\({{E'}_{cell}}\,E_{cell}^ \circ \, - \,\frac{{0.059}}{2}\log \frac{{[10C{u^{ + 2}}]}}{{{{[10A{g^ + }]}^2}}}\)
\( \Rightarrow \,\,{{E'}_{cell}}\, = \,\,E_{cell}^ \circ \, - \,\,0.0295\,V.\)
$H_3PO_4 + OH^- \rightarrow H_2PO_4^- + H_2O$ ;
$H_3PO_4 + 2OH^- \rightarrow HPO_4^{2-} + 2H_2O$ ;
$H_3PO_4 + 3OH^- \rightarrow PO_4^{3-} + 3H_2O$