A PHOTOGRAPHIC STUDY OF T H E GROWTH OF ELECTROLYTIC STRIATIONS BY ROBERT TAFT AND OREN R. BINGHAM~
In a previous paper2 we have reported upon the elect,ro-deposition of copper in the presence of gum arabic. I n that study the effects of the gum arabic upon the form and mass of the cathode deposit are ascribed to the mutual adsorption of gum arabic and copper oxides upon the surfaces of the newly formed copper crystals. Deposits obtained in those cases where the concentration of gum arabic was high or the hydrolysis of copper sulfate was relatively extensive were very markedly st,riated. Such striated deposits have been known for many years, although a satisfactory explanation of their growth has never been made. It has been recognized that the convection currents set up in a “still” electroplating bath, as well as the presence of a foreign substance, were essential for their formation i.e. some material (usually organic in nature) ot’her than the metallic salt and water forming t’he plating bath. The most. serious attempt to explain the formations of such deposits has been t’hat of Rosa, Vinal and NcDanieP in connect,ion w,ith the striations encountered in the silver coulometer. Their explanation is as follows: the presence of certain types of impurities leads to t,he reduction of traces of Ag+, forming colloidal silver. This colloidal silver finds its way to the cathode and is deposited upon silver crystals growing upon its surface. This deposition of colloidal silver distorts the crystal structure of t’he growing silver deposit’; the deposit now “grows by preference upward in the direction favored by the upward movement of the liquid.” These deposits tend to form elongated structures, which in time grow into one another, developing well defined ridges, or striae. In time the striae will disappear by growing into each other laterally. The data of Marie and Buffat4 and of Taft and Messmorej offer a somewhat different explanation of the cause of striations obtained by depositing copper in the presence of gelatin. These authors found that gelatin (or its degradation product’s) was actually present in the copper deposit, the weight of the deposit being materially enhanced by its presence. Further, it seems fairly well established that t>hegelatin finds its way into t’he deposit as a result of adsorption upon the depositing copper crystals. The growth of striations in such solutions has been explained by this assumption, i.e. adsorption of gelatin by copper.6 1 3
6
Presented before the Kansas Academy of Science, McPherson, Kansas. April 16, 1932. J. Ph s. Chem., 36, 2338 (1932). Bur. itandards Bull., 9, 263 (1912). J. Chim. hys 24, 470 (1927). J. Phys. Ehem!, 35, 2585 (1931). Cf. Taft and Messmore: Loc. cit.
GROWTH OF ELECTROLTI'IC STRIATIONS
2447
I n the case of the deposition of copper in the presence of gum arabic under the conditions mentioned in the opening paragraph of this paper, the striations offered such contrast to the base metal as to make it appear feasible to study the growth of such striations photographically. Such a study should furnish information bearing on the cause of the formation of striated deposits as well as furnishing a record of many of the statements already found in the literature concerning their growth, but for which there is no record save description. Our method for recording the growth of such deposits was to construct a wooden cell, some 4 X 4% X 6% cm. in dimensions. One end was fitted with a window made from optical glass and cemented in with de Khotinsky cement.' An anode was made of heavy copper sheet coiled to allow
FIG.I
a clear vision of the far end of the cell through the window. The cathode of platinum ( 2 . 5 om. on a side) was placed a t the end opposite the window and was then illuminated from above by a 400 watt focusing spot light. The arrangement is shown diagrammatically in Fig. I. Powerful illumination was necessary because of the very considerable absorption of light by the electrolyte, copper sulfate. A view camera fitted with a Bausch and Lomb Tessar lens was set up in front of the window and focused upon the cathode. A series of photographs of the cathode was then made as electrolysis continued, without disturbing either the electrolytic cell or the camera. I n order to reduce the time necessary for illumination (which would induce local heating) super-speed panchromatic cut film was used as the recording medium, the exposure necessary being I O seconds at f 16, with a lens of 7 focal length and at bellows extension of 22". Electrolyses were carried out at room temperature, which for the Series I photographs was 32'C, and for the Series 2 set was approximately 4ooC. The Series I photographs were obtained in a solution containing 0.5 M copper sulfate and 1.5 grams of gum The entire interior of the cell was coated with this cement to render it solution-proof.
nomm TAFT A N D onm n. R I N G H A M
2448
nmbic pep 100 cc. of salution a t a current density of 2 anipcres per square decimeter. The form of the deposit obtained from zero time up to 2 % hours is shown in Series I , a t,o i inclusive.
b
d R
h
I
YrniEti 1
The growth of the Cnthodr Ihposit in 0.5 M CuSO,. Current Density, 1 amperes pm quare decimeter; 32%; 1 . 5p m s of &urn m d i e per aolution. The deposit was photographed a t the following time intervals: R, 0.0 minutea; h, 5 miriuteli; e , 2s minutes; d, 40 minutes; e, 50 minute; f, 60 minutes; g, 1.75 horns; I,, 2.25 hourj; i, 2.5 hours. I M ec: of
Our explanation of these deposits is as follows: Due to the form of the anode snd also to t,he well-known tendency of current lines to converge on edges, the initial current density will be considerably greater around the edges of the cathode than upon the main face of the cnthode. This results in an initial greater deposition of capper around the edges of the cnthode, resulting as we have already pointed out' in an increasing hydrolysis. As a result of this hydrolysis, cupric hydroxide is formed, which, together s i t h the gum arabic, is adsorbed upon these areas forming a dark-colored deposit,. The extent of this process is not growt, BS the second photograph (Series I , 8
Tnft and Bingham: h. cit.
GROWTH OF ELECTROLYTIC STRIATIO$JS
2449
No, b) still shows the scratch marks around the edges of the base metal (compare Series I , No. a). There is a deposit of dark color around the edges (with the exception of the bottom edge) which apparently prevents, temporarily, the further deposition of copper. It will be shown later in this paper that the addition of gum arabic produces an increase in the cathode polarization which could account for prevention of copper deposition in this area. The increased polarization around the edges diverts the current lines to the face of the cathode where apparently the correct density is quite uniform and copper (the light-colored area) is laid down. (As we have already pointed out, this copper may contain some copper oxides or gum arabic as a result of adsorption). As electrolysis proceeds, however, the solution around the cathode becomes more dilute for two reasons. First, due to the difference in transport number of sulfate and copper ion, there will be a more or less uniform dilution of the electrolyte in this region; second, due to the deposition of copper upon the cathode and the solution of copper a t the anode, a well established convection current will be set up, the solution becoming progressively more dilute as it rises up the face of the cathode. As a result of these dilution effects, there will be produced more and more cupric hydroxide. When the concentration of this substance reaches a certain value, it is adsorbed simultaneously with the gum arabic,' producing the dark colored area. The formation of this dark-colored area first becomes appreciable in the photopaph of Series I , No. d, although careful examination will show it in Series I , No. c. Series I , No. d is produced after some forty minutes of electrolysis. The time a t which this phenomenon first became apparent could be reproduced to the minute in duplicate trials. Once it has started, the dark area bridges rapidly across the electrode, a t the same time growing upward. The rising electrolyte (after it leaves a given dark area) becomes richer in copper oxide as dilution continues up to a certain value2 and poorer in gum arabic, due to its increased adsorption when in the presence of cupric hydroxide. I n addition to the dilution of the gum arabic caused by its adsorption there is also a diluting tendency due to the migration of arabate ions3 toward the anode. These two effects are sufficient to prevent further deposition of the dark-colored material until the solution behind the advancing front supplies the necessary gum arabic, which, we must assume, is not extensively adsorbed upon the dark-colored area already laid down.' In this way the dark-colored deposit continues to grow upward, the small initial As we have pointed out in our f i s t paper, there is apparently a factor similar to a solubility product involved, i.e. when Cgum X Coopper oxide (C's re resenting concentrations) exceeds a certain value, the dark-colored area is produced. f n fact, the formation of this dark-colored area appears very similar to a supersaturation phenomenon. The original dark areas appear very suddenly and grow quite rapidly durin the first interval of their formation, the rate of growth diminishing after the first few areasaave been formed. I t must be remembered that the concentration of cupric oxide (or hydroxide) is the result of two factors, the extent of hydrolysis of the salt and also of its concentration. Cf. Taft and Malm: J. Phys. Chem., 35, 874 (1931). As we have already pointed out, the formation of the dark-colored deposit also likely v t s the deposition of copper in this region, as long as there are any copper areas present. he prevention of copper deposition in this region increases the effective current density in the remaining region.
GROWTH OF ELECTEOLYTIC STHIATIONS
2451
The mowth of the Cathode Dewit in I MC,rSOI. Current Density, z amperes per square decimeter; 0°C; i 5 grams of gum arabic per IOU CC. solution. The photoginphs s.i .... . ~ ~ i . 5.. minutes: c. I S mmutes: .. ... taken . ~~C .t,ime intorvdn: s. 20 seconds: h. .. d, 20 minutes; e, 2s minutes; f, 45 minutes; g, 60 minutes; h, 1.5 houra; i, z ho%j, 3 h o w ; k, 4 hours; I,5 hours.
thekiollnu-inr ~
2452
ROBERT TAFT AND OREX R. BINGHAY
differences in the height of the dark-colored area (due to slight differences in concentrations of copper oxide and gum arabic) becoming greatly magnified as time proceeds, Series I , Nos. e, f , g. I n order to accept this explanation of the form of the deposits, it is, however, necessary to make the additional assumption that the dark-colored deposit is more readily adsorbed by itself than by the copper crystals,' this adsorption taking place upon the upper edge of the dark-colored deposit already laid down. The rising current of electrolyte then accounts for the striae as suggested above. The striae developed when a higher concentration of copper sulfate ( I 9'1) is employed are more striking than those shown in Series I . It should also be recalled that these, i.e. the Series z photographs, were made a t a higher temperature than the Series I set. Rosa, Vinal and McDanieP have called attention to the fact that increasing the concentration of the electrolyte makes electrolytic striae coarser and more distinct. A comparison of our Series I and 2 photographs shows this very clearly. In the case of our Series z photographs it is evident, as would be expected, that with the increased concent,ration of electrolyte, t,he areas at which cupric oxide and gum arabic are present in sufficient concentration to produce the dark-colored material are more widely separated. Further examination of the photographs shows that the striae result from these initial areas. Thus, in the fifteen minute deposit of Series 2 , small circular areas3 first become visible. The striae grow vertically from these initial areas, finally giving rise to the very distinct striae of Series 2, No. h. Eventually the striae grow together to give a deposit more or less homogeneous in appearance. Further electrolysis (Series 2 , No. 1) does not change the form of t'he deposit. Evidently when the surface has reached this condition, deposition (at a higher negative potential) of both copper and adsorption of copper oxide and gum arabic occurs uniformly over the entire surface. As we have already pointed out, the addition of gum arabic produces an increase in the cat,hode polarization. The method pursued for determining This assumption, we feel, is justified from the form of the striae in the Series z deposits, for example No. g of this series. The rounded bottom of these striae is always thicker than the upper part, indicating the small amount of dark-colored product formed below the striae is adsorbed upon coming in contact with the material already laid down. As the adsorbable material formed is slight, the upper part of the striae can only grow by the formation of new material. This can only take place until deposition of copper above the dark area begins again. 2 Loc. cit. 3 The circular form of this initially deposited dark-colored material and the drop-like form which the bottom of these areas assumes as the areas grow, indicate very clearly that interfacial forces are at work. Free adsorption (Le. spontaneous adsorption) of the dark-colored substance would lead to a reduction of the interfacial tension at the boundary, copper-solution. As we feel that free adsorption has been proved (Taft and Bingham: LOC.cit.), these rounding forms can scarcely arise as a result of an increase in the interfacial tension. Rather, they must be due to an increase in the plasticity of the material in the deposit. As we have already shown (Taft and Bingham: LOC.cit., Fig. s), even small amounts of gum arabic in a copper plating bath produce slipping and displacement of the various crystal surfaces over each other. The large amount of gum arabic and copper oxide present in the solutions from which the Series z deposits were obtained would undoubtedly produce a still more plastic substance.
2453
GROWTH OF ELECTROLYTIC STRIATIONS
this polarization was that of Haring.' The polarization measurements were carried out at a constant temperature of 3oOC. Some of our results are tabulated below in Table I and shown graphically in Fig. 2.
TABLEI Cathode Polarization a t 30' of 0.5
M CuSOa, volts
Current Densit Amperes per sq. jm.
Con. of gum arabic,
grams per IOO cc.
0.16 0.32 0.48 0.64 0.80 0.96
0.022
1.12
0.071
0.083 0,096
0.15
0
0.038 0,054 0.061 0.069
0.029 0.036 0.041 0,047 0.058
0.072
.28
0.085
0.11
I .44
0.096
0.13
I .60
0.11
0.15
.oo
0.I 5
0.19
2.40
0.18
0.22
I
2
FIQ.z
Abscissae, Cathode Polarization, volts; Ordinates, Current Densit ,amperes per square deci0.15 grams of gum meter. Curve NO. I is fo? 0.5 M C~SO,;NO. 2 for 0.5 M CUJO, arabic per 100cc. of solution. 'Trans. Am. Electrochem. SOC., 49, 417 (1926).
+
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ROBERT TAFT AND OREN R. BINGHAM
Similar measurements carried out at lower concentrations of gum arabic gave polarization values intermediate between those given, indicating that polarization becomes greater, at a given current density, the greater the concentration of gum arabic. Incidentally, if the polarisat,ion values of Table I are compared with those obtained during the deposition of copper in the presence of gelatin by Taft and Messmore a marked difference is observable. The polarization curves of Fig. z duplicate each other in form quite closely. On the other hand, in the presence of gelatin a very marked increase in polarization occurs at a current density of 0.8 amperes. They interpreted this increase to be due to the formation of complex cations between gelatin and copper. Its absence in the present' case could be presented as an argument against the existence of any such complex cations of gum arabic and copper. Summary I . A photographic record of the growth of electrolytic striations of copper produced in the presence of gum arabic is presented. The formation and growt,h of these striae can be explained by assuming adsorption of a gum arabic-copper hydroxide complex upon the surfaces of the electrodeposited crystals of copper. 2. The cathode polarization of copper is increased by the presence of gum arabic. Univ. of Kansas, Lawrence.
