In borax $\left(\mathrm{Na}_{2} \mathrm{B}_{4} \mathrm{O}_{7} \cdot 10 \mathrm{H}_{2} \mathrm{O}\right),$ among $10$ water molecules $2$ molecules are part of structure, $i \mathrm{e},$ exist $\mathrm{Na}_{2}\left[\mathrm{B_4}  \mathrm{O}_{5}(\mathrm{OH})_{4}\right] \cdot \mathrm{8H}_{2} \mathrm{O}$

$N{a_2}[{B_4}{O_5}{(OH)_4}].8{H_2}O + 2HCl \to 2NaCl + 4{H_3}B{O_3} + 5{H_2}O$
Methyl orange $(\mathrm{pH}=3.7)$ is used to detectend point. Aq solution of borax acts as buffer, as borax is salt of strong base $\mathrm{NaOH}$ and weak acid $\mathrm{H}_{3} \mathrm{BO}_{3}$.