Question 14.E.D: One glass-membrane sodium ion-selective electrode has a sele...
One glass-membrane sodium ion-selective electrode has a selectivity coefficient K_{Na^{+}, H^{+}}^{Pot} = 36. When this electrode was immersed in 1.00 mM NaCl at pH 8.00, a potential of −38 mV (versus S.C.E.) was recorded.
(a) Neglecting activity coefficients, calculate the potential with Equation 14-10 if the electrode were immersed in 5.00 mM NaCl at pH 8.00.
E = constant ± \frac{0.059 16}{\mathcal{z}_{A}}\log[\mathcal{A}_{A} + \sum\limits_{x}{K_{A,X}^{Pot}}\mathcal{A}_{X}] (14-10)
(b) What would the potential be for 1.00 mM NaCl at pH 3.87? You can see that pH is a critical variable for the sodium electrode.
(a) For 1.00 mM Na^{+} at pH 8.00, we can write
E = constant + 0.059 16 \log ([Na^{+}] + 36[H^{+}]) − 0.038
= constant + 0.059 16 \log [(1.00 × 10^{−3}) + (36 × 10^{−8})]
⇒ constant = +0.139 V
For 5.00 mM Na^{+} at pH 8.00, we have
E = +0.139 + 0.059 16 \log [(5.00 × 10^{−3}) + (36 × 10^{−8})]
= 0.003 V
(b) For 1.00 mM Na^{+} at pH 3.87, we have
E = +0.139 + 0.059 16 \log [(1.00 × 10^{−3}) + (36 × 10^{−3.87})]
= 0.007 V