Optrodes in Dilute Solutions

In the optrode for calcium ion, a so-called optical ionophore is immobilized in a 

It has been suggested that such a sensor can be used for the accurate measurement of Ca2+ activity in dilute sample solutions because, in dilute solutions, activity coefficients tend to unity. Is that statement correct or not  

Does the signal in an opt(r)ode for calcium ion depend on the thickness of the membrane containing the calcium chromophore How does it compare with the thickness dependence of the signal in a calcium ion-selective electrode containing calcium ionophore  

Is there a fundamental difference in the nature of the time response of these two types of sensors  

(1969) Interactions in dipolar aprotic solvents. In Coetzee, J.F. and Ritchie, C.D. (Eds.) 

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