Ferroxyl indicator

Ferroxyl.—An effective method of showing that differences of potential exist between different parts of a piece of iron consists in utilising the ferroxyl indicator devised jointly by Cushman1 and Walker.2 A 1 5 per cent, solution of agar-agar jelly is prepared, a few drops of phenolphthalein added, and the whole rendered perfectly neutral whilst hot by titration either with alkali or acid as occasion requires. A small quantity of potassium ferricyanide solution is now added, and the solution poured into a shallow dish to cool. A clean sample of iron is placed on the solidified jelly and covered with a layer of warm solution, and the whole allowed to cool. After a few hours some very beautiful colour effects will have developed, and may be preserved for several months by keeping the surface of the agar covered with alcohol. 

A steel nail which has a drop of ferroxyl indicator (Figure 9.32) added shows the two important electrochemical aspects of this redox reaction. A blue mass forms showing that iron(ii) ions are present. This anodic process is the oxidation of iron atom to iron(ii) ions ... 

Figure 9.32 A rusting nail in the presence of ferroxyl indicator (an aqueous mixture of potassium hexacyanoferrate(iii) and phenolphthalein)... 

The ferroxyl indicator is a mixture of two indicators used to reveal the nature of surface corrosion on steel. Phenolphthalein in the ferroxyl indicator reveals surface areas that are becoming basic and potassium ferricyanide which turns blue in the presence of the iron (II) ions produced during corrosion. The use of ferroxyl indicator will be discussed in more details in Chap. 7. 

When the pure metal was treated with ferroxyl (see p. 72), unlike ordinary iron, it remained quite bright for an indefinite time, manifesting no tendency to corrode. On applying pressure locally, however, corrosion set in, a pink colour developing round the pressed portion, and Turnbull s blue appearing round the unpressed parts, indicating solution of the metal. 

Pores and active defects in nonmetallic coatings can be revealed by color indication or deposit fomation. On nickel substrates, dimethylglyoxime, or steel, potassium ferricyanide (ferroxyl test) indicator can be applied to surface on filter paper while substrate is made the anode. Alternatively, a substrate immersed in acidic copper sulfate can be made the cathode to form copper nodules at conductive coatings defects. 

The local action cell on a single metal surface may be revealed by placing an ordinary steel nail in the ferroxyl gel (Fig. 7.43). After a short time, a pink area develops aroimd the shank of the nail, indicating the formation of a cathode. Blue areas develop around the cold worked head and point of the nail where the iron goes into solution, indicating that these areas are the anodes. 

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