How to Make a Chromic Acid Plating Solution

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Chrome plating with chromic acid solutions provides a corrosion-resistant metal surface.
Chrome plating with chromic acid solutions provides a corrosion-resistant metal surface. (Image: Jupiterimages/Comstock/Getty Images)

Chromic acid plating solution is used in the electroplating of metals for making surfaces decorative and corrosion resistant. There are several recipes for making the plating solution. The ingredients of the most common electroplating bath include chromium trioxide and sulfuric acid. The ratios of these chemicals in the recipe vary depending on the objective of the plating. There are "hard plating" and "decorative plating" recipes that have different ratios of chromic acid to sulfate. Some bath solutions include flouride catalysts which increase the rate of chromium deposition.

Things You'll Need

  • Sulfuric acid
  • Chromium trioxide crystals
  • Distilled or deionized water
  • Plastic container for the acid bath
  • Container for measuring volume in liters
  • 5 ml pipet
  • Pipet bulb
  • 100 ml graduated cylinder
  • Weighing scale
  • Weighing paper or boat
  • Sulfate test kit
  • Rubber gloves
  • Face shield
  • Rubber or plastic apron

Fill the bath container to the halfway mark with distilled or deionized water and calculate the amount of chromium trioxide and sulfuric acid to be added to the solution. For decorative plating, add 300 grams per liter of chromium trioxide and 3 grams per liter of sulfate. For 50 percent sulfuric acid, you will need to 61 milliliters per liter of bath solution to achieve this sulfate concentration.

Weigh the the amount of chromium oxide needed and add it to the water in the electroplating tank. Measure the amount of 50 percent sulfuric acid that is needed and add it slowly to the tank. Gently stir the bath tank solution to assure that all of the chromium oxide crystals dissolve into the solution.

Transfer by pipet 2.5 milliliters of the electroplating solution to a 100 ml graduated cylinder that has 97.5 ml of deionized water to make a dilution. This dilution is necessary because the testing range of the testing kit for sulfate, given in the references below, is 50-200 milligrams per liter.

Mix the solution in the graduated cylinder with a plastic or glass rod. Filter the solution before testing if there is sediment or suspended particles. Then perform the sulfate test according to the instructions of the test kit.

Multiply the results of the test kit by 40 to arrive at the grams per liter of sulfate in the electroplating solution. Adjust the concentration if needed.

Tips & Warnings

  • Hard plating processes use a chromic acid to sulfuric acid ratio of 155-200:1. Decorative plating use ratios from 100-125:1
  • Convert grams per liter to avoirdupois ounces per gallon by multiplying by 0.133.
  • Addition of fluoride catalysts to the bath will double the rate of chromium deposition.
  • If there are particulates in the solution for sulfate testing, it must be filtered prior to analysis.
  • If the level of sulfate becomes to high, barium carbonate can be used to precipitate out the sulfate. Barium sulfate is formed.
  • Wear protective equipment when working with sulfuric acid and chromium trioxide.
  • Pour acid into water slowly. Never pour water into acid.
  • Ventilation in the area of the electroplating bath is necessary because hydrogen gas is produced in the process.

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