Hydrofluoric-Nitric-Sulfuric Acid Method

Abrade the surface with medium-grit emery paper 

Using equipment constructed of fluoropolymer resins, polyethylene, or polypropylene, prepare the solution given in Table 6.26. 

Blend the hydrofluoric acid and the nitric acid with water. Then slowly add the sulfuric acid, stirring constantly with a polytetrafluoroethy-lene or polyethylene rod. Add a few drops of 20% hydrogen peroxide. 

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Subboiling distillation also is an effective technique for the purification of nitric, hydrochloric, hydrofluoric, perchloric, and sulfuric acids. Typical analyses indicate a 100-fold reduction in the level of metallic impurities.73 While this method is extremely efficient for the removal of metal ions, it offers litde purification from impurities with high vapor pressures, such as organic matter and many of the anions. 

The analysis of mineral acids — sulfuric, nitric, hydrochloric, hydrobromic, hydrofluoric, and phosphoric acids—in air may be performed by NIOSH (1984) Method... 

Chemical Methods. Chemical methods include pickling, which can be used to remove scale and metal oxides. Sulfuric acid and hydrochloric acid are usually used to treat steel and zinc, while nitric acid, nitric acid-hydrofluoric acid, or sulfuric acid-hydrofluoric acid are used for aluminum. Although pickling in phosphoric acid is somewhat slower, it has the advantage that a thin conversion layer of the substrate phosphates is produced, which provides temporary corrosion protection... 

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