76
INDUSTRIAL AND ENGINEERING CHEMISTRY
doing away with the long flexible tube, which is rather expensive. In this early form the hangers were mounted on swivel casters and the apparatus, hydrogen tank and all, could be moved readily from place to place. I n the drawing a thermometer well is shown welded to the side of the tee. If the apparatus is to be used a t fairly constant temperatures, this method of measurement will be moderately satisfactory, but if the exact temperature of the reactants is required, and
VOl. 20, No. 1
particularly if the apparatus is subjected to sudden temperature changes, a thermocouple should be placed inside the shell with the leads coming through the tee. Directions for running these leads through the shell are given by In working with high pressures, make-shift apparatus is in general to be avoided but for pressures such as described in this paper the writers believe that the dictates of safety are fully satisfied by the material herein described.
Tin Plate and the Electrochemical Series' E. F. Kohman and N. H. Sanborn NATIONALCANNERSASSOCIATION, WASHINGTON, D. C.
SING as a basis the generally accepted relative position of tin and iron in the electrochemical series, one can easily set up a serious case against tin plate. It is generally accepted that the grades of tin plate used in the canning industry have microscopic areas of iron which are not coated with tin. Even were it possible to tin base plate in such a way as to avoid exposures of iron, it is doubtful whether it would be possible to form cans with so little strain on the metal that the tin coat would not be fractured. According to the usual electrochemical series tin would be the cathode in any galvanic couple set up between the tin and iron. Since the area of tin is so large in proportion to the anodic iron, corrosion of the iron at the exposed points would be greatly intensified, resulting in excessive solution of iron a t such points and eventually in perforation of the can. I n the types of can in which a cut edge of the tin plate is directly exposed to the contents, it would cause excessive solution of the iron a t this point. I n a series of articles Mantel and Lincoln2 develop such a case. They state that certain foods, such as the non-acid vegetables, do not perforate, while the more acid fruits tend to perforate the can. They further state that in the nonperforating products, although the tin is cathodic when tin and iron electrodes are first introduced into thePe products, the direction of the current shortly reverses itself and the tin becomes anodic. They ascribe this reversal of the current in the less acid vegetables to film effects which render the iron passive. According to them, this film results from the greater conductivity of the electrolyte due to the added salt with which these products are canned, and they suggest that salt might have a similar effect with fruits. That salt renders iron passive is not in harmony with the general effect of chlorides on passivity. Moreover, these authors have failed to refer to extensive experiments3 in which salt has been found not to decrease perforation, or t o the fact that tomatoes and many other vegetables frequently canned without salt show no tendency to perforate. The case against the tin can as outlined above fails to take into account most of the results experienced in its conimercial use. The pitting of a plain tin can resulting in perforations is an unusual type of corrosion. The usual corrosion in such cans is not local but generally spread over the entire surface of the tin. In the case of the type of can having a cut edge exposed to the contents there is no excessive solution of iron.
U
1 Presented before the Division of Industrial and Engineering Chemistry a t the 74th Meeting of the American Chemical Society, Detroit, Mich., September 5 to 10, 1927. * Canntng Age, 7, 847 (1926); Iron Age, 119, 843 (1927), Can Ckem M e t , 11, 29 (1927); Preprint of paper at 51st Meeting of the American Electrochemical Society, Philadelphia, P a , April 28 t o 30, 1927. 9 Clark, Canner, 1923, Convention number, p. 149, Kohman and Sanborn, I n d Eng Chem , 16, 290 (1924).
If a galvanic couple is set up with the iron anode, increasing the thickness of the tin coat (decreasing the iron exposed) should intensify the galvanic action on the iron and perforation of the can should occur a t an earlier date. Contrary to this, a heavier coat of tin has been found to result in less tendency to perforations. I n an enameled can since most of the tin is covered by the enamel, its area relative to the iron is enormously reduced. If tin functions as the cathode, the intensity of its action should be reduced by the enamel. On the contrary, enameled cans tend to perforate enormously more than plain cans. For these reasons and others, some of those connected with the canning industry have realized for some time that there are many things about a tin can which cannot be explained on the basis of tin acting as the cathode while the iron acts as the anode. A number of experiments have therefore been conducted to learn the definite relationship of tin and iron to one another in this respect. Methods Since this is the first of a series of reports, some detail as to the procedure will be given, It is highly important in this connection to conduct all corrosion experiments or electrical measurements in the absence of air, since the corrosion commonly encountered in canned fruits is in the absence of air. The methods employed were devised with this in mind. The experiments have involved the use of various solutions of organic acids commonly found in fruits as well as the fruits themselves. This report will be confined t o the results obtained with fruits only. When fruits were used, they were ground in a meat grinder and diluted with water in the proportion of three parts of fruit to two parts of distilled water, with the exception of grapefruit, which was used without additional water. After the water was added, the fruit was heated to boiling and boiled for a few minutes to expel any gases within the cells of the fruit and to secure sterilization. CORROSION TESTS-The corrosion experiments were made in wide-mouthed glass bottles having a capacity, with a stopper inserted. of 135 cc. The bottles containing the corrosion specimens were filled with the boiling fruit, a onehole rubber stopper firmly inserted, and while pressure was still being exerted on the rubber stopper, a glass rod was introduced into the hole of the stopper. The bottles were then cooled as rapidly as was permissible without danger of breakage. I n most cases they were probably cooled coinpletely within 15 to 20 minutes. The stoppers were then firmly clamped down by means of screw clamps made for the purpose, and thereafter the bottles were held a t room temperature. For corrosion specimens finished base plate and cold rolled
nuary. 1928
.
INDUSTRI.4L AND ENGINEERING CHEMISTRY
77
ELECTRICAL MEASUREMENTS-TO supplement these corsheet tin n.ere used. The specimens were 2.54 by 7.62 cm. (1 by 3 inches), cut on an ordinary hand squaring machine. rosion tests preliminary electrical measurements were made. The edges of the base plate were lightly ground with an emery The potential difference between tin and base-plate electrodes wheel to smooth any rough or frayed edges. .411 corrosion was determined. Likewise their individual potentials against tests here reported were conducted with a strip of the base a standardized tenth-normal calomel cell were measured. plate and of the sheet tin in each bottle. When contact Electrodes of tin and base plate 2.54 by 7.62 em. with narrow of the specimens was not desired, this was avoided by inserting projections extending through the rubber stopper were a glass tube between the two specimens. When contact was pickled 3 minutes in 3 per cent sulfuric acid solution at 80' C. desired, it was secured by fusing the tin specimens to the base and then thoroughly washed free from acid. The electrode., plate. To make this contact by fusion, a small projection, were immediately placed in a bottle of boiled fruit mixture approximately 1 by 2 mm., was left on the end of each similar to that used in the corrosion experiments. d glass specimen and these projections were fused to one another. tube extending through the stopper into the fruit mixture, The projection was so small compared with the entire area of with its external end extending into a small beaker of the the specimen that it introduced an insignificant error. boiling fruit mixture, replaced the glass rod used in the corThe amount of corrosion was determined by weighing the rosion tests. This glass tube consisted of two sections conspecimens before and after the test. In the case of the fused nected with a rubber tube. The bottle containing the elecspecimens, the base-plate specimen was weighed, the tin trodes was then cooled in running water while the beaker into which the open tube exspecimen tlien fused onto ____ tended x i s left uncooled, it, and the combined specimaking it possible t o sterilmens again weighed. After Experiments have been conducted to determine the ize the c o n t e n t s of t h e the corrosion test the commechanism of the corrosion of tin and iron, both when bottle and keep them sterile b i n e d specimens were in contact and when not in contact, in fruits and during cooling. When the weighed. the tin specimen the relation of the results to the electrochemical contents were cool, the glass removed. and the base-plate theory of corrosion. It has been found that in a numt u b e e x t e n d i n g into the s p e c i m e n again weighed. ber of fruits tin is anodic to iron. Although not all beaker of hot fruit mixture To remove the tin from the fruits have been studied, the data indicate that this was transferred into a tube projection of the base plate, is a rather general condition in canned fruits. Many of mercury, which acted as it was melted by holding in a of the facts which did not conform to the older idea a seal to avoid bacterial small flame at the point of that tin is cathodic to iron are readily explained in contamination and exclude fusion and i m m e d i a t e l y accordance with these new findings. Electrical measatmospheric oxygen. Elecwiped off with a dry towel urements have likewise been made and they bear out t r i c a l contact was estabafter being removed from the results of the corrosion tests that tin is anodic to lished between the electhe flame. It was f o u n d iron. The nature of corrosion in fruits is distinctly trodes and the calomel halfthat tin specimens could be different from that in solutions of the fruit acids. cell by removing the tube fused to the base plate in The results are in harmony with commercial experience. from the mercury seal and this manner and again replacing it in a connecting moved without altering the vessel containing potassium weight of the b a s e - p l a t e specimen more than 0.1 mg. chloride. The rubber connecting tube was kept closed with Before weighing, all specimens were pickled in 3 per cent a screw clamp at all times except when making potential sulfuric acid solution for 3 minutes at 80' C. All base- readings, in order to prevent the diffusion of potassium chloplate specimens for any one experiment were pickled simul- ride into the fruit mixture. taneously in a beaker. To avoid their lying against one Results another, glass rods were set up between the individual specimens. Likewise, all the tin specimens were pickled simulI n Table I are given all the results obtained when contact taneously for each experiment. When the pickling was was made by the fusion method. Similar results have been complete, most of the pickling solution was decanted off, obtained with other fruits using other means of contact. a small amount being left in to avoid oxide formation, the The duplicates illustrate the extent to which agreement was specimens covered with warm water, taken out individually, obtainable by this method. No selection of results has been and thoroughly dried by wiping with a clean towel and made in this table. It is therefore evident that satisfactorily weighed. When contact was desired, the fusion was made close checks have resulted, with the exception of cherries, after the pickling. in spite of the fact that the corrosion medium was not homoVarious methods of preparing the specimens were tried. geneous. This medium was not homogeneous for several Polishing reduced the corrosion of the iron, but increased reasons. It contained the fruit pulp as an insoluble constituthe corrosion of the tin. Pickling is simple and gave satis- ent. The bottles were all filled from one large flask and it factory checks on duplicate specimens. was impossible to secure a perfect distribution of fruit pulp It is recognized that it is virtually impossible to start any throughout all the bottles, since some fruit pulps tend to corrosion experiment of this kind without contamination settle while others tend to float. Moreover, as the corrosion by at least minute quantities of oxygen or oxides of' the metals. progressed the fruit colors were gradually bleached around Furthermore, it was considered possible, as corrosion pro- the specimens, causing non-uniformity throughout the corgressed, that the products of corrosion might have some effect rosion medium. The cause for variations in cherries is not on the nature of corrosion. Bottles were opened for the sake definitely known. of convenience a t intervals of 19, 67, 310, and 810 hours. It is evident from Table I that the corrosion of the base The first opening was believed to include the effect of any plate was decreased very materially by being in contact with oxygen or oxides of the metal that might have been intro- tin and that the corrosion of the tin was greatly increased duced in the beginning of the experiment, as well as the because of its contact with base plate. In fact, when in effect of the high initial temperature. The subsequent contact the tin was much the more corroded of the two metals. openings were expected to reveal any change in the nature The question arises-why does the tin show so much less of corrosion as it progressed. corrosion when not in contact with base plate? Polarization
INDUSTRIAL A N D ENGINEERING CHEMISTRY
78 T a b l e I-Loss
Vol. 20, No. 1
in W e i g h t of 19.4 Sq. C m . (1 b y 3 I n c h e s ) S e c i m e n s of Base P l a t e a n d S h e e t Tin in 135 Cc. of F r u i t . B o t h w h e n t h e Two S p e c i m e n s Are &t in C o n t a c t a n d w h e n in C o n t a c t (Figures in milligrams) 19 HOURS
I
67 HOURS
310 HOURS
810 HOURS
FRUIT NO
Grapefruit Av.
Av. Strawberries
I
NO
Contact
contact
Contact
contact
Contact
contact
Contact
contact
1.0 1.3
0.6 0.3
0.2 0.3
2.8 4.6
2.1 1.9
0.4 0.6
0.2 0.2
4.5 4.7
9.3 9.2
0.7 1.0
0.9 1.0
0.51
0.5
0.3
3.7
0.5
1.6
i::
I
2.0
0.51
0.2
1 ::: 1 A:: 2.0
0.9
0.7
::!
4.61
2.7
1
9.3
7.4
!:: i::;
Contact
No contact
pi0
Contact
contact
0.8 0.9
12.2
24.0 24.4 22.4 23.6
1.2 1.0 1.0 1.1
18.9 20.0 20.8 19.9
2.8 3.0
10.3
1.3 1.4 1.4 1.4
21.8
1.0
20.3 20.3 20.4 20.3
i::
2.4 2.5
17.1 17.1
3.3 2.8 3.5 3.1
37.4 35.2
0.91
1.9
1
1.0
12.1 12.3
1.8
0.7
0.7
3.4
3.1
0.7
1.0
6.3
13.7
1.3
2.5
17.1
2.1 2.0
1.2 1.8
1.1 1.2
3.1 3.1
4.1 4.0
1.7 1.5
2.5 2.5
8.2 8.1
16.3 15.5
4.9 5.1
5.5 5.6
30.5 31.3
2.1
1.1
1.2
3.1
4.1
1.6
2.5
8.2
15.9
5.0
5.6
30.9
1.3 1.4
0.8 0.7
0.6 0.6
4.6 4.3
2.6 2.4
0.8 0.8
1.0 1.0
11.3 10.8
8.8 8.1
1.2 1.1
2.5 2.3
32.4 33.2
27.0 33.2 37.6 32.6 49.6 46.0 42.5 46.0
0.7 0.8
2.9 3.7 2.1 2.9
Contact
24.7 23.3
36.3 69.6 71.1
8.4
7.3 7.5 7.1 7.3
1.0 1.2 1.2 1.1
4.6 3.9 4.5 4.3
57.1 57.6 59.1 57.9
3.3 3.7
8.2 8.6
70.4
1.4
0.8
0.6
4.5
2.5
0.8
1.0
11.1
8.5
1.2
2.4
32.8
17.1 17.6 17.6 17.4
27.1 27.1 22,7 25.6
15.8 16.5
0.6
1.8 1.7
25.5 29.3
20.6 20.0
1.6 1.5
4.0 3.9
28.3 34.2
21.7 20.1
3.1 2.8
12.9 14.0
43.9 48.0
16.2
0.9 0.7
1.8
27.4
20.3
1.6
4.0
31.3
20.9
3.0
13.5
46.0
19.0 30.8 27.8 25.9
3.5
14.2 17.5 21.5 17.7
26.9 129.3
8.3 8.2
0.7 0.6
g:2
37.2 125.9
0.9 0.8
60.6 60.6
38.0 47.5
1.2 1.5
9.3 7.1
0.7
2.4
131.6
1.4 1.3 1.2 1.3
7.9 8.7
8.3
5.3 24.4 5.3 1 2 1 . 9 4.8 35.3 5.1 1 2 7 . 2
24.2 18.5
128.1
8.6 11.6 16.5 12.2
8.3
60.6
42.8
1.4
8.2
Rhubarb Av.
Black (Bing) sweet cherties, pitted Av.
0.7
1
I
1 I
0.9
seems to be an important factor and a further study of it in this respect is planned. The relative homogeneity of the two metals must also be considered. The data in Table I1 show the effect of varying the relative size of the two specimens. Increasing the size of the iron relative to the tin increases the rate of solution of the tin and decreases the protection afforded the iron by the tin. of Relative Size of S p e c i m e n s in C o n t a c t for 308 Hours in Blackberries
T a b l e 11-Effect
AREAOF SPECIMEN I Fe
Sn
1
TOTALLoss Fe
Sn
1 I
Loss
PER
Fe
So. C M Sn
~~
Sg. cm.a
Sq. cm.a
0.0 1.6 19.4 38.7 38.7 38.7 38.7
38.7 38.7 38.7 38.7 19.4 1.6 0.0
by
NO
contact
1 i:; ::;I
Av.
NO
Contact
1.3
Blackberries
NO
contact
1.2
Av.
Red sour cherries, uitted . Av.
NO
Sn
Fe
MA=. 0.0 Ma' 0.0 0.9 5.0 8.7 15.8 15.9
5.6 8.5 19.4 30.9 22.1 5.2 0.0
ME. 0.000 0.000 0.016 0.129 0.225 0.408 0.411
Id& 0.145 0.220 0.501 0.799 1.14 3.25 0.00
Specimens of 1.6, 19.4, and 38.7 sq. cm. area were, respectively, '/z inch, 1 by l'/z inches, and 1 by 3 inches in dimension.
The electrolytic potential measurements which can be reported at this time are only preliminary. They are in harmony with the corrosion data given above. It has been found that electrical measurements are very erratic immediately after the experiment is started. I n some cases various reversals occ.ur for several hours. It is not always possible to duplicate the first results with the same products. In some cases the first measurements indicated that iron was momentarily cathodic but very quickly reversed t o become anodic, and later another reversal occurred and the iron remained cathodic. In general, however, the iron is anodic at the beginning, but becomes cathodic within a short period and remains so. It is possible to make either the tin or the base plate cathodic a t will by gently bubbling air around the electrode. As a result of a number of determinations, the average potentials of iron and tin in strawberries were found to be
21.4
I1
-0.039 and -0.081 volt, respectively, I n rhubarb the potential of iron was -0.038 volt and of tin -0.141 volt. Discussion
It is generally accepted that the relative positions of the metals in the electrochemical series do not necessarily indicate their potentials in various mediums. There is no doubt, however, that the discussion of this series in most textbooks gives the impression that the electrolytic potential is largely an intrinsic property of the metal rather than that of a system. The static potentials of two metals in a given medium do not necessarily show the relative rates at which they corrode, as corrosion is a dynamic process. The influence of overvoltage, depolarizers, and other substances, and the physical structure of the metals must also be considered. Measurements of the solution potentials of iron and tin in strawberries, rhubarb, and other foods show that the potential of tin is more negative than that of iron. Table I, however, ihows that the solution of iron is greater than that of tin when the two specimens are not in contact. Overvoltages of hydrogen evolution or of cathodic reduction on iron and tin in these solutions have not yet been determined. It has been hhown that hydrogen has a higher overvoltage on tin than on iron.4 Under these conditions the high overvoltage on tin might prevent its solution when alone, despite its greater solution pressure. However, the electrical contact with a metal of lower hydrogen overvoltage-in this case, ironmould break down the protection afforded by the high overroltage. The experiments described in this paper clearly show that under certain conditions iron may be cathodic to tin. The experience of the canning industry further confirms this view, 4 Caspari, 2. fihysik. Chcm., 30, 89 (1889); Knobel, Trans. Am. EEcctrochem. Soc., 43, 55 (1923); Newberry, Mem. Proc. Manchester Lit. Phil. SOL..61, No. 9 (1917).
I N D U S T R I A L A,VD ENGINEERING CHEMISTRY
January, 1928
for, as previously mentioned, a heavier coating of tin reduces corrosion in the can as measured by its tendency to perforate. Increasing the tin coating decreases the area of the cathodic iron relative to the anodic tin and, in accordance with the electrochemical theory, corrosion of the iron is reduced. Again, commercial experience shows that enameled cans perforate sooner than plain cans. The enamel covers a larger area of tin than of iron, thus reducing the area of the
79
tin relative to that of the‘ iron. This follows from the fact that at points where relatively more iron is exposed owing to forming and closing of the can the enamel is also least continuous. On the basis of the electrochemical theory, corrosion of the anode should increase as the relative area of the cathode increases. The mechanism of corrosion as indicated by these results is in harmony with the corrosion encountered in commercial experience.
An Inexpensive Cell for the Purification of Colloids b y Electrodialysis’ Richard Bradfield DEPARTMENT OF‘ SOILS, UNIVERSITY on MISSOURI, COLUMBIA, Mo.
PIIOPI’Gthe greatest impediments to progress in colloidal chemistry have been the difficultiesinvolved in obtaining pure, reproducible products. Nearly all colloids, synthetic or natural, contain variable quantities of ordinary electrolytes which are either neglected or only partially removed by ordinary dialysis. The purification of colloidal electrolytes in which one ion is of colloidal, the other of crystalloidal, dimensions is even more difficult to accomplish, but no less important for an understanding of the behavior of the sol. The classical dialysis of Thomas Graham is extremely slow and tedious. It has long been knoryn that the rate of purification could be greatly acce1er:tted by the application of an electrical current to electrodes placed outside the dialyzing membrane containing i,he colloid. But not until recently has this method of purification, now commonly termed “electrodialysis,” received the attention it seems to deserve. Freundlich2has found that in the case of serum a degree of purification that would require a week by ordinary dialysis ELECTRO-DIALYSIS CPLi can be accomplished in 10 to 40 minutes by electrodialysis. Dhkr6,3 Paull,4 Bechold,b and numerous others have also called attention to the advantages and limitations of the process, In spite of this increased interest in the subject, so far as the authorisaware, there is no apparatus on the American market for t h i s p u r p o s e . For that reason a description is glr-en herein of simple, inexa I pensire cell which can
A
,
’
I
11
I
1
Received August 20,
1927. 2 B o g u e , “Colloidal B e h a v i o r , ” P 300, McGraw-Hill Book c0 1924. * Kollor6-2, 41, 243
Top V/ru
6
Alexander, “Colloid
Chemistry,” p, 834, ChemF~~~~~ I--Rubber Cell Made from s~~~~~~ B a t t e r y Case
ical Catalog C o , 1926
be easily made from an old rubber storage battery case, Cells of this type have been used in the author’s laboratory for three years and are still giving satisfaction. Description of Cell
The construction of the cell is shown in Figure 1. The battery jar is sawed into three sections, the outside sections about the width desired in the finished cell. The width of the resulting center section will vary with the type of cell used, but it can be sawed again into any desired size. The following dimensions have been found to be satisfactory for routine work: width of each section, 4.0 c m , depth. 14 t o 15 cm., length, 14 to 15 cm. Such a cell will have a working capacity of about 700 CC. in each compartment. The edges of each section are carefully smoothed and squared so that watertight joints may be obtained without the use of gaskets. The three sections of the cell are held together by seven brass rods, three of which are regularly spaced on each side of the cell, and one in the middle of the cell bottom. These rods pass through holes drilled in the cell itself, as shown by the dotted 100 lines i n F i g u r e 1. T h i s a r r ~ ~ g e m e n0t holds the membranes ?*’ t%htly in Place and Q P r e v e n t s warping of E 6 0 the different sections e of t h e cell. Brass C40 strips about 2 wide and t h e s a m e length as the cell, with 0 holes drilled to corre- a spond with those in the cell Proper, serve as WdU3-s and hold Figure 2-Rate of Electrodialysis of Dife r e n t Ions w h e n Placed b e t w e e n Parcheach rod in place. If fment Membranes the edges Of the cell har-e been properly squared, water-tight junctions can be obtained by merely tightening the thumb screws on each rod. Any type of dialyzing membrane can be used. A good grade of parchment paper is satisfactory for most purposes. The cathode may be made of nickel or copper gauze, the
5 3
can be used for continuous electrodialysis by a continuous stream of distilled water to enter a t the bottom
