[mirror05]

Hey there,

If a solution contains both I− and Cl− at about 0.01 M, can we determine the amount of Cl− and I− independently by using gravimetric analysis?

I was thinking of precipitating Cl- perhaps with Ag so as to filter that. However, I cannot think of what is not soluble with I-? 

Thank you. 

[nj_bartel]

lead(II) iodide is moderately insoluble in water.

[mirror05]

I was just thinking though... everything that precipitates with chloride, precipitates with iodide as well. 

[nj_bartel]

And those compounds would have different physical properties.

[mirror05]

So say I add a some Ag into the solution of iodide & chloride, and there is the precipitation of AgI and AgCl. At that point, what method could I use to separate them?

Thx.

[nj_bartel]

I"m not sure =P  How well do the crystals form?  I just knew offhand that PbCl2 was white and PbI2 was yellowy.

[Borek]

I don't remember any reagents that may be used to separately precipitate these anions; everything I can think off will precipitate both.

[AWK]

Silver chloride can be separated from this mixture by ammonia, then precipitated by HNO₃. But the problem comes from photosensibility of AgCl.

[wpenrose]

The amount of I and Cl can be determined by argentometric titration. The AgI precipitates first, followed by the AgCl. A silver wire electrode is used to follow the titration, using a pH meter and standard reference electrode to make the actual measurement.

The example is a hoary old classic found in many analytical chemistry textbooks. You should find it there.

Here's another one. Get a table of solubility products, and find the Ksp's for the chloride and iodide of the same metal.

If the Ksp's are separated by three or more orders of magnitude, it will be easy to separate by fractional precipitation.

Finally, you can convert the iodide to iodine with a weak oxidizing agent, extract into an organic solvent, and reconvert to the iodide again.

Dangerous Bill

[Borek]

The amount of I and Cl can be determined by argentometric titration.

Hardly gravimetric method 

[wpenrose]

Fractional precipitation is gravimetric, but I'm too lazy to look up a good precipitant. I'd be inclined to look at the rare earths first. I think lanthanum iodide is insoluble, but wouldn't bet the farm on it.

Dangerous Bill