POTENTIAL MEASUREMENTS ON T H E COPPERNICKEL SERIES OF ALLOYS, AND SOME OBSERVATIONS ON BRASSGS BY NEWELL
T. GORDON AND DONALD P. SMITH
The present experiments are a continuation of the study of the factors affecting the potential difference between a binary (solid) alloy and an electrolyte containing the corresponding ions, with regard to which a preliminary publication was recently made. With the extension of the experiments upon copper-zinc alloys from the single ingot previously discussed to a much larger number of samples, and particularly with prolongation of the time during which each sample was kept under observation, from a few hours to many days, it developed that no treatment which could be employed was sufficient to insure, with the alloys of this series, the desired degree either of reproducibility or of constancy in the “steady” potential finally attained. The account of these experiments will therefore be deferred until after the discussion of those made upon the copper-nickel alloys, in which a system was found remarkably free from the accidental variations shown by the brasses. The System Copper-Nickel The copper-nickel ingots were prepared from electrolytic copper and the best grade nickel in finely divided form. The weighed portions of the two metals sufficient to make about twenty-five grams of alloy were thoroughly mixed, introduced into a porcelain crucible of test-tube form about one centimeter in diameter, covered with borax, and melted in a carbon resistance furnace of the Tammann type. After the melt had been well stirred for some time with a porcelain rod it was allowed to cool in the furnace. The ingot was then inverted in the crucible and again fused and stirred under borax. Upon the second solidification the alloy was obtained in a state highly homogeneous and free from oxide, as was confirmed by microscopic examination. Prom this ingot small bars 2 X 2 X 5
Copper-Nickel Series of Alloys, Etc.
I95
mm were sawed longitudinally and then cut transversely into halves. The end surfaces produced by this last cut constituted portions of horizontal planes through the middle of the ingot, and were the ones later exposed to the action of the electrolytg. The composition of each ingot was determined by analyzing the middle portions of some of the small bars not employed for potential measurements. To this end copper was deposited electrolytically from sulphuric acid solution, while the nickel was in most cases found by difference. In several instances however, the result was controlled by rendering the solution alkaline with ammonia, after the removal of the copper, and depositing the nickel also electrolytically. The errors of analysis, and those due to differences of composition between the samples analyzed and those taken for experiment, may be taken together as not over 0.3 percent in the composition of the alloy. The six ingots prepared had, respectively, 6.8, 10.9, 65.5, 74.3, 83.4, and 94.7 percent by weight of copper. Each test-piece was annealed in a close-fitting, hard glass tube which was three times filled with nitrogen and evacuated, and finally sealed when the pressure of nitrogen equaled approximately 160 mm of mercury. Commercial nitrogen, which had been freed from oxygen by means of copper and an ammoniacal solution of ammonium carbonate, in an apparatus similar to that described by Van Brunt,2 insured bright annealed test-pieces exhibiting no evidence of oxidation, The alloys of 6.8 and 10.9 percent copper were annealed in naphthalene vapor a t 218’ C, below the magnetic transition of nickel, while all the others were embedded in sand and heated in an electric furnace a t 600’ C. Each annealing continued for one week. The electrolytes employed were 41 I-normal in copper sulphate and nickel sulphate combined, some of them being in addition I-normal in sodium sulphate. They were prepared by mixing and diluting 1.4-normal stock solutions made from the best obtainable “C. P.” salts, with the use of volumetric apparatus calibrated at 20’ C. Figure I shows in cross-section the half-cell in which the
Newell T. Gordon afid Donald P. Smith
196
P.
test-piece was placedTfor observation of the potential. The whole apparatus was 'of glass, and consisted of a cylindrical outer vessel A, closed 'by the ground-glass stopper B, which carried the other parts.' In preparation for a series of observations one end of a fine copper wire was tw$sted tightly around the test-piece, and the latter, together with the adjacent part.of the wire, was heavily lacquered with an alcoholic solution of marine glue, leaving exposed only the end surface of the test-piece, which has previously been referred to as formed by the last cut in the preparation of the sample. The free end of the copper wire was joined by I Ill I I fusion to a small loop of platinum. The lacquer having dried, the stopper B and its attachments were removed, the testpiece C was suspended by its wire from the platinum hook D, so as 'to hang near the bottom of the inner cylinder E, and the one-hole rubber I stopper F was put in place. I After the electrolyte had been Fig. I added to A the part B was replaced, the stopcock G being closed so that the pressure of air in E prevented the electrolyte from entering and coming in contact with the test-piece. Nitrogen, which had been freed from oxygen by the method previously mentioned, was next admitted to the halfcell through the stopcock G and allowed t o pass for five i .
I
Cop,per-Nickel Series o j Alloys, Etc.
I97
minutes, bubbling out through the trap K, which contained a portion of electrolyte. The cell was now put into the thermostat and allowed to remain for fifteen minutes. Thereupon, by opening the stopcock G and applying a slight pressure of nitrogen at K, the electrolyte was caused to enter the inner cylinder E and come into contact with the test-piece. The stopcock L was then at once opened, establishing connection with the calomel electrode through the siphon H, containing the cell-electrolyte, and an intermediate vessel, containing I -normal potassium chloride; the electrical circuit was now completed through the hook D and observations were begun with as little delay as possible. All potentials were measured at 2.5' C against a normal calomel electrode by means of potentiometer and galvanometer. Other experimental arrangements were such as were described in the preliminary publication. From the curves of Figure 2 may be seen the general charac-
~~2 6 8 ' 947. NO4 94.7
,'
005
,
02 03
N O 3
I
. .
-09
,
.07
*L38
' ' "
T!ME I N HOURS
Fig.
2
ter of the results obtained with each of the six alloys of the eopper-nickel series. The observed points are inclosed by
Newel1 T . Gordolz and Dolzald P. Smith
198
1
Curve
No.
I
No.
2
-1
Test-piece
0.01N Cu and 0.99 it‘ Ni Ingot I1 (94.7% Cu) 6.8% CU 7 10.05 T\i Cu and 0.95 N Ni
No. 3 - Ingot I No. 4 _---
No. 5
Composition of electrolyte
(94.7% Cu)
Ingot I (94.7% CU) 74.3% cu
0.2
N Cu and 0.8 N Ni
0.3 N Cu and 0.7 N Ni 0.3 N Cu and 0.7
N Ni
-
The variations at the beginning of the run may be accounted for in two ways: by the process of adjustment of the surface compositions of the two phases; and by irregularities due to superficial conditions such as films of oxide or possibly adsorbed gas. It is evident that the “steady” value of the flat portion of the curve is the important quantity and not the changeable readings of the first hours. This will be referred to as the “steady” value, and has in every instance which follows been derived from curves such as those shown in Figure 2 . It may be pointed out that the constancy of the steady value indicates that the method of measurement with a galvano-
Copper-Nickel Series of Alloys, Etc.
I99
meter as a zero instrument, in place of the electrometer often employed, produced no drift in the values observed. In a search for the best method of treating the alloy to obtain uniform potentials numerous experiments were carried out with sawed, emeried, polished, etched, and annealed surfaces. These experiments show that surfaces carefully annealed while protected from oxidation by an atmosphere of nitrogen yield results which are the most nearly constant in any given experiment and which are reproducible within the n&rowest limits. Test-pieces cut from an ingot containing 94.7 percent copper, and having annealed surfaces, were investigated in electrolytes of different compositions. Table I1 exhibits the characteristic values of the potentials for different electrolytes and also the agreement of the values in electrolytes of the same composition.
TABLE I1 Test-piece
Composition of electrolyte
No. 3
No. No. No. No. No. No.
4 5 6 7
0.2 0.2 0.2 0.2 0.2 0.2
No. No.
IO
so.
I2
KO.
---
K
Cu and 0.9 N Ni Cu and 0.9 Ni 0.1X Cu and 0.9 N Ni
KO.
I 2
8 g I1
0.1
Steady potential Calomel electrode = 0.5642) 0 . 5602
S Cu and 0.8 S Ni
0.5601 0.5587 0.5692 0.5682 0.5687 0.5695 0.5682 0.5666
0.3 i T Cu and 0.7 iV Ni 0.3 S Cu and 0.7 S Ni 0.3 12' Cu and 0.7 . Y Ni
0 5742 0.5782 0.5732
0.1' \ L
X Cu and 0.8 N Ni
N
Cu and 0.8
A? Ni
N Cu and 0.8 X Ni AT Cu and 0.8 A' Ni iL' Cu and 0.8 N Xi
2 00
Mewell T . Gordon and Donald
P.Smith
These experiments showed that for alloys from a given ingot the steady potential was definitely determined by the composition of the electrolyte. An ingot was next prepared showing the same analysis as the foregoing to within 0 . 2 percent, and the values of its potential were compared with those of ingot I. The results are given in Table 111. TABLEI11 Electrolytes: 0 . 2 N in Cu and 0.8 iV in Nil 1.0 N in Na2S04 (Calomel electrode = 0.5642)
-
1
Ingot 11
Initial potential Steady potential
1
Ingot 1
0.5619 0.5680
0.5691
Copper-A’ickel Series o j Alloys, Etc.
201
of alloy potentials in general. However this may be, the results previously cited show that there is no such ambiguity in the potentials of the copper-nickel series. The next experiments related to the influence of the surface condition of the alloy. They were made upon surfaces rubbed with emery or upon those which had been polished with rouge on a broadcloth-covered wheel. The amount of abrading or polishing being very difficult to regulate, it is not surprising that the measurements showed considerable variation. The test-pieces, numbered I to 6 in the following table, were obtained from two ingots of approximately 95 percent copper. The results displayed in Table IV show no regular differences between the steady potentials of annealed and of emeried surfaces, although the latter appear to exhibit the greater accidental variations. Between the annealed and the polished surfaces, however, there is a difference which, while variable in amount, is always of the same quality, the polished surfaces having in every instance a greater tendency to give ions to the solution. This is made more evident by considering the fact that No. I and KO. j were the most highly polished. It is not without interest that this fact seems to be in accord with Beilby’s5“amorphous layer” theory of polished surf aces. When the foregoing experiments had established the constancy and reproducibility, within the limits mentioned, of copper-nickel alloys having annealed surfaces, a new series of experiments was undertaken to show the relation between the potential and the compositions of the alloy and of the electrolyte. Potential measurements of copper-nickel alloys (94.7y0 Cu), immersed in electrolytes normal in copper and nickel sulphates combined and also normal in sodium sulphate, were continued until a steady value had been unmistakably attained. The values for each concentration, shown in Table V, are the average of from three t o six experiments. The greatest deviation from the average obtained in a single experiment was less than three millivolts. Plotting these
202
Newel1 T . Gordon aiid Donald P.Smith N
0
M v)
0
N
0
M v)
0
Q 00
d-
u)
0
N
N
d-
h M
v)
v)
0
0
Q
N
N
v) v)
v)
u,
0
0
L3
r.
a
N
0
0
0
v: 0
-
-
E .-d
E
v)
2 r .
d
.3
2 v)
2 a
a
5
8
.3
d
.3
2
2
9
m
i
0
v)
0 0
-
c? 0
8
d
v)
0 0
v)
0
z
Copper-Nickel Series oj' Alloys, Etc.
203
potentials against the logarithms of the copper concentrations gave the linear relation displayed in curve No. 4, Figure 3.
TABLEV Composition of electrolytes
Steady potentials (averages) calomel electrode = 0.5642
-
-
0.3 N in Cu and 0.7 A' in Xi 0.2 i'i
in Cu and 0.8
in Ni
0,5752 0.5682
0.1N
in Cu and 0.9 1 1: in Ni
0.5602
1 V
IT in Ni
0.5498
in Cu and 0.99 A T in Ni
0.5332
0.05 N in Cu and 0.95 0.01N
A set of experiments conducted in a similar manner, but with the omission of sodium sulphate from the electrolyte, again indicated the same relation. It was found that while, for a given alloy composition, the potentials obtained with electrolytes containing sodium sulphate differed from those observed when sodium sulphate was omitted, the differences corresponded merely to a parallel displacement of the lines representing the dependence of potential upon the composition of the electrolyte. Hence in subsequent experiments sodium sulphate was not employed. The potentials of the series of copper-nickel alloys containing 6.8, 10.9, 65.j,74.3, 83.4, and 94.7 percent copper were investigated in electrolytes 0.3 N in copper and 0.7 N in nickel; 0.1 Nincopperando.9Ninnickel;o.ojNincopperando.9j N in nickel; and 0.01 N in copper andb0.99 N in nickel. Two test-pieces were used for each determination and the values to be recorded are the steady potentials obtained by the method illustrated in Figure 2 . For every pair the steady potentials agreed within three millivolts. In the case of each alloy composition the linear relation between the potential and the logarithm of the copper concentration of the electrolyte was found to apply. Several of these curves are shown in Pigure 3, and the complete summary of values is given later in Table VI, Curve No. I represents the average slope of the
204
Newell T . Gordow awd Dowald
P.Smith
curves corresponding to all the alloy compositions investigated, each slope having been determined by finding graphically the intercept on the axis of potentials and that on the axis of logarithms of copper concentrations. The value for the average slope so determined is 30.14, and curve No. 2 having a slope of 414 3 represents the greatest observed deviation. Curves Nos. 2,3,5, and 6 show, respectively, the dependence of potential on the logarithm of the copper concentration for alloys containing 6.8, 10.9, 74.3, and 83.4 percent copper. The points on curve No. 4 are in each case the average of from three to
Fig. 3
six determinations of the potential of a 94.7 percent copper alloy immersed in an electrolyte normal in copper and nickel combined and normal also in sodium sulphate. In the foregoing paragraphs the linear relation between alloy potential and the logarithm of the copper concentration of the electrolyte has been established by experimental evidence. It is of interest to consider how this relation compares with the requirements of the well-known thermodynamic relation due to Nernst.6 According to his application of the theory to binary alloys the equation
Copp.er-IVicke1 Series o j Alloys, Etc.
7
20.5
holds for metals forming homogeneous solid solutions. We shall designate absolute potentials by E , those referred to a calomel electrode by E and the absolute value of the calomel electrode by E,. Then E E , = E . Instead of Equation I we rnav write
+
Regarding only the copper concentrations, the nickel present being beyond the limits for the exact application of the laws of dilute solutions, we obtain instead of Equation z
RT
(3)
E , = - - In [Cu"], nF
RT + nF - In [Cu"].
R When the substitution of values E' = 0.861 X IO-^, 'I'= 2 9 8 O , n = 2 , and the conversion to Briggs' logarithms are made, Equation 3 takes the form 3 X Io-210g [CU"]. ( 3 ~ ) E = - 3 X Io-210g [CU"], Since the logarithmic terms have throughout a negative value, confusion of signs may be avoided by regarding all quantities as positive and writing Equation 3a as (3b) E = 3 X IO-' 1 log [CU"], I-3 X I Q - ~ ,log [CU"] j , which is of the linear form E = b - - b 1 log [CU"] i .
+
(4)
a
Equations 3 and 4 express the kependence of potential upon electrolyte-composition for a given alloy, if [Cu ' ' lo is the concentration corresponding to the solution pressure for this alloy composition. They are based upon the assumptions :( I ) that the alloy does not alter composition as the result of [CU. .] changes in the electrolyte ; ( 2 ) that the p r o p o r t i o n a l i t y 7 [CU'
'Cu
Pcu. '
I,
is valid. This relation holds so long as the ion concen-
tration is small enough to make the extension of the gas laws to solutions permissible. In applying the equations, [Cu' ' ] is put equal to [CUSO~].Since its dissociation is not complete in the range of concentrations considered, the justifi-
206
Newell T . Gordon and Donald P . Smith
cation for this substitution will be discussed in later paragraphs. The first assumption certainly is not justified, as is shown by the m’anner in which E a t first changes in a new solution but finally, as may be seen by referring to Table VI, becomes independent of renewal of electrolyte. Instead of the constant, log [Cu ’ lo, characteristic of an alloy composition, the function j [Cu ’ ] must be substituted, producing in place of Equation 3b E = 3 X IO-'^ [ C U ” ] - ~ X IO-’ 1 log ~ C U ”1] . (5) The combination of Equation 5 with Equation 4 produces the relation or
+
k ?I2 1 log [ C U , . ] I . I n other words the linear dependence of potential on concentration, empirically found, is consistent with Equation 3b only on the supposition of a linear relation between the logarithm of ion concentration in the electrolyte and the apparent final solution pressure concentration in the alloy. Comparing different alloy compositions we find empirically ;variation in both intercepts and slopes of the curves corresponding to Equation 4. However, taking the calculated average slope (30.14), as.shown in Figure 3 , to redetermine the potentials at the intercepts, and plotting these potentials against the compositions of the alloys, we obtain as represented in Figure 4, the simple relation y.,= c hx, showing the de(6a)
log [CU”],
=
I
+
Fig. 4
Copper-Nickel Series o j Alloys, Etc.
208
n'ewell T . Gordon and DoNald
P.Smith
pendence of intercept in Equation 4 upon the composition x of the alloy. Equation 4 now takes the form (7)
E
= c
+ h~ - 30.14 1 log [CUSO~]I .
The constants c and h are obtained graphically from the slope and intercept of the curve in Figure 4. Upon inserting their values we obtain finally the relation E = 573 0.277 x - 30.14 I log [CUSOI]I (8) which represents the dependence of the potential 'upon the compositions both of the alloy and of the electrolyte. The accuracy with which Equation 8 represents the observed values of potential, for the twenty-four experiments made with electrolytes not containing sodium sulphate, is shown by Table VI. This table contains complete information regarding the composition of the alloys; the potentials calculated from Equation 8 ; the experimentally determined values; the corresponding compositions of the electrolytes; and the percent error. The average error for each composition of alloy ranges from 0 . 2 percent to 1.1 percent which includes all the potentials observed, none having been omitted even when apparently in error. It is here shown that the dependence of potential both upon the composition of the alloys and upon the copper concentration of the electrolyte is represented by Equation 8 within an average error of less than 0.75 percent. Hence the empirical results are in close agreement with theory, and it will be shown, by considering actual ion concentrations, that the constants of Equation 8 compensate for the incomplete dissociation of the copper sulphate. The determination of th.e concentration of copper ions, in the solutions used, is rendered uncertain by the lack of conductivity measurements at 25' C and is furthermore influenced by the presence of nickel sulphate, but calculations were made using the data of Kohlrausch €or conductivity and ionic mobilities at 18' C. Plotting the potentials of the series of alloys against the logarithms of the calculated ion concentrations produced straight line curves similar t o those of Pigure 3 but having different slopes and intercepts from those plotted against copper concentrations. From these curves an equation was
+
'
Copper--V'ickel Series oj' Alloys, Etc.
209
developed in the same manner as Equation 8. This procedure produced t h e equation E = 587 0.4 x - 37.65 1 log [Cu. . ] 1 , (9) which corresponds with Equation 8, the value of the constants being changed and [Cu' ' ] substituted for [CuS04]. Equation g yields results, for the series of copper-nickel alloys, which agree with the observed values within an average error of one percent. Since, theref ore, the employment of the copper sulphate concentration (in Equation 8) in place of th.e cupric ion concentration required by the theory of' Nernst gives such excellent agreement with the observed potentials, it seems practically justifiable to avoid in this way the use of the irnperfectly known concentrations of the cupric ion. At this point a consideration may be inserted to show that the liquid potential between the normal potassium chloride solution of the electrode and the electrolyte normal in copper and nickel combined is not influenced to any disturbing extent by changing the ratio of the copper and nickel concentrations. If the liquid potential were not constant the variation would be reflected in the values of E when the equation E = --o.j642 - 0.3 1 log [Cu' ' 1, I 0.3 I log (Cu 1 1 is applied t o different concentrations of electrolytes. The values of E were found by experiment and the copper concentrations of the electrolytes being known it was possible to calculate the values for log [Cu' ' lo corresponding to each concentration. The calculated values are given in Table VII. TABLE VI1 -___ _ _ ~ _ _ _ - _ _ .
+
+
Electrolyte
0.3 S in Cu and 0.7 X in Ni 0.1S in Cu and 0.9 S in S i 0.05 S in Cu and 0.95 X in Xi 0.01 in Cu and 0.99 S in Ni
j
log 1CU
I,
-19.86 -19.92 -20.00
-19.91
According to the theory of n'ernst the value of log [Cu' . 1, should, for a given alloy, be independent of the composition of the electrolyte. If there were a variation in the liquid potential the values for log [Cu ' ' lo calculated for different com-
Newel1 T . Gordon and Donald P . Smith
2 IO
positions of electrolytes should exhibit a progressive change. The constancy of log [Cu ' * lo in the table shows, therefore, that in this work the liquid potential may be regarded as constant. The results which have been discussed show that the steady potential attained is determined by the composition of the alloy and the composition of the electrolyte jointly. It may be supposed, then, that when an alloy and an electrolyte not having compositions which would be in equilibrium with each other are brought into contact, the surface layers of both change their compositions in such a manner as to approach the equilibrium conditions. As a consequence concentration gradients and corresponding rates of diffusion are established in both phases. The steady potential will then be attained when a balance is established between the rates of diffusion in the two phases. On the basis of this conception it might be expected that the rate at which the potential changes on first immersion of the alloy would be determined by the comparatively slow rate of diffusion in the solid alloy and not by the more rapid processes in the electrolyte. This seems to be confirmed by results which will be described. In these experiments, after the attainment of an unmistakable steady potential, the electrolyte was removed and replaced by a fresh portion having the same composition. Nitrogen was again bubbled through the apparatus and the potential observations were repeated. In one such experiment the steady potential upon first immersion was attained only after eight hours, while with renewal of electrolyte the steady value was reached before the first measurement, which was taken in less than three minutes. The conduct of the potential in this and in two other cases is shown in Table VIII. TABLE VI11 Alloy
No. I No.2
No. 3
!
I I
Elapsed time
1st steady
1I I
Steady with renewed electrolyte
Elapsed time
3 min.
8 hrs.
0.5452
0.5672
19 hrs.
0.5675
hr. -I-
0.5678
19 hrs.
0.5678
I
0.5410
-
hr.
Copper-Nickel Series of Alloys, Etc.
211
It is of interest to note that similar conduct was shown by a pure copper wire immersed in a copper-zinc solution. The foregoing experiments include the entire range of alloy compositions and the middle range of copper concentrations of electrolyte for the copper-nickel series, and it seems a t first somewhat surprising that a single relation between alloy composiiion and potential (Equation 8) applies both to the right and to the left of the heterogeneous region due to the transition of nickel a t 320' C, which is represented in the diagram of Guertler and T a m m ~ n n . This ~ is not improbably accounted for by the enrichment of the alloy-surface in copper. Before the attainment of the steady potential electrolytes of such relatively high copper concentrations as those employed may be supposed to have converted even a test-piece having originally only five percent of copper into one having superficially a copper content higher than that which corresponds to the limit of the region of non-miscibility in the equilibrium diagram. The steady potentials would in that case all correspond to equilibria with the homogeneous solid solutions of the copper side of the diagram. The appearance of the test-pieces on removal from the electrolyte in some degree supports this view; for with solutions 0.1N in copper, or higher, the alloys presented the appearance of having been copper plated and must therefore have acquired a surface composition of not less than 85 percent copper, this being approximately the point at which the copper color begins to dominate in the alloq s of this series ; and while with electrolytes 0.05 AT and 0.01 X in copper the test-pieces were not reddened, but only somewhat dulled in appearance, by contact with the solution, it is likely that the surfaces had acquired compositions higher in copper than the 45 percents or less a t which the transition interval ends. It may a t least be said that the results of the potential measurements are not inconsistent with those which have been obtained by other methods. The lower range of electrolyte compositions, below 0.01 iV in copper, has not been included in this investigation.
212
Newell T . Gordon awd Donald
P.Smith
Unfortunately the extension of the measurements to this interesting region, within which the equilibrium electrolytes of the entire series of alloys are doubtless to be found, has been rendered impossible by the very considerable loss of time experienced in endeavoring to obtain steady and reproducible results with the copper-zinc alloys.
The System Copper-Zinc As was explained a t the beginning of the paper, the continuation of the study of the binary brasses &owed these alloys to be highly variable in conduct and unsuitable for an investigation of the kind contemplated. A brief account of the results may neverth.eless be of interest, since they appear to conform to indications obtained by other workers that brasses are subject to changes of a sort not accounted for by th.e equilibrium diagrams hitherto proposed. The materials used for the preparation of alloys and electrolytes, the methods of analysis, and the means for obtaining uniform test-pieces were identical with those described in the preliminary publication. Xumerous conditions for annealing were investigated, with the employment both of an atmosphere of nitrogen and of other means of excluding air. Annealing in sealed tubes filled with nitrogen was conducted both at the temperature of boiling sulphur, and a t 560' C. The alloys treated at the lower temperature appeared entirely bright and clean, but were neverth.eless unsatisfactory with respect to the constancy and reproducibility of potential, while those annealed at the higher temperature sometimes interacted with the glass, becoming covered either with a red scale or with a thin film resembling black dust. It was shown that this was not due to the presence of oxygen in the tube; but the introduction of alundum thimbles or calcined magnesium oxide between brass and glass, although preventing the formation of the red scale, did not obviate the production of the black film or cause the alloys to give satisfactory results in the subsequent potential measurements. If the tube in-
a
Copper-Nickel Series o j Alloys, Etc.
213
stead of being filled with nitrogen was evacuated before sealing to a pressure of less than 0.01 mm of mercury, as indicated by a McLeod gage, the volatilization of zinc during annealing was sufficient to roughen the surface of the test-piece perceptibly, and hence to alter the surface composition of the alloy beyond permissible limits. Attempts to anneal by plunging the testpieces into molten borax were equally unsuccessful because of the rapid loss of zinc. On trying to eliminate the irregularities, shown by pieces annealed in nitrogen, through subsequent treatment with nitric acid, or by grinding or polishing, highly variable potentials were still obtained. Xor did specimens of several commercial brasses, which were known to have lain undisturbed for more than twentyfive years, and hence might be regarded as annealed at atmospheric temperatures, show more regular conduct. These irregularities were shown not to be due to variations in the illumination of the electrode, since they occurred in much the same degree with an open thermostat and with one from which light was carefully excluded. That the variations were not due to differing orientation of the crystal grains seemed to be shown by the results of experiments made with. a tapping electrode which made it possible to explore the potential at the faces of different crystals in the surface of the alloy, for the differences found in this way were of an altogether smaller order The range of variation between test-pieces which had formed adjacent portions of the same ingot, and had been annealed together in the same tube, was from 5 to 180 millivolts, and appeared to render necessary the conclusion that the copper-zinc alloys of the solid solution series cannot be brought to a definite reproducible condition by any ordinary process of annealing Furthermore, the continued irregular drift of the potential in the case of each alloy indicated that even after being annealed for from one to two weeks the system was still subject to a slow, progressive change; and this was observed even with alloys having as much as 80 percent (Y
214
Mewell T . G o r d m and Donald
P.Smith
of copper, which could hardly be supposed to contain anything but the pure a constituent. It seems probable khat these irregularities may be connected with various observations of other workers which appear to indicate that the a brasses are not so simple in conduct as would be inferred from the generally accepted equilibrium diagrams of Shepherd and of Carpenter. Among such previous observations may be mentioned the finding by Roberts-Austeng of a thermal effect at 473 O C in an alloy with 75.6 percent copper and the peculiar effects of aging and subsequent annealing upon the mechanical properties of brasses which led Carpenter and EdwardslO to make special assumptions as to the nature of these alloys, and of solid solutions in general. Whether there is, in the region of the equilibrium diagram assigned to these alloys, a heterogeneous region not yet recognized or whether the changes indicated are to be accounted for merely by a slow attainment of equilibrium cannot be decided upon the basis of existing data.
Summary A continuation of previous experiments upon the CopperZinc alloys shows these to be variable as regards electromotive force, but lends confirmation to indications obtained by others regarding unexplained changes to which these alloys are subject. The Copper-Nickel alloys are shown to be very regular in electro-chemical conduct, it being possible to obtain values for their potentials which are reproducible and constant to within three millivolts and which remain constant, within the same limits, for many hours. They exhibit potential differences against mixed solutions of cupric sulphate and nickel sulphate of moderate copper concentrations expressible by a relation which is linear both with respect to alloy composition and to the logarithm of electrolyte composition. It is shown that copper sulphate 'concentration may be employed without any sacrifice of accuracy in the representation of the empirical results, in place of the cupric ion concen-
. Copper-Nickel Series o j Alloys, Etc.
21 j
tration required by the thermodynamic theory of Nernst, provided the constants of the equation be altered. The tendency to give ions to the solution is increased as a result of polishing the surfaces of the alloys, while by coarser grinding only small accidental variations are produced from the steady values found with annealed surfaces. The data for the copper-nickel alloys are of a higher order of constancy and reproducibility than those formerly obtained for solid alloys, and th.e experiments show that the employment of a calomel electrode and other refinements generally used in the study of amalgams is justified in application to this field of investigation. REFERENCES
Jour. Phys. Chem., 20, 2 2 8 (1916). *Jour. Am. Chem. SOC.,36, 1448 (1914). Zeit. anorg. Chem., 56, I (1908). Ann. Chim. Phys., 2 5 , I (1912). Proc. Roy. Soc., 72, 227 (1903). E Zeit. phys. Chem., 22, 539 (1897). Zeit. anorg. Chem., 5 2 , 2 5 (1907). * Guertler and Tammann: LOC.cit. Proc. Inst. Mech. Eng., 31 (1897); 4th Rept. Alloys Research Committee. Jour. Inst. Metals, 5, 140-3 (1911).
