Regeneration of Chromic Acid Cleaning Solutions
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Spent chromic acid solutions present a waste disposal problem in many laboratories. Chromic sulfate (Cr2(SO& nH~O),sulfuricacid, and water are the typical components. Distillation followed by filtration separates anhydrous chromic free SO$), and water, simplifying disposal or recovery. sulfate, oleum (fuming sulfuric acid-HzS04 The first step is to assemble a distillation apparatus using a flask, distillation head, and condensation column. The spent cleaning solution is distilled a t 260-300 'C with continuous stirring. The first condensate is water with the quantity produced being dependent upon the original solution composition. After sufficientheating fuming sulfuric acid will form in the distillation flask (copious quantities of white fumes are evident). At this time it is prudent to change the receiver flask to prevent an overly vigorous reaction of oleum with water. (The first fraction may be discarded.) As the distillation continues, the chromic sulfate is dehydrated and changes the solution color from the original green to violet. Finally, a peach colored precipitate of anhydrous chromic sulfate forms. When precipitation is complete the apparatus is cooled. The precipitate can now be filtered through a glass fiber filter (anhydrous chromic sulfate is practically insoluble in acids and water). The chromic sulfate can be bottled for storage or ignited to form the oxide (CIZOQ)and then reoxidized to sodium chromate (NaCr04) using a sodium peroxide fusion. Storage is recommended because of the time-eonsumingnature of the reoxidizine moeedure and the relativelv low cost of sodium chromate. ~, The filtrate and o h m fractiuns can be carefully combined. Regeneration of concentrated sulfuric acid ir acromplished hy adding the oleum to 7UT sulfuric acid in a 1:l ratio. The regenerated arid is used for the preparation of the next batch of cleaning solution. Use caution when handling oleum and particularly when diluting the oleum.
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Davhl Schwab and Pal MartInez New Mexico lnstltute of Mining and Technology S O C O ~NM . 87801
528
Journal of Chemical Education
