Titration Methods for Sulfuric Acid Anodizing Baths


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Q. Do you have a recommended analytical method to determine the sulfur and aluminum concentrations in sulfuric anodizing solutions? —W.W.

A. Here are three good methods that can be used for titrating the sulfuric acid anodizing electrolyte. Take your pick.

Sulfuric Acid Anodize Titration Procedure Principle:

One sample is titrated with caustic soda to determine the sum of acid and aluminum. Another is treated with potassium fluoride to precipitate aluminum and titrated to determine acid. The difference is calculated as aluminum.
Reagents: Standard sodium hydroxide solution, 1.0 N.
Potassium fluoride solution. Dissolve 50 grams of KF:2H2O in 100 milliliters of water and neutralize to phenolphthalein. Store in a plastic container. (KF will etch glass.)
Procedure: Take two separate 10-milliliter samples, dilute each to about 150 milliliters with DI or distilled water and add phenolphthalein. While stirring vigorously, titrate one portion with standard caustic to the indicator end point. Record the titrant volume as ‘A.’
Titrate the other portion slowly with standard caustic until the alumina that forms with the addition of each drop is only slowly redissolved. Now add 20 milliliters of potassium fluoride solution (or about five grams of KF powder – a rounded plastic spoon full) and continue the titration to the phenolphthalein end point. Record the titrant volume as ‘B.’
Calculation: Free H2SO4, g/l = B × normality of NaOH × 4.9
Aluminum, g/l = (A – B) × normality of NaOH × 0.9.

Titration Method for Sulfuric Acid Anodizing Bath Using Methyl Orange as Indicator:

  1. Pipette a 5-ml sample of the bath into a 250-ml Erlenmeyer flask
  2. Dilute to about 50 ml with DI water
  3. Add a few drops of Methyl Orange indicator
  4. Titrate with “B” ml of 1.0 N NaOH from orange to a yellow end point.

Calculation: “B” ml × 9.8 = g/l H2SO4

  1. Add a few drops of phenolphthalein indicator
  2. Titrate to a total of “A” ml with 1.0 N NaOH from colorless to pink end point. Slightest pink color that persists is the proper end point.)

Calculation: “A” ml – “B” ml of NaOH × 1.8 = g/l dissolved aluminum.

pH Method:

A 5-ml sample of the electrolyte is pipetted into a 400-ml glass beaker and diluted to approximately 200 ml. With the pH electrode inserted and stirring the solution continuously, the diluted sample is titrated using 1.0 N sodium hydroxide solution until a pH of 3.6 is reached. Record the amount of NaOH used as “A.”
Continue titrating until a pH of 10.0 is reached. Record the total amount of sodium hydroxide used as “B.”
Calculation: “A” × normality of sodium hydroxide × 9.8 = g/l sulfuric acid.
(“B” – “A”) × normality of sodium hydroxide × 1.35 = g/l aluminum.