Potentiometric conundrum

I'm trying to develop a simple method of analysis based on the half-reaction:


$$\ce{Zn^2+(aq) + 2e- -> Zn(s)}$$


Making up the cell is a $\ce{Ag/AgCl}$ reference electrode. The cell is only $\pu{5 ml}$.


**Dress rehearsal:**


[![ZnAgAgCl cell](https://i.stack.imgur.com/ufnQq.jpg)](https://i.stack.imgur.com/ufnQq.jpg)


I measured the EMF of $4$ zinc sulfate stock solutions from $\pu{0.5 M}$ to $\pu{1.6E-3 M}$. Each point was replicated once. I obtained the following correlation:


[![Potentiometry](https://i.stack.imgur.com/Yvocu.png)](https://i.stack.imgur.com/Yvocu.png)


Using $\log\_{10}$ I got a slope of:


$$\pu{8.2\pm 2.2 mV}$$


But using Nernst's Equation at $\pu{298 K}$ the value should be about $\pu{28 mV}$, **so the discrepancy is enormous!** And this means reduced sensitivity, of course.


Additional problems which may or may not be related to the first problem are:


* reproducibility wasn't great
* long equilibrium times (over $5\text{min}$)
* $\ce{ZnSO4}$ turned out of poor quality


**Does anyone know what causes the low slope problem?**


