How to dispose of oxalic acid solution

At some point, you may have to use oxalic acid for a cleaning project in your home (it is especially useful for cleaning rust stains out of concrete). Should this be the case, properly dispose of the acid or acid solution you have made. Oxalic acid is a Class A organic chemical, a carboxylic acid. Although it is an acid, it is readily biodegradable when properly diluted and neutralised.

Dilute the oxalic acid. Pour the acid into a larger container of ice water. Pouring acid into water eliminates the risk of boiling and splashing that can result from pouring water into acid. Always pour acid into water and not the other way around.

Neutralise the oxalic acid. Slowly pour baking soda into the dilute. It will begin to bubble. Continue adding baking soda until the bubbling stops.

Test the pH. Using litmus paper, dip the strip into the solution until it reaches a pH of at least 5.5. It is best to continue adding baking soda until the solution reaches a pH of 7 (neutral on the pH scale), but a pH of at least 5.5 is safe to pour down the drain.

Pour down the drain. At this point, the solution has been neutralised and no longer poses a toxicity hazard. Pour down the drain while flushing with lots of excess water. Up to 100g or 100ml at a time are suitable for flushing down the drain while flushing with excess water.