- According to the regulations("Indirekteinleiterverordnung") waste water must not contain more than 0,2 mg/l. (Whereby the oxidation state is irrelevant.)
Do not treat pure componds but dispose in a separate bottle.
Concentration of Chromium Salts from diluted Solutions
Generally oxidation reactions of organic componds with chromium oxide yield lots of diluted aqueous chromium salt solutions. Disposing of these solutions is quite expensive because of the large quntities. Precipitating the chrome therefore can save money. But be careful: If the aqueous solution contains acetic acid it may happen, that a precipitation as described below is not complete!
- Reduce all chromium(VI)-componds to chromium(III)-compounds by adding disulfite. Calculate the necessary amount or use an excess.
- Adjust the pH to 7-11 to precipitate the chromium(III) as chromium(III) hydroxide. Be careful! Chromium(III) hydroxide is amphoteric! It will dissolve again if the solution becomes too basic. It is quite difficult to proper fix the pH since the mixture will not buffer the pH between 7 and 11 but it will do outside this range!
- Filter the green bulky precipitate and dispose it without further treatment (= without drying) in a separate bottle!
- Check the filtrate if all of the chromium has been removed. You may be sure that this holds true if the solution is colorless. Otherwise check if further chromium will precipitate on adding barium chloride. The solution must first be prepared to be neutral using an acetic acid / sodium acetate buffer.
- Immediately clean all used glassware, to avoid dust developed by dried material.
Chromosulfuric acid should not be in use any more. If there are still some old charges, dispose them as they are in the "Materialverwaltung" (=equipment counter). A deactivation as described above would need much effort, because the acid has to be highly diluted and lots of alkali would be needed. Additionally sodium sulfate would precipitate which is only poor soluble in water.