317
Anal. Chem. 1904, 56,317-319
ppm added Na,EDTA, the sample was found to contain 1.2 ppm Fe(EDTA)-.
i
ACKNOWLEDGMENT We thank Ramanathan Panayappan for performing the elemental analyses of the simulated boiler water sample and John Liebermann for many helpful discussions.
Registry No. EDTA, 60-00-4; water, 7732-18-5. LITERATURE CITED
Figure 2. Chromatograms obtained with a sample containing boller water "spiked" with Na,EDTA to a final added concentration of (1) 0 ppm, (2) 0.5 ppm, (3) 1.3 ppm, (4) 2.3 ppm, (5) 4.6 ppm, and (6) 9.1 ppm. Retention volume ( V , ) of Fe(EDTA)- is 4.20 mL. The detector setting is 0.04 aufs.
2 ppm phosphate ion was analyzed for total EDTA. By use of the method of standard additions, the sample was spiked with N%EDTA. See Figure 2. After the data were linearized from five different spiked samples, and extrapolated to 0.0
(1) Motekaitis, Ramunas J., Cox, X. B., 111; Taylor, Patrick; Martell, A. E.; Miles, Brad; Tvedt, Tory J., Jr. Can. J . Chem. 1982, 60, 1207-1213. (2) Venezky, David L.; Dausuel, Robert L., Jr. "EDTA as a Boiler Water Treatment"; Proceedings of the 40th International Water Conference, Pittsburgh, PA, 1979; Paper No. 1WC-79-6. (3) Venezky, David L.; Moniz, William B. Anal. Chem. 1969, 41, 11-16. (4) Annu. Book ASTM Stand. 1982, ASTM 031 13-80 Part 31 (Water), 727-731. (5) Sniegoski, Paul J.; Venezky, David L. J. Chromatogr. Sci. 1974, 12, 359-361. (6) Chen, S. G.; Cheng, K. L.; Vogt, Corazon, R. Mikrochim. Acta 1983, 473-481. (7) . . Perfetti. Gracia A.; Warner, Charles R. J . Assoc. Off. Anal. Chem. 1979, 62, 1092-1095. (8) Parkes, D. G.; Caruso, M. G.; Spradling, J. E., I11 Anal. Chem. 1981, 53, 2154-2156. (9) "Keys to Chelation"; Dow Chemical Co.: Midland, MI, 1969. (10) Martell, Arthur; Sillen, Lars G. Chem. SOC., Spec. Pub/. 1964, No. 17,Section 11.
RECEIVED for review August 18, 1983. Accepted October 4, 1983. W. Rudzinski wishes to thank the American Society of Engineering Education (ASEE) for a Summer Fellowship at the Naval Research Laboratory.
Iridium/Iridium Oxide Electrode for Potentiometric Determination of Proton Activity in Hydroorganic Solutions at Sub-Zero Temperatures Silvano Bordi,* Marcello Carll, and Giorgio Papeschi Department of Chemistry, University of Florence, Via Gin0 Capponi 9, 50121 Florence, Italy
Sergio Pinzauti Department of Pharmaceutical Sciences, University of Florence, Via Gino Capponi 9, 50121 Florence, Italy At present spectrophotometry is the most widespread method for paH+ determination in cryobiochemistry work. However, this technique is rather complex, cumbersome, and time-consuming for routine work (1). In an attempt to setup a simpler and quicker method for measuring proton activity a t sub-zero temperatures, some researchers have modified (2) the glass electrode, replacing the internal solution with a hydroorganic mixture. The modified glass electrode however exhibits several drawbacks including increase in electrode impedance and decrease in response speed as the temperature decreases. The aim of this work is to verify the feasibility and performance of the Ir/Ir02 electrode for paH+potentiometric measurements in hydroorganic solutions in the -20 to +20 "C temperature range. The use of iridium-thick iridium oxide electrodes with a view to producing practical pH sensors has so far been limited to few investigations in aqueous solutions ( 3 , 4 )or in biological
fluids (5,6) but they are particularly attractive candidates for a future generation of pH sensors because of their stability, their low impendance, and their fast response.
EXPERIMENTAL SECTION Iridium wire of 99.9% purity, 0.5 mm in diameter,was supplied by Engelhard Industries, Newark, NJ. Platinum wire of 99.9% purity, 0.2 mm in diameter, was supplied by Metalli Preziosi, Milan, Italy. All reagents used throughout were of analytical grade. Bidistilled water was used for preparing the solutions. A combined Radiometer GK 232C glass electrode was employed for measuring the pH of aqueous solutions, in conjunction with a pH meter with resolution of 0.01 pH (SEAC Model 106,Florence, Italy). The iridium/iridium oxide electrode was prepared by welding 1 cm of Ir wire t o 10 cm long Pt wire, and it was oxidized by heating in an electric oven at 600 "C, the iridium wire being wetted with saturated NaHC03 solution. The oxidation process was
0003-2700/84/0356-0317$01.50/00 1984 American Chemical Society
318
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
Table I. Aqueous Buffer Solutions (25 ' C ; 0 . 0 5 Ionic Strength) Acetate Buffer x mL of 1 M acetic acid 4. y mL of 1 M NaOH for 100 mL of solution PH 3.7 4.8 5.3
X
Y
42.8 8.7 5.4
5 5 5
Cacodylate Buffer x mL of 1 M cacodylic acid + y mL 1 M NaOH for 100 mL of solution PI1 5.2 6.0 7.1
X
Y
52.2 12.5 5.7
5 5 5
Phosphate Buffer x mL of 0.5 M KH,PO, + y mL 0.5 M NaOH for 100 mL of solution PH 6.5 7.1 8.0
Y
X
6.9 4.8 3.6
1.5 2.6 5.2
The response time was less than 1 s in the 1.0-8.0 pH range but it increased as the medium became more basic, and at pH 12 the response time was 30 s. The stability in aqueous solutions was good with a drift less than 1 mV/day. For preparing the hydroorganic solutions we proceeded as follows: First of all aqueous buffer solutions were prepared (Table I) and their pH values were checked with the glass electrode. Hydroorganicbuffer solutions were prepared from these by adding an identical volume of dimethyl sulfoxide to aqueous solutions of different pH. The iridium/iridium oxide electrode response times in the hydroorganic solutions were larger than in the aqueous solutions. In the acidic range, response times were observed up to 5 s but they reached values of 60 s at the higher values of the pH. No significant changes of the response time were observed when the temperature was decreased. The paH+values of the resulting hydroorganic buffer solutions were obtained with spectrophotometric measurements (7) and are recorded in Table 11. RESULTS AND DISCUSSION The iridium/iridium oxide electrode potential values measured in the hydroorganic buffer solutions (Me,SO 50%) a t temperatures of +20,0, and -20 "C are shown in Table I1 (mV columns) for the various paH+values. The potential/paH+ relationship is linear and of the Nernstian type, with slopes of 67, 57, and 48 mV. Respectively a t +20,0, and -20 "C the findings set out in Table I1 yield the following relationships:
+ 283 at +20 "C mV = -57 paH++ 279 at 0 "C mV = -48 paH++ 267 a t -20 "C
repeated until a blue-black coating was formed. The electrodes were then soaked in water for 3 days before using (3). For the measurements of proton activity at sub-zero temperatures, the electrodic chain was as follows:
m V = -67 paH+
Ir-IrO,/test solutioniibuffer (paH+3,8)/Ir-1rO2 B
A
with a reference Ir/Ir02 half cell (B), prepared by introducing an Ir/Ir02 electrode into a small glass tubing closed with a ceramic plug for the liquid junction and containing a paH+3.8 hydroorganic buffer solution. Two digital voltmeters with 100 pV resolution and A input current were used for electrode potential measurements. The electrodes and the solution were contained in a cylindrical Pyrex glass cell inserted in a copper cube contacting at the bottom a Peltier element. The temperature of the copper cube was regulated by a thermostatic system consisting of a Pt 100 element in a Wheatstone bridge, a chopped amplifier, a current booster, and the Peltier battery. This system reached an accuracy of f0.2 "C over the range -20 to +20 "C and a response time of about 10 min in upward and downward temperature jumps. The behavior of the iridium/iridium oxide electrodes was previously (3)compared in aqueous buffer solutions (2.6-11.0 pH range) to a glass electrode.
(1)
The above potentials are measured against an iridium/ iridium oxide electrode in contact with a hydroorganic solution of paH+ 3.8. Statistical analysis of the findings shows that the mean square error on each individual measurement is about 0.14 p H units. CONCLUSIONS Potentiometric measurements of proton activity a t sub-zero temperatures are undoubtedly simpler and faster than spectrophotometric ones when an Ir/Ir02 electrode shows a linear potential/pH relationship in the explored p H range of 3 to 8. The Ir/Ir02 electrode offers the following advantages as compared to the glass electrode: low impedance; ruggedness and ease of construction; compactness; stability in aqueous as well as in hydroorganic solution; fast response time; good linearity.
Table 11. paH+Values of the Hydroorganic Buffer Solutions as Obtained from Spectrophotometric Measurements ( 7 ) and Iridium/Iridium Oxide Potentia1 (Ir-IrO,/test solution//Buffer paH+3.8/1r-1rOZ)in the Same Solutions, at the Temperatures of + 2 0 . 0 , 0.0, -20.0 t 0.2 " C t 20 "C
buffer acetate cacodylate phosphate
0 "C
-
-20 "C
PaH+
mV
PaH+
mV
PaH+
4.40 5.39 5.77 5.68 6.43 7.52 7.75 8.26 9.07
-4.0 -68.2 -106.0 -116.0 -167.3 -222.4 -239.1 --273.9 -320.4
4.60 5.54 6.97 5.93 6.68 7.76 8.10 8.65 9.41
+ 19.9
4.80 5.79 6.22 6.22 7.00 8.06 8.50 9.05 9.86
-30.3 --62.7 -72.6 -114.1 -163.6 -188.8 -220.5 -356.3
mV -t 40.0 -7.4 -34.8 -44.2 -82.7 -124.1 -149.5 -171.9 -20 6 .O
319
Anal. Chem. 1984, 56, 319-320
ACKNOWLEDGMENT
(3) Papeschi, G.; Bordi, S.; Beni, C.; Ventura, L.; Biochim. Biopbys. Acta 1978. 453. 192-199. (4) Katsube, T.: Lanks, I.; and Zemel, J. N. Sens. Actuators 1982, 2, 399-4 10. (5) Macur, R. A. German Patent 2 121 047, 1971; Apr. 30, 1970 U S . APPl. (6) Papeschi, G.; Bordi, S . ; Carll, M.; Criscione, L.; Ledda, F. J . Med. Eng. Technoi. 1981, 5 , 84-86. (7) Douzou, P. ”Cryobiochemistry”; Academic Press: Lonon, New York. and San Francisco, 1977.
We thank Michele Perella (Institute of Enzimology, University of Milan, Italy) for his interest in this work. Registry No. Ir, 7439-88-5; IrOz, 12030-49-8.
LITERATURE CITED (1) Maurei, P.; Travers, F.; Douzou, P. Anal. Biocbem. 1974, 57,
555-563. (2) Larroque, C.; Maurei, P.; Balny, C.; Douzou, P. Anal. Biocbem. 1978, 73, 9-19.
RECEIVED for review July 1, 1983. Accepted September 21, 1983.
Modified Valve Seat for the Static Mercury Drop Electrode R. E. Morton
Instrument Shop, University
of
Georgia, Athens, Georgia 30602
0. M. Evans*
Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, Georgia 30613 The introduction of the static mercury drop electrode (SMDE) (1) was welcomed as a solution to our problem of producing constant-area mercury drops. Our mission is to develop and evaluate new techniques for trace metal speciation. Therefore we use the instrument in both the HMDE and DME modes. Over the past several years, however, we have encountered some difficulty with air getting into the system. The problems were recognizable as drops with varying areas, breaks in the mercury causing discontinuity, and failure of drops to remain on the capillary-all of which led to distorted, undersirable wave forms. We have attempted many solutions, but only temporary corrections were achieved. Modified Valve Seat. Sturrock and William’s (2) approach appeared to offer a viable alternative that could be applied to our situation. However, with the introduction of the modified capillary we decided to redesign the valve seat in a manner to eliminate or a t least minimize air leakage into the system. Figure 1A shows the design of the original valve seat with the modified capillary inserted. This new capillary performed much better than the original and clogging was virtually eliminated. Air entrapment and leakage remained as difficulties, however. A helium leak detector attached to the system revealed that air was diffusing into the system. Probable points of entry were around the capillary nut (upper portion) and the point where the capillary projects from the capillary nut (Teflon tape wrapped around the threads of the valve seat can prevent air from entering at this point). Our conclusion was that the “0” ring was not providing an effective seal. When the capillary nut is tightened the “0” ring seal tends to flatten, perhaps pucker, and become distorted depending on the degree of tightness. In our design we retained the exact height and dimensions of the original valve seat but included a retaining shoulder (-0.056 in. in height) to prevent flattening of the “0”-ring seal. The top of the ferrule is buttressed against this shoulder resulting in an effective “four-point’’seal. We further modified the valve seat (Figure 1B) to obviate electrical conductance problems with the tin oxide film. We used the measurements of ref 2 to provide a basis for comparison. The “0”-ringseal retaining wall is shown (Figure 1B). We have evaluated the performance of the new designs and
I l l
1h I
Figure 1. (A) Original configurations of valve seat and “modified” electrode: (a) valve seat, (b) solenoM valve stem, (c) valve stem rubber tip, (d) capillary, (e) “0”-ring seal, (f) metal ferrule. Figure at right shows compressed condition of “0”-ring seal. (B)Modified valve seat: (a)-(f) same as A, (9) mercury-filled cavity, (h) valve seat retaining shoulder or “0”-ring seal retaining wall. (C) Original valve seat configuration with “modified” capillary: (a)-(f) same as 6,(h) groove in ferrule, (i) ferrule retaining wall.
0003-2700/84/0356-0319501.50/00 1984 American Chemical Society
