a
DEMONSTRATIONS USING THE FERROXYL REAGENT WILLIAM B. MELDRUM Haverford College, Haverford, Pennsylvania
Tm
ferroz$ reagent was developed by Allerton S. Cushman, of the U. S. Department of Agriculture, in 1909.' It may be used effectively and convincingly to demonstrate the mechanism of the oxidationreduction reaction occurring when iron or steel corrodes. It may also be used to show the nature of the protection against corrosion furnished by coatings of metals more and less active than iron, and to study other factors affecting the corrosion process. The electrolytic theory of corrosion was advanced by Willis R. Whitney and others only a few years after the enunciation of the dissociation theory by Arrhenius in 1887. It has long been accepted as representing a t least the initial stages of the process. The theory, in brief. is as follows. If a piece of iron or steel, such as an ordinary wire nail, is placed in a dish of water, or even in a drop or a film of water, corrosion occur%made evident by the appearance of the familiar iron rust. It is assumed that at some point, A, as in Figure the solution tension of the iron, the tendency of the iron atom to lose electrons and pass into solution as the ferrous other point, B, and the ion, 'is greater than at change occurs, Fe + F e + +
+ 2e
The metal thus assumes a negative charge because of t,he electrons deposited and the solution a corresponding positive charge due to the ferrous ions formed. Unless these charges are somehow neutralized the process quickly comes to a halt. The means for the neutralization are provided by the water itself. The hydrogen ions formed by its slight ionization, 2H20
* H30C+ OH-
deposit a t the point B, 2HaO+
+ 2e
-
H,
+ 2H,O
removing the electrons from the iron and diminishing the positive charge of the solution. Thus the iron a t A may continuously pass into solution and simulof B the hydroxyl taueously in the concentratior~ increases. The ferrous and hydroxyl ions diffuse through the solution, interact, and in a series of reactions into which both the water and the dissolved oxygen enter, produce ultimately iron rust, I Cr~sr1x.wA. S., U. S. Department of Agriculture, Department of Roads, Bulletin No. 35, 5,909; "The Corrosion and Preservation of Iron and Steel," MoGraw-Hill Book Co., New York, 1910.
F i 6
----
H30
+
Hz0
Fe++
9"
0 2
4 F~~O&O)X outline08 the comosion proce= the composition of which may be represented by F~~O~(BO),. The initial steps in the process may readily be tested in twoways, using either the familiar lecture table meter or a box galvanometer, the sensitiveness of which is cut down by a 2- or a-ohm shunt, it may be shown that any two pieces of iron, selected a t random, placed in a solution of, say, ferrous sulfate, show a difference of potential, indicating a difference in solution tension. Further, by keeping one such electrode fixed in position, it may likewise be shown that the two ends of nail or other article have potentials, The chemical reactions involved in the theory may be demonstrated using the fenoxyl reagent, In preparing the fmoxyl reagent to 20 g, of agar is added to liter of water, the mixture boiled gently for about hour to peptize the agar and filtered through a cloth. Two ml. of phenolphthalein indicator solution and 5 to 7 ml. of 1 per cent potassium ferricyanide solution are added for each 100 ml. of the agar solution and the solution brought to a pH of about 8.0, just short of the appearance of the red color, with dilute sodium hydroxide solution. The reagent is then ready for use. Enough of the agar solution, still hot, is poured into a suitable container, such as a Petri dish, to form a foundation layer which is allowed to cool until it has gelled. The nail, or other iron article, is pressed into place in this layer and a t once sufficient agar solution is added to cover it completely. In this step the agar solution should not be too hot as distortion of the M ~the~ petri , dish may be may floated On ice water to hasten the gelling. The effects, illustrated in Figure 2, are observable
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~ i g u mI.
MAY. 1948
within a few minutes and develop to a maximum in two or three days. At those points where the iron goes into solution the ferrous ions react with ferricyanide ions,
When a tinned nail is cut through a t one or more points with the emery wheel and placed in the ferroxyl reagent it is found that the iron goes into solution, the red color appearing on the tin. The prot,ection afforded by tin, copper, cadmium, or other less active 3FeCC FC(CN)~'+ FeaFc(CN), ( metal than iron, is merely mechanical; the coating producing the blue ferrous ferricyanide precipit,ate. protects only when it is complete. Where the hydrogen ions discharge,
+
hydroxyl ion remains in excess, the p H rises, and the phenolphthalein turns red. For demonstration purposes wire hishing nails give the most satisfactory results. Nodes of the blue precipitate appear at the head and the point because at these points the solution tension of the iron is greatest. Along the stem the red phenolphthalein appears. By touching various points along the nail with the emery wheel, strains are set up and in the ferroxyl reagent nodes of blue precipitate appear alternating with red zones. For variety other articles may ,be used. Unequal solution tensions depending upon inequalities in strains or in compositions are indicated by the nodes appearing in the ferroxyl reagent. A wood screw will show "corrosion" along the thread, the red developing around the upper part where the strain is less. A washer, having been stamped from the hot metal, has greater strain around the hole and so the hole soon fills up with the blue precipitate. The effectsof a magnetic field,of adiiect current, and of various other factors, have been examined but so far with inconclusive results. The nature of the protection against corrosion afforded by metallic coatings on iron may also be demonstrated using the ferroxyl reagent. The metals, in this respect, fall into two categories: Those with lower solution tension than iron, like tin, copper, nickel and cadmium, and those with higher solution tension, like zinc. Tin, on the one hand, and zinc, on the other, serve most readily for demonstration purposes. In order to coat an iron nail with tin or zinc, pickle the nail by dipping in concentrated hydrochloric acid and then dip it, first one end and then the other, into the molten metal contained in a large porcelain or pyrex brand vycor crucible.
rigur.2. Nail. "R".ti"." in F.rro*yl R.a..nt? Zon.. a : Blue pr.sipitat. of ferrous forrisyanide. Zen.. b ; R.d phenolphthal.in at p H -.tar than 8.3
On the other hand, a galvanized nail, with the eoating cut through at several points, shows no blue nodes when immersed in the ferroxyl reagent. Instead the zinc goes into solution, a white precipitate of zinc ferricyanide appears, and the red color develops on the exposed spots of iron. The protection in such a case is electrolytic. Ferroxyl jellies may be kept for some time if placed in a desiccator over 90 per cent alcohol. If kept in the open they dry out and usually, within a week or so, develop spots of mold. In conclusion it should be pointed out that the study of the rusting process using the ferroxyl reagent merits consideration as a laboratory exercise in the general chemistry course. Lieut. R. J. Perry, of the U. S. Coast -Guard Academy, has adapted the reagent very successfully to this purpose. a For an actud photograph of the effect see MELDRUM, W. B., AND F. T. GUCKER, "An Introduction to Theoretical Chemistry,"
American Book Company, New York. 1936, p. 402.
