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Calculating solubility of aluminum in aluminum sulfate (AL2(SO4)3) at various pH levels?

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Alum (aluminum sulfate) is used as a flocculant to floc an anionic material in a water solution at pH = 4.0 and temperature = 130 degrees F. The solubility of the alum is noted as 87 grams per 100 cc's of liquid (assuming std temp & pressure).

Assume 1 liter of solution....based on this, how would I determine the actual solubility of the aluminum ion (how much dissolved vs. not) at say a pH of 3.0 vs. 4.0 vs. 5.0.

Typical aluminum chemistry suggests that lower pH increases solubility given a fixed temperature of 130 degrees F ---- I also heretell that the optimal pH for alum chemistry is in the range of 4.5 to 5.0.

But I do not understand this and want to know how to derive the soluble vs. insoluble Al at these various pH ranges.

I am ultimately trying to drive the following:

Al(SO4)3 + 6H2O = 2Al(OH)3 + 3H2SO4

My theory being that if I have more insoluble alum sulfate, I will produce more aluminum hydroxide (assuming sufficient OH)

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Alum (aluminum sulfate) is used as a flocculant to floc an anionic material in a water solution at pH = 4.0 and temperature = 130 degrees F. The solubility of the alum is noted as 87 grams per 100 cc's of liquid (assuming std temp & pressure).

Assume 1 liter of solution....based on this, how would I determine the actual solubility of the aluminum ion (how much dissolved vs. not) at say a pH of 3.0 vs. 4.0 vs. 5.0.

Typical aluminum chemistry suggests that lower pH increases solubility given a fixed temperature of 130 degrees ...

Solution Summary

This solution describes how to calculate solubility of aluminum in aluminum sulfate at various pH levels.

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See Also This Related BrainMass Solution

Solubility of Zinc Oxide, Boric Acid, and Aluminum Sulfate

Hope you can help....I've always struggled calculating solubilities of compounds at different pH's. I have 3 separate but related questions:

1) 1000 g of zinc oxide is added to a 1 liter container of pH neutral water (pH 7.0); the solubility of the ZnO is given as .29 g/L at pH 7.0; how much more ZnO would dissolve if the pH dropped to 6.0?

2) 1000 g of boric acid H3BO3 is added to a 1 liter solution of pH neutral water; the solubility of the boric acid is given as 57 g/L; again, how much more boric acid would dissolve if the pH dropped to 6.0?

3) Paper-maker's alum (Al2(SO4)3-14H2O has a pH of 3.0; the solution of alum used is given as 38% alum solids; it's solubility is unknown; 1000 g of the alum is then added to 1 liter of pH neutral (7.0) water; a) how much alum dissolves? What is the resultant change to the pH of the overall solution?

I really want to understand how these solubility calcs work, so do your best to make the analysis complete but yet simple.

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