June, 1930
INDUSTRIBL AND ENGINEERING CHEMISTRY
These results also checked those in which the separated cream of tartar was determined. Determinations on other samples of juice prepared from lluscat and from Thompson seedless grapes showed that practically all of the separable cream of tartar could be removed by storage a t -18" C. until the product was thoroughly frozen and then subsequently thawing. Thus, where the juice was sorted in small bottles in which it was completely frozen in the course of 4 hours, practically as much cream of tartar was removed upon storage a t -18" C. for 24 hours as upon prolonged storage for 2 weeks or more. Although in the case of fresh juice freezing storage was just as efficient and more rapid than storage at 0" C., this mas not the case where juice had been previously stored a t room temperature. Thus, in the case of Muscat juice which had been stored a t 20" C. for a period of about 3 months, less cream of tartar was precipitated in the samples which were frozen and subsequently thawed than where the samples were stored a t 0" C. and then brought to room
615
temperature. This was due to the greater solubility of the smaller crystals produced by freezing. Other experiments have shown definitely that the conditions of thawing play a marked role in cases where the juice is initially low in cream of tartar content, but are not so important where fresh juice fairly high in cream of tartar is tested. Conclusion The separation of excess cream of tartar from grape juice by freezing storage and subsequent thawing is as thorough as by storage a t 0" C., and if suitable containers are used in which freezing is rapid the method can be made much more rapid. However, owing to the increased solubility of the smaller crystals formed, this method may not be so efficient for juice lour in cream of tartar content. Literature Cited (1) Cruess, "Commercial Fruit and Vegetable Products," p. 223, McGrawHill, 1924. (2) Hartmann and Tolman, U.S. Dept. Agr., Bull. 656 (1918).
Acidity and Corrosion in Canned Fruits' E. F. Kohman and N. H. Sanborn RESEARCH L.4BOR.4TORY, NATIONAL CANNERS ASSOCIATION, WASHINGTON, D.C .
Data are presented to illustrate the effect of pH on This tendency is somewhat HE writers have previthe single and relative potentials of iron (base Plate) greater w i t h t h e c o l o r e d ously ( 3 ) shown that tin may be distinctly and tin. It is probable that the effect of the pH is fruits than with the nonmore or less influenced by the acid radical of the acid colored fruits, but if the anodic to the base plate in a tin can when canned fruit used. Nevertheless, there is a definite tendency for non-colored fruits, such as the tin to become less and less noble compared with a p p l e s , are canned with a is the electrolyte. I n a subsequent publication ( 4 ) some iron as the PH becomes l ~ w e r . This explains the persmall quantity of oxidizing foration tendency of the less acid fruits in enameled factors which tend to bring agent, such as m e t h y l e n e about this relationship were cans. On the basis of iron being the more noble of blue, anthocyan pigment, or pointed out. Among other the two metals in a fruit can, the greater tendency of gaseous oxygen, they show factors, it was shown that perforations and hydrogen swells to Occur in enameled the same tendency to perforacidity or hydrogen-ion concans is plausibly explained. ate as the naturally colored centrations may influence the fruits do. relative potential of tin and iron. The data presented On the basis of the tin being the anodic metal, the following showed a greater effect on the tin than on the iron. We picture of corrosion in plain and enameled cans may be given. have recently obtained data which show that the single I n a plain can the area of tin is enormous as compared with potential of iron may likewise be materially affected by the the area of iron. If the tin were the more noble i t should acidity under certain conditions. cause verv ratid Derforation of the iron where the iron is exposed. Inasmuih as tin is less noble, even though the Corrosion in Plain and Enameled Cans single potential varies only slightly from that of iron, the Prior to the demonstration that tin is less noble than base protection that the anodic tin affords the iron is very effective plate in canned fruits, the relative development of perfora- because of the relatively enormous area. This explains, in tions and the formation of hydrogen in plain and enameled part, the non-perforating tendency of plain cans. I n an enameled can also both iron and tin are exposed to cans was little understood and seemed contradictory. Fruits containing anthocyan pigments are canned in enameled cans the contents, due to the rupture of the enamel coat, particuto preserve the color. The effect of this color upon the cor- larly in the seam. The relative area of tin to that of iron is rosion in a can has been pointed out (1, 2 ) . The first im- very greatly reduced, however, because it is very largely pression of those not familiar with the situation is that covered by the enamel. Nevertheless, the area of tin exenamel should act as a protection for the can and inhibit posed is still very much greater than that of the iron exposed perforations and the formation of hydrogen. The contrary and affords a fair degree of protection for the iron a t the is true. Plain cans do not perforate under practical con- beginning. The relative area of the cathodic iron, however, ditions, whether the colorless fruits, such as apples, peaches, is now great enough to have an appreciable effect on the and pears, or the colored fruits are canned in them. I n corrosion of the tin which is exposed, whereas in the plain the latter case, however, the colors are bleached. I n can this effect was spread over the entire inner surface of the enameled cans there is a tendency both for the cans to per- can and resulted in no local difficulty. I n the enameled can forate and for the development of hydrogen to swell them. this effect of the cathodic iron is localized to the small area of tin exposed and results in detinning until the area of iron 1 Received March 13, 1930. Presented before the Division of Indusexposed actually becomes greater than the area of tin extrial and Engineering Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, Ga., April 7 to 11, 1930. posed. The tin, then, although still anodic, ceases to be an
T
I
*
INDUSTRIAL A-VD ENGINEERING CHEMISTRY
616
adequate protection for the iron, upon which local couples form and rapid corrosion begins. As has been previously shown (,4, b ) , tin in solution very greatly inhibits iron corrosion. I n a plain can this is a material factor as there is a constant supply of tin. In an enameled can this probably is a considerable factor in inhibiting the corrosion of the iron while there is still a relatively large area of tin exposed. After extensive detinning of this exposed tin, this inhibiting effect of dissolved tin becomes less and less, since the tin which corrodes eventually is absorbed and taken out of the solution by some component of the fruit (4). Table I-Perforations w i t h Black Sweet Cherries (Bine) in Enameled Cans, C o n t a i n i n g Varying A m o u n t s of Added Citric Acid
1 1
L O S S AFTER:
CITRIC ACID
pH
152 days
196 days
238 days
270 days
302 days
Per cent
Per cenf
_ _ _ ~ - - _ _ _ _ _ _ Per cent
~
~
Per cenf Per cenl
Per cent
T a b l e IV-E. m . f . of Base P l a t e a n d Tin Measured a g a i n s t S a t u r a t e d Calomel ,Half-Cell in O n e L o t of Fresh P r u n e s a n d O n e Lot of Dried Prunes, t h e p H of W h i c h Was Varied t o Approximately t h e S a m e Value by Various Acids a n d S o d i u m Hydroxide (The values were so constant over a period of 8 days that the average of eight daily measurements is given.)
1
pH
- =
3.8
DRIEDPRUNES, P H = 4.4
EX-
Fe
POSURE
FIours _ .
Sn 1.01:
T'Oll
0 6074 0 6038 0 6044 0 6066 o 6031
Difference
0 6274 0 6267 0 6271 0 6297 o 6259
0 0 0 0
o
Fe
I.olt 0200 0229 0227 0231 022s
VOlf
0 0 0 0
o
6664 6637 6612 6618 6572
Difference
Sn
0 0 0 0
o
voir 6664 6698 6730 6752 6705
VOlf
0 0 0 0
o
0000 0061 0118 0134 0133
T a b l e 111-E. m. f. of Base P l a t e a n d T i n in Dried a n d Fresh P r u n e s Measured a g a i n s t a S a t u r a t e d Calomel Half-Cell The values were so constant over a period of 5 days that the average of all measurements is given. Each value represents four measurements on duplicate specimens-i. e., the average of eight measurements. PH
Fe
Sn I'ol1
5.011
0 643 0 637 0 613
3.5 3.6 3.5 3.6
0.601 0,598 0.586 0,598
0 596 0 638 0 611
Fe
Difference
Sn
T.Olf roll 3 6 0.598 0.610 3 . 1 0.584 0 . 5 8 s 3 . 1 0.569 0.608 3 . 0 0 , 5 6 1 0.600 3 0 0.561 0.594 5 . 1 0 681 0 . 6 3 2
None Malic acid Citric acid Tartaric acid Maleic acid Sodium hydroxide
DRIEDPRUNES
Fe
pH
~
1.oli 0.012
Sn
T'oli 0.613 0.572 0.553 0.542 0,572 5 . 5 0.696
4.0 3.0 3.0 2.8 2.9
0.004
0,039 0.038 0,033 0.029"
Difference
I*Olf 1'oli 0.611 0.002a 0 . 6 1 2 0 040 0.5SO 0.027 0,611 0.070 0.592 0.019 0,684 0,012"
Table V-Loss of Specimens of Base P l a t e a n d S h e e t T i n 2.5 X 4.0 c m . Exposed 720 H o u r s a t R o o m T e m p e r a t u r e in D r i e d P r u n e s , p H of W h i c h Was Varied w i t h Citric Acid a n d S o d i u m Hydroxide N o CONTACT
ELECTRICAL ADDITION
DIFFERENCE T'olt
DRIED PRLXES
4 1 4 2 4 0
1
FRESH PRUNES
I
FRESH PRUKEP, PH
KO.6
made in the summer of 1928 in which the fresh and dried prunes represented the same lot of fruit and were canned similarly with the same number of prunes per can and nothing was added but water, the dried prunes having first been soaked and the usual precautions taken to exclude oxygen, every can of the dried prunes had either perforated or was swelled by hydrogen within a year, whereas every can containing fresh prunes was still normal after this period. I n the dehydration of prunes they are lye-dipped for a fraction of a minute in order to check the skin so as to permit rapid evaporation. The prunes are thoroughly washed after this lye-dipping but, nevertheless, the pH is somewhat affected. Table I1 gives single potential measurements of base plate and sheet tin, together with the pH values in the fresh and dehydrated prunes. It is to be noted that both the base plate and the tin are less noble in the dehydrated
Table 11-E. m. f. of Base P l a t e a n d T i n in C a n n e d Fresh a n d C a n n e d Dried P r u n e s Measured a g a i n s t a S a t u r a t e d Calomel Half-Cell, a f t e r Varvina T i m e s of Exnosure
Tb,E
Vol. 22,
0 046R 0 001 0 002a
Citric acid Citric acid None
21.4 6 .5
5.5 12.2
FRESHPRUNES
0
0,623 0.623 0.612 0.610
0,023 0.026 0.026 0.012
Tin is cathodic.
T a b l e VI-Loss of Specimens of Base P l a t e a n d S h e e t Tin 2.5 X 4.0 c m . Ex osed 336 H o u r s a t R o o m T e m p e r a t u r e in Dried Prunes, p H of 3 h i c h W a s Varied w i t h Malic Acid a n d S o d i u m Hydroside ADDITION
Effect of Acidity on Corrosion
The writers have previously pointed out that the worst perforating fruits are those with the lowest acidity or highest pH. After the conviction that a tin can is only possible as a container for fruits because the tin is the less noble metal, it seemed logical that if the tin could be made relatively still less noble in the perforating fruits this difficulty could be corrected. The addition of foreign elements t o a canned product is contrary t o the ethics of the industry and the principles of the Pure Food Law unless the product added itself is a food and, furthermore, tends to improve the quality. These conditions are met by the addition of small quantities of citric acid to sweet black cherries. I n an experimental pack of such cherries made last summer it has been found that the tendency of these cherries to perforate the can is actually inhibited as was predicted. (Table I) It has been found that dehydrated prunes are far more corrosive in a can than fresh Drunes. For examde, in a pack
1
hT0 COhTACT
I
pH
3.05 3 33 3.58
Malic acid hIalic acid Malic acid A-one Sodium hydroxide Sodium hydroxide Sodium hydroxide
4 98 6.13
1
I
Fe
Sn
'
.\Ig
Mg
j
8.7 6.5 5.6 3 6 3.3 3.5 3.2
0.9 0.6 0.6 0.5 0.4 0.3 0.3
ELECTRICAL COXTACT Fe
Sn
Mg
Jig.
4.7 3 3 2.2 2.9 2 9 3.0 1.3
7.8 5 7 4 7 2.0 1.4 1.4 2 2
T a b l e VII-Loss of Specimens of Base P l a t e a n d S h e e t T i n 2.5 X 4.0 c m . Exposed 264 H o u r s a t R o o m T e m p e r a t u r e i n 17 Per C e n t S u g a r Solution C o n t a i n i n g 2 Per C e n t Citric Acid Buffered by P a r t i a l Neutralization w i t h Varying A m o u n t s of S o d i u m Hydroxide pH
2 3 3 4 4 4
70 22 70 00 35 66
I
S o CONTACT
Fe
Sn
Xg. 8 2 6 2 5 4 4 3 3 6 3 0
.!IC.
1 1
O S
ELECTRICAL CONTACT ~
Fe ,Vl g . 0 5
Sn
Mg. 8 8 9 9 8 6
7 9 5 7 4 6
ISDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1930
prunes than in the fresh prunes and that the tin is anodic in both cases but more strongly so in the fresh prunes than in the dehydrated prunes. Table I11 gives similar measurements for several lots of dried and fresh prunes as purchased on the grocers' shelves, with the same general results. I n Table IV are given similar measurements with one lot of fresh prunes and one of dried prunes in which the p H has been changed t o approximately the same w l u e by the addition of various acids and has also been materially raised by the addition of sodium hydroxide. It is to be noted that in this lot of dried prunes tin is actually more noble than the base plate and that with the addition of sodium hydroxide this relationship is more pronounced; moreover, the relative potential of the tin and iron is reversed in the fresh prunes by the addition of sodium hydroxide. The effect of the different acids is not exactly comparable, although a similar hydrogen-ion concentration exists. Chemically pure acids were used with the exception of malic, which was a commercial grade. In Tables T' and VI are given corrosion experiments in which citric acid and the same commercial grade of malic acid are compared under similar conditions. It is a t once evident that citric acid has a more pronounced effect in rendering tin anodic t o iron than this malic acid. Table VIII-Loss of Specimens of Base Plate a n d Sheet Tin 2.5 X 4.0 c m . Exposed 264 Hours a t R o o m Temperature i n 17 Per Cent Sugar Solution Containing Malic Acid Equivalent t o 2 Per Cent Citric .4cid a n d Buffered b y Partial Neutralization with Varying A m o u n t s of S o d i u m Hydroxide KO CONTACT
Sn Mg.
2.85
~
3.31 3 60 4.21 4.47
1 1
10 7 8 7 7 6
0 7 0.7
6 0 4.2
0.I
ELECTRlC.41. CONTACT ~
Fe
Sn
Jf g
1.1
1.1
0.7
I n Tables T'II and 1-111are presented corrosion data with these two acids in a simple medium of l i per cent sugar n-hich was inverted to a large extent by boiling for 30 minutes with the acid before buffering with sodium hydroxide. It should be noted that, although the same p H is covered by the two acids, in the citric acid the tin is distinctly more anodic to the iron than in the commercial malic acid.
617
Table IX-E. m. f. of Base Plate a n d Tin Measured against a Saturated Calomel Half-Cell i n Dehydrated Prunes (7.5 kg. Prunes Diluted t o 30 1.) with a n d without Varying Additions of Acids i n Equivalent A m o u n t s a n d of Alkali, a n d Corrosion Loss of Similar Specimens The potential values are the average of duplicate specimens measured seven times during 11 days. The corrosion loss is the average of triplicates over the same period. When Fe and Sn specimens were not in contact, they were nevertheless in the same bottle.
I ADDITION
CORROSION LOSS
ELECTRIC 4 L CONTACT
PH
Fe
None Citric acid Citric acid Citric acid Malic acid Malic acid Malic acid Tartaric acid Tartaric acid Tartaric acid Oxalic acid Oxalic acid Oxalic acid Lactic acid Lactic acid Lactic acid Sodium hydroxide Sodium hydroxide
4 3 3 2 3 3 2 3 2 2
10 42 12
3 2 4 4
10 83 35 86
80
39
13 85 19 86 55 2 57 1 78 1 35 3 33
-
0.633 0.573 0,354 0,543 0.569 0.559 0.547 0.558 0.545 0.533 0.519 0,492 0,464 0 574 0.557 0.545 0.648 0.672
0 0 0 0 0 0 0 0 0 0 0
647 637 596 617 618 616 601 615 612 605
617 0 649
0 643 0 619 0 616 0 613 0 654 0 675
'
3.4 0 5 3.3 0.7 4.9 0.8 3.7 0.6 4.2 0.8 5.4 1.3 4 . 4 1.1 4 . 5 1.1 6.0 1.4 2.7 1.5 6 . 5 1.8 9.1 2.0 4.2 0.8 4 . 2 0.9 6 . 0 1.1 2.7 0 . 4
Sn
M g
Mg
1 0 0 0
2 6 8 9 6
2 3 2 2
0 0 0 0 0
7 5 2 5 3
0 0 0 0 0
1 0 1 0 7
2 9 1 1 5
6 8 9 6
7 1 8 3 107 201 325 404 5 5 0 7 7 1 0 5 9 1 1 6 1 6 1 1 1 0
Table IX, for which only c. P. acids were used, gives further evidence t,hat all the acids have the same general tendency. I n this experiment, however, the tin did not become cathodic with the addibion of sodium hydroxide, even though a higher p H resulted than in other lots of prunes in which blie t,in was cathodic. It is apparent that, although the effect of p H on either the tin or the base plate may be definite and material, it is not possible to predict the specific effect in any other medium. However, there is the general tendency for tin to become relatively less noble than iron with decrease in pH within t,he range encountered in fruits and probably beyond this range. Literature Cited (1) Kohman, IND. ENG.CHEM,,15, 527 (1923); Canning .Age. 6, 308, 310, 314, 385, 389 (1924); 6, 191 (1925); Canner, 60, (11, pt. 2 ) , 151 (1925); Canning Age, 7, 187 (1926). (2) Kohman and Sanborn, I N D . ENG.CHEM.,16, 290 (1924). (3) Kohman and Sanborn, Ibid., 20, 76 (1928). (4) Kohman and Sanborn, Ibid., 20, 1373 (1928). (5) Kohman and Sanborn, Trans. A m , Elecfrochem. SOL..54, 279 (1928).
Specific Heat of Pyrex Glass from 25" to 175" C.' Thos. De Vries PURDUE UNIVERSITY, LAFAYETTE, IND.
HE specific heat of Pyrex glass was determined by the Nernst method (1) over the temperature range 25" t o 175" c.
T
Preparation of Glass Slug
The glass was taken from the regular stock-room supply and was made into a slug of about 25 grams, containing a two-junction copper-constantan thermocouple and a nichrome heating element (both KO. 30 wire). -4 piece of heavy capillary tube (2-mm. bore, 4-mm. wall) was placed inside a tube of Pyrex and the heating element inserted between the walls. This was placed inside a second tube, with the thermocouples between the walls, one a t the end and 1
Received March 22, 1930.
the other a t the center of the slug. Both of these outer tubes were drawn down snug by the use of suction after softening the glass in a flame. The finished slug was 1.5 cm. in diameter and 6.5 em. long. It weighed 27.75 grams and with the enclosed wires, whose weight was 0.62 gram, the corrected weight taken for specific-heat calculations was 28.02 grams. This is assuming a specific heat of 0.09 for the metal and 0.20 for the Pyrex. To minimize heat losses the slug was silvered and highly polished. It was suspended by a fine wire in a n evacuated container. Charcoal with liquid air was used to improve the vacuum obtained by the usual methods. The container was immersed in a n oil bath thermostatically controlled to less than a half degree. The lead-out wires were
