H*Oi-Fe(III) ratio increases. The same effect has been observed by Kolthoff and Parry (30, 40). The numerical discrepancy between the calculated and literature values of the bimolecular rate constant may be attributed to the slight heterogeneous reduction of hydrogen peroxide, providing an additional source of free radicals which increases the rates of Reactions 25 and 26.
Although the demonstration system is complicated by these side reactions, the agreement between theory and experiment is excellent.
RECEIVED for review September 1, 1967. Accepted December 11, 1967. Work supported in part by the National Science Foundation, through Grant No. GP-3190, and in part by the Atomic Energy Commission, through Grant No. AT(11-1-)1082. A fellowship to Leon N. Klatt from the American Oil
Study of Impregnated Silicone Rubber Membranes for Potential Indicating Electrodes E. B. Buchanan, Jr., and J a m e s L. Seago’ Department of Chemistry, Uniuersity of Iowa, Iowa C i t y , Iowa
Electrodes fabricated from silicone rubber membranes and impregnated with various insoluble transition metal salts and chelates were investigated for their responses toward transition metal cations. Only electrodes fabricated from membranes impregnated with metal salts, and not chelates, responded. The crystalline form, the degree of hydration, and the associated anion of the imbedded salt had no effect on a membrane’s response or selectivity. These electrodes were nonspecific in their responses toward cations. The presence of additional electrolytes in the test solutions leveled the responses of the electrodes to the concentrations of the added salts. The primary purpose of the imbedded material is to adsorb water and ions during the conditioning process and to provide sites for rapid metal ion exchange at the membrane interface. The transition metal responsive electrodes were found to possess suitable mechanical properties but the desired selectivity was not attained.
SUCCESSFUL DEVELOPMENTof ion-selective electrodes for alkali and alkaline-earth metal cations has created a renewed interest in direct potentiometry as an analytical scheme. Few studies have been made of electrodes responsive to transition metal ions. Electrodes of this latter type would have direct application for the determination of transition metal ions found in biological specimens and industrial wastes. Rechnitz ( I ) , Hill (2), Lakshminarayanaiah (3),and Pungor (4)have reviewed the various types of membranes employed in the attempted development of a specific ion electrode. Most reported works have dealt with the responses of these membranes in solutions of alkali or alkaline-earth metals. Chatterjee ( 5 ) prepared clay membranes that responded to 1 Present address, Procter & Gamble, Ivorydale Technical Center, Cincinnati, Ohio.
(1) G. A. Rechnitz, Chem. Eng. News, 45 (25), 146 (1967). (2) G. J. Hill, in “Reference Electrodes,” Ives and G. J. Janz, Eds., Academic Press, New York, 1961, chapter 9. (3) N. Lakshrninarayanaiah, Chem. Rel;., 65, 491 (1965). (4) E. Pungor, ANAL. CHEM., 39, 28A (1967). (5) B. Chatterjee and D. K. Mitra, J . Indian Chem. Soc., 32, 751 (1955).
52240 copper, molybdenum, and cobalt. Recently, MorazzaniPelletier and Baffier (6) published work on the use of certain insoluble transition metal salts embedded in collodion and paraffin membranes. They found membrane response was affected by the number and nature of the anions in the solution, the ionic strength of the solution, and hydrostatic effects on the membrane. These membranes possessed a high rate of water transference and the resultant potentials were subject to drift and were not selective. Shatkay (7) has reinvestigated the paraffin electrodes and has found that they are not completely permselective nor specific in their responses, At least two companies, Orion Research Laboratories and Corning Glass Works, are marketing specific liquid ion-exchange electrodes. Pungor (8-13) developed impregnated silicone rubber membrane electrodes which show selective responses to chloride, bromide, iodide, and sulfate ions. Rechnitz (14, 1 9 , utilizing Pungor’s silicone rubber membrane electrodes impregnated with the silver halides, characterized the responses of these electrodes toward chloride, bromide, and iodide ions. The present study is an attempt to evaluate those factors that contribute to the selectivity and sensitivity of impregnated membrane electrodes toward transition metal ions. The suitabilities of paraffin and silicone rubber as matrix materials are compared. Membranes containing transition metal compounds (chelates and ionic salts) in varying degrees of (6) S. Morazzani-Pelletier and M. A. Baffier, J . Chim. Phys., 62,429 (1965). 39, 1056 (1967). (7) A. Shatkay, ANAL.CHEM., (8) E. Pungor and K. Toth, Acta Chim. Acad. Sci. Hung., 41, 239 (1964). (9) E . Pungor, J. Havas, and G. Madarasz, Fr. Patent No. 1,402,343 (1965). (IO) E. Pungor, J. Havas, and K. Toth, Z . Chem., 5,9 (1965). (11) E. Pungor, J. Havas, and K. Toth, Inst. Control Systems, 38, 105 (1965). (12) E. Pungor, J. Havas, and K. Toth, Acta Chim. Acad. Sci. Hung., 48, 17 (1966). (13) E. Pungor, J. Havas, and K. Toth, Mikrochim. Acta, 1966, p 689. (14) G. A. Rechnitz, M. R. Kresz, and S. B. Zamochnick, ANAL. CHEM., 38, 973 (1966). (15) G. A. Rechnitz and M. R. Kresz, Ibid., p 1786. VOL. 40, NO. 3, MARCH 1968
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Figure 1. Membrane cell assembly hydration and crystalline form are tested to determine the nature of the effects produced by the embedded materials. Also, changes in the electrode's responses as a function of previous conditioning are examined.
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EXPERIMENTAL Apparatus. A Cary Model 31 vibrating reed electrometer was used to make the potential measurements. Readings were recorded to 0.1 mV and subsequently rounded to the nearest mV. Reagents. Bis(acety1acetonato) copper(11) and bis(acety1acetonato) nickel(I1) dihydrate were prepared by the addition of a sufficient amount of acetylacetone to an ammonical solution of the appropriate metal ion. The resulting precipitate was filtered, washed, and air dried. Bis(acety1acetonato) nickel(I1) was formed by heating the dihydrate to 100" C in a vacuum-drying pistol overnight. Bis(dimethylg1yoximato) nickel(I1) was prepared by the standard method and recrystallized from boiling nitrobenzene. The anhydrous insoluble metal salts were formed by heating the hydrated form at 200" C for 12 hours. Crystalline anhydrous cobalt(I1) orthophosphate was formed by heating the amorphous anhydrous cobalt(I1) orthophosphate (secured by the dehydration of the octahydrate at 200" C) at 600" C for four days. All solutions were prepared from readily available analytical grade salts of the metals. Membrane Preparation. Melted paraffin was poured onto wax paper covering a warm (approximately 80" C) glass plate. While the paraffin was still fluid, an 18- X 150-mm borosilicate tube was carefully inserted into the paraffin and clamped in position. After the assembly was cooled, the paraffin was stripped from the plate and paper and trimmed to an appropriate size. By this procedure, a membrane approximately 1.5 mm thick was secured to the tube. The electrode was soaked in distilled water prior to use. Impregnated paraffin membranes were prepared in a similar fashion. A 200-mg portion of the finely powdered metal compound was mixed with 1 ml of the melted paraffin. The resulting membrane was soaked from two to four days in a 0.01M solution of the corresponding metal chloride salt. Impregnated silicone rubber membranes utilized a roomtemperature vulcanized silicone rubber (G.E. "Clear Seal") as the matrix. A sufficient amount of silicone rubber was mixed with a finely powdered sample of the insoluble metal compound to obtain a coherent slurry which contained at least 50% by weight of embedded compound. The resultant 518
ANALYTICAL CHEMISTRY
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c
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solFigure 2. Response of silicone rubber membranes impregnated with BaS04 toward Sod243- Response of the NazS04-conditionedmembrane in Na2S04; slope : 36 mV/pSOa2-3- Response of the NazS04-conditionedmembrane in NazSOd 5 % NaNOa -A- Response of the heated membrane in Na2S04; slope: 13 mV/ pso2-0- Response of the alkali-treated membrane in NazS04;slope: 11 mV/ pSO2-
+
slurry was placed between a heavy polyethylene plate and a polyvinyl chloride sheet and pressed into a uniform membrane of 0.5-mm thickness. This assembly was allowed to cure in air. The membrane was conditioned by placing it for a week in a hot (90" C), concentrated (1-2M) solution of the appropriate metal salt. A suitable section of the membrane was selected and secured to the end of a 18- X 150-mm borosilicate tube by the use of additional silicone rubber. The electrodes were stored in solutions of the appropriate metal salt or distilled water. The membrane electrodes were assembled as shown in Figure 1. The cell potential was taken as the potential difference between the two matched SCE'S. The SCE in the reference compartment was connected to the positive terminal and the SCE in the test solution was connected to the negative terminal of the vibrating reed electrometer. The steady state potential was usually attained after 5 minutes. The observed potential was within 1 mV of the steady state value after 1 or 2 minutes. All experiments were performed at 25 " C. RESULTS AND DISCUSSION Paraffin Membranes. Several paraffin membranes, both pure and impregnated, were prepared and tested as metal-ion indicating electrodes. Only those pure paraffin membrane electrodes that showed evidence of leakage were capable of
2t
Metal ion
PCO
Figure 3. Response of silicone rubber membranes impregnated with amorphous and crystalline C O ~ ( P Otoward ~ ) ~ Co2
Figure 4. Responses of silicone rubber membranes impregnated with metal phosphates with varying degrees of hydration
42- Response of crystalline Co3(P04)2 in CoSO4; slope: 18 mV/
-6Response of C03(P04)2.8H20in COCIZ;slope: 22 mV/pCo2+ -0- Response of Co3(PO4)2 in CoCL; slope: 34 mV/pCo2+ -0- Response of CuHPO4.2H20 in Cu(N03):!; slope: 18 mV/ pcua+ -M- Response of CuHP04 in Cu(N03)~ -A- Response of Ni3(P01)2.7H20 in NiCI?; slope: 19 mV/pNiZ+ -A- Response of Ni3(P04)* in NiCI2; slope: 29 mV/pNi2+
+
pcoa+ -3- Response of amorphous Coa(PO& in CoSOa; slope: 16 mV/ pco*+
responding t o concentration differences. These responses agree with the results obtained by other investigators. If there was no leakage, the membrane functioned as an insulator and the electrode provided no meaningful response. Leakage is probably the result of microscopic cracks at the paraffin-glass seal. These cracks are caused by the extensive contraction of the paraffin during the cooling process. No known procedure ensures a watertight membrane. A large majority of the paraffin membrane electrodes fabricated for this study leaked. In the case of paraffin membranes impregnated with an insoluble metal salt, the paraffin-salt boundary increases the probability of microscopic cracks. However, if the paraffin is capable of wetting the surface of the embedded material, this source of cracks is removed. A paraffin membrane impregnated with nickel dimethylglyoximate functioned as an insulating barrier. The electrode fabricated from this membrane gave no results when tested in a nickel chloride concentration cell. Silicone Rubber Membranes. In contrast to paraffin membranes, watertight silicone rubber membranes are easily prepared. Silicone rubber, because of its flexibility, toughness, and elasticity, forms an effective membrane matrix that is neither porous nor susceptible t o the development of cracks. Membranes of pure silicone rubber were found t o possess a n extremely high resistance (over 11,000 MQ), even after
exposure t o a boiling 8M sodium hydroxide solution and after prolonged soaking in distilled water. Potentials from these electrodes were unsteady and sensitive t o the presence of static charges. Material transport through a n impregnated silicone rubber membrane was extremely slow as compared to that through a paraffin membrane. A silicone rubber membrane impregnated with dimethylglyoxime was placed in distilled water, and the reference compartment was filled with a 1M nickel sulfate solution. After a period of six months, a faint red coloration, caused by nickel dimethylglyoximate, appeared on the exterior surface of the membrane. This coloration is the result of the passage of nickel ions through the membrane. Impregnation of a silicone rubber matrix with a n insoluble salt followed by proper conditioning resulted in a considerable decrease in the membrane’s resistance. An electrode fabricated from such a membrane is capable of responding t o the ion of the embedded metal salt. Conditioning. The previous history of the electrode has a pronounced effect upon its response. Figure 2 shows the responses obtained from membranes which were impregnated with barium sulfate. Three different methods of conditioning the electrodes were examined. The best response VOL. 40, NO. 3, MARCH 1968
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Figure 5. Responses of silicone rubber membranes impregnated with metal carbonates and phosphates
0
was obtained from an electrode soaked in a 1M sodium sulfate solution for approximately two weeks. This electrode provided linear response toward sulfate ions over the concentration range of 1M t o 1 X 10-jM sodium sulfate with a n observed slope of 36 mV/pS04-* ( P S O ~ -= ~ -log [SOa-z]). The presence of an additional electrolyte in the test solution leveled the response of the electrode toward sulfate ions at the concentration of the added salt. Less satisfactory results were obtained from electrodes which were either boiled in sodium hydroxide or heated t o 200" C. In both of the latter treatments the electrodes were subsequently soaked in distilled water to hydrate the membrane prior t o testing. Embedded Materials. With the silicone rubber acting as an inert membrane matrix, it is then the nature of the embedded material that controls the response of the electrode. Anhydrous cobaltous orthophosphate, both the crystalline and amorphous forms, were embedded in membranes in order to investigate the influence of the crystalline forms of the impregnating material upon the electrodes' responses toward cobalt ions. Each membrane was identically conditioned and fabricated into an electrode. Response curves (Figure 3) from both electrodes were essentially the same. In a n attempt to study the effects produced by varying the degree of hydration of the embedded metal salt, electrodes were fabricated from membranes containing cupric mono520
ANALYTICAL CHEMISTRY
3
2
4
5
pMetal ion
Figure 6. Responses of silicone rubber membranes impregnated ~ metal salt solutions with C O ~ ( P Oin~ )different 4- Response in COSOI; slope: 22 mV/pCo2' -E-
-0- Response of MnC03 in MnCI2; slope: 29 mV/pMn2+ -0-Response of MnHP04.3 H ~ in 0 MnCl2 ; slope: 24 mV/pMn2 + -A- Response of NiCOI in NiCI2; slope: 30 mV/pNi2+ -0- Response of Ni3(P0&.7H20 in NiClz; slope: 19 mV/pNi2'
I
Response in CoSOa
+ 0.01M NaOAc
-A- Response in Cu(N03)2; slope: 31 mV/pCu2 -0-Response in NiS04; slope: 12 mV/pNi2+
+
hydrogen phosphate, both its heptahydrate and anhydrous forms; and cobaltous orthophosphate, both its octahydrate and anhydrous forms. Each pair of membranes was conditioned in a n identical manner prior to use. Figure 4 compares the responses of the hydrated and anhydrous salts for each of these electrodes. In order to compare the effects of the embedded anion, responses were taken from pairs of membranes each of which contained a different salt of the same metal. The salts were cupric carbonate and cupric biphosphate dihydrate, manganous carbonate and manganous biphosphate trihydrate, nickel carbonate and nickel orthophosphate heptahydrate, and cobaltous carbonate and cobaltous orthophosphate octahydrate, Cupric carbonate and cobaltous carbonate failed to provide reproducible responses. Data from the remaining electrodes are presented in Figures 4 and 5 . The specificity provided by chelating agents in the precipitation of transition metal cations suggests the use of these chelates in silicone rubber membranes for specific ion response, Nickel dimethylglyoximate, copper acetylacetonate, and nickel acetylacetonate, both its dihydrate and anhydrous forms, were investigated as possible impregnating compounds. Membranes containing nickel dimethylglyoximate, nickel acetylacetonate dihydrate, and copper acetylacetonate gave no response. Copper acetylacetonate dissolved during the conditioning process. A sluggish response toward nickel
ions was obtained from the nickel acetylacetonate membrane. This response is thought to be caused by the presence of ionic compounds formed by the partial hydrolysis of the chelate during the conditioning period. Selectivity. Figure 6 shows the responses obtained from a silicone rubber membrane impregnated with cobaltous orthophosphate toward solutions of cobalt, nickel, and copper ions. The response curves obtained from this electrode are typical of those obtained from other electrodes in similar solutions. Figure 6 also shows that the presence of an additional electrolyte, sodium acetate, leveled the response of the electrode toward its own ions at the concentration of the added salt, Response curves from solutions of cupric nitrate were similar to those obtained from cobaltous chloride (Figure 4). Nickel sulfate and cobaltous sulfate response curves (Figure 6) are essentially the same. Response curves from all membrane electrodes in metal sulfate solutions are of the type shown in Figure 6. CONCLUSIONS
It was found that crystalline form, the degree of hydration, and the associated anion of the embedded material had no pronounced effect on the membranes’ responses. Satisfactory responses toward cations were provided only by membranes containing ionic compounds that were capable of undergoing rapid metal ion exchange and the adsorption of water at the membrane surface. Chelates are not capable of such rapid ion exchange. For this reason, membranes containing chelates, such as nickel dimethylglyoximate or nickel acetylacetonate dihydrate, failed to show responses toward cations. The dissolution of copper acetylacetonate from a silicone rubber membrane during the conditioning process indicated that solvent and ions could enter the membrane via the embedded material. Nickel acetylacetonate was found to display a sluggish response toward nickel ions. This response is thought to be caused by the presence in the membrane of ionic compounds formed by the partial hydrolysis of nickel acetylacetonate during the conditioning period. While the membrane is being conditioned, water enters the membrane via the embedded particles and is bound on their surface. It is by this process that the membrane’s resistance is decreased. Conduction is probably established by point to point contact of the embedded particles and is enhanced by the bound surface water. Only the presence of ionic compounds can render the membrane conductive and
capable of providing sites for adsorption and rapid metal ion exchange at the membrane interface. The impregnated silicone rubber membranes are not specific in their response toward any one kind of cation. The ern. bedded ionic salt, which is necessary for response, is incapable of providing selective response toward its own ions, but rather it provides response to the total free cations in solution. This is shown by the leveling effect of an additional electrolyte in the test solution and the similarity of the response curves obtained from the cobaltous orthophosphate electrode tested in nickel sulfate solutions, cupric nitrate solutions, and the corresponding cobaltous salt solutions. Values of slopes and ranges of linear responses for all membrane electrodes were greatest when responses were evaluated in test solutions of metal chlorides or nitrates. In metal sulfate solutions, values of slopes and ranges of linear response decreased. In high concentrations of metal sulfates responses of the membranes usually displayed a positive deviation from linearity. These results are contrary to what would be expected if the silicone rubber membranes functioned on the bases of liquid-junction potentials. It is believed that this positive deviation results from the fact that the membrane adsorbs sulfate ions from the test solution and is rendered partially responsive toward sulfate ions. Normal response is again displayed when the membrane is leached of adsorbed sulfate ions by placing it in distilled water and subsequently reconditioning the membrane surface in molar solutions of the metal chloride or nitrate. The electrodes will respond only to free metal ions, not to metal ions in complexed states. The types of responses these membrane electrodes display are dependent upon conditioning and the adsorptive properties of the impregnated compounds. Because chelometric and precipitiometric titrations are usually performed in alkaline solutions with the transition metal ion present in a weakly complexed state, the electrode will not provide satisfactory response for potentiometric titrations. The presence of buffers and the increase of cationic strength by the titrant would level the response of the electrode. It is doubtful if these electrodes will be of value in physiological or biological systems because of the presence of buffers and the complexed forms of the metal ions. RECEIVED for review October 4, 1967. Accepted December 18, 1967.
VOL. 40, NO. 3, MARCH 1968
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