The figure shows the effect of pHpH on the response of a nitrite (NO2-) (NO^-₂) ion-selective electrode immersed in 1 μM NaNO2+1 mM NaCLl. 1 μM NaNO2+1 mM NaCLl. The authors recommend using the flat region near _pH5.5pH5.5 for measuring nitrite. To a first approximation, we expect the electrode to respond to [NO2-][NO^-₂] by the equation E=constant-(59 mV) log [NO2-].E = constant - (59 mV) log [NO^-2].
a. Look up _pKapK_a for nitrous acid and explain the behavior of the curve at _pH<4.pH < 4.
b. Suggest an explanation for the behavior at _pH>6.pH > 6.

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Electrode potential (mV) 500 450 400 350 300 250 3 4 5 6 pH 7 8 9