Anal. Chem. 1980, 52, 861-864
86 1
Modification of a Platinum Electrode Surface by Irreversible Adsorption of Adenosine-5‘-Monophosphate and Metallation by Iron James A. Cox’ and Marcin Majda Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 6290 1
Cycling a Pt electrode between -0.05 and 0.40 V vs. SCE in a 2 mM adenosine-5’-monophosphate, 0.1 M chloroacetate solution at pH 3.5 produces a stable, modified surface with a coverage of 8.5 X lo-’’ mol/cm2. By contact with a solution of Fe(NH4)2(S04)2or a comparable Fe(II1) solution In pH 3.5 buffer, the surface is metallated. Linearity of cyclic voltammetric peak currents with scan rate, peak persistence after transfer of the metallated surface, and the peak shapes demonstrate the latter. Changes in scan rate cause a shift in the Fe(1II) reduction peak potential but do not significantly change that of Fe(I1) oxidation which suggests a difference in the bonding with the surface. An interaction of Fe(I1) with a purine ring nitrogen In addition to the phosphate bond is proposed to account for the more facile charge transfer. When the Pt-adenoslne-5’-monophosphate electrode Is contacted to K,Fe( CN), or K,Fe(CN),, similar evidence for metallation is obtained.
Modification o f electrode surfaces by attachment of electroactive species has become an important technique for the study of charge transfer reactions and a means of devising surfaces with selective voltammetric properties. Examples of modification procedures include irreversible adsorption on Pt ( 1 ) and graphite ( 2 ) ;silanization of S n 0 2 ( 3 ) ,other metal oxides (4-61, and carbon (7);condensation of amines with acid or acid chloride groups generated at the electrode surfaces (8, 9); covalent attachment to deoxygenated graphite (10-13); cyanuric chloride linkage (14); and polymer coating (15). T h e electroactive species may be attached subsequent to modification of a surface by a non-electroactive one. For example, metallation of modified electrodes by ruthenium complexes via ligand exchange reactions has been reported ( 1 1 , 12, 16-19). Silanized SnOz was metallated by Ru(II1) chloride (20). Metallation of glassy carbon which was modified by covalent bonding of tetra(aminopheny1)porphyrin (21,22) by Mn, Fe, Co, Ni, Cu, and Zn was demonstrated (22). T h e adsorption of adenosine nucleotides, including adenosine-5’-monophosphate (AMP), on Hg has been recently studied (23). Flat adsorption of the purine ring of AMP was reported. In the present paper, the modification of Pt by irreversible adsorption of AMP is described and evidence for subsequent metallation by Fe is presented. The results suggest that Pt-AMP may be useful as an electrode a t which Fe can be determined by cathodic stripping voltammetry. EXPERIMENTAL The voltammetric experiments were performed with a PAR 170 Electrochemistry System. Positive feedback was usually employed to minimize resistance effects, but its use changed peak potential differences by only about 4 mV. All potentials were measured and reported vs. the SCE. The reference was contacted to the solution with a luggin capillary salt bridge filled with deaerated supporting electrolyte. Iron solutions were prepared from the following ACS Reagent Grade salts which were used without purification: Fe(NH4)*0003-2700/80/0352-0861$01 .OO/O
(S04)2.6H20, Fe(NH4)(S04)2.12H20, K,Fe(CN)6, and K,Fe(CN),. Monochloracetic acid was recrystallized from water and distilled. The 81-82 “C fraction was collected. The 0.1 M buffers at various pH’s were prepared by adjusting the acid solutions with KOH to the appropriate value. The water was doubly distilled from alkaline KMn04. The adenosine-5’-monophophate was obtained as a 99% assay, Sigma Grade, sodium salt from Sigma Chemical Co. Adenosine was likewise purchased. They were stored at -18 “C except for 30 min prior to weighing. The solutions were freshly prepared immediately prior to an electrode modification. The Pt substrates were prepared by heat-sealing wire into soft glass tubing. The electrode was cleaned for 15 min in chromic acid, rinsed, and transferred to deaerated 0.5 M HC104. The electrode was stepped between -0.3 and 1.3 V, 10 s at each value, for 50-100 full cycles. The electrode was subsequently scanned continuously between -0.26 and 1.05 V at 50 mV s-l for 20-50 cycles. The potential scan was terminated at 0.1 V at which the electrode is free of adsorbed hydrogen and platinum oxide. This treatment was based upon previous reports ( I , 24). The pretreated electrode was rinsed and placed immediately into a deaerated, M AMP (or adenosine) solution. buffered 2 X Several modification treatments were attempted. They involved combinations of dipping of the Pt in AMP at open circuit, cycling the potential, varying the pH from 3.2 to 5.8, and including Fe(I1) in the AMP solution. The resulting electrodes were evaluated in terms of the development of cyclic voltammetric peaks in an Fe(I1) test solution and the reproducibility of the background current. The recommended procedure is to cycle the electrode potential from -0.05 to 0.40 V for 5 min at 200 mV s-’ in a 2 X M AMP solution buffered at pH 3.5 with chloroacetate. The electrodes modified in this manner can be stored for at least 2 weeks in distilled water without significant loss of AMP. The working range of the Pt-AMP electrodes depends upon solution pH. Typical limits are as follows: pH 1.5, 0.63 to 0.02 V; pH 2.5, 0.55 to -0.04 V; pH 3.5, 0.45 to -0.12 V. Outside of these limits, proton reduction and Pt oxidation occur. The former causes irreversible changes in the Pt-AMP behavior. The electrode areas were taken as the experimentally determined values of the Pt substrates. These areas were determined by potential step chronoamperometry of 1.195 mM Fe(CN)63-in 0.1 M KCl using the linear diffusion version of the Cottrell equation. The measured value was typically 0.039 0.001 cm2. It did not vary for a given substrate used repeatedly for several months.
RESULTS A N D DISCUSSION Adsorption of AMP onto Pt yields an electrode a t which the electrochemical behavior of the Fe(III)/Fe(lI) is markedly different from that a t Pt alone. As shown by the initial scan voltammograms in Figure 1,the primary electrode processes occur at more negative potentials a t Pt-AMP. T h a t AMP decreases the double-layer charging current, which is not an unusual result of adsorption of a high molecular weight organic compound (23),is also shown. When Pt is modified by adsorption of adenosine rather than AMP, no faradaic process is observed under the conditions of Figure 1. Apparently the adsorption of either adenosine or AMP blocks the oxidation of Fe(I1) from the bulk solution phase; however, the phosphate group of AMP facilitates a redox process in which Fe(I1) and CZ 1980 American Chemical Society
862
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
L--.-A--
0.3E/V
0.1
Cyclic voltammetry of Fe(III)/(II) at R and R-AMP electrodes. Pt (---), Pt-AMP (---). Solution, 0.4 mM Fe(NH,),(SO,), in 0.1 M chloroacetate at pH 3.5; v , 100 mV s-’ Figure 1.
Fe(II1) are bound to the surface of the modified electrode. The AMP is monobasic at p H 3.5 and could undergo an ion-exchange reaction with Fe. The slight current increase at 0.4 V at Pt-AMP is due to the onset of the oxidation of solution-phase Fe(I1). Investigation of the effect of scan rate, u, on the anodic and cathodic peak currents, i, and ipc, under the conditions of Figure 1 provides evidence for the surface nature of the reactions. Over the u-range of 10 to 200 mV s-l, plots of the peak currents vs. u are linear and have the following characteristics: i, vs. u (slope, 9.57 PA s; standard deviation, 2.1%; correlation coefficient, 0.9994) and i, vs. L) (slope, 18.2 pA V-’ s; standard deviation, 0.9 % ; correlation coefficient, 0.99997). Here, the switching potential, EA,is 0.5 V. Stirring the solution during the cyclic voltammetric experiments does not alter the peak current ratios. An important characteristic of the above data is that the peak current ratio, ipc/ipa, is much greater than unity (the actual value is 1.9 in the above experiments). Further, integration of the peak areas and direct charge measurement by double potential step chronocoulometry (2.5) both demonstrate that the cathodic charge is 5.3 times the anodic charge. However, if EA is decreased to 0.2 V, the cathodic and anodic processes become charge-equivalent. Apparently the AMP is not saturated by Fe(I1). The oxidation of solution-phase Fe(I1) beyond 0.4 V therefore yields surface-bound Fe(II1). Medium transfer experiments agree with this interpretation. When a cyclic voltammetric scan under the Figure 1conditions is interrupted at 0.5 V and the electrode is removed from the solution, rinsed, and inserted into an Fe-free supporting electrolyte, the cathodic current which is observed during the scan from 0.5 to 4.1 V is identical to that in Figure 1. If the experiment is performed by interrupting the scan at 4.1 V, removing the electrode from solution, rinsing, and inserting into an Fe-free electrolyte, the first cycle is characterized by a unity peak current ratio even when EA is 0.5 V. In this case i, is decreased to the value of i,, in Figure 1. With either experimental design, continuous potential cycling after medium transfer results in the disappearance of the peaks after 5-10 cycles. However in the former design if the electrode is held at 0.5 V after medium transfer, the initial cyclic scan shows the Figure 1behavior even after several hours of contact with stirred electrolyte. With 10 h between transfer and initiation of the cyclic voltammetric experiment, the peak currents are only attenuated by 20%. On the other hand, the medium transfer with Fe(I1) on the electrode surface will not tolerate long delays prior to performing the cyclic voltammetric experiments. Even with a 1-min delay at 0.0 V, the peaks are not developed. The medium transfer experiments and the effect of EAon i,,/i,, suggest that the Fe(I1)-AMP complex is labile and is much weaker than Fe(II1)-AMP. As Fe(II1) is a “harder” cation than Fe(I1) and phosphate is a “hard” ligand on the
Effect of scan rate on t h e relative peak potentials for Fe(III)/Fe(II)at a R-AMP electrode. Outer curve v , 200 mV s-’;inner curve v , 50 mV s-’: initial potential, -0.1 V; solution, 1 mM Fe(NH,),(SO,), in 0.1 M chloroacetate at pH 3.5
Figure 2.
Nn2
cj Representation of possible Fe(I1) interaction with the phosphate and N(7) of AMP-5’ Figure 3.
Pearson scale (26),it is predictable that Fe(II1)-AMP complexes would be the more stable. A significant difference in the complexation was also indicated by the effect of u on the anodic and cathodic peak potentials, E,, and E,,. Under the Figure 1 conditions with u varied from 10-200 mV s-l, aE,,/a log u was -85 f 2 mV/decade, and aE,,/a log u was 0 4 mV/decade. The correlation coefficients were 0.9995 for the &point plots. As shown in Figure 2, the unusual behavior that the reduction of Fe(II1)-AMP occurs at a more positive potential than the oxidation of Fe(I1)-AMP is found at low scan rates. The complexes must therefore have different formal potentials, so a chemical (or structural) change must occur in the Fe(II1)-AMP when it is electrolytically formed and when it is again reduced. This change allows the difference in the anodic and cathodic peak potential shifts with u to be interpreted as being due to a difference in the charge transfer rate constants for the oxidation and reduction processes. Differences in the bonding in Fe(I1)-AMP and Fe(II1)AMP may be attributed to greater interaction between Fe(I1) and a purine nitrogen. Such a scheme is shown in Figure 3. This structure is favored by the syn conformation of AMP which is observed when AMP is adsorbed a t Hg (23). Nitrogen-type ligands preferentially react with Fe(1I) (27). In the case of AMP, Mossbauer studies have demonstrated that Fe complexes undergo mixed interactions with N(7) of Figure 3 and phosphate (28). This interpretation further allows a hypothesis regarding the relative rates of the charge transfer reactions as indicated by the effect of u on the peak potentials. The purine ring is presumably adsorbed flat against the Pt. Charge transfer with the Fe(I1)-AMP is thus facilitated by the Fe-N interaction, so the oxidation has a relatively high charge transfer rate constant. The virtual independence of E , of u is a result. On the other hand, with Fe(II1) the metal is isolated from the electrode by the Fe-0-P-0-ribose path except for possible weak interaction with N(7) of Figure 3.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
863
Table I. Cyclic Voltammetric Parameters for Fe( CN),”/Fe(CN),4- Immobilized at €%-AMPa vlmV s-’
200 100 50 20 10
i,lpA 5.00 3.00 1.75 0.86 0.48
ipc/pA E p J V 9.25 6.31 4.31 2.53 1.66
0.239 0.238 0.236 0.235 0.233
EpclV 0.122 0.129 0.133 0.139 0.142
6,lzal
6,/2CJ
84 76 73 72 72
47 36 30 22 19
mV
mV
a Data were obtained after transferring Pt-AMP from 1 mM Fe(CN),3-, pH 3.5, 0.1 M chloroacetate buffer into an Fe-free buffer. Positive feedback was used to minimize resistance effects.
Flgure 4. Steady-statecyclic voltammogram of Fe(III)/Fe(II) immobilized
by contact by K,Fe(CN), to R-AMP. R-AMP contacted to 1 mM K,Fe(CN), in 0.1 M chloroacetate at pH 3.5 for 2 h, then transferred to Fe-free buffer in which the voltammogram was obtained; v , 200
mV
s-I
A low charge transfer rate constant for the reduction can therefore be anticipated. T h e coverage of the electrode by AMP was estimated by performing experiments which first saturated the phosphate sites with Fe(II1) and then used linear potential scan stripping voltammetry to develop a cathodic peak. Integration yielded the area under the peak which was converted to surface charge. Assuming a 1:l stoichiometry of Fe:AMP the charge was related to AMP coverage. The site-saturation was accomplished by controlled potential electrolysis at 0.45 V in stirred solution. With Fe(I1) above 0.30 mM a t p H 2.9, the electrolysis time did not influence the peak area. For example, with 2 mM Fe(II), 9 values ranging from 5-600 s yielded surface charges of 2.7 f 0.03 pC. Below 3 mM Fe(II), the measured charge depended upon concentration. With 0.1, 1, 2, 5, 10, and 20 mM Fe(I1) at pH 2.9, the measured charges were 1.7, 2.4, 2.7, 3.2, 3.2, and 3.2 pC, respectively. The plateau value of 3.2 pC converted to 82 pC/cm2 or 8.5 X mol AMP/cm2 using the electrochemically measured area of the Pt substrate. A theoretical surface coverage of 2.7 X mol/cm2 can be calculated by assuming flat adsorption of the purine rings of AMP and 62 A2 as the adsorbed molecule area (23). The discrepancy of the values probably reflects surface roughness of the Pt. The cylindrical configuration of the electrode makes polishing quite difficult, and the pretreatment method causes some addition roughening. As mentioned earlier, chronocoulometry yielded 82 pC/cm2 for the surface charge of Fe(II1). For Fe(I1) this method gave 16 pC/cm2. T o further characterize the Pt-AMP electrode, the cyclic voltammetry of Fe(CN)63-was investigated at that surface. With 1 mM solutions at pH 3.5 in 0.1 M chloroacetate buffer, the voltammetric peaks are narrower than at a Pt electrode. Unlike the case of the simple Fe(III)/Fe(II) couple, it is not clear whether the electrode process is surface or partially diffusional in nature in this solution. Medium transfer experiments were performed to establish the basic mechanism. Figure 4 illustrates the cyclic voltammogram which is obtained after Pt-AMP is contacted to 1 mM Fe(CN)6*, rinsed, and inserted into an Fe-free 0.1 M chloroacetate buffer at pH 3.5. The peaks are highly persistent. Continuous cycling for 1 h (over 700 cycles) causes only a 35% decrease in the peak
currents. Integration of the peaks shows that the ratio of cathodic-to-anodic charge is 1.25; with a contact time of 2 h prior to medium transfer, the cathodic charge is 79 pC/cm2, a value in agreement with that calculated previously for the simple Fe(II1) reduction a t Pt-AMP. Further, when the medium transfer experiment was performed at a Pt-adenosine electrode, no faradaic current was developed in the potential region of Figure 4. The peak persistence demonstrates that both the oxidized and reduced complexes are attached to the surface. Unlike the simple Fe(III)/Fe(II) case, the interactions of both Fe(CN)63-and Fe(CN)64-with Pt-AMP yield low lability species although the charge ratio indicates a somewhat greater lability of the product of the Fe(CN):--AMP interaction. The relative labilities are verified by a modified medium transfer experiment. If the electrode is held at -0.1 V for 1min during which the buffer is stirred, the subsequently obtained cyclic voltammetric peak areas are decreased by 20%; however, if the electrode is instead held at 0.4 V, the peaks are not decreased by time and stirring. Comparison of the behavior of Pt-AMP to Pt-adenosine electrodes and the agreement of the surface charge calculated from the Fe(CN)63-and the simple Fe(II1) studies indicates metallation of the phosphate site of the AMP by Fe(CN)63-. Further, a 1:lstoichiometry is suggested, subject to the same limitations as in the simple Fe(II1) case. An attempt to further elucidate the nature of electrode process was made by cyclic voltammetry. As shown in Table I, the results were not definitive. The peak half-widths, especially the cathodic peak value, were less than the theoretical prediction of 90.6/n mV for a surface process (29, 30). A similar result on a study of adsorbed tran~-4’,4’’-dipyridyl1,2-ethylene at Hg was attributed to the “deblocking” effect of strongly interacting electroactive centers (31). The narrower than theoretical peak widths are especially interesting in that generally the departure which is observed on modified electrodes is toward broadening (32). The peak potentials vary little with scan rate which indicates that the CN- groups promote the charge transfer at Pt-AMP in the same manner as in the case of the highly reversible, hexacyano system on Pt. However, the peak potential differences in Table I are much greater than the theoretical value (29). Also, the peak shapes of Figure 4 are not symmetrical. These observations may indicate that the oxidized and reduced forms interact in a different manner with Pt-AMP. Further, the peak currents correspond more closely to v1f2-dependencethan to direct proportionality with u; thus, the present study was only sufficient to demonstrate that metallation occurs with Fe(CN)63-/Fe(CN)64-.Further work is required to elucidate the nature of these interactions with Pt-AMP. That Pt-AMP can be metallated by iron in a controlled manner has analytical utility. We have, for example, developed a cathodic stripping voltammetric method for Fe based
Anal. Chem. 1980, 52,864-869
upon the results of this study (33).
LITERATURE CITED (1) Lane, R. F.; Hubbard, A. T. J. Pbys. Cbem. 1973, 77, 1401-1410. (2) Brown, A. P.; Koval, C.; Anson, F. C. J. Nectroanal. Cbem. 1978, 72, 379-387. (3) Moses, P. R.; Wier, L.; Murray, R. W. Anal. Cbem. 1975, 47, 1882-1886. (4) Moses, P. R.; Murray, R. W. J. Am. Chem. SGC. 1978, 98, 7435-7436. (5) Moses, P. R.; Murray, R. W. J. Nectroanal. Cbem. 1977, 77, 393-399. (6) Lenhard, J. R.; Murray, R. W. J. €kcfroanal. Cbem. 1977, 78, 195-201. (7) Elliott, C. M.: Murray, R. W. Anal. Chem. 1978, 48, 1247-1254. (8) Evans, J. F.; Kuwana, T. Anal. Cbem. 1977, 49, 1632-1635. (9) Watkins, B. F.; Behling, J. R.; Kariv, E.; Miller, L. L. J. Am. Cbem. Soc. 1975, 97, 3549-3550. (10) Mazur, S.;Matusinovic, T.; Cammann, K. J. Am. Chem. SOC.1977, 99, 3888-3890. (11) Oyama, N.; Brown, A. P.; Anson, F. C. J. Nectroanal. Cbem. 1978, 87, 435-441. (12) Nowak, R.; Schultz, F. A,; Umana, M.; Abruna, H.; Murray, R. W. J. Nectroanal. Cbem. 1978, 94, 219-225. (13) Oyama, N.; Yap. K. B.; Anson, F. C. J. Nectroanal. Cbem. 1979, 100, 233-245. (14) Lin, A. W. C.; Yeh, P.; Yacynch, A. M.; Kuwana, T. J. Nectroanal. Cbem. 1977, 84, 411-419. (15) Miller. L. L.; Van de Mark, M. R. J. Am. Cbem. Soc. 1978, 700, 639-640. (16) Brown, A. P.; Anson, F. C. J. Nectroanal. Chem. 1977, 83, 203-206. (17) Koval, C. A.; Anson, F. C. Anal. Cbem. 1978, 50, 223-229. (18) Oyama, N.; Anson, F. C. J. Am. Cbem. SOC.1979, 107, 1634-1635.
(19) Oyama, N.; Anson, F. C. J. Am. Chem. Soc. 1979, 101, 739-741. (20) Wier, L. W.; Murray, R. W. J. Necfrochem. SOC.1979, 126, 617-623. (21) Lennox, J. C.; Murray, R. W. J. Am. Chem. SOC. 1978, 100, 37 10-37 14. (22) Rocklin, R. D.; Murray, R. W. J . Electroanal. Cbem. 1979, 100, 271-282. (23) Brabec, V.; Kim, M. H.; Christian, S. D.; Dryhurst, G. J. Nectroanal. Cbem. 1979, 100, 111-133. (24) Angerstein-Kozlowska, H.; Conway, B. E.; Sharp, W. B. A. J. Nectroanal. Cbem. 1973, 43, 9-35. (25) Christie, J. H.; Osteryoung, R. A,; Anson, F. C. J. Electroanal. Cbem. 1987, 13, 236-244. (26) Pearson, R. G. J. Cbem. Educ. 1988, 45, 581-587, 643-648. (27) Martin, R. B.; Mariam. Y. H. In "Metal Ions in Biological Systems", Vol. 8, Sigel, H. Ed.; Marcel Dekker: New York, 1979; Chapter 2. (28) Rabinowitz, I. N.; Davis, F. F.; Herbert, R. H. J. Am. Cbem. Soc. 1988, 88, 4346-4354. (29) Laviron, E. Bull. SOC.Cblm. Fr. 1987, 3717-3721. (30) Laviron. E. J. Electroanal. Cbem. 1979, 100, 263-270. (31) Laviron, E. J. Electroanal. Cbem. 1974, 52, 355-393. (32) Kuo, K.; Moses, P. R.; Lenhard, J. R.; Green, D. C.; Murray, R. W. Anal. Cbem. 1979, 51, 745-748. (33) Cox, J. A.; Majda, M.; unpublished results, 1979.
RECEIVED for review November 19, 1979. Accepted January 29, 1980. Partial support for this work was received from the Eastern European Universities Exchange Program grant from the U.S. State Department to SIU-C.
Fiber Optic pH Probe for Physiological Use John I. Peterson,* Seth
R. Goldstein, and
Raphael V. Fitzgerald
Biomedical Engineering and Instrumentation Branch, Division of Research Services, Building 13, Room 3 W- 13, National Institutes of Health, Bethesda, Maryland 20205
Delwin K. Buckhold Section on Laboratory Medicine and Surgery, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland
20205
A flber optic probe has been developed for pH monitoring, based on the use of a dye indicator. Microspheres of polyacrylamlde containing bound phenol red and smaller polystyrene microspheres for light scattering are packed In an envelope of cellulosic dialysis tublng at the end of a pair of plastic optical flbers. The probe measures pH over the physiological pH range of 7.0 to 7.4 to the nearest 0.01 pH unit. It is of flexible construction and is about 0.4 mm in diameter.
A non-electrical p H probe, based on fiber optics and a dye sensor, offers some advantages in physiological applications over the conventional micro-pH electrodes which are available. An important safety feature for human use is that no possible electrical connection to the body is involved. The use of a plastic optical fiber allows a high degree of mechanical flexibility combined with very small size and low-cost, disposable construction. A probe has been designed which is suitable for tissue and blood p H measurements over the physiological range of pH 7.0 to 7.4 with an accuracy of 0.01 p H unit. The probe can be inserted into tissue or a blood vessel through a 22-gauge (0.41-mm i.d.) hypodermic needle. It can be used for monitoring pH in studies of respiration and tissue oxygenation, and was originally developed for obtaining the pH value necessary
for fixing the position of the blood oxygen saturation curve, which shifts with p H (Bohr effect). The soft construction makes the probe potentially suitable for muscle implantation in exercise experiments. It can also be inserted into a blood vessel for monitoring p H during operative procedures. Principle of pH Measurement. The probe is based on the use of the indicator dye phenol red (phenolsulfonphthalein). In the p H range of interest, this dye behaves as a weak acid of pK 7.9 and exists in two tautomeric forms, each having a different light absorption spectrum (1). As the p H of the solution varies, the relative size of each tautomer's optical absorption peak varies in proportion to the changing relative concentrations of the acid and base forms of the dye. Thus the optical absorption of the dye solution at one of these peak wavelengths can be used for measuring pH. pH can be expressed as a function of the pK of the indicator, the total dye concentration (T),and the concentration of the base form of the indicator (A-): PH = p K
-
log[ -
11
The base form is chosen because its optical density is greater than that of the acid form, thus providing a better optical sensitivity to p H changes. For the purpose of making a fiber optic probe measuring instrument, it was desirable to extend this familiar theory to
This article not subject to U.S. Copyright. Published 1980 by the American Chemical Society
