Semi-active optodes

Dye Theoretical pA", pK measured with passive optode Semi-active optode References... 

The techniques most frequently employed are absorption, reflectance and fluorescence, with some attempts to exploit the variation in refractive index of the sensitive polymer acting as the cladding for the fiber. The general principle of semi-active and active optodes (Chapter 12) is applied. The optical spectrum of an optode is modified by chemical action between a reagent (indicator), immobilized on the fiber or an inert support at its end, and the activity of the H ions (Oh )=... 

The pH range of the dyes is narrow (3 pH units). The best performance (accuracy <0.01) is obtained in a narrow zone, generally 0.02 units around the pK, . The accuracy decreases sharply with increasing distance from the pK (the relative intensity curves f(pH) are nonlinear). In the outer zones ( 1.5 pK units), the accuracy hardly exceeds 0.1 pH units. The use of several p J, values with the same indicators requires differentiated spectral responses for the two species present [12, 14, 36). New dyes are under development [58, 59] to broaden the pH range with a single indicator for several pIC values and for wider spectra. Also, the pIC is different in solution and for a semi-active optode as presented in Figure 17-6 [35]. A summary of theoretical pK and pK values for some optodes is given in Thble 17-3. 

Ihble 17-3. values for passive (without immobilization of dyes) and semi-active (active-extrinsic) optodes... 

With active and semi-active optodes, the chemical species (S) to be measured (analyte) reacts with a reagent phase (R. This reaction forms another species (SR according to the reaction... 

The main research projects for determining cations are on semi-active optodes. The measurement of trace species (Pu , UO, rare earths) in the environment is dealt with in Chapter 18 and optodes for continuous monitoring (Cu"), U and Pu valencies) in Section 17.3. 

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