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with three inhibitors-isopropyl, isobutyl, and benzyl alcoholin a given concentration of sodium sulfite solution, the amount of alcohol oxidized per unit time is constant over a wide range in concentration of inhibitor, this range varying with the inhibitor chosen; the number of molecules of inhibitor oxidized, however, in unit time remains identical from case to case. The experiments also show that, over the range of inhibitor concentrations studied, the amount of sulfite oxidized is inversely proportional to the inhibitor concentration. This means that the inhibitor does not influence the number of reaction chains started and that each chain started is ended by an inhibitor molecule. The amount of inhibitor oxidized is also significant. It is approximately one fifty-thousandth of the amount of the sulfite which would have been oxidized under the given conditions had the inhibitor not been present. This has demanded a special technic of measurement of such slight amounts of oxidation. The examples chosen are those for which sensitive quantitative colorimetric methods of estimating the oxidation product could be developed. This has proved to be possible in the case of acetone, methylethylketone, and benzaldehyde, the three oxidation products of the several alcohols studied. The data obtained are summarized in Table 11.
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These results are obvious on the chain-reaction theory. The constant amount of oxidized inhibitor means that substantially every chain is broken by a n alcohol molecule being oxidized once a certain alcohol concentration is exceeded. The minute amount of alcohol oxidized is also understandable on the chain theory. The varying efficiency of the inhibitors, as evidenced by the varying lower limits of concentration a t which substantially complete inhibition is secured, is however a matter which calls for more than passing attention. It is a problem full of significance in the general question of reaction mechanism to ascertain why the destruction of reaction chains in equivalent alcohol concentrations occurs thirty-seven times more frequently with benzyl alcohol than with secondary isobutyl alcohol. It is a matter, too, of high scientific interest to learn by what mechanism the energy-rich reaction products of the sulfite oxidation transfer that energy to unoxidized sulfite molecules even in aqueous solutions less than 1 molar in concentration, in which, therefore, a t least fifty-five collisions with water molecules occur for every collision with a sulfite molecule. Conclusion
There are those who imagine that industrial problems are lacking in thrills for the pure scientist. They surely cannot have Table 11-Action of Alcohols as Inhibitors of Sodium Sulfite realized the inspiration and stimulus to fundamental research Oxidation which achievements in the field of industrial catalysis have so RATIO INHIBITOR abundantly produced. Over the fireplace in the new chemical MOLS0x1DIZED t0 laboratories now under construction in Princeton there will be CONCN. INCREASERELATIVE ALCOHOLSULFITE MOLS RANCG OF IN ALCO- INHIBITION 0x1NORMALLY inscribed the motto “Felix qui potuit rerum cognoscere causas.’’ POWER OF DIZED A OXIDIZED* HOL ALcoiroL It is that happiness in learning the cause of things which has ALCOHOL C V - K * l o % C O N C N . ALCOHOL 15% 15% been the chief joy of these studies and which has brought in its Mols Mols/hour Isopropyl 0 , 0 2 5 to 2 , 5 100-fold 3 0.000046 1:54,000 train the additional joy of the appreciation of which this occasion sec-Isobutyl 0 . 1 5 t o 1 . 8 12-fold 1 0.000049 1:51,000 is a testimony. Benzyl 0.005 to 0 . 1 7 34-fold 37 0.000049 1:51,000
NOTES AND CORRESPONDENCE Tin Plate and the Electrochemical Series Editor of Industrial and Engineering Chemistry: I n a recent paper Kohman and Sanbornl have shown that under certain conditions in canned fruit juices the electrochemical relations usually assigned to the tin and iron comprising tin plate are reversed, the tin becoming anodic t o the iron. This confirms the results of experiments carried out in the writers’ laboratory in the winter of 1924 and 1925. Like Kohman and Sanborn, the writers cannot subscribe to the views recently published on this subject by Mantel1 and Lincoln,2 who have stated that, in fruit juices in general, tin continues in a cathodic relation t o iron, thereby accelerating the corrosion of iron with which it is in electrical contact. The writers have demonstrated that immediately after contact with many fruit juices or a water solution of citric and malic acids tin and iron exhibit their normal electrochemical relations; i. e., iron is the anodic metal. However, this condition is quickly reversed and the iron ceases to be corroded a t an appreciable rate. This reversal of the polarity in the tin-iron cell under suitable conditions is the direct result of the large difference in = 0.53 volt3 the hydrogen overvoltage of the two metals-tin and iron = 0.175 v o l t 4 The results of the writers’ investigations are now being prepared for publication. I n d . Eng. Chem., 20, 76 (1928). (1926); Zron Age, 119, 843 (1927); Can. Chem. Met., 11, 29 (1927); A m . Metal. Markef Monthly Rev., 33, 242 (1926). a Caspari, Z . phys. Chem., 30, 89 (1899). 4 Thiel and Breuning, Z . anorg. Chem., 83, 329 (1913). 1
* Canning Age, 7, 847
Although the observations of Kohman and Sanborn confirm those made in this laboratory a t an earlier date, we cannot agree with these authors in their attempt t o apply their results to the commercial aspects of corrosion in the tin container. As far as the canner is concerned, one of the most serious results of corrosion in the tin can is the perforation of the tin plate. Kohman and Sanborn have stated that perforations develop at points where the iron base is exposed by imperfections or fractures in the tin coating. It has been conclusively demonstrated both by experiment and commercial practice that the rate at which tin plate is perforated decreases with increase in the weight of tin coating. I n attempting to explain this fact on the basis of their results, Kohman and Sanborn assume that increasing the thickness of tin coating decreases the total area of iron base relative to the area of the tin coating exposed t o the contents of the can, and state: “Increasing the tin coating decreases the area of t h e cathodic iron relative t o the anodic tin and, in accordance with the electrochemical theory, corrosion of the iron is reduced.” On the basis of this statement one would expect that any increase in the area of iron base relative to that of the tin coating exposed in the can should bring about an increase in the rate a t which a can is perforated. This view is not in harmony with results previously published by the same authors,6 in which it was shown that in enameled cans free from tin coating perforations develop a t a much slower rate than in enameled cans carrying a normal weight of tin coating. It appears that Kohman and Sanborn have fallen into the 5
Kohman and Sanborn, Ind. Eng. Chcm., 19, 514 (1927).
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error of attempting t o explain commercial observations by the results of laboratory experiments made under conditions not closely related t o those met in commercial practice. Strictly speaking, the corrosion of the iron in an iron-tin couple should be inappreciable as long as i t bears a cathodic relation to the tin. Except in electrolytes possessing a high conductivity, however, this condition is not attained unless all portions of the cathodic iron are in very close proximity t o the anodic tin. I n case parts or all of the iron element are located at appreciable distances from the anodic tin, the protective influence of the tin is lost and corrosion of the cathodic iron occurs as a result of “local action.” This is the situation encountered in the type of iron-tin couples which Kohman and Sanborn used in their experiments except in those cases where the area of the iron element was very small as compared with the area of the tin element. Kohman and Sanborn’s results show that where the ratio of the area of iron t o the area of tin was 0.0413 or less there was no corrosion of the iron. Had they altered their technic so that, regardless of the relative area of the iron element, all portions of it would be at no greater distance from the tin element than that existing in the couple of the above-mentioned ratio of iron t o tin, they would have found that variations in the relative areas of cathodic iron t o anodic tin have no i d u e n c e on the corrosion of the cathodic iron. In no case would there have been any appreciable corrosion of the iron. As stated above, Kohman and Sanborn found no corrosion of the iron where the ratio of area of iron t o area of tin was 0.0413 or less. I n a normal can this ratio is even very many times less than the smallest ratio employed by these authors. Consequently, any variation in the relative areas of iron base and tin coating exposed in a tin can t h a t would be met in commercial tin plate would not be expected t o influence the rate of corrosion of any microscopic areas of exposed iron. Obviously, the observed effect of increased weight of tin coating in delaying the perforation of the tin cannot be explained by a cathodic relation of iron base t o the tin coating. Experiments in the writers’ laboratory have indicated that, under conditions favoring the corrosion and perforation of tin plate, the iron base retains its normal anodic relation to the tin coating. On this basis there is no difficulty in explaining perforation of tin plate. It is a common observation in the canning industry that inside-enameled cans perforate more rapidly than plain cans. Kohman and Sanborn have attempted to explain this on the assumption t h a t in the enameled can the area of tin coating relative to that of the exposed iron base is reduced. This view is open t o the same objections as their attempt to explain the effect which increase in weight of tin coating has in delaying the perforation rate. There is a strong possibility that the enamel film itself may behave electrochemically towards areas of exposed iron base. Walkers pointed this out for the first time in 1909. He showed that in potassium chloride solutions can-enamel films are definitely cathodic t o adjacent areas of uncoated iron and consequently intensify the corrosion of exposed iron with which they are in contact. Walker’s work is being repeated in this laboratory in solutions of the fruit acids having approximately the same hydrogen-ion concentration as the fruit juices. Can i t be demonstrated that commercial enamel films behave cathodically t o adjacent areas of uncoated tin plate or black iron under the conditions existent in a can of fruit, it becomes a simple matter t o explain the fact that enameled cans perforate more rapidly than plain cans. ROGERH. LUECK HAROLD T. BLAIR AMERICAN CAN COMPANY SANFRANCISCO, CALIF.,AND MAYWOOD, ILL. January 17, 1928 6
Walker and Lewis, J. Ind. Eng. Chem., 1, 754 (1909).
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Editor of Industrial and Engineering Chemistry: Corrosion in canned foods is an exceedingly complex process, influenced by numerous factors. It is not likely that any one factor is dominating throughout the life of a can. In our paper, “Tin Plate and the Electrochemical Series,” t o which Messrs. Lueck and Blair refer, we had occasion t o discuss but one factor. Our data and our procedure in securing it are in sufficient detail‘ t o speak for themselves. We will await with interest the promised publication of t h e data sustaining the various points h e c k and Blair have made. E. F. KOHMAN NATIONAL CANNERSASSOCIATION N. H. SANBORN 1739 H ST., N . W. WASHINGTON, D. c. February 28, 1928
Definition of “Atomic Volume” Editor of Industrial and Engineering Chemistry: I read with great interest and appreciation the article entitled “Surface Tension of Metals with Reference t o Soldering Conditions,” by A. W. Coffman and s. W. Parr, which appeared i n the December issue [Ind. Eng. Chem., 19,1308 (1927)l. The authors quote Smith for the hypothetical statement t h a t “the larger the difference in atomic volumes and surface tensions between two metals, the more of a decrease will be obtained in surface tension upon alloying the two.” They also give a table of surface tensions and (supposedly) atomic volumes of some of the metals and from it deduce t h a t thallium should form better soldering alloys when alloyed with tin, bismuth, or antimony than our existing solders. I have added the word “supposedly” advisedly, as the term “atomic volume” which they employ is not the same as the one generally understood and which was used by Smith, being by definition “atomic weight divided by specXc gravity.” I feel that the authors should have given their reasons for this substitution of terms, as it is by no means sure t h a t Smith’s hypothesis holds good when atomic volume is considered as “the approximate volume of any atom, denoting that portion of space required for its existence, which is doubtless equal t o the space within the outermast shell of electrons.” [The words quoted are taken from a n exposition of Bragg’s theory b y Winchell, Science, 61, 553 (1925)l. The new term is usually called “atomic domain” and there are several objections to its use in connection with Smith‘s theory: 1-There are possible variations of 25 to 30 per cent between calculated and some of the observed volumes of the atom taken from the supposed diameters of the atoms, the diameters being the distance between the centers of the positive nuclei of adjacent atoms. 2-“It is shown that the relation is less accurate when applied to crystals of metals, which in Langmuir’s theory consist of a n assemblage of positive ions held together by electrons which have no fixed positions in the structure.” [Bragg, Phi!. Mag., 40, 169 (1920)l. The following table of calculated atomic volumes as employed by Smith in stating his hypothesis is given for several of the metallic elements. Upon this basis the variation of atomic volume between thallium and tin, bismuth, or antimony is slight, disproving the belief that binary alloys of thallium with the above metals will have greatly reduced surface tensions and consequent superior solder qualities. At the side are given the authors’ figures for atomic volume. METAL Lead Tin Antimony Bismuth Thallium
ATOMIC WEIGHT SPECIFIC GRAVITY 18.2
17 18 21-22 17.2
NATIONAL LSADCOMPANY 106 YORKSr., BROOKLYN, N. Y. December 29, 1927
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USED BY COFPMAN AND PA-
29 12 12 13 48
FRANK H. EDWARDS
