Walter R. Carmody and Jack Wiersmal Seattle University Seattle, Washington
II
A S t d 01 ~ the Silver Tree Experiment
The production of silver "trees" by the reaction of copper with solutions of silver salts has fascinated chemists for more than one hundred years.* More recently this reaction has been adapted for use as an experiment in the CHEMS program now widely adopted by the high schools of the country. During the past four or five years a number of high school teachers taking NSF Summer Institute courses a t Seattle University have expressed a good deal of dissatisfaction with the results obtained using this experiment in their classes. Their main cause of dissatisfaction is in the determination of the number of atoms of silver, designated by R, displaced by one atom of copper; the results obtained by students for the value of R tend t o cluster around 1.85 rather than 2.0. Also, in many clitsses the proportion of values of R that are greater than 1.9 is quite low. One of the dissatisfied teachers started this work under the direction of the principal author, who later carried it on to completion. I t was evident from the reports of the high school teachers that the experiment involved one or more systematic errors of considerable size. The work was begun by making a number of runs varying the conditions within the framework of the directions written for the experiment, noting possible sources of error and how the errors change with varyiug conditions. In order to prevent the introduction of statistical errors which might cloud the interpretation of the results, all measurements and calculations were carried out with p r o cisions a t least ten times better than those normally used in high school work. Analysis of Conventional Method According to the directions giveu in the CHEMS laboratory m a n ~ a lapproximately ,~ 4 g of silver nitrate are dissolved in 125-150 ml of water in a 250-m1 beaker. I n this solution is immersed a weighed coil of copper wire, and the assembly is allowed to stand until the next laboratory period. The mass of crystals formed during the course of the reaction is shaken off the coil which is then lifted from the solution, washed with water, dried, and weighed. The solution is decanted from the residue, 5 ml of 0.2 M silver nitrate solution is added, and the mixture is then "stirred gently until the flecks of copper disappear," and the solution is again decanted. The residue is finally washed several times by decantaThis work was partially supported by the National Science Summer Institute at Seattle University. ' Present address is 1210 Highland Avenue, Iowa. City, Iowa. GLEIM,DAVIDI., J. CHEM.EDUC., 30, 151 (1953). S M ~LLOYD ~ ~E.,, "Chemistry: An Experimental Science Laboratory manual," W. H. Freeman and Co., San Francisco, 1963.
tion, after which it is dried and weighed. At this point in the directions appears the statement: "You may neglect the few particles that may float over with the wash water since the amount is usually not weighable." A number of preliminary runs served to expose the qualitative aspects of the reaction. As the coil of copper wire was put into the silver nitrate solution, there was rapidly formed a coating of very fine amorphous particles. In a very few minutes this deposit was covered by a dense layer of coarse white crystals of silver. Within a half-hour there was deposited long needlelike white crystals (whiskers) projecting a t right angles t o the wire. Duriug the next few hours the system was filled in with a large number of very fine grey needle-like crystals. Finally the whole deposit became covered with first an amorphous grey and fiually a black deposit. Upon removal from the mixture the copper wire was found t o be covered with a red substance, and bits of a substance of the same color were noticed in the deposit. During the stirriug aud washing operations it was observed that after the residue was stirred with the silver nitrate solution decantates were cloudy with suspended material, and the amounts of residue floated off appeared to be not only weighable, but significant. On the basis of these observations it was decided to obtain more quantitative data concerning the loss of fines as well as the nature of the red substance, and t o what extent its presence affects the value of the ratio, R, obtained. Samples of the red substance collected from the residue and from the surface of the copper wire were found to be soluble in HCI, producing a yellow solution. Upon dilution of this solution with water a white precipitate resulted. This was readily soluble in ammonia forming a clear solution. Upon standing in air the solution took on a deep blue color. It appears that the red substance is not copper, as has been previously assumed, but is copper(1) oxide. Several series of quantitative runs were made in which the directions given in the Laboratory Manual were followed. Each reaction was allowed t o proceed for 24 hr. (The results of preliminary runs had indicated that this was sufficienttime for the reaction t o be essentially complete.) The silver fines (except for the colloidal suspensions) carried over with the decantate were collected on fiberglas filters and weighed. The amounts of copper(1) oxide adhering to the wires and the total copper in the residues were determined using a colorimetric method precise to 2y0. The variations in the results in the runs of the several series were relatively low. The averages of runs in a representative series is shown as Series 1 in Table 1. Several runs were made also in which the treatmeut (stirring) of the residue with Volume 44, Number 7, July 1967
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Table 1. Average Results Obtained in Series of Runs Following Directions Given in CHEMS Laboratory Manual
Series 1
Time, Days Wt. of Cu in Res., g Wt. of Cu.0 on wire, g Wt. of fines, g Ratio. R
1 0.04 0.02 0.06 1.95
2
3
0.19 0.15 0.04 1.88
3 018 0.04 0.04 1.80
3
silver nitrate solution was omitted. The resulk of these runs differed only slightly from those of Series 1. It would appear that any gain in the weight of the residue resulting from the chemical reaction is pretty well counterbalanced by a loss of colloidal material resulting from the stirring operation. Quite by chance a series of runs was allowed to proceed for three days before the copper wires were removed. The results proved t o be so different, particularly in the amounts of Cu10 produced and the value of R obtained, that several other series of 3-day runs were made over a period of several weeks. The runs of a particular series checked well; however, the runs of two series made several weeks apart differed considerably, particularly in the matter of the amount of CunO remaining on the copper wire. No explanation for these differences has been found. The results of two such series, nos. 2 and 3 are shown in Table 1. The copper content of the final residues is included in the data of the table. In order to get an idea of the Cu10 content of the original silver deposit, a series of three-day runs was carried out in which the final treatment with silver nitrate was omitted. The average copper content of the deposits was found to he 0.23 g. This corresponds t o a copper(1) oxide content of 0.25 g. I n order to interpret the results shown in Tahle 1, it is necessary to identify the two side reactions involved and to develop the quantitative relationships that exist between the products of these reactions and the resulting changes in the experimental value of R. From the data shown in Table 1 it is apparent that CuzO continues to be produced long after the silver ion has been removed from the solution. One may conclude that the reaction by which the Cu20is formed does not involve silver ions. Geloso and Giordano-Orsini4have reported that copper(I1) nitrate solution in contact with copper plate gave CuzO "after several days." Possible oxidizing agents present in the mixture include copper(I1) ion, nitrate ion, and atmospheric oxygen. Runs were made in which all atmospheric oxygen had been removed from the solution and also a series OF runs in which the silver nitrate was replaced by silver acetate (dissolved in alcoholic solution). The amounts of copper(1) oxide produced in both these series differed little from that produced in the regular runs. From these results one may conclude that the CuGforming reaction may be written: Roughly quantitative studies were made to elucidate the nature of the reaction between silver ion and Cu20. When a sample of copper(1) oxide was treated with an excess of silver ion in solution, half of the copper in the sample was found in the final solution as copper(I1) ion while the remaining half was found in the silver residue 418
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Journal o f Chemical Education
as copper(I1) oxide. On the basis of these results we concluded that the reaction may be formulated: Cu10
+ 2 Ag+
-
CuO
+ Cua++ 2 Ag
Analysis of Errors
The loss of fine material during the decantation processes naturally results in a reduction in the value of the ratio obtained. On the basis of an original sample of 4.0 g of silver nitrate, the weight of copper reacting is 0.75 g and that of the silver residue is 2.5 g. Accordingly, the factor for converting the loss of fines in grams t o the error produced in the value of R is:
The effect on the value of R resulting from the production of copper(1) oxide depends not only on where the copper(1) oxide is deposited, hut also upon what fraction of the copper(1) oxide deposited in the silver residue is reacted upon during the second treatment with silver nitrate solution. The 5 ml of 0.20 M solution of silver nitrate used contains 10 milliequivalents of silver ion. This will react with 0.07 g of CuzO. From the data in Table 1 it appears that this amount would be sufficient t o react with all of the Cu10 contained in the residue of a one-day run, but with only a small fraction of that in the residue of a three-day run. Any copper(1) oxide remaining in the silver residue after treatment with the 0.2 M silver nitrate solution is there at the expense of or in place of two atoms of silver per molecule of CuzO. The factor for converting the weight of Cu,O t o a negative error in the value of R is:
For each copper(1) oxide molecule in the residue reacted on by silver ion, one molecule of copper(I1) oxide is produced. The factor to convert the weight of copper (I) oxide reacted upon to an error in the value of R is:
Accordingly, the error in R produced by the reaction of CuzO in the silver residue with 5 ml of 0.2 M silver nitrate solution would be: (0.07) (0.44) = 0.03
Finally the deposit of copper(1) oxide remaining on the copper wire results in a positive error in the value of the ratio R. The factor to convert the weight of CuzO on the wire t o an error in the value of R is:
On the basis of the above discussion it is possible to interpret, a t least roughly, the results shown in Tahle 1. It is apparent that the main error in one-day runs is due to a loss in fines (both filterable and colloidal) as the result of the stirring and decanting operations. This is partially balanced by positive errors due to the presence of small quantities of Cu10 left on the copper wire and the presence of CuO in the final residue. The lower values of R obtained in the three-day runs of Series 2 GELOSO,Max, AND GIOBDANO-ORBINI, EVELINE, Compl. rend. 204,588 (1937).
and Series 3 may be accounted for by the large amount of copper (mainly as Cu20) contained in the silver residues. The difference in the values of R obtained in the two series may be explained by the fact that the quantities of CuzO retained on the wire in Series 2 is much greater than that retained on the wire in Series 3. Possible Methods for Improving Results
It appeared that any improvement of the results would depend upon: first, a reduction of the losses of fines, both filterable and colloidal, and second, a reduction in the formation of CutO to a point a t which the operation of stirring the residue with silver nitrate solution would be unnecessary. It turned out that the procedure that was designed to accomplish the first effect was successful also in accomplishing the second. When the copper is first introduced into the solution of silver nitrate the reaction is purely chemical, in that it takes place upon the direct contact of silver ions with the copper wire. The tan, amorphous product sticks to and soon entirely covers the wire. From this point on the reaction becomes and remains electrochemical in nature. The copper(I1) ion is freed at the copper wire (anode) while the silver is deposited on the outer surfaces of the silver deposit (cathode). The mass of silver deposit serves as the metallic conductor between the two half cells. As the reaction takes place concentration polarization occurs at the electrodes; the concentration of the copper(I1) ion increases a t the anode while that of the silver ion decreases at the cathode. As the reaction proceeds the silver ion concentration at the surface of the cathode in the unstirred cell decreases very rapidly since the silver ion is furnished to the electrode from areas of decreasing concentration and farther and farther away. The last traces of silver ion will be deposited from concentrations approaching zero. The decrease in the concentration of the silver ion causes a corresponding decrease in the voltage of the electrochemical cell. Assuming that the activity of the copper(I1) ion a t the surface of the copper wire is of the order of magnitude of unity near the end of the reaction, we may write: &I,
=
0.46
1.0 - 0.030 log (Apt)*
Stirring the solution (to reduce concentration polarization) Increasing the original concentration of silver ion (by reduotion of volume) so that a smaller fraction of the total silver will he deposited from very dilute solutions Raising the cell voltage Reduction of the distance aver which the silver ion has to diffuse
Although electric stirrers are not available t o most high school classes, it was thought that the results obtained when the mixturemas stirred might be of theoretical, if not practical, interest t o high school teachers. A magnetic stirred reaction was found t o be complete within less than an hour. For the first half hour dense white crystals were deposited; after this the deposit turned slightly grey and finally a trace of black was formed. Only slight traces of CurO were found in the deposit, and in a series of runs the values of R obtained varied from 1.98-2.00 with an average of 1.993. It appears that most of the errors associated with the conventional method are eliminated by the use of stirring. An increase in cell voltage by reducing the concentration of the copper(I1) ion at the copper electrode seemed to offer some prospect of improving the results. Runs were made in which potassium acid tartrate was added in amounts sufficient to complex all of the copper(I1) ions produced. The silver deposit resulting was white and dense, hut so closely adherent that the reaction stopped before it was half complete. When the length of the wire was increased to 30 inches the reaction did go to completion. However, the deposit adhered t o the wire so strongly that the process of scraping off the product was difficult and was judged to be impracticable for student work. New Method
On the basis of this equation the voltage of the cell will approach zero as the concentration of the silver ion a t the electrode approaches 2 X 10-8 .It. The relationship between the crystal shape and size of electrolytically deposited silver and the concentration of the silver ion in the electrolytic bath is well known. At higher concentrations massive crystals are deposited. As the concentration is progressively lowered the crystals change to "whiskers" at right angles to the electrode, then to fine grey crystals and finally to a black amorphous deposit. This suggests that the factor that determines the crystal size in the present case also is the concentration of the silver ion at the surface of the deposit. Schottly and Bever5 have demonstrated that it requires slightly more energy to deposit silver continuously as the size of the crystal progressively changes from compact crystals through 5 SCHOTTLY, W. F., AND BEVER,M. B., A C ~ Q ~ 199 (1959).
"whiskers" and fine needles to amorphous particles. This would connect the degradation of the deposit encountered as the reaction proceeded with the lowering of the potential of the reaction cell. At very low potentials the deposition of the silver as amorphous particles would naturally be preferred to that of silver as crystals. On the basis of these principles it appeared that the reduction of fines in the silver deposit could possibly be effectedby:
~
~ 7,
The method described below was designed to accomplish three of the four possible improvements outlined above and yet remain within the capacities of the average high school laboratory. The cell comprises a vessel 18 mm od and long enough t o hold approximately 30 ml of solution. A 25-ml tall-form Pyrex graduated cylinder may be used, or the cell may be made from a 10-in. piece of 18-ml glass tubing. The cell is fitted with a one-hole stopper. Copper wire (no. 14 or heavier) is cut t o a length 1 in. longer than the cell and carefully straightened. I t is then wound helically from the bottom to within 2 in. of the top with fine (no. 23) copper wire, the successive turns being roughly in. apart. (This is t o prevent the deposit from sliding down the wire which would result in the deposition of colloidal silver on the newly exposed uarts.1 The wire is then weighed as ~reciselv as possible. The sample of silver nitrate (a~proximkely ~4 g)uis dissolved ~ ~ in 25 ~ ml iof distilled ~ ~water, , and the solution is poured into the cell. The copper wire is Volume 44, Number
7,July 1967 / 41 9
lowered into the vessel and the lower end centered a t the bottom. Finally the rubber stopper is inserted so that the upper end of the wire is centered at the top. After hr the stopper is removed, and the wire is shaken free of the silver deposit, removed from the cell, washed and dried, and again carefully weighed. The contents of the cell are removed by inverting the cell (quickly) over a small (100-ml) weighed beaker. The residue is washed several times and dried, and the beaker with its contents weighed. During the washing operations, the loss of fines may he kept to a minimum by keeping the residue in a single mass. (Before the final washing a vial or similar tube with a flat bottom may be used to press the soft mass of crystals to a dense mat on the bottom of the beaker.) Two advantages resulting from the new cell design are obvious. The concentration of the silver nitrate solution has been increased by a factor of five, and the maximum diffusion distance has been reduced by a factor probably as large as eight or ten. A third and less obvious advantage is that, as the reaction proceeds, a circulation system is set up in the cell that brings about the continuous renewal of the solution at the surface of the cathode. This circulation depends upon the fact that a solution of silver nitrate is more dense than a solution of copper nitrate of the same normality. Soon after the copper wire is inserted into the cell a thin layer of solution may be noted rising along the surface-of the wire. Silver-free blue solution cdlects a t the top of the tube, and within 30 minutes the edge of the blue color has moved down and envelopes the whole solution. At this time the reaction is, for practical purposes, complete. The appearance of the two types of cells at this point is shown in the figure. The almost total lack of dark colloidal material on the surface of the silver deposit can be noted. In addition t o the merits of the new cell design in reducing markedly the production of fine colloidal material during the course of the reaction, it has an additional if not more important advantage. The new cell design permits the reaction time t o be reduced from one or more days to 30 minutes. As a result, the CuzO content of the silver residue is markedly reduced6 and also, no CuzO remains on the copper wire. Results obtained from runs made according to the new directions are quite satisfying. The residue recovered is decidedly more crystalline in appearance, and losses (except colloidal particles) occurring during the washing operations have been reduced practically t o zero. The quantitative results of two series of runs T h e CurO content may he reduced to zero by slightly acidifying the silver nitrate solution. However, the amount is so small that its effect on the value of the ratio R is negligible.
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/ Journal o f Chemical Education
React'on
Of
reoctionr.
are s h o n ~in Table 2. The ruus of Series I I ~ J3. mere performed by the principle author. Those of Series Xo. 1 were performed by fourteen high school teachers attending the NSF Summer Institute at, Seattle University. The results indicate that the use of the new method markedly reduces syst,ematic errors. Those remaining may be accounted for by the loss of very fine mat,erial during the washing operations. Table 2.
Average
Time, min Wt. of Cu in res., g Wt. of fines, g Ratio, K Avg. Dev., R
Results Obtained
30 0.005 0.004 1.988
0.002
Using
New Cell
30
... 2.00 0.02
a Series no. T, comprise average results ahfaiued by 14 high school teachers.
I t is not to be expected that the precision of the results obtained by high school student,^ would be particularly better with this method than that obtained with the old method. However, it is t,o be expected that class averages for the rat,io R mould more closely approximate the value 2.00.
