Sciencemadness Discussion Board

Copper and Iron Double Salts

The Volatile Chemist - 26-3-2014 at 18:49

Recently, I've been working on a lot of reactions which involve double salts. There seem to be two main problems with them.

1. They can decay to their two counterparts (ex. Double salt A+IIB+IIX-II2 >> AX + BX)

2. There seem to be problems discerning forming a complex versus forming a double salt.

The first one is rather easy to determine. If Ferric Ammonium Sulfate decays to Ferric Sulfate and Ammonium Sulfate, it would be visible that it turned to Ferric Sulfate and Ammonium Sulfate. The other problem is more difficult. It begins with how the theoretical double salt is formed. One method involves using pure ammonia, when making ammonium double salts. The other mixes two chemicals with either identical anions or cations. It seems sometimes a complex can form (Correct me if I'm wrong) when the two ions that share charge in the double salt are cations. An example complex in this situation could be the tetraamminecopper complex, which could form when attempting to make a cupric ammonium double salt. I would like to make a list of copper and iron double salts on here, but it would be necessary to find a way to determine if a chemical formed was a complex or a double salt. Got any Ideas or definite double salts?

The Volatile Chemist - 26-3-2014 at 18:56

This is my attempt at calcium copper carbonate. I'm not sure if it turned to a complex or stayed a double salt:

I first made a solution of calcium sulfate and copper (II) sulfate. I then added sodium Bicarbonate which did nothing, being a pale green mix of calcium carbonate and copper(II) carbonate. At the time of the reaction, I thought nothing had happened, but I wasn't sure. I left it to sit, and after about 20 hrs. I noticed a blue layer had formed between the settled calcium carbonate and copper carbonate; and the water in my beaker:
See here: http://ptp.x10.mx/PTDC0211.JPG
(Sorry for the High Res...) Upon stirring, All three compounds (The leftover calcium carbonate and copper carbonate, and the new compound) all suspended in the water. The new compound was actually small beady nodules (crystals) that fell out of suspension immediately, so I was able to pour off the calcium carbonate and copper carbonate. As if to prove a reaction had occurred, the extra material I had poured off reacted over a few more (5) hours, to form more of the blue-ish compound and excess copper carbonate (Which i had predicted, as I had added too much initially). I'm pretty sure this is the double salt Calcium copper carbonate.


Metacelsus - 26-3-2014 at 19:04

Can you prove it contains an integer ratio of calcium and copper?

The Volatile Chemist - 26-3-2014 at 19:09

Quote: Originally posted by Cheddite Cheese  
Can you prove it contains an integer ratio of calcium and copper?

Ah, good Idea. I suppose I'll have to do the reactions again stoiochemically.

DraconicAcid - 26-3-2014 at 19:21

Quote: Originally posted by The Volatile Chemist  
The other problem is more difficult. It begins with how the theoretical double salt is formed. One method involves using pure ammonia, when making ammonium double salts. The other mixes two chemicals with either identical anions or cations. It seems sometimes a complex can form (Correct me if I'm wrong) when the two ions that share charge in the double salt are cations. An example complex in this situation could be the tetraamminecopper complex, which could form when attempting to make a cupric ammonium double salt.


If you want to control whether you get the tetramminecopper(II) complex or an ammonium/copper double salt, control the pH. In an acidic solution, the ammonia will form ammonium ions, which cannot act as ligands. In a basic solution, you will have ammonia molecules, which can act as ligands, but cannot act as cations in double salts. If you're having trouble remembering the difference between ammonia and ammonium, I suggest doing more reading before returning to the lab.

You will only get a complex when you have something that can act as a ligand to one (or both) of the cations involved. K2SnCl6 is a complex, because the chlorides can act as a ligand to the tin cation. NH4MgPO4 is a double salt because phosphate is not much of a ligand, and neither magnesium nor ammonium will form complex ions (usually).

The Volatile Chemist - 27-3-2014 at 15:04

Quote: Originally posted by DraconicAcid  
Quote: Originally posted by The Volatile Chemist  
The other problem is more difficult. It begins with how the theoretical double salt is formed. One method involves using pure ammonia, when making ammonium double salts. The other mixes two chemicals with either identical anions or cations. It seems sometimes a complex can form (Correct me if I'm wrong) when the two ions that share charge in the double salt are cations. An example complex in this situation could be the tetraamminecopper complex, which could form when attempting to make a cupric ammonium double salt.


If you want to control whether you get the tetramminecopper(II) complex or an ammonium/copper double salt, control the pH. In an acidic solution, the ammonia will form ammonium ions, which cannot act as ligands. In a basic solution, you will have ammonia molecules, which can act as ligands, but cannot act as cations in double salts. If you're having trouble remembering the difference between ammonia and ammonium, I suggest doing more reading before returning to the lab.

You will only get a complex when you have something that can act as a ligand to one (or both) of the cations involved. K2SnCl6 is a complex, because the chlorides can act as a ligand to the tin cation. NH4MgPO4 is a double salt because phosphate is not much of a ligand, and neither magnesium nor ammonium will form complex ions (usually).

Thanks! I guess I need to do a bit of reading, but that's perfectly fine :)