When making up a brand-new sulfuric acid anodizing bath (Type II & Type III), it is important to have at least a small amount of dissolved aluminum in the bath to act as a buffer to help prevent the burning of the parts.
Larry ChesterfieldAluminum sulfate is the preferred method of providing that aluminum. In North America, it is normally supplied as aluminum sulfate in the hydrated form as Al2(SO4)3•14.3H2O. In some parts of the world, aluminum sulfate is only available in its non-hydrated form, Al2(SO4)3. The amounts of hydrated or non-hydrated aluminum sulfate to add to the bath are different. Here are the math and the chemistry.
For hydrated aluminum sulfate, start with the atomic weights of each of the elements in Al2(SO4)3•14.3H2O. Don’t confuse the atomic number for the atomic weight of these elements; they are not the same. Atomic weight is also known as molecular weight. In a chemical compound, the sum of the atomic weights of each of the elements in the compound is the molecular weight of that compound.
The atomic weights (rounded off) of each of the elements in hydrated aluminum sulfate are aluminum 27, sulfur 32, oxygen 16, and hydrogen 1.
Looks Complicated, but Simple Math
It’s actually simple math; it just looks complicated:
Al2 + (SO4)3 + H2O x 14.3 = 2(27) + 3((32 + (4x 16)) + (2(1) + 16))14.3 =
54 + 3(32 + 64) + (18)14.3 = 54 + 288 + 257 = 599
The atomic weight, also known as the gram molecular weight, of aluminum sulfate with 14.3 waters attached is 599. Since we want to know the ratio of aluminum to aluminum sulfate, we divide the atomic weight of the aluminum by the atomic weight of the entire aluminum sulfate molecule: 54 ÷ 599 = 0.0902.
So, there are 0.0902-gram atoms of aluminum per molecule of the hydrated aluminum sulfate compound.
Suppose we invert the result: 1 ÷ 0.0902 = 11.11. This means there are 11.11 grams of hydrated aluminum sulfate per gram of aluminum. Adding 11.11 grams of the hydrated compound to one liter of water will raise the aluminum concentration by 1 gram per liter.
A more practical way of saying this in metric terms: Add 1,111 grams (1.11 kilograms) of hydrated aluminum sulfate per 100 liters of the sulfuric acid bath to raise the dissolved aluminum by approximately 1 gram per liter.
In English terms: to raise the concentration of 100 gallons of sulfuric acid bath by one gram per liter using hydrated aluminum sulfate:
- 11.11 x 379 (liters per 100 gal) = 4211 grams or 4.2 kilograms
- 4211 grams ÷ 454 grams per pound = 9.28 lbs of hydrated aluminum sulfate per 100 gallons of a new anodizing bath to raise the aluminum concentration by approximately 1 gram per liter.
For Non-Hydrated Aluminum Sulfate
Follow the same procedure, but leave off the weight of the water in the molecule.
- Al2 + (SO4)3
- 2(27) + 3((32 + (4x 16)) = 54 + 3(32 + 64) = 54 + 3(96)
- 54 + 3(96) = 342
This gives the result that the aluminum sulfate molecule (no water attached) has an atomic (or molecular) weight of 342. So we divide the aluminum by the aluminum sulfate to find the ratio:
- 54 ÷ 342 = 0.158
So 1 ÷ 0.158 = 6.33, or 6.33 grams of Al2(SO4)3 (anhydrous) added to a one-liter bath of brand-new sulfuric acid will raise the concentration of dissolved aluminum by 1 gram per liter.
In metric terms: Add 633 grams of anhydrous aluminum sulfate per 100 liters of the sulfuric acid bath to raise the dissolved aluminum by approximately 1 gram per liter.
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 x 379 = 2399 grams or 2.4 kilograms
- 2400 grams ÷ 454 grams per pound = 5.3 lbs of anhydrous aluminum sulfate per 100 gallons of the new anodizing bath to raise the aluminum concentration by approximately 1 gram per liter.
- 379 liters = 100 gallons and one pound = 454 grams
Larry Chesterfield is President and CEO of Anodizing Technologies, Inc. He has experience in a job shop performing anodizing, painting, and fabrication and now offers equipment systems design and build, anodizing technical expertise, operations / process troubleshooting. Visit anodizingtechnologies.com or email him at larry@anodizingtechnologies.com
