FUNDAMENTALS I N APPLIED ELECTROCHEMISTRY' COLIN G. FINK Columbia University, New York
AT THE VERY start in the very first lecture it is impor- have attended courses in physical chemistry. Unfortant to emphasize the fact that electrical methods and tunately their experience has been confined to very electrical reactions are usually of decided advantage dilute solutions-so dilute that you can hardly detect over older mechanical and chemical methods. A very by taste the presence of any salt. The laws of these very convincing curve is the one for the cost of electricity dilute solutions do not, in general, apply to the solutions over the period of the last twenty-five years as against employed in applied electrochemistry. the cost of labor. As the price of labor goes up with A hundred books have been published on dilute soluthe years the price of electricity goes down. "Do it tions but not a single acceptable volume on concenelectrically" is the slogan to bear in mind throughout trated solutions. The simple relation PV = RT the course in electrochemistry. We divide our course breaks down for concentrated solutions. The aqueous into three chapters: (1) Reactions in Gases; (2) Re- solutions we meet in electrometallurgy frequently are actions in solutions; and (3) Reactions in Fused Salts. so concentrated that a lo-degree drop in temperature will cause one or another salt to crystallise out-a very REACTIONS IN GASES disturbing factor when it does occur in any electroThe electric discharge through gases is a most fasci- chemical process. Aqueous solutions with but one nating field of electrochemistry. Due to the student's constituent are practically never encountered in pracacquaintance with the radio tuhe he is interested from tice. On the other hand, the student must appreciate the start. Personally, we are convinced that, for ex- from the start that recoveries are not to he confined to ample, the electric fixation of nitrogen will eventually the main product, say copper, but must include a numhe cheaper than any other fixation process. Even her of by-products, such as silver, gold, platinum, arthough the Bradley and Lovejoy process and its sue- senic, and selenium in the copper-sulfate electrolyte. cessor, the Birkeland and Eyde process, for the fixation Our approach to the electro-hydrometallurgy of c o p of atmospheric nitrogen are not in use today, it is good per, zinc, nickel, and many other metals is that of a to outline the processes briefly and point out the weak- problem to be tackled and solved by the class. For exnesses-why they failed commercially. But do not ample, take the electro-winning of copper from the bring the session to a close without indicating the new Chuquicamata copper ore assaying 1 per cent copper. developments in the offing and emphasize the fact that A number of problems had to be solved: supply of nitrogen is not the inert gas of our freshman chemistry. water, power, engineers, low-cost insoluble anode and A number of our best research electrical engineers have new ideas. , tackled the nitrogen fixation problem and i t has been The mountain of ore is situated in the desert not far established beyond the slightest doubt that nitrogen is from the Chile nitrate deposits. The mineral is largely one of the easiest gases to fix in the electric discharge hrochantite, a basic copper sulfate. This fact was one tube. A neon tube will glow for 3000 hours with its of the most fortunate items in the many problems eoncharacteristic red color. Take out the neon and put fronting the chemical engineers. Sulfate solutions have nitrogen into the tuhe in its place and the nitrogen will been used in chemical factories for many years and sulglow for about 30 hours only and after that no current fate-resistant equipment is available in various forms will pass through the tuhe because the nitrogen has dis- and appliances-tanks, launders, pipes, pumps, etc. appeared. It has been "fixed"; the tuhe has grown As you present one problem after another to the clam "hard." William J. Cotton superimposes a high-fre- you will find considerable rivalry in suggesting the dequency discharge upon the low-frequency discharge sired solution. Eventually when the complete process through nitrogen and turns out nitric oxide in quantity has been presented and discussed the class then appremore than double the amount ever obtained by any of ciates the "whys and wherefors" of various procedures. the older processes for the same amount of electricity TERMINOLOGY consumed. The world-wide accepted terminology of electr* AQUEOUS ELECTROLYTES chemistry is usually not accepted by students without a Most of the students in our classes in electrochemistry struggle. Thus the designation of the electrodes of a cell often causes confusion. Accordingly, we have Addres.8 delivered at the luncheon of the Division of made the terms in several different chemical ~ d ~at the ~ 112th ~ meeting i ~ of~ the, ~ ~ ~ , . it ai point ~ to ~ define ~ ways. For example, the cathode is that pole or elecChemical Society, September, 1947.
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tmde: 1, that 'emits electrons; 2, to which positive charged ions migrate under the influence of the electric current; 3, a t which metals are deposited; 4, a t which hydrogen is evolved; 5, a t which reduction takes place; 6, at which negative charged hydroxyl ions (OH-) are formed; 7, that has an alkaline surface film. Definition 1 is usually approved by the class without hesitation. This is attributable to familiarity with the operation of the radio tube.' Definitions 2, 3, and 4 are admitted by the students to be correct. But definition 5 leads to a little confusion because only a small proportion of the class can visualize reduction without hydrogen. Even more confusing are definitions 6 and 7. That the surface of the cathode has an alkaline or hydroxyl film, hydroxyl ions with negative charges, seems definitely preposterous. So, rather than discuss the dissociation of water and the accumulation of hydroxyl ions (with negative charges) at the cathode, a few experiment! will clear up the situation in shorter time than a series of drawings on the blackboard. In the first experiment current is passed through a 1g./L. sulfuric acid solution using platinum electrodes. Hydrogen is shown to be evolved a t the cathode and oxygen at the anode. By means of potassium-iodide paper the polarity is easily established and the iodine stain on the paper at the anode can be seen by the whole class. We next proceed to show the presence of a layer of hydroxyl ions (OH-) at the cathode. This is done very simply by adding a drop of phenolphthalein solution to the sulfuric acid solution. Tipon electrolysis the reddish layer of solution next the cathode surface is readily visible. Another experiment performed in class to establish the presence of the hydroxyl layer next to the cathode is the electrodeposition of gold from very dilute chloride solutions. We use a rotating nickel disc. When this is stationary no gold deposit appears uponselectrolysis. But when the disc is rotated a t high speed a bright deposit appears. The interpretation is simple. Rotating the cathode, the hydroxyl film is reduced to a minimum and gold ions reach the cathode surface.
JOURNAL OF CHEMICAL EDUCATION
experiment introduces to the student (almost almays for the first time in his career) the measurable effect of dissolved oxygen in the electrolyte, obtained from the air in contact with the electrolyte. For years he may have studied properties of solutions, and simply because he could not easily detect the presence of oxygen in his electrolytes he, like thousands of others, has ignored it. In the case of the copper coulometer we can eliminate (or almost so) the effect of dissolved oxygen by the addition of a substance more readily oxidized by dissolved oxygen than the cuprous ion (Cu+). The reagent usually added to the copper coulometer is ethylalcohol. To be sure, this is not a direct method for detecting the oxygen. Another indirect method is the preparation of cuprous chloride crystals in the absence of air and moisture. The salt obtained is white. Now admit a little air and the mass of crystals turns green. These class demonstrations indicate very forcibly the difficulties encountered in the electrodeposition of magnesium and aluminum from aqueous solutions. And yet, these difficulties are practically overcome upon eliminating most of the dissolved oxygen by substituting an organic solvent for the aqueous solvent, the magnesium and aluminum salts. Beautiful bright crystalline metal deposits of magnesium and aluminum are then obtained. Returning to the laws of Faraday, there is one question that arises perennially and persistently and that is, "What voltage am I to apply to my cell used to demonstrate the laws of Faraday?" This leads to a discussion of the decomposition voltage and a discussion of the socalled "standard" potentials. A few experiments performed in class soon clarify a rather puzzling conception and interpretation. What surprises most students is that the polarity of a metal electrode can be either cathodic or anodic. In an alkaline solution tungsten is anodic and copper cathodic, whereas in an acid solution copper is anodic and tungsten cathodic. Similarly, in a cyanide solution gold is anodic and iron is cathodic. Is there such a thing as a standard potential? FUSED ELECTROLYTES
The laws of Faraday apply rigidly to the fused salts. The cathode product is crystalline, as in aqueous elecTHE LAWS OF FARADAY trolytes and organic electrolytes. Class demonstraConsiderable time is devoted to demonstrations in tions with fused silver nitrate are easily carried out. support of Faraday's laws. These lams are most fund* Beautiful silver crystals are obtained. A recent dismental and apply to aqueous solutions as well as to covery in our laboratory is the presence of a cathode surfused salt baths. It is most important for students to face film in fused electrolytes. This is usually an alkali appreciate the exact relation between the total electric metal film. The thickness of this film is regulated and current that has passed through a cell and the weight of controlled by voltage, current density, and bath temcathode and anode products. There are hut few dis- perature. The surface film must not be too thick nor coveries in the physical science field that can compare in too thin. The findings are very similar to those we disrevolutionary effect and world-wide importance with covered for the essential hydrogen film in the case of the electrochemical discoveries of Faraday. For dem- chromium plating from aqueous solutions: "The atomic onstration purposes we use the coulometers, in particu- chromium hydrogen film must not be too thick nor too lar the copper coulometer. The copper coulometer, thin." We found that the conditions for bright chrowe can show, always gives.lower values than the silver mium plate are readily obtained if the sulfate concentracoulometer. But there is a means by vhich the results tion is maintained at a ratio of about one to a hundred from the two coulometers may be made identical. This of chromate ion. That very similar conditions prevail
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tuents, such as glue and certain other colloids. Another interesting observation is the change in crystalline structure when depositing one metal onto anoth~r. The crystal forces active in the first or basis metal will make the second or outer metal assume the same crystal shape as that of the basis metal. A crystalline deposit at the cathode is the usual and natural occurrence. An amorphous metal deposit is practically nonexistent. From all of our observations a metal ion upon discharge a t the cathode forms a unit colloid metal particle and this in turn is quickly (almost instantly) transformed into a unit metal crystal. At times it may be desirable to produce relatively fine crystals, as in the electrolytic parting of gold from silver, or in the electrodeposition of tin. Certain additions to the solution are necessary. These facts can be easily demonstrated in class. In closing, may we say that it is much more imporCATHODE CRYSTALS tant to drive home a few basic principles in electroOne of the most fascinating andintriguing phenomenon chemistry than to have students try to memorize dozens in electrochemistry is the formation of crystals of metals of relatively inconsequential facts. Stress the marvels upon the electrodeposition of these metals from any one of electrochemistry. Arouse and establish the student's of several types of electrolytes; aqueous, fused salt, interest in electrochemistry; get it firmly rooted in his organic, gaseous, etc. The underlying crystallization mind and it will remain with him forever, no matter forccs are very pronounced and metal crystals may be what division of science or technology he makes his life made to grow to almost any large size providing the work. He will find frequent occasions to apply one or solution is kept free from growth-interfering consti- the other of the principles of electrorhemistry. in the case of fused salt baths was most gratifying. It is well to emphasise that fused salts are simpler than salts in aqueous solutions. Dissociation is usually near 100 per cent. Complex formation is unusual. Cathode deposits are purer. The three commercial metals produced by fused electrolytes are the cheapest metals (per unit volume) next to iron: sodium, magnesium, and aluminum. I t is possible to produce sodium metal without the aid of electricity, and this sodium metal can he used to produce aluminum metal without the aid of electricity. But these processes are no longer used commercially since electrochemical methods are so very much cheaper. Furthermore, products can be turned out electrochemically which are impossible by older methods based on coal or oil heating or based on simple chemical reactions.
