Anal. Chem. 2007, 79, 2184-2185
Response to a Comment on “Disposable Receptor-Based Optical Sensor for Nitrate” In a comment on our paper,1 Drs. Hu and Pelletier suggest that it contains some problems related to the calculation of the coextraction constant Ke2. In short, this constant Ke2 justifies the observed extraction of nitrate to the plasticized polymeric sensing membrane in the absence of ionophore, due to the coextraction of nitrate and proton from the solution in the presence of the basic chromoionophore present in the membrane. To justify the experimental results without nitrate ionophore, we used eq 1. with R being the deprotonation
Ke2 )
1-R
(1)
RaH+aNO3-
degree of chromoionophore. This equation comes from the supposition that a single species is present in the membrane, according to +
-
+
-
C h + H + NO3 T HC NO3
Figure 1. Disposable sensor response of membranes containing ionophore at (a) pH 4.0; (b) pH 5.0; (c) pH 6.0; (d) pH 7.0; (e) pH 8.0; and (f) pH 9.0. Dotted line corresponds to equation proposed by Hu and Pelletier (eq 4).
the two proposals. Namely, eq 3 would explain experimental data for R values between 1 > R > 0.8 and eq 4 for R < 0.8 according to Pelletier’s proposal. In the case that our mixed model works, eq 4 would be valid in the whole R range.
Ke3 )
(1 - R)4CC
(2)
Drs. Hu and Pelletier agree with us at R < 0.8 values, but they suggest that, for the range 1 > R > 0.8 (i.e., low nitrate activity), the process that occurs is
R
2
Ke3 ) R2
(
1 CL 2 2 - a + - (1 - R) aNO H 3 CC
(
)
(1 - R)3
)
CL 2 2 - a + - (1 - R) aNO H 3 CC
(3)
(4)
C h + H + + NO3- T HC+ + NO3which implies a quadratic dependence on 1 - R (eq 2). Indeed,
Ke2 )
(1 - R)2CC 1 ‚ + R aH aNO3-
(2)
its seems that, at low nitrate activity, the experimental data fit the new equilibrium suggested better, as shown in Figure 1 of Pelletier’s comment based on our experimental data. This possibility, along with others, was considered, tested, and finally rejected by us when we were working on this sensing nitrate system. However, if the proposed hypothesis were true, in the presence of nitrate ionophore in membrane, according to the mixed model proposed in our paper,1 the dependence of 1 R with the logarithm of nitrate activity would be different in both cases and would make it possible to distinguish between (1) Capita´n-Vallvey, L. F.; Arroyo-Guerrero, E.; Fernandez-Ramos, M. D.; SantoyoGonzalez, F. Anal. Chem. 2005, 77, 4459-66.
2184 Analytical Chemistry, Vol. 79, No. 5, March 1, 2007
Data obtained from the experiments, already presented in the paper,1 with membranes containing the nitrate ionophore, clearly show (Figure 1) a good fit to eq 4 in the whole activity range studied at 4 and 5 pH values. For this reason, we do not consider it necessary to accept the more complicated two-step model suggested or to modify the Ke2 and Ke3 constants. In the conclusion to their comments, Drs. Hu and Pelletier point out that, for application of the proposed sensor, especially for the determination of low nitrate concentrations, it is necessary to consider the two-step model proposed by them and the equations derived from it. However, the analytical procedure for nitrate proposed by us works at pH 6 and does not require the use of the mixed model or, consequently, the two-step model, to explain the experimental data. As we indicated in our paper, at pH values from 6 on, the extraction of nitrate to membrane in the absence of ionophore only occurs at high nitrate activity, so it is not necessary to consider the mixed model at pH g6, and a simple coextraction model given by eq 5 works well. The mixed model is only necessary to justify experimental data at pH e5. 10.1021/ac061866b CCC: $37.00
© 2007 American Chemical Society Published on Web 11/17/2006
Ke1 ) R
(
(1 - R)2
)
CL - (1 - R) aNO3-aH+ CC
(5)
We use this pH value (6.0) as the working pH to determine nitrate in water because the sensing system shows enough sensitivity together with minimization of nitrate coextraction without ionophore. Consequently, it is not necessary, in our opinion, to modify the coextraction constant Ke1 (logKe1: 9.06 ( 0.04) calculated from experimental data working at pH 6 because it fits the simple coextraction model (eq 5). The goodness of this simple coextraction model at pH 6 was later checked in a study of linearization of the sigmoidal calibration curve for nitrate determination based on a decimal logistic transformation.2 In this case, a good fit of the linearized theoretical model to the experimental data was observed. There is a typewriting error in the definition of the analytical parameter R used with this type of sensor. In the paper, we (2) Capita´n-Vallvey, L. F.; Arroyo-Guerrero, E.; Fernandez-Ramos, M. D.; CuadrosRodriguez, L. Anal. Chim. Acta 2006, 561, 156-63.
define the deprotonation degree R correctly, but it appears written in text as the protonation degree. ACKNOWLEDGMENT We thank Drs. Hu and Pelletier for their interest in our work and their efforts to reach a better understanding of the underlying equilibriums in this bulk membrane sensor.
Luis Fermı´n Capita´n-Vallvey,*,† Eduardo Arroyo-Guerrero,† Marı´a Dolores Ferna´ndez-Ramos,† and F. Santoyo-Gonzalez‡
Departments of Analytical Chemistry and Organic Chemistry, Faculty of Sciences, Campus Fuentenueva, University of Granada, Granada 18071, Spain Received for review October 4, 2006. Accepted October 11, 2006. AC061866B * Corresponding author. E-mail:
[email protected]. † Department of Analytical Chemistry. ‡ Department of Organic Chemistry.
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