A simpler but more hazardous approach since it involves hydrofluoric acid is to dissolve the substance in the hot ferric chloride-hydrofluoric acid reagent. T h e main difficulty encountered here was the selection of the appropriate conditions. A t high temperatures, solution was rapid but volume losses occurred during digestion in the plastic containers. This problem was overcome by using a relatively low temperature (75 “ C ) for 24 hours. All samples were soluble under these conditions and good results were obtained without the necessity to correct for the change in volume of the solution during hydrolysis. Nevertheless, f o r the routine procedure the samples were transferred to volumetric flasks and made u p t o a standard volume before analysis. Both the hydrochloric acid and the hydrofluoric acid procedure for dissolving the tanned hide require only a small amount of operator time and have the advantage of convenience over repeated fuming and ashing operations as required for the sulfuric acid-nitric acid procedure (4). Analysis of zirconium samples by direct solution into hot HFFeC13 reagent overcomes the solubility problems normally associated with zirconium dioxide and crystalline zirconiumchromium complexes. Application of the Method. Analytical results obtained with the atomic absorption method are shown in Table I and compared with those obtained with the same samples after ashing and titration with EDTA (4). In general, agreement was excellent for the different procedures. Zirconium solutions were found to be stable in the hydrogen fluoride-ferric chloride mixture and precipitates did not form on standing. Absorbance readings obtained are quick t o reach a steady maximum, thus amounts of sample used may be reduced by decreasing the volume of solution from 5 ml to 2 ml o r less. Since 18 mg of a treated hide sample in 5 ml of stock solution usually gives a n absorbance reading of approximately 0.25, the recommended sample size may be reduced considerably for a homogeneous sample. In this work, it was considered desirable to work with 10-30 mg of hide to ensure representative sampling. The sensitivity of the method described here is no greater than that previously found by Bond (7) in ammonium fluoride solutions. However, the method is applicable to a wider range of compounds a t this sensitivity as exemplified by the
I
Table I. Analytical Results for Zirconium by Atomic Absorption Method
Z Zr found ZrOn Zr(SO&. 4H20 Zirconium salts containing variable sulfated
73.9O 26.0 32.2 25.7 35.3 35.6 24. 4c 22, 8,*mC23.
Z Zr expected 74.0 25.7
32.5ie 25,le 35,4e 3 5 . 6e
Compound A I 24. Compound A containing glutamic 22. 3e acid Compound A containing glycine 25. 2,b’c2 5 . 3a,c 24. 5e 21 . 9m ; C 21 , 9*lr Compound A containing aspartic 22, Oe acid Hide (1) 5 . w 5.4e Hide (2) 6. 6.4e a HF digestion. * HCl digestion. Contains Cr within the limits of 4-9z by wt. Sulfate contents for these salts and the hides are unknown but they would be expected to be less than the sulfate content of Zr(SO&. 4H20. e Values found by Montgomery and Scroggie method ( 4 ) . f A basic chromium-zirconium sulfate complex characterized by Montgomery and Scroggie (10).
results for zirconium sulfate solutions and for zirconyl chloride solutions containing amino acids. The fact that all substances examined in the hydrofluoric acid-ferric chloride solution gave virtually identical calibration curves suggests that complexing of the zirconium by hydrofluoric acid is optimal in this mixture. One would also conclude that either higher temperatures o r complexing agents not involving oxygen links are necessary if the sensitivity of the atomic absorption method is to be increased substantially. ACKNOWLEDGMENT
The authors thank Dr. J. B. Willis for helpful discussion and Mr. P. Mc. Strike for his skilled technical assistance.
RECEIVED for review May 10,1971. Accepted July 30,1971.
CORRESPONDENCE
I
Coated Wire Ion Selective Electrodes SIR: Ion selective electrodes of both solid state and liquid membrane types are now available for a wide variety of cations and anions ( I ) . The electrodes typically consist of a reference electrode (Ag/AgCl) immersed in a n aqueous reference solution in contact with a “membrane” which in turn contacts the solution under test. The commercially available electrode assemblies are fairly bulky and, because of their fairly complex structures, rather expensive. We have devoted some time t o designing simple and compact electrodes that might permit measurement on micro samples. (1) J. W. Ross in “Ion Selective Electrodes,” R. A. Durst, Ed., Nat. Bur. Stand., US.,Spec. Publ., 314, 57 (1969).
Among the smallest and simplest electrodes are those of the “first type,” namely fine metallic wires such as Ag and Cu that respond to changes in and uCu2+,respectively. These, unfortunately, are available for very few cations. A wider variety of ions give potentiometric responses to electrodes of the “second type” in which a metal wire is coated with a poorly soluble salt of that metal, as with the Ag/AgCI electrode which responds t o changes in U C I - . Although the salt layer is usually deposited by anodizing the metal electrode in a suitable electrolyte, recently it has been shown possible (2) to incorporate the poorly soluble salt (2) H. Hirata and K. Date, Talunta, 17,883 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1 9 7 1
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a
Table I. Selectivity Coefficients,Ki,. of Various Divalent Cations Interferent Orion electrode Coated wire electrode Nil' 0.026 0.0039 cuz+ 0.24 0.15 Mga+ 0.033 0.014 Ba2+ 0.016 0.0036 Sr I + 0.029 0.021 Pb2+ 0.23 1.86 Zn2+ 1.44 32.3 Calculated from A E = 30 log (1 K i a i / a c J
+
in a polymeric matrix, affixed to the metal wire, provided that the salt particles are in continuous contact from the outer surface of the polymer film to the metal surface underneath. Hirata and Date ( 2 ) have produced a functioning Cu-responsive electrode by attaching a Cu*S-1mpregnated silicone rubber or epoxy resin film to either a copper foil or platinum. If a molecular dispersion, or solution, of a salt or complex in a polymeric matrix such as polyvinyl chloride, epoxy resin, or methyl methacrylate painted on a platinum wire should function as an electrode responsive to one of the ions of the salt, then the whole range of so-called liquid membrane electrodes could be produced in inexpensive, compact form. Preliminary indications of the efficacy of this hypothesis are very promising. One example of the effectiveness of this approach was shown by coating a 0.018-in. platinum wire with a 6 : l mixture of 5 z polyvinyl chloride dissolved in cyclohexanone and 0.1M calcium didecylphosphate in dioctyl phosphonate (Orion No. 92-20-01). The wire was dipped in the mixture several times and allowed to set overnight in air. The remainder of the exposed wire was covered by wrapping it tightly with a paraffin film to prevent direct contact of the metal surface with the test solution. Although the coated wire electrode prepared in this manner gave potential responses to changes in calcium ion activities when placed in solutions immediately, best results were obtained after a n initial conditioning, accomplished by soaking the electrode for about a n hour in either distilled water or in a dilute (ca. 10-4M) CaCh solution. The performance characteristics of the Ca-coated wire electrode were compared side by side with those of the Orion Ca-electrode (Model 92-20). In all cases, the Beckman saturated calomel electrode was used as the reference electrode, The response curve to concentration and activity show two interesting differences. The Orion electrode exhibits a linear response to log aCa2+(slope 30 mV) in the range of lo-' to lO-*M Ca, but changes only about 12 mV from 10-4M to lO-jM Ca2-. The coated wire electrode shows steeper linear responses to both the logarithmic variation of C a concentration (slope 31 mV) and of C a activity (slope 38 mV) than does the Orion electrode. Moreover, the electrode response in the lo-' to 10-jM range, while '
1906
not Nernstian, is about 25 mV, large enough to extend the useful range to the more dilute solutions. The p H range of effective use of the coated wire electrode (pH 4.0 t o 9.5) is somewhat larger than that of the Orion electrode (4.5-9.0). The coated wire electrode gives a very rapid response time (-10-15 sec), can be stored in air, a n d has a long useful life (some are still functioning after 3 months' use). A complexometric titration of 10-2M Ca2+ with 0.05M EDTA at pH 9.0 using the coated wire electrode resulted in a sharply defined curve that resembled the one obtained with the Orion electrode, except that the potential reversal beyond the end point found in the latter case was absent. This advantage of the coated wire electrode probably results from its lower sensitivity to sodium ions. The response of the Orion electrode in a 0.01M Ca2+ solution changed 5 mV when the solution was made 1M in NaCI, whereas the coated wire electrode response changed only 0.6 mV, indicating a much higher Ca/Na selectivity ratio. Interferences of other cations were determined in the following way. For the large interferences (Cu, Pb, Zn) lO-3M CaCI2 and 10-2M of the interfering ion were used. For small interferences (Ni, Mg, Ba, Sr) 10-4M CaCh and 10-2M of the interfering ion were used. In all cases the standard 10-3M or lO-'M CaCL solutions were adjusted to the same ionic strength as the test solution with NaCI. The interferences of several cations on the calcium electrodes are shown in Table I where values of the selectivity coefficients, calculated in the usual manner from the Eisenman equation, are listed. For several cations the coated wire electrode shows considerably less interference than the Orion liquid membrane electrode. I n particular, the interference of Naf and Mg*', NiZ-, Ba2', and Sr2+ with the coated wire electrode is very small. The interference of Pb*+ and more particularly Zn2+are significantly greater than that observed with the Orion electrode. The behavior of the Ca-responsive coated wire electrode demonstrates the possibility of developing a wide variety of miniaturized, inexpensive, sturdy, and reliable ion selective electrodes. Preliminary work in the laboratory dealing with various inorganic and organic anion selective electrodes has proved that the coated wire electrode is effective for many ions and that the internal reference solution is not needed. R. W. CATTRALL' HENRY FREISER
Chemistry Department University of Arizona Tucson, Ariz. 85721
RECEIVED for review August 6, 1971. Accepted September 7, 1971. The authors acknowledge the assistance of a N S F Science Development Program grant. 1 On study leave from La Trobe University, Bundoora, Victoria 3083, Australia.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
