[JadenErius]

Hi guys, couldnt find a reference to this.
In a lab experiment we are required to test for Zinc, Lead (ii), aluminium and calcium ions. The procedure is that we test all 4 solutions with sodium hydroxide and ammonia solution.
Results are as such : Test with NaOH                              Test with Ammonia Solution
Zinc ion                   white ppt soluble in excess       white ppt insoluble in excess

Aluminium Ion           white ppt soluble in excess        white ppt insoluble in excess

Lead (ii) ion              white ppt soluble in excess        white ppt soluble in excess

Calcium ion                white ppt insoluble in excess      no ppt.

Can someone supply me the chemical equations involved in these test and what complex ions are formed when the ppt is soluble in excess solution. (Not homework)

[cliverlong]

the following, although about transition metals, may contain relevant information about your reactions

http://www.chemguide.co.uk/inorganic/complexions/aquaoh.html#top

http://www.chemguide.co.uk/inorganic/complexions/ligandexch.html#top

http://www.chemguide.co.uk/inorganic/complexions/aquanh3.html#top

The tests results you have gven suggest zinc and aluminium cannot be distinguished using sodium hydroxide or excess ammonia

[JadenErius]

its taking me a long time to read on the three links u have given me. Still reading them btw.
But anyway, Am i correct to assume that those that dissolve in ammonia form a tetraamine metal complex ion which subsequently dissolves in the solution therefore no ppt is formed?
And what about calcium. Does calcium form the same tetraamine metal complex ion? or something to that end? And doesnt comlpex ions normally only form when the solution is in excess?



(P.S. to differentiate lead and aluminium ions, sodium chloride solution is added. A white ppt is indicative of lead ion)

[cliverlong]

The key idea (I think) is this

the (+) metal ions in solution are hydrated. This is achieved by the metal ion attracting the negative end of the polar water molecule (the oxygen)

Six water molecules can fit around the metal ion (may not be true for very small metal ions) in an octohedral arrangement

The positive metal ion slightly weakens the O-H bond in liganded water molecules. Now maybe the analogy with hydrated transition metal ions breaks down here because electron lone pairs from the oxygen need a d-orbital to occupy - and may not be accessible in your metal ions

Pressing on. A base will remove one, two etc H⁺ ions from the liganded water until the charge on the complex is zero. At this point the complex ion precipitates. Both hydroxide and ammonia are bases and should be able to do this deprotonation.

the next bit is not completely clear in my mind. Sometimes (often?) adding more (or concentrated) base will either

remove more hydrogen ions from the precipitated complex so making the complex negatively charged and thus will become soluble again
or
in case of ammonia some of the water moleculess will be replaced by ammonia molecules; ligand exchange. For one-to-one replacement, ammonia for water, there is no change in charge on the complex since both molecules are neutral

I think all these deprotonations and ligand exchanges are in fact equlibria and this is discussed a bit in the Jim Clarke web pages

Clive

[AWK]

Hi guys, couldnt find a reference to this.
In a lab experiment we are required to test for Zinc, Lead (ii), aluminium and calcium ions. The procedure is that we test all 4 solutions with sodium hydroxide and ammonia solution.
Results are as such : Test with NaOH                              Test with Ammonia Solution
Zinc ion                   white ppt soluble in excess       white ppt insoluble in excess

Aluminium Ion           white ppt soluble in excess        white ppt insoluble in excess

Lead (ii) ion              white ppt soluble in excess        white ppt soluble in excess

Calcium ion                white ppt insoluble in excess      no ppt.

Can someone supply me the chemical equations involved in these test and what complex ions are formed when the ppt is soluble in excess solution. (Not homework)

Most answers are wrong

[cliverlong]

Hi guys, couldnt find a reference to this.
In a lab experiment we are required to test for Zinc, Lead (ii), aluminium and calcium ions. The procedure is that we test all 4 solutions with sodium hydroxide and ammonia solution.
Results are as such : Test with NaOH                              Test with Ammonia Solution
Zinc ion                   white ppt soluble in excess       white ppt insoluble in excess

Aluminium Ion           white ppt soluble in excess        white ppt insoluble in excess

Lead (ii) ion              white ppt soluble in excess        white ppt soluble in excess

Calcium ion                white ppt insoluble in excess      no ppt.

Can someone supply me the chemical equations involved in these test and what complex ions are formed when the ppt is soluble in excess solution. (Not homework)

Most answers are wrong

Which ones? From a quick glance at the links I provided, quite a few are correct.

Maybe the original poster can do some searching and reading and check all the answers and come up with an updated list?

Clive

[JadenErius]

Results are as such : Test with NaOH                              Test with Ammonia Solution
Zinc ion                   white ppt soluble in excess       white ppt soluble in excess

Aluminium Ion           white ppt soluble in excess        white ppt insoluble in excess

Lead (ii) ion              white ppt soluble in excess        white ppt insoluble in excess

Calcium ion                white ppt insoluble in excess      no ppt.

Sorry my bad. This should be the correct list. Lead (ii) ions and aluminium ions are tested with sodium chloride solution. white ppt formed is lead (ii) ion and no ppt is aluminium.

[AWK]

now OK

[JadenErius]

lol so what are the complex ions formed, does calcium for a complex ion? Can anyone give me the chemical equations that explain the reactions in this analysis?

[AWK]

Complex ions are formed when the precipitate dissolves in the excess of reagent

[JadenErius]

lol ok i understand that part. But what about calcium ions. Why does it dissolve in ammonia? shouldnt it form calcium hydroxide which is also insoluble in water (at least 95% of it is insoluble) ?

[AWK]

Calcium hydroxide is soluble in water  and in water ammonia solution.
When NaOH solution is used a common ion effect works

see: http://en.wikipedia.org/wiki/Solubility_equilibrium
ionic compounds

[cliverlong]

Calcium hydroxide is soluble in water  << snip >>

http://en.wikipedia.org/wiki/Lime_water

[JadenErius]

ok so from what i can deduce from what you guys are saying.
calcium hydroxide formed from the test with NaOH precipitates because the saturation point has been reached for the solubility of calcium hydroxide therefore causing most of the Ca(OH)2 to ppt out.
For ammonia's case, since ammonia produces only a spare amount of hydroxide ions. the solubility equilibrium is on the side of calcium hydroxide dissolving. Therefore, calcium hydroxide does not ppt out cause the saturation point has not been reached?