Electrodeposition of Silver from Iodide Solutions CHARLESW. FLEETWOOD AND L. E'. YNTEMA St. Louis University, St. Louis, Ma.
F I G ~ 1.E PnoTor;n*pii
OF
APPAR.ATUS
M
ANY attcmpts have been made to find a siiitablesub-
stitute for the cyanide bath for the deposition of silver since its discovery by John Wright (6) in 1840. The perforniance oi the cyanide bath can hardly be criticized, but a nonpoisonous bath would be welcomed by the plating industry. Xumerous refercnccs to attcmpts to discover such a bath are summarized in an article by Frary ( 3 ) . &fore recent investigations are those of Mather and Kuehler (.9), Mather and Blue (a), Sanigar (If), Simpson and Withrow ( I S ) , McKec, Mann, and Montillon (7), Taft and Barham ( I d ) , Aodrieth and Yntemn (l), nnd Sclildtter et al. (18). These various haths include many silver complexes and various silver salts with or vritliout addition aeents. , , , in ameons or nonauueous solvents. It is noteworthv that not one of these baths gave depo8its comparabie t o those of the cyanide bath. The most satisiaetory are those of Sanigar a.nd of Scliliitt,er who secured bright deposits of silver from hatiis of silver iodide dissolved in ~otassiuinor sodium iodide solution. Colloids m-ere employid as addition agents. Silver deposited from these batlls contained silver iodide as n d l as free iodinc. The object of this investigation mas to prepare a bath which would be a t least as satisfactory as the cyanide hath.
amperes per six. dm. xas employed. The deposits werc easily polished but were adherent only when very thin. Continued electrolysis resulted in the decomposition of the ba.th. A concentrated solut.ion of hydrochloric acid saturated with silver chloride and containing 50 grams per liter of ethyl alcolrol, when electrolyzed at 1 to 5 amperes per sq. dm. and 20" to 80" C., gave thin deposits on copper. They were adlirrent and could be polished. Excellent deposits were obtained from a bath containing 16 grams per liter of silver iodide, 160 oi sodium iodide, and 32 of tartaric acid, electrolyzed at 1 to 1.4 amperes per sq. dm. over a temperature range of 25O t o 80' C . The cream-yellow deposits were adherent and % e r eeasily polished to a bright finish. Mimoscupic examination showed them to be finegrained and iinifom in textnre. Other coilcentrations of tartaric acid and silver iodide gave good results. The total iodide concentration had to be high enough to prevent the precipitation of silver iodide. €Iellwig's data (6) for the soluhility of silver iodide in potassium iodide solutions in moles Der liter are as follows: KI AB1
0.335 0.586 1.008 1.018 1.406 1 . 4 s 1.634 1.937 0.0148 0 . 0 ~ 50.0658 0,102 0.198 o.ooo383 o . o ~ ~ l0,0141 s
The results from the iodide bath seemed bo show that a satisfactory silver-plating bath could be prepared byelectrolyzing \\.itli a silver anode a concentrated solution of alkali iodide containing a small amount of tartaric or citric acid. When solutions containing various concentrations of sodium iodide
Exmm&Eh"rS ON KONCYANIDE BATHS A preliminary investigation of various types of silver
baths favorable for silver plating was carried out. Silver halides, sulfate, and nitrate were employed in the iollowing solvents: acetic acid, liquid nrea, acetamide, pyridine, phenol, dimethyl aniline, and glycerol. These same solvents were used as addition agents in aqueous solutions of silver salts. Deposition from solutions containing silver salts in the presence of thiosulfate, citrate, tartrate, and aminoacetate ions was also investigated, as well as from baths containing a high concentration of lialide ion. It was concluded that those solutions containing eomplexes of silver are the most suitable for the electrodeposition of silver. These baths were further investigated, but no large-scale plating wm attempted. A bath containing 36 grams per liter of silver chloride and 62 grams per liter oi Na&Os.5E.D gave fair deposits on nickel when a current, of 1 to 2
A.
Ammeter
B . Ba*h H . IIRIf-eel1 C. Galvanometer
c.
Commutator
P. Potentiometer
FEGURE 2. D~AORAM OF APFARATUS
and tartaric acid or potassium iodide and citric acid were electrolyzed with silver anodes at current densities of 2 to 4 amperes per sq. dm. for 10 to 15 minutes, deposition of silver occurred, showing that silver ions had been introduced by anode corrosion. After electrolyzing a solution containing
March, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
60 grams per liter of citric acid and 520 of sodium iodide with a silver anode for 2 to 3 hours, good deposits were produced on copper or brass. The results secured from this bath warranted its further investigation.
QUANTITATIVE STUDY O F THE SODIUM SILVER IODIDE-CITRIC ACIDBATH A more detailed study was made of a bath containing initially 60 grams per liter of citric acid and 520 of sodium iodide. However, these concentrations may be varied over a wide range with equally satisfactory results. The reagents used mere sodium iodide and citric acid (Mallinckrodt's malytical reagent) and a commercial grade of pure silver. The apparatus shown in Figures 1 and 2 consists of an electrolytic bath and circuits for determining single electrode potentials. A mechanical stirrer was used. The intermittent method (IO)was employed to measure the single electrode potentials. A double commutator (2400 r. p. m.) was used t o break the circuits. The cathodic polarization is the difference between the dynamic and the static potential, the latter being the potential of a silver plate suspended in the bath. Anodic polarization is measured in a similar manner. The amount of silver deposited was found by weighing a copper cathode (15 X 1 X 0.005 cm.) before and after plating. As the purpose was to determine the efficiency of the bath in depositing adherent bright silver, the dull surface layer was removed before weighing by light polishing.
O.LO0
0.400 0.100
0
0.2.
0.b
1.4
1.0
1.8
AMPERU/D H:
FIGURE3. ELECTRODE POLARIZATION AT SILVERCONCENTRATION OF 4.23 GRAMSPER LITER Cathode polarization, umtirred bath, 25' C. Cathode polarizat/on, unstjrred bath, 50" C. Cathode polar!zat!on, unstirred bath, 75' C. Cathode polarization etirred bath 25' C. 5: Cathode polarization,' stirred bath,'5O0 and 75' C. 6. Anode polarization
1. 2. 3. 4
Since the silver concentration of the bath increased with use, it was necessary to analyze the bath before experimental data were taken. The following method of analysis was employed: A 10-cc. portion of the bath was heated with an equal volume of concentrated sulfuric acid until all iodine was removed. The iodide-free solution was diluted to 150 cc. and the silver mas precipitated as the chloride. After solution in ammonium hydroxide and reprecipitation with nitric acid and a few drops of hydrochloric acid, the silver chloride was weighed. The silver-ion concentration was determined from electromotive force data. The calomel half-cell (I N potassium chloride) employed gave a value against a hydrogen electrode of 0.283 volt a t 25" C. as compared to the standard value of 0.281 volt. The silver half-cell value is given by the folowing equation :
E
=
Eo
RT In - FF
[Ag+]
341
when [Ag+] is the concentration (or activity) of silver ions. The normal electrode potential Eo for Ag 7Ag' is -0.7978 a t 25" C. The voltage of the cell (E,u) is the sum of voltages of the two half-cells and is determined by experiment: Eceii =
=
RT EO- dTF in [Ag+] - .0.7978
+ 0.283
- 0.059 N log
[Ag+]
+ 0.283
The final equation enables one to calculate the silver-ion concentration. POLARIZATION. It was f o u n d that the a n o d e polarization is independent of t e m perature and agitation. Curve 6 (Figure 3) indicates its slight variation with current denFIGURE 4. CATHODE EFFICIENCY s i t y . T h e c a t h o d e AT SILVER CONCENTRATION OF polarization is reduced 6.84 GRAMSPER LITER as the temperature is increased (curves 1, 2, and 3). Stirring the solution decreases the cathode polarization (curve 4). When the bath is stirred, a change in temperature has little effect as indicated by the coincidence of the data for 50" and 75" C. (curve 5 ) , which is only slightly below curve 4 (for 25' C.). Egeberg and Promise1 ( 2 ) state that the maximum anodic and cathodic polarization in a standard cyanide bath is about 0.5 volt. They report a cathodic polarization of 0.35 volt at 22" C. and 0.6 ampere per sq. dm. The sodium silver iodide-citric acid bath under similar conditions gave a value of 0.60 volt. DEPOSITED SILVER. The silver deposits, before polishing, mere cream-white to yellow in color, resembling those obtained from a cyanide bath. Under those conditions resulting in cathode efficiencies less than 80 per cent, the silver was covered by a yellow, slightly granular, loo nonadherent coating , but, when the current efficiency was high, there appeared a cream-white adherent 60 layer. In every case 0.L 1.0 1.4 1.8 polishing an FIGURE5. CATHODE EFFICIENCY underlying deposit of AT SILVER CONCENTRATION OF good quality. Bending 23.8 GRAMSPER LITER tests showed an excellent bond between the base metal and the silver, and microscopic examination showed a smooth, continuous deposit. A number of copper and brass objects were successfully plated. The silver took a good polish and appeared in every way as satisfactory as the deposits obtained from the cyanide bath. CATHODE EFFICIEXCIES. In Figures 4 and 5 the cathode efficiencies are plotted against current densities for different temperatures and concentrations of silver. The time of each deposition was 20 minutes. Figure 5 indicates that the cathode efficiency of a bath that is high in silver is slightly affected by temperature or by variation of current density from 1.0 to 1.8 amperes per sq. dm. However, current densities below 1.0 ampere per sq. dm. gave lower cathode efficiencies. Data obtained with deposition periods of 5. 10, or 15 minutes gave curves almost identical with those of Figure 5 .
INDUSTRIAL AND ENGINEERING CHEMISTRY
342
Figure 4 indicates that the cathode efficiency of a bath that is low in silver is markedly affected by both temperature and current density. At a high temperature the cathode efficiency increases with current density (within the range studied), but a t lower temperatures the cathode efficiency decreases with current density. If the temperature is approximately 75" C., the efficiencies a t current densities of 1 to 1.8amperes per sq. dm. are similar to those of the more concentrated bath. TABLEI. STATIC
POTENTIALS IN
SILVER SOLUTIOXS
(All baths contained 520 grams per lite: of sodium iodide per liter of citric acid) Ag Ed' OBSVD. lAg+] x 10-12 K Gra?ns/liter 23.78 0.389 9.55 11.21 0.402 7.06 4.63 0.412 5.31 2.15 0.423 2.46 1.07 0.438 1.47
and GO grams
x
10-15
1.42 1.78 4.60 4.03 5.00
SILVER-ION CONCEKTRATION. Table I lists the measured silver-ion concentration of baths whose total silver content had been determined. It should be noted that the respective concentrations of sodium iodide and citric acid are the same in each of the baths. In calculating the instability constant values of the AgI; ion from the equation
no attempt was made to evaluate the effect of the citrate ion, whose presence may partly explain the lack of constancy of K .
Vol. 27, No. 3
CONCLUSIONS The sodium iodide-citrate bath satisfies the requirements for a good plating solution: 1. The deposit is fine-grained and adherent. No quantitative study was made of the throwing power, but there appeared t o be uniform deposition even on irregularly shaped articles. 2. The permanency of the bath is indicated by the fact that it gave as good deposits after standing exposed to the atmosphere for 4 months as when newly prepared. 3. The anode corrosion is good. When operated at cathode efficiencies of 80 per cent and higher, the silver content of the bath increased. 4. The bath is easily prepared. The silver content, as well as the silver-ion concentration of the more concentrated bath, approximates the values of a cyanide bath as found by Frary and Porter (4). LITERATURE CITED Audrieth and Yntema, J . Phus. Chem., 34, 1903 (1930). Egeberg and Promisel, Trans. EZectrochem Soc., 59, 287 (1931). Frary, Ihid., 23, 64 (1913). Frary and Porter, Ihid., 28, 307 (1915). Hellwig, Z . anorg. Chem., 25, 157 (1900). Leader, J . Inst. of Metals, 22, 11, 305 (1919). McKee, Mann, and Montillon, Trans. Am. Electrochem. Soc., 53, 333 (1928).
Mather and Blue, Ibid., 31, 285 (1917). Mather and Kuebler, Ibid., 28, 417 (1915). Newbery, E., J . Chem. Soc., 105, 2419 (1914). Sanigar, Trans. Electrochem. Soc., 59, 307 (1931). Schlotter e t al., Z . Metalllcunde, 25, 107 (1933). Simpson and Withrow, J . Am. Chem. Soc., 32, 1571 (1910). Taft and Barham, J . Phys. Chem., 34, 929 (1930). RECEIVED November 30, 1934. Originally submitted Derember 9, 1933.
Double Decomposition and Oxidation of Inorganic Compounds under Pressure Transformation of Heavy Spar into Barium Carbonate V. N. IPATIEFF~ AND C. FREITAG, Bayerische Stickstoff-Werke, A,-G., Berlin, Germany The reaction of double decomposition of barium I p a t i e f f bomb, and air was OLUTIONS of chromium sulfate and soda, under pressure and certain inpumped in until the pressure salts are transformed in the presence of air under reached 100 atmospheres. The dicated conditions, proceeds to completion at reaction was allowed to proceed pressure into salts of chromic 320" I n order to obtain g7 Per cent bansacid and, conversely, salts of for 12 hours a t 300" to 350" C. formation of barium sulfate into carbonate under At the end of the reaction the chromic acid under pressure of the conditions, it is necessary to use twice the whole of the chromium oxide hydrogen are reduced to chrowas oxidized to the calcium salt mium salts, forming under cerequivalent amount of soda. It may be concluded, of chromic acid, CaCr04. t a i n conditions complex comtherefore,f r o m the data obtained that other metallic These experiments showed p o u n d s ( 1 ) . The reaction of sulfates which are insoluble in water can be that, under pressure, even stable oxidation of chromium hydroxide in an alkaline medium has been transformed completely info the corresponding and very little reactive oxides are capable of readily entering variinvestigated (2)and the optimum carbonates. ous reactions and yielding deconditions have been found for transforming this hydroxide into sirable products. In order to salts of chromic acid-potassium chromate and potassium verify such a conclusion, the reactions of double decomposition of barium sulfate and soda were investigated. The re-. dichromate. Oxidation experiments were made on commercial green sults completely corroborated the authors' assumptions. chromium oxide (insoluble in acids) by means of air under The first experiments were made with powdered heavy spar pressure in the presence of the hydrate of calcium oxide (from the firm of de Haen, Germany) and solutions of soda (milk of lime). One hundred grams of chromium oxide and saturated a t boiling temperature. In each test 58.4 grams of an excess of calcium hydroxide were placed in a rotating heavy spar were used, and the amount of soda used was at but later the quangreater than its 1 Present address, Universal Oil Products Company, 310 South Michigan tity was doubled, The reactions were carried out a t 320" to Ave., Chicago, Ill.
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