What is the equation for solving the concentration of the unknown from a Single-point Standard Addition Calibration for Potentiometry?
The little problem with the standard addition in potentiometry is, that the response to the addition is not linear, but logarithmic.

If we consider the simplified Nernst equation in context of parameters $A, B$,

and if we consider $c, c_\mathrm{0}$ as unknown concentration and  concentration increment, respectively, 

we can follow this derivation:

$$\begin{align}
E_\mathrm{1} &amp;= A + B \cdot \log c\\
E_\mathrm{2} &amp;= A + B \cdot \log ( c + c_\mathrm{0} )\\
E_\mathrm{2}- E_\mathrm{1} &amp;= B . \log( 1 + \frac {c_\mathrm{0}}{c} )\\
10^{\frac{E_\mathrm{2}- E_\mathrm{1}}{B}} &amp;= 1 + \frac {c_\mathrm{0}}{c}\\
c &amp;= \frac {c_\mathrm{0}}{10^{\frac{E_\mathrm{2}- E_\mathrm{1}}{B}} -1}
\end{align}$$ 

where $B = \frac{RT}{nF} \cdot \ln(10)$