Indicators

Fundamentals of Acid-Base Indicators

• Definition of Acid-Base Indicators:

  • Acid-base indicators are chemical substances that undergo a distinct color change depending on the $\text{pH}$ of the surrounding liquid.
  • They are used to determine whether a solution is acidic, neutral, or basic, and to estimate its specific $\text{pH}$ value.

• Litmus Paper Example:

  • Litmus paper is a classic indicator that changes color depending on whether the solution $\text{pH}$ is above or below $7.0$.
  • Red litmus paper turns blue in alkaline conditions ($\text{pH} > 7.0$).
  • Blue litmus paper turns red in acidic conditions ($\text{pH} < 7.0$).      

• Phenolphthalein Example:

  • Phenolphthalein remains completely colorless when dissolved in a liquid with a $\text{pH}$ less than $8.2$
  • Phenolphthalein turns pink (fuchsia) when dissolved in a liquid with a $\text{pH}$ greater than $10.0$

• Diversity of Indicators in Chemistry Laboratories:

  • There are dozens of different chemical indicators available.
  • Standard laboratory inventory includes:
  • Red litmus paper
  • Blue litmus paper
  • Universal indicator paper
  • Universal indicator solution
  • Phenolphthalein
  • Bromothymol blue
  • Methyl orange
  • Bromocresol green
  • Bromophenol blue
  • Most chemistry laboratories maintain stock of between $5$ and $20$ different indicators because each indicator undergoes its characteristic color change across a different, unique $\text{pH}$ range.

Specific Indicator Characteristics and Transition Ranges

• Comprehensive Reference List of Common Indicators:
  • Gentian violet (Methyl violet):
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $0.0$ to $2.0$
  • High $\text{pH}$ color: blue-violet
  • Yellow when $\text{pH} < 0.0$; blue-violet when $\text{pH} > 2.0$
  • Leucomalachite green (first transition):
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $0.0$ to $2.0$
  • High $\text{pH}$ color: green
  • Thymol blue (first transition):
  • Low $\text{pH}$ color: red
  • Transition $\text{pH}$ range: $1.2$ to $2.8$
  • High $\text{pH}$ color: yellow
  • Orange IV:
  • Low $\text{pH}$ color: red
  • Transition $\text{pH}$ range: $1.4$ to $2.8$
  • High $\text{pH}$ color: yellow
  • Methyl yellow:
  • Low $\text{pH}$ color: red
  • Transition $\text{pH}$ range: $2.9$ to $4.0$
  • High $\text{pH}$ color: yellow
  • Bromophenol blue:
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $3.0$ to $4.6$
  • High $\text{pH}$ color: purple / blue
  • Congo red:
  • Low $\text{pH}$ color: blue-violet
  • Transition $\text{pH}$ range: $3.0$ to $5.0$
  • High $\text{pH}$ color: red
  • Methyl orange:
  • Low $\text{pH}$ color: red
  • Transition $\text{pH}$ range: $3.1$ to $4.4$
  • High $\text{pH}$ color: orange / yellow
  • Bromocresol green:
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $3.8$ to $5.4$
  • High $\text{pH}$ color: blue-green
  • Methyl red:
  • Low $\text{pH}$ color: red
  • Transition $\text{pH}$ range: $4.4$ to $6.2$
  • High $\text{pH}$ color: yellow
  • Methyl red / Bromocresol green:
  • Low $\text{pH}$ color: red
  • Transition $\text{pH}$ range: $4.5$ to $5.2$
  • High $\text{pH}$ color: green
  • Azolitmin:
  • Low $\text{pH}$ color: red
  • Transition $\text{pH}$ range: $4.5$ to $8.3$
  • High $\text{pH}$ color: blue
  • Bromocresol purple:
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $5.2$ to $6.8$
  • High $\text{pH}$ color: purple
  • Bromothymol blue:
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $6.0$ to $7.6$
  • High $\text{pH}$ color: blue
  • Phenol red:
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $6.6$ to $8.0$
  • High $\text{pH}$ color: red
  • Neutral red:
  • Low $\text{pH}$ color: red
  • Transition $\text{pH}$ range: $6.8$ to $8.0$
  • High $\text{pH}$ color: yellow
  • Naphtholphthalein:
  • Low $\text{pH}$ color: colorless to reddish
  • Transition $\text{pH}$ range: $7.3$ to $8.7$
  • High $\text{pH}$ color: greenish to blue
  • Cresol Red:
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $7.2$ to $8.8$
  • High $\text{pH}$ color: reddish-purple
  • Is yellow when $\text{pH} < 7.2$
  • Thymol blue (second transition):
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $8.0$ to $9.6$
  • High $\text{pH}$ color: blue
  • Phenolphthalein:
  • Low $\text{pH}$ color: colorless
  • Transition $\text{pH}$ range: $8.3$ to $10.0$
  • High $\text{pH}$ color: fuchsia
  • Thymolphthalein:
  • Low $\text{pH}$ color: colorless
  • Transition $\text{pH}$ range: $9.3$ to $10.5$
  • High $\text{pH}$ color: blue
  • Alizarine Yellow R:
  • Low $\text{pH}$ color: yellow
  • Transition $\text{pH}$ range: $10.2$ to $12.0$
  • High $\text{pH}$ color: red
  • Indigo carmine:
  • Low $\text{pH}$ color: blue
  • Transition $\text{pH}$ range: $11.4$ to $13.0$
  • High $\text{pH}$ color: yellow
  • Leucomalachite green (second transition):
  • Low $\text{pH}$ color: green
  • Transition $\text{pH}$ range: $11.6$ to $14.0$
  • High $\text{pH}$ color: colorless

Analysis of IB Chemistry Data Booklet (Section 18)

• Data Booklet Indicator Table Values:

  • methyl orange: $\text{p}K_a = 3.7$; $\text{pH}$ range $3.1\text{--}4.4$; Acid color: red; Alkali color: yellow
  • bromophenol blue: $\text{p}K_a = 4.2$; $\text{pH}$ range $3.0\text{--}4.6$; Acid color: yellow; Alkali color: blue
  • bromocresol green: $\text{p}K_a = 4.7$; $\text{pH}$ range $4.0\text{--}5.6$; Acid color: yellow; Alkali color: blue
  • methyl red: $\text{p}K_a = 5.1$; $\text{pH}$ range $4.4\text{--}6.2$; Acid color: red; Alkali color: yellow
  • bromothymol blue: $\text{p}K_a = 7.0$; $\text{pH}$ range $6.0\text{--}7.6$; Acid color: yellow; Alkali color: blue
  • phenol red: $\text{p}K_a = 7.9$; $\text{pH}$ range $6.4\text{--}8.0$; Acid color: yellow; Alkali color: red
  • phenolphthalein: $\text{p}K_a = 9.6$; $\text{pH}$ range $8.0\text{--}10.0$; Acid color: colourless; Alkali color: pink

• Misleading Headings and Common Pitfalls:

  • The table column headings "Acid" and "Alkali" in standardized data booklets are often misleading to students.
  • An indicator does NOT turn its "Acid" color simply by being added to any acid ($\text{pH} < 7.0$).
  • Example: Methyl orange is not red in every acidic solution. If added to a solution with a $\text{pH}$ of $5.0$ (which is acidic), methyl orange will be yellow, because yellow appears at any $\text{pH} > 4.4$. It is only red when $\text{pH} < 3.1$
  • An indicator does NOT turn its "Alkali" color simply by being added to any base ($\text{pH} > 7.0$).
  • Example: Phenolphthalein is not pink in every basic solution. In a solution with a $\text{pH}$ of $7.5$ or $8.5$ (which are alkaline), phenolphthalein remains completely colorless. It only becomes pink when $\text{pH} > 10.0$

Spectrum Analysis and Intermediate Colors

• Color Theory at Transition Midpoints:

  • Data tables state the lower and upper limits of color transitions, but do not state the color observed within the intermediate transition range.
  • According to color theory, the color at the midpoint of an indicator's transition is an equal mixture (50:50 blend) of its low-$\text{pH}$ and high-$\text{pH}$ colors.
  • Example: Bromophenol blue is yellow at $\text{pH} < 3.0$ and blue at $\text{pH} > 4.6$. Since green is halfway between yellow and blue, bromophenol blue is green when in a solution with $\text{pH} \approx 4.0$

• Limitations of Data Booklet Cutoff Figures:

  • Text tables make it appear as though indicators change color abruptly at specific discrete numbers (suggesting bromophenol blue would not be yellow at $\text{pH} = 3.05$).
  • Continuous graphical spectrum tables demonstrate that indicators shift color gradually across a continuous range.
  • Bromophenol blue begins changing from yellow to green at $\text{pH} \approx 3.0$, remains mostly yellow at $\text{pH} = 3.05$, passes through green at $\text{pH} \approx 3.7\text{--}4.0$, and completes its transition to blue at $\text{pH} > 4.1\text{--}4.6$

Practical Problem Solving and Deductions

• Single Indicator Conditions:

  • Bromothymol blue is yellow when $\text{pH} < 6.0$
  • Phenol red is red when $\text{pH} > 8.0$
  • Methyl orange is yellow when $\text{pH} > 4.4$
  • Cresol purple is purple when $\text{pH} > 8.4$

• Deduction Problems using Multiple Indicators:

  • Problem 1: A liquid turns blue upon adding bromophenol blue and turns yellow upon adding bromothymol blue. What is its $\text{pH}$?
  • Bromophenol blue turns blue when $\text{pH} > 4.6$
  • Bromothymol blue stays yellow when $\text{pH} < 6.0$
  • Conclusion: The $\text{pH}$ of the liquid is between $4.6$ and $6.0$ ($4.6 < \text{pH} < 6.0$).
  • Problem 2: A liquid turns red upon adding methyl orange and turns red upon adding methyl red. What is its $\text{pH}$?
  • Methyl orange turns red when $\text{pH} < 3.1$
  • Methyl red turns red when $\text{pH} < 4.4$
  • Conclusion: To satisfy both conditions, the $\text{pH}$ must be below $3.1$ ($\text{pH} < 3.1$).
  • Problem 3: To experimentally confirm that a colorless liquid has a $\text{pH}$ close to $5.8$, select valid indicators and predict the observed color:
  • Option A (Bromocresol green): Since $5.8 > 5.4$, adding bromocresol green will make the mixture turn blue.
  • Option B (Bromothymol blue): Since $5.8 < 6.0$, adding bromothymol blue will make the mixture turn yellow.
  • Option C (Methyl red): Since $5.8$ is within the transition range ($4.4\text{--}6.2$), adding methyl red will make the mixture turn orangy-yellow.

Non-pH Indicators

• Indicators also exist for measuring chemical properties other than $\text{pH}$:
  • Humidity Indicators: Dyes that change color based on environmental moisture or humidity levels rather than acidity.
  • Metallochromic Indicators: Indicators that alter color based on the specific concentration of metal ions present in solution.

Higher Level (HL): Chemical Equilibrium and Molecular Structure of Indicators

• Indicator Acid-Base Equilibrium:

  • Indicators are weak organic acids.
  • The general un-ionized acid formula is represented as $\text{HInd}$.
  • The conjugate base anion formula is represented as $\text{Ind}^-$.
  • The equilibrium reaction in aqueous solution is:     $\text{HInd}_{(aq)} \rightleftharpoons \text{Ind}^-_{(aq)} + \text{H}^+_{(aq)}$     or written with water:     $\text{HInd}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{Ind}^-_{(aq)} + \text{H}_3\text{O}^+_{(aq)}$
  • When $\text{pH}$ is below the color transmission range, $\text{HInd}$ is the dominant species.
  • When $\text{pH}$ is above the color transmission range, $\text{Ind}^-$ is the dominant species.

• Application of Le Chatelier's Principle:

  • In Acidic Solutions (High $[\text{H}^+]$): Adding acid increases $[\text{H}^+]$. The equilibrium shifts to the left to consume excess $\text{H}^+$, increasing the relative concentration of $\text{HInd}$. The solution displays the color of $\text{HInd}$.
  • In Alkaline Solutions (High $[\text{OH}^-]$): Hydroxide ions react with hydrogen ions ($\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$), lowering $[\text{H}^+]$. The equilibrium shifts to the right to produce more $\text{H}^+$, increasing the relative concentration of $\text{Ind}^-$. The solution displays the color of $\text{Ind}^-$.

• Structural Changes in Indicator Molecules:

  • The $\text{HInd}$ molecular form contains one extra $\text{H}^+$ ion compared to the $\text{Ind}^-$ form.

  • Phenolphthalein Structural Shift:

  • In its protonated form ($\text{HInd}$), phenolphthalein has a closed lactone ring structure and is colorless.

  • Upon deprotonation to form $\text{Ind}^-$ (carboxylate anion form with an open ring structure), phenolphthalein turns pink.          

  • Methyl Orange Structural Shift:

  • $\text{HInd}$ (protonated form, dominant at low $\text{pH}$) is red.

  • $\text{Ind}^-$ (deprotonated form, dominant at high $\text{pH}$) is yellow.

  • Adding $\text{H}^+$ in strong acid attaches a proton to the azo nitrogen system, converting the yellow $\text{Ind}^-$ molecule into the red $\text{HInd}$ molecule.

• Mathematical Relationship Between Midpoint $\text{pH}$ and Indicator $\text{p}K_a$:

  • The acid dissociation constant ($K_a$) expression for an indicator is:     $K_a = \frac{[\text{H}^+][\text{Ind}^-]}{[\text{HInd}]}$
  • At the exact midpoint of the indicator's color transition, the concentration of the acidic form equals the concentration of the conjugate base form:     $[\text{HInd}] = [\text{Ind}^-]$
  • Substituting $[\text{HInd}] = [\text{Ind}^-]$ into the $ $K_a expression:\n    K_a = \frac{[\text{H}^+][\text{Ind}^-]}{[\text{Ind}^-]} = [\text{H}^+]\n - Taking the negative logarithm (-\log_{10}) of both sides:\n    \text{p}K_a = \text{pH}\n - **Rule:** The middle of an indicator's color change occurs precisely at a \text{pH}$equal to its$\text{p}K_a\n\n- **Estimating \text{p}K_a Values from Transition Data:**\n - Methyl orange has a \text{p}K_a = 3.7$; its transition range is$3.1 ext{--}4.4$, with its midpoint at$\text{pH} = 3.7\n - Bromophenol blue has a \text{p}K_a = 3.85$(or$4.2$in standard data tables); its transition range is$3.0 ext{--}4.6\n - Thymolphthalein has a transition range of 9.3 ext{--}10.5$; its$\text{p}K_a$is estimated to be approximately$10.0$$