an addition to the analysis of seawater lab

[mevans]

I recently did the part in this lab where you use silver nitrate to prcipitate the chloride ions out of solution. After i was finished with the titration, i got rid of the the silver chloride and then, i had an idea for an addition to this lab.
Insted of just stopping at this point, would it be possible to take this one step further. Once you have precipitated all of the chloride out of solution, could you then filter it, let it dry, weigh it and then add the dry silver chloride to a solution of 6M aqueous amonia? This would dissolve all of the silver chloride but not the silver bromide and iodide. Then you could filter it again and weigh it. This mass would be a mix of silver iodide and bromide. Then you could add this prcipitate to a 15M solution of aqueous ammonia which would dissolve the Silver bromide leaving only silver iodide. In this way you could more accurately measure the chloride ion concentration as well as the bromide and iondide concentration. The only problem i see with this is the small amount of silver chromate that was fomed when the chloride ion in solution was used up. Is there any possible way to get rid of silver chromate before you filter and dry the silver chloride Or like silver chloride, would silver chromate form a coordination complex with aqueous ammonia?
Thanks,
Mevans

[thompson]

Good idea, in principle. The problem is that the iodide and bromide ion concentration in seawater is so much lower than the chloride concentration that you'd need a gigantic aliquot to get accurately massable amounts of silver iodide and bromide. The relative solubilities of silver iodide and silver bromide in aqueous ammonia would probably also be a problem. Whatever the scale you did the experiment on, I think it's likely that the bromide and iodide salts would go into solution right along with the chloride, simply because there's so little of them.

[thompson]

Incidentally, all that assumes that the iodide and bromide ions would even precipitate as the silver salts. Remember that "insoluble" doesn't mean that none of it dissolves, just that only a small amount goes into solution. The iodide and bromide concentration in seawater may be low enough that they won't be precipitated by silver ions. To get a rough idea of whether this will happen, look up the Ksp for each of those salts and run the numbers. (It's simpler because both salts are diatomic.) The numbers you get won't be a completely accurate indication of true solubility because the halide ions interfere with each other.