Question

Methyl orange changes from red at pH 3.1 to yellow at pH 4.4. Would methyl orange be a suitable indicator for the titration of propanoic acid (pKa=4.87, Ka=1.34x10^-5) with 0.01 M NaOH? Explain. Phenolphthalein changes from colorless at pH 8.0 to pink at pH 9.6. Would phenolphthalein be a suitable indicator for the titration of propanoic acid with 0.01 M NaOH? Explain.

Answer 1

Indicator- is a dye(substance) which changes colour over a short pH range.

an indicator is usually weak acid or weak base for acid-base titration.the ccolour change occurs when a proton transferred to form a conjugate base or a conjugate acids of a different colour

Methyl orange (MeOH) is represents as

MeOH Me⁺ + OH^-

(Yellow in base) (Red in acid)

When a strong acid is added,equilibrium of MeOH shifts towards the right and the colour change Red is detected.

Phenolphthalein (PhH)

PhH    Ph⁺ + H⁺

(Colourless) (pink in base)

PhH is a colourless in acid solution and pink in basic solution. When a strong base such as NaOH is added it displaces the equilibrium towards right direction and there is a colur change.

As in given question titration of propanoic acid(weak acid as organic acid mostly weak) and NaOH(strong base),the pH range near the end point is 8-10.since the pH lies on alkaline side.phenolphthalein (pH range 8-9.6 or 9.8) and thymol blue(pH range 8.6-9.6)are suitable indicators for such titrations.methyl orange (pH range 3.1-4.4)cannot be used for such titrations.also propanoic acid is weak acid does not furnish sufficient number of H⁺ ions.these H⁺ ions is not sufficient to combine with sufficient number of OH^- ions of methyl orange to form water and to shift the equlibrium in favour of red colouration Me⁺ ions.

MeOH Me⁺ + OH^-

CH₃CH₂COOH   CH₃CH₂COO^- + H ⁺

CH₃CH₂COO^- + Me⁺ CH₃CH₂COOMe

H⁺ + OH^-    H₂O

However,the salt of CH₃CH₂COOMe does not furnish sufficient red ions because of hydrolysis

CH₃CH₂COOMe + H₂O CH₃CH₂COOH + MeOH