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# Sulfuric Acid Anodizing Bath Makeup

Q. Please explain to me the ratio of acid in the anodizing baths; usually the ratio is from 15 to18 percent, but I don’t know if this ratio is calculated by volume or by weight.

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Q. I’m a beginner in the anodizing field. Please explain to me the ratio of acid in the anodizing baths; usually the ratio is from 15 to18 percent, but I don’t know if this ratio is calculated by volume or by weight. —H.N.T.

A. Here is the formula for calculating the amount of sulfuric acid to use for the anodizing bath makeup. Please note that this formula is for using 66° Baumé (93 percent) sulfuric acid. If you're not using 66°, you will have to adjust the formula accordingly. The bath is always made up by weight of sulfuric acid, not volume. If you make up the bath by volume, it will give results almost twice the concentration you want, because 93-percent sulfuric acid weighs almost twice as much as water.

Metric calculation:
Liters of 66° Bé H2SO4 per 378 liters of anodizing bath = 100 × % by weight bath desired × 2.37.
Example: Liters of H2SO4 required for a 500-liter, 15 percent by weight anodizing bath = 100 × 0.15 × 2.37 × (500/378) = 47 liters.
English calculation:
Gallons of 66° Bé H2SO4 per 100 gallons of anodizing bath = 100 × % by weight bath desired × 0.625.
Example: Gallons of H2SO4 required for a 500-gallon, 15 percent by weight anodizing bath = 100 × 0.15 × 0.625 × (500/100) = 47 gallons.

The concentration of the anodizing bath should be expressed in grams per liter (g/l). The amount of dissolved aluminum in the bath should be controlled between 5 g/l and 15 g/l for “general purpose,” Type II anodizing baths. Some specifications call for less than 5 g/l of aluminum. It’s best to follow the specification being used. Add aluminum sulfate to a brand new bath in order to get some aluminum in the bath. It’s best to have about 2 g/l of aluminum to start with. This “buffers” the bath and helps prevent burning of the parts. Aluminum sulfate is usually sold in the hydrated form with 13 molecules of water attached to each molecule of aluminum sulfate. The formula for adding Al2(SO4)3 * 13H2O is: 11 g Al2(SO4)3 * 13H2O per liter of bath to raise the aluminum concentration by one g/l.

Metric calculation:
To raise the aluminum to 2 g/l in a brand new 500-liter bath, it would require 2 g/l × 11 g/l × 500 liters = 11,000 g, or 11 kg of aluminum sulfate.
English calculation:
To raise the aluminum to 2 g/l in a brand new 500-gallon bath, it would require 2 g/l × 11 g/l × 500 gal × 3.785 liters/gal = 41,635 g, or about 92 pounds of aluminum sulfate. It helps to remember that 3.785 liters = 1 gallon and 454 grams = 1 pound.

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