Anal. Chem. 2007, 79, 2186

Comment on “Disposable Receptor-Based Optical Sensor for Nitrate” One of the major points of this paper1 is on the combination of the coextraction equilibriums with/without ionophore in the nitrate-selective optode sensor. The authors attempted to describe a mixed model in the membranes containing process without ionophore resulting in dissociated nitrate, which provides a new concept for the coextraction reaction taking place in the optode sensor. Unfortunately, a number of problems in the deduction of the equations have been detected. One of these errors is the definition of the symbol R. Since the authors have illustrated that R is the concentration ratio of deprotonated chromoionophore to total chromoionophore ([C]/Cc), R denotes the degree of deprotonation instead of the protonated degree the authors defined. The second and more important problem appears in the calculation of a co-extraction constant (Ke2). According to the equilibrium C h + H+ + NO3- T HC+ + NO3- T HC+NO3-, this constant should be calculated in two steps. Step 1: Protonation of the Chromoionophore and Coextraction of Nitrate Ion. In this step, the coextracted nitrate ion is in free form. The coextraction constant (Ke2) is calculcated according to

Ke2 )

[HC+][NO3-] [C][H+][NO3-]

f Ke2 )

[(1 - R)Cc]2 f RCcaH+aNO3-

Ke2 )

(1 - R)2Cc 1 (1) R aH+aNO3-

Step 2: Combination of Free Nitrate Ion and Protonated Chromoionophore. When the protonation degree of the chromoionophore is higher than 0.2 (i.e., R <0.8), the equilibrium between the free nitrate ion coextracted into the optical sensor and a complex of protonated chromoionophorenitrate needs to be considered. The coextraction constant (Ke2) equals the one that Capitan-Vallvey et al. reported, i.e. Ke2 ) (1 - R)/RaH+aNO3-. Obviously, Figure 1 shows that data (obtained from Figure 2B in the original paper) fit the solid line (0.8 < R < 1) and dotted line (0 < R < 0.8) in terms of the two-step mechanism very well, especially at pH 6. As mentioned in the Preparation of Disposable Membranes and Measurement Setup section, the concentration of total * Corresponding author. E-mail: [email protected]. Tel.: +1 418 7231986-1764. Fax: +1 418 7241842. (1) Capita´n-Valley, L. F.; Arroyo-Guerrero, E.; Ferna´ndez-Ramos, M.; SantoyoGonzalez, F. Anal. Chem. 2005, 77, 4459-4466.

2186 Analytical Chemistry, Vol. 79, No. 5, March 1, 2007

Figure 1. Response of the optical sensor without ionophore at pH 4, 5, and 6.

chromoionophore (Cc) is 1.71 × 10-3 mol/kg (i.e., 1.95 × 10-3 mol/L considering the average density of the membrane). At low protonation degree (R > 0.8), log Ke2 of the optical sensor without ionophore is 3.71. When the protonation degree of chromoionophore is higher than 0.2 (R < 0.8), log Ke2 is 6.90 as reported. By the way the results of Ke1 and Ke2 reported in the discussion section (p 4463) should correctly be the logarithmic forms of Ke1 and Ke2; otherwise, the performance of optical sensor is incredibly different. Consequently, to explain the mix behavior of the nitrate optical sensor containing ionophore, our calculation of Ke2 (eq 1) leads to a different expression of Ke3 (see eq 2) when the protonation degree of the chromoionophore is lower than 0.2.

Ke3 )

(1 - R)4Cc 1 ‚ 2 C R L - (1 - R) aH+2aNO3-2 Cc

(

)

(2)

Considering the application of the optical sensor and the concentration of nitrate ion in real samples that is generally lower than 1 mM, the response of the optical sensor to low concentrations (i.e., low protonation degree of chromoionophore) is more important. Therefore, it is necessary to set a reasonable equilibrium that can exactly fit the data in the whole region and to present a precise coextraction constant to estimate the performance of the optical sensor.

Ying Yu Hu and EÄ milien Pelletier*

Institut des sciences de la mer (ISMER), Universite´ du Que´ bec a` Rimouski, Rimouski, QC G5L 3A1, Canada AC061012R 10.1021/ac061012r CCC: $37.00

© 2007 American Chemical Society Published on Web 02/06/2007