Why is methyl orange used as an indicator in the titration of HCl and borax
Methyl orange is used as an indicator in the titration of HCl and borax because it undergoes a color change at the pH range of the equivalence point of this titration.
In the HCl and borax titration, HCl is used as the titrant and borax is the analyte. HCl is a strong acid, and when it reacts with borax, it forms boric acid and sodium chloride:
HCl + Na2B4O7 → NaCl + H3BO3
Boric acid is a weak acid, and its conjugate base, borate ion, is a weak base. At the pH range near the equivalence point, the borate ion is partially ionized, resulting in a slightly basic solution.
Methyl orange is a pH indicator that has a transition range of pH 3.1-4.4. It is yellow in acidic solutions (pH<3.1) and red in basic solutions (pH>4.4).
During the titration, as HCl is added to the borax solution, it reacts with the borate ion, thus increasing the acidity of the solution. The solution starts off basic (due to the presence of a weak base), but as more HCl is added, it becomes increasingly acidic. At the equivalence point, when the moles of HCl added equals the moles of borax, the solution is acidic enough to cause a color change in methyl orange from yellow to red.
Therefore, methyl orange is used as an indicator in the titration of HCl and borax because it allows us to visually detect when the equivalence point is reached, making it easier to determine the concentration of borax in solution.
Methyl orange is often used as an indicator in the titration of HCl (hydrochloric acid) and borax (sodium borate) because it undergoes a color change within the pH range of this titration.
In the presence of an acidic solution, such as HCl, methyl orange appears red. This is because the acidic solution provides an excess of H+ ions, which react with the indicator to form the red-colored form of methyl orange.
On the other hand, in the presence of a basic solution, such as borax, the solution becomes alkaline and the excess of OH- ions causes methyl orange to adopt a yellow color.
Therefore, during the titration process, as the HCl is gradually added to the solution containing borax, a point is reached where all the borax has been neutralized and the solution becomes slightly acidic. At this point, a color change from yellow to red is observed, indicating the end point of the titration.
Methyl orange is a suitable indicator for this titration because its color change occurs in the pH range where the solution changes from alkaline to slightly acidic, which corresponds to the point of neutralization of borax by HCl.