Calcium Ion-Selective Electrode in Which a Membrane Contacts Graphite Directly Anthony Ansaldi and Samuel I. Epstein Lever Brothers Research and Decelopment Dioision, 45 Ricer Road, Edgewater, N.J. 07020
IN RECENT YEARS there has been a sharp increase in the use of ion-selective electrodes as an analytical tool. This growth was aided by the development of several commercially available ion-selective electrodes which all utilize an ion-exchanging barrier material (1-8). The barrier may be a solid and consist of an inorganic crystal, a precipitate-impregnated membrane, or a thin layer of electroactive material. The barrier may also be a liquid ion-exchanger (LIE). The LIE type offers broad possibilities for sensing almost any ion, both cationic and anionic, although frequently ion selectivity is only moderate. In an effort to utilize liquid ionexchangers, but eliminate stirring and pressure effects which are found with many LIE electrodes, Moody et al. (6) developed an electrode in which the LIE was incorporated into a poly(vinylch1oride) (PVC) membrane. The use of this membrane eliminated the need for a reservoir of liquid ion-exchanger. Several workers have been able, in addition, to construct electrodes which function without internal reference solutions. For example, Hirata and Date ( 5 ) made a Cu-selective electrode by attaching a Cu&impregnated silicone rubber or epoxy resin film to either copper foil or platinum. Ruzicka et al. (7,8) have described electrodes in which the ion-sensitive material, which may be either an electroactive solid or a solution of an electroactive species dissolved in a water-immiscible organic solvent, coats the surface of graphite. Cattrall and Freiser (9) have described a Ca-selective electrode, which was made by coating a platinum wire with a mixture of the Orion Ca-liquid ion-exchanger (Orion No. 92-2GO1) and PVC. This paper describes a Ca-selective electrode in which a membrane made according to Moody et al. (6) is in direct contact with the end of a graphite rod, most of which was covered with a hydrophobic material. This electrode functions without an internal reference solution. EXPERIMENTAL
Construction of Electrode. The membrane was prepared by mixing 0.4 gram of liquid ion-exchanger (Orion No. 9220-01) and 0.17 gram of PVC (Chromatographic Grade obtained from Polysciences Inc.) in 6 ml of reagent grade tetrahydrofuran (6). The mixture was then poured into a polypropylene cap (0.d. 29 mm) from a 50-ml centrifuge tube (Nalgene No. 3111). The cap was covered with several pieces of filter paper and left in the hood for a few days, during which time the solvent evaporated leaving a membrane approximately 0.5 mm thick. A disk was cut from the (1) E. Pungor, ANAL. CHEM.,39(13), 28A (1967). (2) G. A. Rechnitz, Cltem. E m . News. 45(241. 146. (19671. (3) G. J. Moody, R. B. Oke,and J. D. R. Thomas, Lab. Pract.. 18, 1056 (1969). (4) A. K . Covington, Chem. Brit., 5, 388 (1969). (5) H. Hirata and K. Date, Talaizia, 17, 883 (1970). (6) G. J. Moody, R. B. Oke, and J. D. R. Thomas, A/iulyst (Loiidoiz), 95,910(1970). (7) J. Ruzicka and K . Rald, A d . Chim. Actu., 53, 1 (1971). (8) J. Ruzicka and C . G. Lamin, ibid.. 54, l(1971). 43, 1905 (1971). (9) R. W. Cattrall and H. Freiser, ANAL.CHEM.,
graphite r o d
Tygon tubing
/
P V C membrane incorporating liquid ion exchanger
Figure 1. Cutaway view of PVC/ graphite electrode
membrane, using a No. 7 cork borer, and cemented to one end of a piece of Tygon (Norton Co.) tubing (10 mm 0.d.. 5 mm i.d. X 11 mm) with a mixture of PVC in tetrahydrofuran (6). A spectrochemical grade graphite rod (5-mm diameter x 90 mm) was force fitted into the Tygon tubing. When the membrane bulged slightly, good contact between the graphite rod and the membrane was indicated. The wire lead was press fitted into a hole which had previously been drilled in the other end of the rod. The exposed portion of graphite was covered with Calectro heat shrinkable tubing, a hydrophobic material. The complete electrode assembly is shown in Figure 1. Reagents. Solutions of known calcium concentration were prepared by dissolving primary standard calcium carbonate in reagent grade hydrochloric acid. The excess acid was neutralized using reagent grade sodium hydroxide. A stock solution of 0.1M calcium chloride was made in this way and all other solutions were made by serial dilution from the stock. All solutions were prepared using water which was redistilled from alkaline permanganate solution. Apparatus. Measurements for the cell calcium electrode 1 test solution ' 1 reference electrode were made at 25 "C using both the PVCjgraphite and the Orion calcium electrodes. In both cases an Orion single junction reference electrode (Model No. 90-01) was used. The potential was measured using a Corning Digital 112 Research pH meter which reads to 0.1 mV. Electrode Calibration. In the absence of interfering ions, the response of the calcium electrode relative to a reference electrode is given by the Nernst equation,
E=Eo+-
2.3RT log Ac,'+ nF
where E is the measured potential, EOis a constant which includes the effect of the reference electrode, R is the gas constant, T i s the temperature, F the Faraday, n the charge on ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
595
Table I. Selectivity Coefficients, Kij, for Various Divalent Cations Selectivity coefficient using: Orion PVC/graphite Ion electrode electrode Mg 0.009 0,0006 Ba 0.007 0.0018 Ni Zn
0.005
0.003
0.29
Pb
2.1
0.27 2.9
the ion being measured (2+ in this case), and AC.2+ is the calcium activity. The Nernst factor 2.3RTlnF has a theoretical value of 29.58 mV a t 25 "C for calcium ion. In this work, the value of E was determined for each of seven standard solutions in the concentration range from 10-'M to lO-5M Ca2+. The slope of a plot of E us. log AcB2+was then determined for comparison with the theoretical value. Determination of Selectivity Coefficients. The selectivity coefficient, K,,, of several divalent cations, was determined from a curve of E us. log AcZ2+,which was obtained in a constant background of interfering ion according to the procedure described in reference 10. This procedure makes use of the relationship
K,, A ,
=
A,
(2)
where A , is the activity of the primary ion (in this case Ca2+) and A , is the activity of the divalent interfering ion present at a constant background level. In the case of such ions as Mgz+, Ba2+, and Ni2+, which interfered relatively little, 10-2M interfering ion was used and in the cases where the interference was large (ZnZ+and Pb'+), 10-4Minterfering ion was present. RESULTS AND DISCUSSION
The performance of the PVC/graphite electrode was evaluated on the basis of a side by side comparison with the Orion (10) Orion Research Inc., Newslerrer, 1, 29 (1969).
Ca-selective electrode (Model 92-20). The electrode potential, E, relative to the reference electrode, of the PVCL graphite electrode, like that of the Orion electrode, varied linearly with log AcaZAin the range 10-lMto 10-5MCa2T,although the response of the PVC/graphite electrode was about 10% lower than that of the Orion (27 mV/log Aca2+ us. 30 mV/log Aca2+). The PVC/graphite electrode was also sensitive below lO-jM, with AE = 15 mV between lO-jM and 1 0 P M Ca'+, whereas the particular Orion electrode used here exhibited A E = 7 mV in this range. The PVC/graphite and the Orion electrodes were both useful in the p H range 5.5 to 10.5 at Ca2+ concentrations as low as lO-4M, indicating that the two electrodes have similar response to H+ in this range. The PVC/graphite electrode was less responsive to Na+ than the Orion electrode. This was indicated by the determination of the potential for each electrode in 0.01MCaC12;when the solution was made l M i n NaC1, the change in potential was 1-3.0 mV in the case of the Orion electrode, but only -0.4 mV in the case of the PVC/graphite electrode. The selectivity coefficient which was obtained for each of several divalent cations is shown in Table I. The response of the PVC,'graphite electrode to Mg2+ and Ba2+ was considerably less than that of the Orion electrode. A similar response of both electrodes to Ni2+ and Zn2+ was observed, while the PVC/graphite electrode was slightly more sensitive to Pb2+ than the Orion electrode. CONCLUSION
In general the PVC/graphite electrode described here is similar to other electrodes referred to above which function without an internal reference solution, though it differs in the combination of materials used and the method of construction. It would be expected that with membranes containing appropriate exchangers, the PVC/graphite electrode system would be adaptable to the determinations of ions other than calcium. This possibility is currently being investigated. RECEIVED for review August 23, 1972. Accepted November 13,1972.
Scanning Calorimetric Determination of Vapor-Phase Kinetics Data Raymond N. Rogers and G . William Daub University of California, Los Alamos Scientific Laboratory, Los Alamos, N . M . 87544
DURING THE PRODUCTION of isothermal rate curves by use of a scanning calorimeter ( I ) , it was observed that several compounds gave two first-order curves a t each temperature. The second curve was normally smaller than the first, and the rate constant obtained from the second curve was larger. The relative sizes of the two parts of the rate curve could be varied by changing the volume of the sample cell. Figure 1 shows a comparison between two rate curves for the explosive RDX (hexahydro-l,3,5-trinitro-s-triazine), obtained at different cell volumes. The working hypothesis that seemed best to (1) R . N. Rogers, Thermocliim.Acta, 3, 437 (1972). 596
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
explain the observations was that the "tail" of the rate curve was the result of decomposition in the vapor phase. The stability given to an organic compound by a crystal lattice, especially one involving multiple hydrogen bonds, can be considerable. It is often observed that a given compound decomposes much more rapidly in the liquid or vapor phase than in the solid phase at any constant temperature. It is also true that the vapor-phase decomposition of a compound can result in products that lower the melting point of the system, forming an unexpected liquid phase. The apparent stability of some materials is found to be a function of the amount of free volume of confining containers, i.e., the
