25) was approximately 30% full scale. The data in Table I show the detection limits for the various aromatic amines investigated. The limit is arbitrarily defined as that amine concentration which produces a fluorescence response three times greater than the processed blank. All of the amines studied could be readily detected within the 2 to 6 nanogramper-milliliter level. The complete fluorometric procedure for aryl amines responded with a 6 relative standard deviation. It is anticipated that all compounds that can generate a primary aromatic amine such as those listed in Table I may be determined by fluorometry. For example, the production of sulfanilamide from Neoprontosil permits measurement of as little as 30 parts per billion of the latter drug. In order to gain an insight into the specificity of DAP for determining aryl amines, two other structurally related re-
agents were examined for potential application. It was found that both 2-aminopyridine (AP) and m-phenylrnediamine (MPD) coupled with diazotized aniline to give the azo product. However, after treatment of the azo product with ammoniacal cupric sulfate, it was observed that only the derivative derived from MPD gave any measurable fluorescence (excit. max. 325 mp; fl. max. 390 mp). The sensitivity in this latter case was considerably less than that achieved with DAP. The inability of AP and MPD to sensitively detect aryl amines demonstrates the importance of both the 6-amino group and the pyridine nitrogen in DAP. Both of these functions are necessary for the production of good fluorescence sensitivity. RECEIVED May 7, 1970. Accepted March 15, 1971.
Barium Ion-Selective Electrode Based on a Neutral Carrier Complex Robert J. Levins Philip Morris, U.S.A.,Research Center, P. 0. Box 26583, Richmond, Va. 23261
A barium ion-selective electrode has been made which shows linear, near Nernstian response from lo-’ to 10-5M of BaZ+. The exchanger consists of a neutral carrier complex of a polyethylene glycol derivative which contains 12 ethylene oxide units per mole of Ba2+ and 2 moles of tetraphenylborate ion (12 EOU*Ba* 2TPB). A typical electrode “ion exchanger” IS a saturated solution of lgepal C0880. Ba02TPB in p-nitroethylbenzene. Selectivity ratios of greater than 10,000 to 1 for Ba2+ over Cat+, MgZ+, and most other common cations have been obtained. The electrode may be used as an indicator electrode for the potentiometric titration of aqueous solutions of Sod2-with Ba2+. NEUTRAL CARRIERS are electrically neutral organic molecules which carry a sequence of localized charges (usually lone pair electrons) of sufficient energy to form ion-dipole ligands with select cations. The conformation of the molecule is such that it forms a solvation shell around a cation of appropriate charge and atomic radius, effectively replacing the ion hydration shell. The charged cationic complex thus formed is electrically balanced by an accompanying anion. The neutral carrier complex dissolves in various solvents, wherein an equilibrium is set up between the neutral carrier, the complexed cation, and the free cations and anions. The equilibrium strongly favors the formation of the complexed cation, so that the concentration of the free ions is extremely low. The charged complex formed by the neutral carrier and the cation is mobile, thus providing a path for cation transfer through the organic medium-Le., an ion-selective membrane is formed. The theory and mode of operation of ion-selective membranes based on neutral carriers has been well described by Eisenman, Ciani, and Szab6 (1-4). (1) G. Eisenman in “Ion-Selective Electrodes,” R. A. Durst, Ed., National Bureau of Standards, Spec. Pub]. 314, Washington, D. C., 1969, Chapter 1. (2) S. Ciani, G. Eisenman, and G. Szab6, J. Membrane Biol., 1, 1 (1969). (3) G. Eisenman, S. Ciani, and G. Szab6, ibid., p 294. (4) G. Szab6, G. Eisenman, and S. Ciani, ibid., p 346.
Potassium ion-selective electrodes based on neutral carrier complexes have been made from neutral macrocyclic antibiotics (J), from the cyclic polyethers synthesized by Pederson (6-8), and from the cyclodepsipeptide antibiotic valinomycin (9, 10). Shemyakin and coworkers (11) have brilliantly elucidated the principle of conformation-dependent cation binding through ion-dipole interactions for valinomycin and enniatin. The valinomycin based electrodes of Frant and Ross (10) and of Simon (9) are the only practical sensors so far developed which have been made from neutral carriers. They are extremely selective for potassium ion, having selectivity ratios of about 10,000: 1 for K+ over Na+. We report here a practical electrode for barium ion, which has a selectivity ratio of greater than 10,000 to 1 for Ba2+ over Ca*+, Mgz+, and most other common cations. Polyethylene glycol derivatives serve as the neutral carrier for the barium ion. EXPERIMENTAL
Chemicals. All chemicals used were reagent grade, except for the polyethylene glycol type polymers. The polyethylene glycols were obtained from Union Carbide Corp. The nonyl phenoxy polyoxyethylene ethanol (Igepal CO-880) was obtained from General Aniline and Film Corp. Igepal CO-880 has a molecular weight of about 1540 and contains 30 ethylene oxide units per mole. Neutral Carrier Complex. The Ba2+ complexes were prepared by dissolving the polymer in water, adding barium chloride solution, and then precipitating the oxonium ion thus formed by adding an excess of aqueous sodium tetra(5) Z. Stefanac and W. Simon, Microchem. J . , 12,125 (1967). (1970). (6) C. J. Pedersen. J. Amer. Chem. SOC.. . 92.386 . . . (71 Ibid.,p391. (8) C. J. Pederson, Chem. Eng. News, 48 (9), 26 (1970). (9) L. Pioda, B. Stankova, and W. Simon, Anal. Lett., 2, 665 (1969). (10) M. S. Frant and J. W. Ross, Science, 167,987 (1970). (1 1) M. M. Shemyakin et al., J. Membrane Biol., 1,402 (1969). ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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0
I
I
1
I
Table I.
BaC12 concentration (moleiliter) 10-1
10-2 10-3 10-4 10-6
Ba2+ Activity Coefficients for Aqueous BaClz Solutions
Total ionic strength
Ba2+ ion activity coefficient
3 x lo-’ 3 x 10-2 3 x 10-3 3 x 10-4 3 x 10-5
0.37 0.59 0.80 0.93 1.00
Bat+ ion activity (mole/liter) 3.7 x 5.9 x 8.0 x 9.3 x 1.0 x
10-2 10-3 10-4 10-6 10-5
Bo‘’ ACTIVITY ( moles/liter)
Figure 1. Electrode response to aqueous BaC& solutions (us. double junction calomel electrode, 3M lithium trichloroacetateinternal bridge)
phenylboron. The precipitated complex was washed well with water and filtered through a medium frit glass funnel. The polyethylene glycol complexes were vacuum dried for several hours at 100 “C. The Igepal CO-880 complex was vacuum dried over anhydrous PzOsin a desiccator; alternatively, it may be vacuum dried at 50 “C. This complex is stable at room temperature but slowly decomposes above 50 “C. The “ion-exchanger” for the electrode was prepared by making a saturated solution of the complex in p-nitroethylbenzene. Electrodes. An Orion Research, Inc. liquid membrane electrode (type 90-20) equipped with a Gelman Instrument Co. Metricel type VF-6 membrane (polyvinylidene fluoride, 0.45 micron pore size) was used for the sensing electrode. The outer chamber of the electrode was filled with the “ionexchanger.” The internal reference chamber was filled with 0.1M BaC12, which had been saturated with AgCl. Early work was done with disposable electrodes made by cementing various membranes t o the ends of disposable glass pipets with silicone rubber adhesive (General Electric RTV-112, pourable). The tips of the pipets had been cut off to give a 2- t o 3-mm opening. Double junction reference electrodes were either Orion Research, Inc. type 90-02 or Arthur H. Thomas Co. type 4857-H10. The outer junction was filled with 3 M lithium trichloroacetate (12, 13). Other equitransferent junction solutions or single junction reference electrodes could probably be used as well. Apparatus. A Photovolt Digicord (Model 120) digital pH meter was used ia- the millivolt mode. The meter was calibrated against a Leeds and Northrup potentiometer. Samples and calibration solutions were measured at ambient room temperature (22-23 “C) while being slowly stirred. RESULTS AND DISCUSSION
Valinomycin, enniatin, and the macrocyclic polyethers form neutral carrier complexes with a monovalent ion by iondipole interaction between the ion and six appropriately spaced oxygen atoms. The selectivity of the carrier for an ion depends on how well it fits into the ring or cage formed by the oxygen atoms-Le., selectivity is mainly a function of ionic radius and ring size. Similar constraints would hold for divalent ions, except that twelve oxygen atoms would be required for charge compensation. The work described here indicates that twelve oxygen atoms in the polymer enclose the Ba2+to form an extremely stable neutral carrier complex. (12) Orion Research Inc. Newsletter/Specific Ion Technology, 1, 23 (1969). (13) Ibid., 2, 10 (1970). 1046
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
The conversion of polyethylene glycols (PEG’s), their esters and adducts to cationic (oxonium) form by the addition of barium chloride has been described by Neu (14, 15). The charged complex is readily precipitated by tetraphenylborate ion (TPB-). Uno and Miyajima (16, 17), working mainly with the nonylphenol adducts of polyethylene glycol (NP’s), established that NP’s which have 10 to 40 ethylene oxide units (EOU) form complex precipitates containing 12 EOU per mole of BaZf and 2 moles of TPB- (12 EOU.Ba. 2TPB). For PEG’s the EOU/Ba2+ ratio was slightly less. Levins and Ikeda (18) demonstrated that PEG’s 600 to 4000 form complex precipitates having the composition 10.4 EOU.Ba.2TPB. Early work was done with PEG.Ba.2TPB complexes dissolved in p-nitroethylbenzene as the electrode ion-exchanger. The solvent was the best one found that was both water insoluble and had a reasonably high dielectric constant. The high selectivity of the electrode for Ba2+ over Ca2+, Mgz+, and other cations was quickly confirmed. However, electrodes made with the PEG complexes exhibited considerable (about 30 mV) day-to-day drift. This was attributed t o the poor solubility of the complexes in the organic solvent. The NP.Ba.2TPB complex was then tried on the supposition that the aromatic ring in the polymeric part of the complex would enhance its solubility in the aromatic solvent. An electrode was made using a saturated solution of Igepal CO-880.Ba.2TPB in p-nitroethylbenzene as the ion exchanger; the internal reference consisted of a Ag/AgCl wire immersed in 0.1M BaC12 which had been saturated with AgC1. This electrode gives linear, near Nernstian response (26.6 mV/decade) from 10-1 to 10-6M BaC1,. Electrode response is reasonably fast and stable (10 to 180 seconds) and reproducible to k0.2 mV. Long term drift is between 1 and 2 mV/ day. Electrode response a t the 10-6M level is slow and somewhat erratic, although useful data could probably be obtained under carefully controlled conditions. The response curve for Ba2+is shown in Figure 1. Activity coefficients for Ba2+ are given in Table I ; they were interpolated from a plot made from Kielland’s data (19), which gives the individual activity coefficients of ions in water at 25 “C calculated from the Debye-Huckel theory. The selectivity ratios in Table I1 were calculated using the simplified Eisenman equation or from the activity ratios of the two ions which produce the same potential when present (14) R.Neu, Armeim. Forsch.,9,585 (1959). (15) R. Neu, Ferre, Seifen, Anstrichm., 61,980 (1959). (16) T. Uno and K. Miyajima, Chem. Pharm. Bull. (Tokyo), 11, 75 (1963). (17) Ibid., p 80. (18) R. J. Levins and R. M. Ikeda, ANAL.CHEM., 37,671 (1965). (19) J. Kielland, J. Amer. Chem. SOC.,59,1675 (1937).
4
:'
t
x
0 K
b,
.
I.",
d
1
1 1 1 1 1 , , 1 I
2
4
6
8
12
IO
14
ml of 0.1000M BaCI,
Figure 2. Titration of 10.00 ml of 0.1000M N a 2 S 0 4 15 ml H 2 0 with 0.1000M BaCh
+
Table 11. Approximate Selectivity Ratios for Various Cations
Ion Baz+ cu2+ Sr2+
Ca2+ Mgz+ Ni2+
co2+ Zn2+ Fen+ K+
Na+ NH4+
Li+ H+
KB,,M
1.0
x 10-3 1 x 10-3 < i x 10-4 < i x 10-4 < I x 10-4 < I x 10-4 < I x 10-4 < I x 10-4 1 x 10-3 < i x 10-4 < I x 10-4 < I x 10-4 < i x 10-4 2
separately (20). Figure 2 shows the potentiometric titration curve obtained with the electrode when Na2S04is titrated with 0.1M BaCL The reverse titration gives an equally sharp break. Most common anions (Cl-, Br- , I-, NO3-, S042-) do not affect the electrode's response. The lack of response to 0.1M NazS04is quite remarkable, because the electrode must function by supplying a finite amount of Ba2+to the sample solution. The electrode gave practically no response to small amounts of PEG added to the sample solution. The electrode must also supply a finite amount of TPBto the sample solution, which probably accounts for the somewhat greater response to K+ than Na+. Attempts were (20) K. Srinivasan and G. A. Rechnitz, ANAL.CHEM., 41, 1203 (1969).
made to use the electrode as a TPB- sensor in the titration of K+ with NaTPB. Electrode response to added TPBwas pronounced, but considerable drift occurred, which led to ill-defined and nonreproducible end points. The source of the problem is probably the relatively high solubility of NaTPB in the solvent. Therefore, the electrode should not be used in the presence of added TPB-. The electrode can be used from pH 2 to about pH 10, the upper limit being set by the solubility product of Ba(OH)2. Neutral or slightly acidic samples are preferred. The corresponding Sr2+complex was also made and used as an ion exchanger in an electrode with a SrClz internal solution. This electrode gave good response to both Ba2+ and Srt+, but IIO further studies were made. The use of ethylene oxide based polymers as neutral carriers is also of interest because they are the first reported example of neutral carriers which are not per se macrocyclic. In the crystalline state poly(ethy1ene oxide) has been reported to have a helical structure containing 7 EOU and two turns in the fiber identity period of 19.3A (21). It is interesting to note that the ionic radii of BaZ+and K+ are nearly identical (1.35 us. 1.33 A), which suggests that the size of the neutral carrier rings accommodating these ions should be nearly the same. The six C-C-0 links in dicyclohexyl-l8-crown-6 form a planar ring having an internal radius of about 1.3 8, (6). This suggests that the 12 EOU of the NP complex with Ba2f assume a helical conformation having a ring radius of about 1.3 A in which the Ba2+ is held by a cage of 12 oxygen atoms (6 in each loop) through ion-dipole interaction. The high specificity of the complex for Ba2+ is probably due to the tightness of the ring fit and to the high charge density of the ion. The presence of Ba2+in these complexes should make them ideally suited for X-ray diffraction studies of their absolute conformation. The electrode should find use in the direct potentiometric determination of Ba2+ and as an indicator electrode in the titration of various anions which form precipitates with Ba2+. It should also find application in the indirect determination of sulfur compounds which can be oxidized to S042-. Since the solubility product of BaS04is 1.3 x 10-10, the electrode should be capable of measuring SOn at the low parts per million level after conversion to S 0 4 2 - and by using the known analate subtraction technique (22). RECEIVED for review March 11, 1971. Accepted April 7, 1971.
(21) H. Tadokoro, Y. Chatani, T. Yoshihara, S. Tahara, and S. Murahashi, Makromol. Chem., 73, 109 (1964). (22) Orion Research Inc. Newsletter/Specific Ion Technology, 2, 22 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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