[armin]

Hi there,
I was to draw titration curve of Ca²⁺ with EDTA in excel, everything was ok until I wanted to do the same with pH=6
Here is what I've got as inputs:


I've ignored C_t and some others in higher pH but here, in pH=6 it ruins the curve and I don't know how to calculate and draw curve based on Vol. EDTA and pCa. As we now equivalent point is at Vol. EDTA=25
Thanks everyone 

[Borek]

Not enough information.

In general all you need is to find the correct stability constant (taking into account side reactions).

[armin]

well that's all I've got beside what I forgot! α₄ for EDTA at pH6 = 2.25×10^-5

[Borek]

I don't see where the problem is. Especially as you have still failed to explain what you are doing and where and how it is failing.

[armin]

This is what I've done in excel, you can see what formulas I've used to calculate [Ca²⁺] But you see in second sheet, for pH 6, near equivalent point, pCa goes wrong and curve is like zigzag! (sorry for my bad English)
Link of my excel: https://dl.dropboxusercontent.com/u/88704510/1.xlsx

[Borek]

I don't use Excell, sorry.

[armin]

I wrote down what I did to calculate [Ca²⁺] with input that I wrote before in my first post. But as I said before, this doesn't work for pH=6 and only works for higher pH. Thanks a lot my friend 
http://img.ir/ZGwc.jpg

[Borek]

Before equivalence it is OK. At equivalence and after - I am not so sure, I have problem following what you are doing. IMHO at equivalence point concentration of Ca²⁺ should be

[tex][Ca^{2+}] = \sqrt{\frac{[CaY]}{K'}}[/tex]

but perhaps you did some assumptions that get you to a different formula.

Can you elaborate a little bit? And finally explain what C_T is?

Also a screenshot of the wrong titration curve won't hurt.

[armin]

C_T is Molar of all EDTA that is not complexed! (If I wrote in proper English!)
I mean CT=[Y^4-]+[HY^3-]+[H₂Y^2-]+[H₃Y^-]+[H₄Y]
Using formulas I wrote before, my pCa has an unexpected decrease in equivalent point:

And titration curve is like:


Using the same formula in other pH doesn't make the same problem. One of my friends said that you should calculate dissociations too but I have no idea how!
THanks again :X