7.3.4Ferroxyl Test for Free Iron is a highly sensitive test and should be used only when even traces of free iron or iron oxide might be objectionable. It should be made only by personnel familiar with its limitations.The test can be used on stainless steel to detect iron contamination,including iron-tool marks, residual-iron salts from pickling solutions,irondust,iron deposits in welds,embedded iron or ironoxide,etc.The test solution is prepared by first adding nitric acid to distilled water and the naddingpotassium ferricyanide,in thefollowing proportions:
Distilledwater 94 weight% 1000cm3 1gal
Nitricacid (60–67%)3weight% 20cm3 1⁄5pt
Potassiumferricyanide 3weight% 30g 4oz
Apply solution with an aluminum,plastic,glass,or rubber atomizer having no iron or steel parts ,or a swab (atomizer spray is preferred).
7.3.4.1The appearance of a blue stain(within 15s of application)is evidence of surface iron contamination(several minutes maybe required for detection of oxide scale).The solution should be removed from the surface as quickly as possible after testing using water or,if necessary, white vinegar or a solution of 5 to 20 weight% acetic acid and scrubbing with a fiber brush. Flush the surface with water several times after use of vinegar or acetic acid.6
NOTE4— Potassium ferricyanide is not a dangerous poison as are the simple
cyanides. However, when heated to decomposition or in contact with concentrated acid , it emits highly toxic cyanide fumes .
NOTE5—Rubber gloves, clothing, and face shields should be worn when applying the test solution ,and inhalation of the atomized spray should be avoided .
NOTE6— The test is not recommended for process- surface so requip-ment that will be used for processing food,beverages,pharmaceuticals,or other products for human consumption unless all traces of the test solution can be thoroughly removed.
NOTE7—The test solution will change color on standing and must be mixed fresh prior to each use
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