[Alan713ch]

Hi... right now I'm taking an Analytics Lab course, and I have to quantify Calcium, Cadmium and Zinc from a real sample (pH acid). When I did qualitative, I found that I also have Fe, and Carbonate, Nitrate and Chloride ions. So far, I have this to Identify the cations:
1.- As my ph is acid, use AgNO3 to get rid of the chlorides
2.- Take the pH to basic conditions (should I get rid of the carbonates adding acid before? Which would be better , HCl or H2SO4?)
3.- Take pH to basic (I fear the calcium precipitates if I use sulphuric acid, or if i don't get rid of carbonates)
4.- Mask all but Ca with Cyanide
5.- Use EDTA to valorate Ca
6.- Unmask (formaldehyde, does it unmask Fe as well?) Zn and Cd.
7.- Titrate (ammonia medium) to obtain Cd and Zn together. Use diethyltiocarbamate (DDC) to mask Cd, and calculate de EDTA liberated.

I know it's kinda rough, but is what I've got so far. I hope anyone could help, at least telling me if it is OK so far, or by giving me any ideas. Thank you!!!

[Borek]

Why do you want to remove chlorides?

Is it iron II or III?

High pH - what about hydroxides solubility?

[Alan713ch]

I'd like to remove the chlorides so i don't have any interference... but i'm not so sure it will help. The pH I'm seeking is to be 7-8, alcalinic enough to form the cyanide complexes but not high enough to start precipitation.
It war Iron III, what I found.
Thank you!!!

[Borek]

Fe(OH)₃ pKso = 38.6, you need low pH to be sure it won't precipitate (or some complexing agent).

[Alan713ch]

That's why i want to use cyanide, to complex the iron, zinc and cadmium... or would you suggest citric or tartatric acids? Then again, another issue I have is that if I unmask Zn and Cd, would Fe unmask as well?
Another thinking that came into my mind was to mask Ca, Zn and Cd then, get rid of Fe (hydroxides) and then unmask, but now what would be a good complexing agent?
Thank you!!