Treating and disposing of exhausted rhodium plating solution

By Mike Melvin, Stuller Inc.

Rhodium plating solutions contain dilute sulfuric and/or phosphoric acids. When the solutions are exhausted, you can treat and dispose of them using a simple process that neutralizes the acid and removes any remaining metals.

Equipment

• 2 plastic containers of adequate size to hold the solution

• 1 plastic colander large enough to hold a coffee filter and fit the top of a plastic container

• 1 package of coffee filters. (Larger filters are available from restaurant supply companies.)

• 1 long-handled wooden or plastic spoon

• 1 pair of household rubber gloves

• 1 pair of plastic safety glasses

• 1 large box of baking soda (sodium bicarbonate)

Process

• Put on the rubber gloves and safety glasses.

• Carefully pour the spent rhodium plating solution into one of the plastic containers until it is half full. You will need some space in the top of the container for the bubbling action of this process.

• Put the long-handled spoon into the solution.

• Open the box of baking soda and very slowly add a small amount at a time to the spent rhodium solution. The addition of baking soda will cause a bubbling action, so you need to add it slowly. Stir the solution with the long-handled spoon regularly during this process. (Note: the gas given off during by the bubbling is harmless carbon dioxide.)

• After a while, the baking soda additions will no longer cause bubbling. This means that the solution has a neutral pH. When bubbling has ceased, add some more baking soda (about ½ cup to 1 cup, depending on the amount of solution) to make the solution alkaline, and stir it well. Allow it to settle for an hour or so.

• Place the plastic colander in the top of the second bucket lined with three layers of coffee filters. It is helpful to wet the filters with water first. Carefully pour the contents of the first container through the filter-lined colander into the second container. Any residue left in the first container should be rinsed out with water and run through the filter. The solution coming through the filter should be clear.

• When completed, the liquid in the second container can be washed down the drain, and the solids in the filter can be put in with your sweeps. Allow the filter and the solids to air dry before adding to the sweeps material.

• Store all of the materials used in the process in a box labeled Spent Rhodium Solution Kit in your plating area for future use.