Anodizing Q&A: Calculation for Adding Aluminum to a New Anodizing Bath

Q. We can only buy non-hydrated aluminum sulfate [Al2(SO4)3] to add dissolved aluminum to a new sulfuric acid anodizing bath. I have read your article in the Products Finishing archive (short.pfonline.com/scabm) about the amount of aluminum sulfate to add in the hydrated form [Al2(SO4)3 • 13 H2O]. Would I have to add the same amount of non-hydrated aluminum sulfate, or would it be a different formula? How is that calculation done?

A. The amounts of hydrated versus non-hydrated aluminum sulfate to add to the bath would definitely be different. Here is how the calculation goes:

• Start with the atomic weights for each element in Al2(SO4)3

Aluminum= 27

Sulfur= 32

Oxygen= 16

• Al2 + (SO4)3

2(27) + 3[(32 + (4 × 16)] = 54 + 3(32 + 64) = 54 + 3(96) = 342

The result is that the aluminum sulfate molecule (no water attached) has an atomic (or molecular) weight of 342. We want to know the ratio of the aluminum to aluminum sulfate.

• The aluminum in this molecule is 54 and the entire aluminum sulfate molecule is 342.
• Divide the aluminum by the aluminum sulfate to find the ratio:

54 ÷ 342 = 0.158

So 1 ÷ 0.158 = 6.33, or 6.33 g of Al2(SO4)3 added to a one liter bath of brand new sulfuric acid will raise the concentration of dissolved aluminum by 1 g/l.

• In metric terms: Add 633 grams of non-hydrated aluminum sulfate per 100 liters of sulfuric acid bath to raise the dissolved aluminum by approximately 1 g/l.
• In English terms: to raise the concentration of 100 gallons of sulfuric acid bath by one gram per liter using non-hydrated aluminum sulfate:

6.33 × 379 = 2399 g or 2.4 kg

2400 g ÷ 454 g/lb = 5.3 lbs

Remember: 379 liters = 100 gal and 1 lb = 454 g

Originally published in the September 2015 issue.