The indicator constant for an acidic indicator, HIn, is 5 ×10^-6 M. This indicator appears only

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The indicator constant for an acidic indicator, HIn, is 5...

The indicator constant for an acidic indicator, HIn, is $5 \times 10^{-6}$ M. This indicator appears only in the colour of acidic form when $\frac{\left[\mathrm{In}^{-}\right]}{[\mathrm{HIn}]} \leq \frac{1}{20}$ and it appears only in the colour of basic form when $\frac{[\mathrm{HIn}]}{\left[\mathrm{In}^{-}\right]} \leq 40 .$ The pH range of indicator is $(\log 2=0.3)$ (a) $4.3-6.3$ (b) $4.0-6.6$ (c) $4.0-6.9$ (d) $3.7-6.6$

The indicator constant for an acidic indicator, HIn, is $5 \times 10^{-6}$ M. This indicator appears only in the colour of acidic form when $\frac{\left[\mathrm{In}^{-}\right]}{[\mathrm{HIn}]} \leq \frac{1}{20}$ and
it appears only in the colour of basic form when $\frac{[\mathrm{HIn}]}{\left[\mathrm{In}^{-}\right]} \leq 40 .$ The pH range of indicator is $(\log 2=0.3)$
(a) $4.3-6.3$
(b) $4.0-6.6$
(c) $4.0-6.9$
(d) $3.7-6.6$

Key Concepts

pH Range of an Indicator

The pH range of an indicator is the interval over which the indicator exhibits a noticeable color change. This range is determined by the ratio of the two forms of the indicator and is typically centered around the indicator’s constant (pKa). The specific ratios at which only one color is visible define the limits of the pH range where the indicator is effective.

Acid?Base Indicators

Acid–base indicators are substances that change color when they undergo protonation or deprotonation, thereby signaling changes in the pH of the environment. They exist in at least two forms, each with a distinct color, and the visible color depends on the relative amounts of these forms present in the solution.

Indicator Constant

The indicator constant, analogous to the pKa in a weak acid, is a measure of the acid–base equilibrium for the indicator. It defines the equilibrium concentration of the two forms, acidic (HIn) and basic (In?), and is crucial in determining the pH at which the indicator changes color.

Henderson–Hasselbalch Equation

The Henderson–Hasselbalch equation relates the pH of the solution to the pKa (or indicator constant) and the ratio of the concentrations of the basic and acidic forms of the indicator. This relationship allows one to calculate the pH at which a particular ratio of the two forms is present, which is key to understanding the indicator’s color transition.

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