414
ANALYTICAL EDITION
(e) Silver is somewhat soluble in strong hydrochloric acid, and therefore the rubbing with it should be done quickly. (f)The residue in hydrochloric acid solution should be inspected for dark undecomposed particles, but all the sands tried have been thoroughly decomposed without the need of a pyrosulfate fusion. If there is an odor of chlorine, it is probable that all organic matter has been destroyed. The ammonium hydrate precipitate will contain ordinarily not only iron hydroxide, but silica and the hydroxides of aluminum, zirconium, and titanium, and possibly slight amounts of vanadium and copper. Should the latter two be present, they are not ordinarily in sufficient amounts to affect the thiosulfate titration. (g) The “copper” flask is of glass, and its shape has been developed for ease of handling in making copper and other titrations in the West. After use, it may become coated inside by a white film. This may be removed by boiling a 50 per cent caustic soda solution in it and washing. (h) The filtering of the dilute hydrochloric acid solution leaves much of the impurities, except aluminum, behind on the paper, The titration is not affected by what passes through. (i) Although it is an annoying separation to make, sodium sulfide precipitation is used because no manipulation which has been noted or which was tried provides for a sufficiently complete precipitation of iron hydroxide by caustic soda. Further, the greater the excess, the more the iron found in the filtrate. It has occurred to the author to add sufficient ferric chloride solution to the sodium hydroxide reagent to saturate it and provide a slight excess of ferric hydroxide, to allow the latter to settle, and to pipet clear supernatant liquor for fusions, for the purpose of doing away with the objectionable use of sodium sulfide and permitting filtering the sodium hydroxide solution almost immediately, but time to try it out and check it has not been available. The residue containing the iron hydroxide should be dissolved and run t o dryness to dehydrate silicic acid,
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filtered, and the solution treated as described for precipitation by ammonia in paragraph ( m ) of Method. ( j ) ,Slight amounts of vanadium seem to have the effect of carrying iron into the filtrate, thereby causing the green-colored solution. Long standing only will permit of complete precipitation and a yellowish filtrate. A single sodium sulfide precipitation will not remove iron sulfide from aluminum, but does free it of enough to avoid the nuisance of having an excessive amount present in the iron hydroxide precipitate formed later (paragraph ( m )of Method). ( k ) The iron sulfide is readily dissolved. There should be no more than a film of sulfides left on the filter paper anyway, provided the precipitate has not been allowed to become dry before removal by the jet. ( I ) , When alumina is relatively high, it is difficult to remove all the silica by a hydrofluoric acid rundown, but the caustic soda fusion will take care of this. The gelatinous silicic acid formed by acidification is objectionable later on in the filtering of iron hydroxide and in the titrating by thiosulfate, so it is best to dehydrate it a t this point and filter it off. (m) No particular time of standing for complete precipitation by ammonia is necessary, provided the solution is brought to a boil. (n) Were a sodium sulfide separation not made to free a small amount of iron from a relatively large quantity of aluminum, and were silica not removed, this titration would be prolonged so greatly, owing to entanglement, that uncertainty would be created as to when an end point is reached. Literature Cited (1) Bur. Standards, “Certificate Analyses, Standard Sample No. 81.” (2) Low, A. H., “Technical Methods of Ore Analysis,” 9th ed., p. 80, Wiley, 1922. (3) Lundell, G. E. F., and Rnowles, H. B.,Jr., J . Am. Cerum. SOC.,11, 119 (1928). (4) Yoshida, I., J . Ckem. SOC.Jupan, 45, 26 (1927).
Residual-Current Measurements in Control of Metal Solution in Milk’*’ H. T. Gebhardta and H. H. Sommer UNIVERSITY OF WISCONSIN, MADISON, WIS~
The solution of copper in milk occurs through t h e action of local elements in which the cathode consists of passive areas. The corrosion elements which are active in copper solution in milk m a y be studied by means of model elements, using copper anodes, a n d platinum or gold cathodes, or by using bright a n d oxidized copper electrodes. For copper, the current gives a qualitative measure of the solution rate under various conditions. From the current the amount of copper dissolving in milk m a y be calculated w i t h fair approximation to the gravimetrically determined values.
The solution of nickel in all probability m a y be studied
in a similar m a n n e r as copper. For other metals the curr e n t measurements give a qualitative indication of the probable solution rate, but the correct interpretation of the observed current requires more investigation. The principle of the oxidimeter of Toedt, which was used mainly to measure the corrosion r a t e of iron and steel, m a y also be used successfully for measuring the solution rate of copper. Therefore, residual-current measurements m a y be used in t h e s t u d y of copper solution in practice.
. . .. , . . .. .. .. HE problem of metal contamination in dairy products is important, as it involves not only the durability of dairy equipment, but also the flavor and keeping quality of the products. I n recent years investigators have studied this problem, with the result that manufacturers of equipment have introduced new alloys designed to be more resistant to corrosion. The industry in general
T
1 Received April 16, 1931. Presented before the Division of Agricultural and Food Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931. 2 Published with the permission of the Director of the Wisconsin Agricultural Experiment Station. The work was supported in part by a grant from the Special Research Fund of the University of Wisconsin. a Present address, Control Laboratory, NestlB’s Milk Products, Inc., Marysville, Ohio.
is in an expectant mood for more information on the problem. The corrosiveneps of many metals under various conditions has been investigated in laboratory corrosion tests, but useful data can be obtained only if in the laboratory experiment the corrosion factors are carefully controlled. Such tests require elaborate apparatus and consume appreciable time (8). On the other hand, laboratory corrosion tests are often criticized (3,9) because the results of such tests are not directly applicable in practice. Corrosion conditions in practice are difficult to simulate in the laboratory. Various compromising methods have been suggested to meet the conditions of special practical problems, Unfortunately
October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
most of these methods involve the use of special apparatus and require much time. Among such methods, that of Toedt (12-15) appeared to be the simplest and most applicable for both laboratory and practical tests. Toedt devised an apparatus for measuring metal corrosion by residual-current measurements. He attempted to show that the current measurements give results which agree with the gravimetrically determined losses in weight of iron (14). These, however, were determined by a method which failed to provide for control of the velocity, and was open to other criticisms.
415
devised by Toedt represents such a model element, this term will be used throughout this paper. I n the past year the solubility of metals in milk has been studied extensively in this laboratory (4). Abundant quantitative data on the corrosion rate of various metals were made available and appeared especially suited to serve as a basis for comparison with results furnished by residualcurrent measurements. Most of the work up to the present time concerned the solution of copper in milk, but interesting observations were made also with other metals. Method The method for exposing the metal test pieces and the cathode metal (noble metal electrode) is illustrated in Figure 1. It is the same method as was used for gravimetric solubility tests ( 4 ) . In Figure 1, number 1 indicates container for milk (quart bottle); 2, thermometer to control temperature of milk; 7, stirring device; 6, metal test piece; 3, platinum, gold, etc., electrodes; 4, potassium chloride-agar bridge to measure single potentials; 9, mercury commutator; and 8, “take-off’’ wires to galvanometer and potentiometer. The arrangement of the apparatus for the current and e. m. f . measurements is shown in Figure 2, where a, b, indicates connection with the electrodes; Sc, Sa, switches; A , amperemeter; and R, resistance box. On the right side is shown the potentiometer. The test metals were samples of commercial sheet metal. They were lightly scoured with Bon Ami powder before being immersed in the milk. Solubility of Copper in Milk
Platinum, either plain or gold-plated, or gold electrodes were used as the cathodes in connection with copper. The preparation of the electrodes and the cleaning required special care (the best results were obtained with the method described by Livingston et al. (6),in slightly modified form). GENERAL OBSERVATIONS-when the circuit was first closed, a very high current was observed, but it decreased rapidly during the fist fey minutes. After this, the current decrease continued for some time a t a slow rate, which varied with individual electrodes. Agitating the milk or the cathode caused the current to increase. Agitating the copper without agitating the milk in contact with the cathode had no effect on the magnitude of the current.
Figure 1-Sketch
of Apparatus
This method of Toedt is based on the electrochemical theory (10) of metal solution, which assumes that the surface of the corroding metal consists of small local areas where the metal dissolves-the anode areas-and that the major part of the surface is passive and serves as the cathode in the corrosion element. The factors which cause the formation of corrosion elements have been studied by various investigators ( I , 9, 7, 8, 11). Model elements which simulate the corrosion element can be prepared in the laboratory, and permit the study of the activity of the corrosion elements by measuring the (residual) ~ u r r e n t . ~Since the apparatus The term “residual current” heretofore was applied to the current which, apparently independent of Ohms law, flows between two electrodes, when an external e. m. f. is applied which is below the decomposition potential ( 5 ) . The current of the local corrosion elements has not been named. Toedt proposed the term “oxygen-residual current” (Sauerstoff Reststrom) to be given to the current which is generated by model corrosion elements, Such as platinum-iron, in an electrolyte solution. The current of similar elements in the literature occasionally is termed “depolarization current.” The present investigation on the nature of the current of model elements has revealed the fact that the current is identical in its physicochemical nature with the original residual current. For this reason it is termed briefly “residual current’’ in this paper. 4
Figure 2-Arrangement
of Apparatus for Current a n d E. m. f . Measurements
EFFECT OF AcIDITY-The copper and the platinum electrodes (about 1.2 sq. em. surface area), attached to the stirring device, were agitated in the milk a t 23” C. Two such model elements were prepared. The current observations are given in Graph 1. After 23 minutes, 10 cc. of dilute lactic acid solution were added to the milk in one of the two samples. The current increased slightly after the first addition (the acid was added slowly), but soon the current decreased rapidly, and was smaller than in the control. After 27
AN A L Y TI CA L EDITION
416
minutes it was 39.5 X as compared to 47.0 X in the control, and the pH of the acid milk was 5.1 as compared to 6.61 for the control milk sample. At this moment 10 cc. more acid was added. Immediately a similar drop of the current, as before, was produced. After the second addition of acid, the pH of the milk was 4.2.
Tempoatwe
of
Milh 23°C
Time M mind-
.
The current decrease is in agreement with the results of gravimetric corrosion tests as previously reported (4, where it was found that the acid milk dissolved appreciably less copper than sweet milk, especially at higher temperatures. The objection has been raised that possibly the decreased copper solution in milk acidified with lactic acid would be due to the reducing effect of lactic acid, and is not due to the increased hydrogen-ion concentration. To examine this objection, the above experiment was repeated using dilute sulfuric acid instead of the lactic acid. The current as indicated in Graph 2 was observed. This shows that the effect of acid is the same whether lactic acid or sulfuric acid is used-i. e., the effect is due to the increased hydrogen-ion concentration. EFFECTOF DrssoLvm Oxudm-Copper solution in milk had previously (4) been found to increase appreciably when oxygen was bubbled through the milk during the exposure. On the other hand, when carbon dioxide was bubbled through the milk, copper solution decreased. This effect was attributed to the rinsing out of the dissolved oxygen from the milk. Two experiments conducted to study the effect of dissolved oxygen on the current are reported in Graph 3. Curve a' illustrates the effect of increased oxygen concentration on the current; a", the effect of rinsing out the dissolved oxygen with pitrogen (not purified commercial), and later of redissolving oxygen, on the current. The current observed in these experiments and the solubility of copper as shown in the gravimetric tests are parallel when influenced by change of the oxygen content of the .milk. Curve a' decreased rapidly from its maximum about 12 minutes after the bubbling of oxygen through the milk had been started. This drop of the current was in all probability due to the beginning of passivity of the copper caused by the high oxygen concentration. This copper test piece showed a faint bluish interference color when removed from the milk, whereas the control test piece was untarnished, a
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phenomenon which had also been observed in previous gravimetric tests (4). EFFECTOF TEMPERATURE-The temperature of the water bath in these experiments increased gradually, and the temperature and current changes a t certain times were noted, as shown in Graph 4. When the current was plotted against the temperature, curve c was obtained. The amounts of copper dissolving were calculated (in terms of milligrams of copper dissolved, per square decimeter, per hour) from the cathode current density, as observed at certain temperatures. Comparison of the two curves, a for the gravimetrically determined. values, and b for, the calculated values, shows fair agreement of the two. In previous work it had been found that the copper solubility increased at a logarithmic rate with the temperature up to 50" C. Similarly it was found that the current increases a t a logarithmic rate up to 45" C., as shown in Graph 5. Above this temperature the observed current is less. The calculation of the dissolved copper from this experiment gives somewhat higher values than the gravimetrically determined values, as obtained after exposure for one hour. The calculated values are about as high as those obtained gravimetrically after exposure for one-half hour. The calculation of the rate of copper solution is based on the assumption that equal amounts of hydrogen are plated out per unit of surface area (12-1.4) of the cathode in the model element, as on the cathodic surface areas of the copper. The total amount of hydrogen plated out is equivalent to the amount of copper which goes into solution, and for this reason one would be justified in calculating directly the amount of copper dissolving by applying Faraday's law: mg./dm.*/hr. Cu =
1.186 grams Cu X current surface of cathode
Although the above-calculated values come quite close to the gravimetric valued, the calculations involve some uncertainty because of the change of the current with time, whereas all the controllable factors remain constant (on the other hand, it is known that copper solution in milk de-
creases with time), but especially because the current is not strictly proportional to the measured surface area of the cathode metd. The cause of these discrepancies has been studied more in detail and will be discussed at another place. These observations with copper model elements evidently prove that the magnitude of the current is an indicator of the comparative rate of copper solution in milk under various conditions. However, they also present strong evidence that copper solution in milk takes place through the action
i)
INDUXTRIAL A N D ENGINEERING CHEMIh'TRY
October 15, 1931 Graph 9
k
417
I n Graph 6 the changes of the current with the temperature, as observed with one of the above model elements, is illustrated. The current is seen to increase a t a logarithmic rate. The maximum current was obtained at 58" C,, and above 65" C. it decreased rapidly, reached zero a t 78" C., and immediately began t o increase in the opposite direction. Apparently the high cathodic current density at 65" C. activated the previously cathodic surface areas on the copper and caused this test piece t o become the anode in the model elements. No doubt this change of the roles of the anode and cathode metals is a periodic phenomenon. This mechanism evidently is of great practical importance
-.-
of local corrosion elements. The activity of these corrosion elements can be observed with fair accuracy on the model elements of copper combined with platinum or gold. A MODEL ELEMENT OF COPPERELECTRODES-The existence and the activity of corrosion elements, however, can be demonstrated in a more simple and striking manner, It was found that two copper test pieces, only one of which was freshly scoured with Bon Ami powder, when immersed in the milk and connected through a galvanometer! produced a strong current which was almost constant during an indefinite number of hours. This current had the same characteristics as the residual current of the model elements previously described. It is affected by agitation, dissolved oxygefi, and temperature. But the current density on the cathodic test piece is less than on the noble metal electrodes. Obviously this is due to the fact that not the entire surface of the cathode test piece serves as cathode, and that in certain localities copper dissolves from it. The current density on the cathode areas, therefore, is greater than on the single test piece.
Temperature
when copper serves as a heat-transmitting medium, as was indicated by a special experiment. This suggests that the rate of copper solution under such conditions must be very high. The activation of the copper due to high temperature (or high cathodic current density) is further indicated by
IO"
T
w
&
M
The difference between two copper test pieces consisted in a faint oxide film produced by exposure to air which produced passivity of the copper surface. This effect had been removed by scouring on the other copper blade.5 The change of the copper surface due t o the exposure to air sometimes goes parallel with the production of a visible oxide film (which may be less soluble, or possibly non-conducting), but it appears that the passivity is not
W
the drop of the single potentials of the copper at temperatures above 70" C.
-
produced by this visible film, because the film could hard1 y explain why the single potential in milk is more positive. The production of a visible film (bluish) by immersion into dilute sodium sulfide solution renders the potential more positive, and the rate of solution, as indicated by the current, is great? from the coated metal than from the bright copper.
ANALYTICAL EDITION
418
VOl. 3, No. 4
in combination with platinum in milksat 62.8” C., when oxygen was bubbled through the milk, are reported. For comparison with the gravimetrically determined weight losses, these are reported to the right. The magnitude of the current and the corresponding weight losses are not in agreement. Nevertheless, the changes of the current during the experiments furnish valuable information concerning the corrosion rate and its variations with time, Thus the current indicates (1) for nickel, the ‘disappearance of passivity (an oxide film); (2) for monel metal, the production of passivity; (3) for tinned copper and solder-coated copper, slight increase in the solution rate; (4) for German silver, only slight variations in the solution rate; ( 5 ) for brass, increased solution rate, and in all probability more rapid solution of copper, so that the relative surface of zinc increased, and thus zinc became the dominant metal determining the current.
V
c
d
i.I
‘C Temperature
Solubility of Other Metals in Milk
The currents produced by various other metals in milk when connected with platinum or gold electrodes were studied. At the present time none of these have been sufficiently investigated to permit definite conclusions. A few genera1 observations are here reported. NIcKEL-The currents observed with nickel in milk under various conditions were very similar to those obtained with copper. In almost all cases higher currents were observed, the currents fluctuating more with time and the surface conditions of the nickel. These observations indicate the parallelism of the current and the actual weight losses, so that the current measurements may give information concerning the relative solution rate of nickel in the same manner as was possible for copper. ALUMINUM, ZINC, AND TIN-These metals differ as a group from the metals already mentioned and from the alloys in that they produce very high residual currents, although they actually dissolve but very little in milk. The current, therefore, is in no proportion to the amount of metal dissolving in the milk. At present no satisfactory explanation for this phenomenon can be given, but it appears that probably all the hydrogen, in amounts equivalent to the metal going into solution, is plated out at the noble metal surface. This was found to occur when zinc was dissolved in hydrochloric acid under hydrogen evolution when connected with a platinum electrode, as reported by Centnerszwer et al. (1). Further, the behavior of these metals in solutioq is largely changed when connected with more noble metals. Thus, aluminum, which otherwise is practically not dissolved by sweet milk, suffers appreciable pitting, and zinc dissolves more rapidly. Tin coatings apparently are not affected by the connection with platinum electrodes. ’ VARIOUSALLOYS-Various alloys were studied in preliminary experiments. Obviously the interpretation of the current in this case is more difficult than with single metals, but from the results it appears that further study of residual currents will lead to a better understanding of the complicated corrosion mechanism of metal alloys. It is believed that residual-current measurements offer the best means for elucidating corrosion mechanisms. I n Graph 7 the current observations with various metals
Weqht-Lars .f Metals in mg/dm?/hr
n”
Literature Cited Centnerszwer, W.,and Straumanis, M., 2. physik. Chem., 128, 415, (1925). Evans, U. R., IND. ENG.CHEM.,17, 383 (1925). Frazer, 0.B. O., Ackerman, D. E., and Sands, J. W.,I b i d . , 19, 332 (1927). Gebhardt, H. T., and Sommer, H. H., “The Solubility of Metals in Milk,” unpublished manuscript, to appear in J . Dairy Sci. (1931) Grube, G., “Grundzuege der theoretisrhen und angewandten Elektrochemie,” Steinkopff, 1930. Livingston, J., Morgan, R., Lammert, 0. M., and Campbell, Margaret A,, J . Am. Chem. Soc., 68, 454 (1931). McKay, R.J., Trans. A m . Electrochem. Soc, 41,201 (1922). McKay, R. J., IND. ENO.CHRM.,17, 23 (lQ26). McKay, R. J., Am. Inst. of Mining and Met. Eng., Tech. Pub 19% Class E, Inst. Metals No. 73. Speller, F. N., IND. ENG.CREM,,17, 339 (1925). Thiel, A., and Eckell, J., 2. Elektrochem., SS, 370 (1927). Toedt, F.,Ibid., 84, 687 (1928). Toedt, F.,Ibid., 84, 691 (1928). Toedt, F.,I b i d . , 84, 863 (1928). Toedt, F.,Chem.-Ztg., 68, 667 (1929).
