Cleaning chromic acid

The most widely used cleansing agent is the chromic acid cleaning mixture. It is essentially a mixture of chromic acid (CrOj) and concentrated sulphuric acid, and possesses powerful oxidising and solvent properties. Two methods of preparation are available —... 

Acid pickles Some of the acid pickles used to clean and etch aluminium alloy surfaces and remove oxide and anodic films, such as the chromic/ sulphuric acid pickle (method O of DEF STAN 03-2) and other chromic-acid bearing pickles (App. Foi DEF-151) probably leave on the surface traces of absorbed or combined chromate which will give at least some protection against mild atmospheres. 

A method which is frequently used consists in filling the apparatus with chromic acid cleaning mixture (CARE), a nearly saturated solution of powdered sodium dichromate or potassium dichromate in concentrated sulphuric acid, and allowing it to stand for several hours, preferably overnight the acid is then poured off, the apparatus thoroughly rinsed with distilled water, and allowed to drain until dry. [It may be mentioned that potassium dichromate is not very soluble in concentrated sulphuric acid (about 5 g per litre), whereas... 

Hydrocarbons and carbonized or coke deposits can be removed by chromic acid. The chromic acid oxidizes the binders holding the deposits together. Use a 10 to 20% solution for 12 to 24 hours at 190 to 200 °F. Chromic acid cannot be effectively inhibited and is not suitable for cleaning copper, brass, aluminum, zinc, or cast iron because these are all rapidly attacked. 

Electrode chemically cleaned with chromic acid. 

This left something else as the possible cause of the variability. When we considered the nature of the test, which was sensitive to parts per million of organic materials, we realized that one possibility was contamination of the glassware by the soap used to clean it. We next cleaned all glassware with chromic acid cleaning solution, and reran the tests, with the result as shown in Table 9-5. 

Table 9-5 Results after cleaning glassware with chromic acid...

Pesticides Glass 1100 g None Unlimited Bottle chromic acid cleaned, ground-glass stoppered and wrapped in polyethylene bag. 

Oils Glass 1100 g None Unlimited Chromic acid cleaned. Separate bottle from main sample, bottle 2/3 filled with sample. 

It is helpful to empty the brown tarry residue from the distillation flask while it is still hot. The flask can then be cleaned by using a sulfuric acid-chromic acid solution. 

This flask must be cleaned with h ot chromic acid solution and then, along with all other glassware used in this preparation, soaked in a base solution, rinsed with distilled waiter, and oven dried. Thermal rearrangement of the intermediate vinyl ether in a new (untreated) flask resulted in elimination. 

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