although calculated recipe will be slightly off, unless you take ionic strength of the solution into account.

About EBT indicator, I read that EBT indicator is make a red wine complex with Mg2+ which will release after all Ca form a complex with EDTA. But, when I standardized my EDTA solution using calcium carbonate, I don't any of Mg in both of EDTA and calcium carbonate solution and it still give me a red wine complex. How can it happen?

EBT changes color reacting with both calcium and magnesium.

[Kudo89]

Sorry for the long reply...
Thank you so much Mr. Borek

May I ask a little more?

I have read that if Mg is precipitated some amount of Ca will also co-precipitated with Mg..
Is there any trick to precipitate Mg without decrease amount of Ca?
...
If I recall, Mg will precipitate in pH 12 but EBT's pH range is between 7 and 11. Is there any suitable indicator for this titration or can I still use EBT for titration at pH 12?
...
About NaOH adding; if I use disodium EDTA salt to make EDTA solution, adding NaOH won't make any sense, will it? Because solubility of a substance is decreased in same ion solution..

Thank you for your reply..

Regards
Kudo89

[Borek]

I have read that if Mg is precipitated some amount of Ca will also co-precipitated with Mg..
Is there any trick to precipitate Mg without decrease amount of Ca?

None that I am aware of, but I don't remember reading about coprecipitation being a problem here.

If I recall, Mg will precipitate in pH 12 but EBT's pH range is between 7 and 11. Is there any suitable indicator for this titration or can I still use EBT for titration at pH 12?

Murexide.

About NaOH adding; if I use disodium EDTA salt to make EDTA solution, adding NaOH won't make any sense, will it? Because solubility of a substance is decreased in same ion solution..

Shouldn't matter, it is soluble enough.

[Kudo89]

Thank you Mr Borek 
It really help me to solve my problem...

So this is what I should do : adjust pH higher than 10 (I read that Mg is precipitated in Mg(OH)2 at pH 12), use murexide instead of EBT (Because EBT isn't working at pH 12), filter Mg precipitate and then titrate the filtrate using EDTA. About EDTA solution, it isn't necessary to add some NaOH if I use disodium salt.

[Borek]

Don't filter Mg(OH)₂, you can safely titrate mixture.

http://www.titrations.info/EDTA-titration-calcium

[Kudo89]

Thanks Mr. Borex 
...
If you don't mind, I have more question....

First: it is a common knowledge that sucrose can increase solubility of lime in water, but which compound of lime? If sucrose only increase some of compound in lime (for example CaO only), can it used as masking agent for other compound (Mg, Fe, Al, etc)?

Second: this is only my wild idea, but if I don't have murexide can I still use EBT with this condition below?
Adjust pH at 12 to create Mg(OH)2 which will precipitate then adjust it back to 10 so I can still use EBT. If necessary, filtration can be used to anticipate the redissolve of Mg(OH)2.

Third: about co-precipitation we've discussed before, if some of Ca is precipitated, can we determined actual content of Ca in lime (amount of Ca which reacts with EDTA is decreased as co-precipitation happen)?

Thanks for your time..

regards
Kudo89