# Aluminium ions are acidic?

1. Jan 2, 2007

### pivoxa15

In a paragraph in the textbook "When CSIRO scientists were called in to investigate, they discovered that the cause was the oxidation of iron pyrites in these area, much of which had been exposed as a result of draining swamps for grazing cattle and the use of floodgates to drain cane fields. This released sulfuric acid, which in turn mobilised the aluminium ions in the clays. These ions are also acidic."

How can aluminium ions be acidic? Is it saying Al(3+) is an acid?

2. Jan 2, 2007

### Gokul43201

Staff Emeritus
I'm not sure exactly what the passage means, but it's probably just saying that the aluminates (or oxides or hydroxides) in the clay are acidic. Many aluminium compounds are known to be amphoteric (notably the oxide and hydroxide) - they will neutralize acids as well as bases, by complexing with the free H+ or OH- in solution. Others are slightly acidic and some are slightly basic.

And when it says "aluminium ions", I'm not sure it necessarily means Al(3+) but it might. Furthermore, in acidic media the ions in solution are probably more like [Al(H2O)n]3+. In any case, that passage is written too casually (lacking any kind of precision of description) to be taken seriously.

3. Jan 2, 2007

### dichotomy

i think it implies $$[Al(H_{2}O)_{6-n}(OH)_{n}]^{3-n}$$

the way i understood it, the high charge density of the "Al3+" ion dragged in electron density off the water ligands, so the water ligands could be deprotonated to form the above complex (though the number of deprotonations depends on the sulphuric acid concentration) and hydronium ions.

Last edited: Jan 2, 2007
4. Jan 3, 2007

### pivoxa15

So it is talking about Aluminium when part of a compound and that compound being an acid? I know that Al(3+) cannot donate a proton (hence not an acid) unless its a nuclear reaction which I don't think was on the author's mind.

5. Jan 5, 2007

### lightarrow

Look for Lewis definition of acidity (more general).