Diphenylamine indicator is used in the titration of ${\mathrm{Fe}}^{2+}$ ions versus ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}\text{ ion }$ in the phesence of phosphoric acid. The function of the latter is to 

Phosphoric acid forms complex with ${\mathrm{Fe}}^{3+}$ ions. Its concentration in solution is decreased. Consequently, $E^{\circ }\left({\mathrm{Fe}}^{3+},{\mathrm{Fe}}^{2+}\mid \mathrm{Pt}\right)$ is decreased as required by the equation

$E_{{\mathrm{Fe}}^{3+},{\mathrm{Fe}}^{2+}\mid \mathrm{Pt}}=E_{{\mathrm{Fe}}^{3+},{\mathrm{Fe}}^{2+}\mid \mathrm{Pt}}^{\circ }-\frac{RT}{nF}\ln \left(\frac{\left[{\mathrm{Fe}}^{2+}\right]}{\left[{\mathrm{Fe}}^{3+}\right]}\right)$