11.02 Indicators
1. Indicators
1.1 What are Indicators?
- Indicator: A substance that changes colour when added to acidic or alkaline (basic) solutions.
- Purpose: To detect whether a solution is acidic, neutral, or alkaline based on observable colour changes.
1.2 Common Indicators and Their Colour Changes
| Indicator | Colour in Acidic Solutions | Colour in Neutral Solutions | Colour in Alkaline Solutions |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Thymolphthalein | Colourless | Colourless | Blue |
| Methyl Orange | Red | Orange | Yellow |
| Universal Indicator | Red (highly acidic) to Yellow (weakly acidic) | Green (neutral) | Blue to Violet (alkaline) |
1.3 Detailed Descriptions of Common Indicators
1.3.1 Litmus
- Source: Extracted from lichens.
- Forms: Available as a liquid solution and as litmus paper (blue and red).
- Colour Changes:
- Blue Litmus Paper: Turns red in acidic solutions.
- Red Litmus Paper: Turns blue in alkaline solutions.
- Usage Note: Litmus provides a single colour change, making it simple but limited in precision.
1.3.2 Thymolphthalein
- Colour Changes:
- Acidic Solutions: Colourless.
- Alkaline Solutions: Blue.
- Usage: Useful in titrations where a clear colour change is needed.
1.3.3 Methyl Orange
- Colour Changes:
- Acidic Solutions: Red.
- Alkaline Solutions: Yellow.
- Usage: Provides distinct colour changes, making it easier to detect pH changes compared to litmus.
1.3.4 Universal Indicator
- Composition: A mixture of several indicator dyes.
- Colour Spectrum: Displays a range of colours corresponding to different pH levels.
- Highly Acidic (pH 0-3): Red.
- Weakly Acidic (pH 4-6): Orange to Yellow.
- Neutral (pH 7): Green.
- Weakly Alkaline (pH 8-10): Blue.
- Highly Alkaline (pH 11-14): Violet.
- Usage: Allows for precise determination of the pH level based on the observed colour.
1.4 Practical Considerations
- Presence of Water: Essential for indicators to function correctly. For example, when testing gases with litmus paper, the paper must be damp to allow the gas to dissolve and interact with the indicator.
- Limitations: Litmus only provides a single colour change, whereas indicators like methyl orange and universal indicator offer a range of colours for more accurate pH determination.
2. The pH Scale
2.1 Understanding the pH Scale
- Definition: A logarithmic scale ranging from 0 to 14 used to measure the acidity or alkalinity of a solution.
- Reference Point: Neutral pH is 7.
- pH < 7: Acidic.
- pH > 7: Alkaline (Basic).
- Logarithmic Nature: Each whole pH unit represents a tenfold change in hydrogen ion (H⁺) concentration.
- Example: pH 1 is ten times more acidic than pH 2.
2.2 pH Scale Classification
| pH Range | Nature of Solution | Universal Indicator Colour |
|---|---|---|
| 0-3 | Highly Acidic | Red |
| 4-6 | Weakly Acidic | Orange to Yellow |
| 7 | Neutral | Green |
| 8-10 | Weakly Alkaline | Blue |
| 11-14 | Highly Alkaline | Violet |
2.3 Measuring pH
- Indicator Papers:
- Universal Indicator Paper: Sensitive over the full pH range, showing a spectrum of colours.
- Narrow-Range Indicator Papers: Provide more accurate pH measurements within a specific range.
- pH Meter:
- Description: An electronic device that measures the pH of a solution using a special electrode.
- Advantages: Highly accurate and precise, suitable for laboratory use.
- Practical Tips:
- Always use a neutral reference point (pH 7) when measuring.
- Ensure indicator papers or solutions are fresh and uncontaminated for accurate results.
2.4 Examples of pH Values in Common Solutions
| Solution | pH Value | Nature |
|---|---|---|
| Highly Acidic: | ||
| Hydrochloric Acid (HCl) | 0.0 | Highly Acidic |
| Gastric Juices | 1.0 | Highly Acidic |
| Acidic Solutions: | ||
| Lemon Juice | 2.5 | Weakly Acidic |
| Vinegar | 3.0 | Weakly Acidic |
| Acid Rain | 4.4 | Weakly Acidic |
| Rainwater | 5.6 | Slightly Acidic |
| Urine | 6.0 | Slightly Acidic |
| Sour Milk | 6.5 | Weakly Acidic |
| Neutral Solutions: | ||
| Pure Water | 7.0 | Neutral |
| Sugar Solution | 7.0 | Neutral |
| Alkaline Solutions: | ||
| Blood (poorly alkaline) | 7.4 | Slightly Alkaline |
| Baking Soda Solution | 8.5 | Moderately Alkaline |
| Toothpaste | 9.0 | Moderately Alkaline |
| Limewater | 11.0 | Strongly Alkaline |
| Household Ammonia | 12.0 | Strongly Alkaline |
| Sodium Hydroxide (NaOH) | 14.0 | Highly Alkaline |
Note: The pH scale is logarithmic. Each unit change represents a tenfold difference in H⁺ ion concentration. For example, a solution with pH 1.0 is ten times more acidic than one with pH 2.0.
3. Key Vocabulary
| Term | Definition |
|---|---|
| Indicator | A substance that changes colour when added to acidic or alkaline solutions. |
| Litmus | A common indicator extracted from lichens; turns red in acids and blue in bases. |
| Thymolphthalein | An acid-base indicator that is colourless in acidic solutions and blue in alkaline solutions. |
| Methyl Orange | An acid-base indicator that is red in acidic solutions and yellow in alkaline solutions. |
| Universal Indicator | A mixture of indicators that changes colour across a wide pH range, allowing for precise pH determination. |
| pH Scale | A scale from 0 to 14 used to measure the acidity or alkalinity of a solution; neutral solution has a pH of 7. |
| Acid | A substance that donates H⁺ ions in water, turns blue litmus red, and has a pH below 7. |
| Base (Alkali) | A substance that accepts H⁺ ions in water, turns red litmus blue, and has a pH above 7. |
| Alkali | Soluble bases that produce OH⁻ ions in water. |
| Corrosive | A substance that can dissolve or “eat away” at materials, including metals, skin, and clothing. |
| Proton Donor | Another term for acids, referring to their ability to donate H⁺ ions. |
| Proton Acceptor | Another term for bases, referring to their ability to accept H⁺ ions. |
| Logarithmic Scale | A scale in which each unit represents a tenfold increase or decrease. |
4. Practical Applications of Indicators and the pH Scale
4.1 Detecting Acidity and Alkalinity
- Using Litmus Paper:
- Blue Litmus Paper: Turns red in acidic solutions.
- Red Litmus Paper: Turns blue in alkaline solutions.
- Using Thymolphthalein and Methyl Orange:
- Thymolphthalein: Useful for titrations requiring clear colour changes.
- Methyl Orange: Offers distinct transitions, aiding in accurate pH determination.
- Using Universal Indicator:
- Full Spectrum Detection: Allows for precise pH measurement by matching the observed colour to the pH scale.
4.2 Importance in Daily Life and Industry
- Everyday Uses:
- Cooking: Use of vinegar (ethanoic acid) and lemon juice (citric acid) for flavouring.
- Cleaning: Use of strong mineral acids like hydrochloric acid for cleaning metal surfaces.
- Health: Stomach acid (hydrochloric acid) aids in digestion; toothpaste contains mild bases to neutralize mouth acids.
- Industrial Applications:
- Manufacturing Fertilizers: Sulfuric acid and nitric acid are key components.
- Batteries: Sulfuric acid is used in car batteries.
- Detergents and Paints: Bases are used in formulations for cleaning and anti-rust properties.
Examples:
Question 1
a. What do you understand by the word corrosive?
Answer:
- Corrosive: A substance that can dissolve or “eat away” at other materials, such as metals, wood, skin, or clothing. Both strong acids and strong bases are typically corrosive.
b. Which acid is present in orange or lemon juice?
Answer:
- Citric Acid (C₆H₈O₇): Present in citrus fruits like oranges and lemons, giving them their sour taste.
c. What acid is present in vinegar?
Answer:
- Ethanoic Acid (CH₃COOH): The primary acid in vinegar, responsible for its characteristic sour taste.
e acidic than pH 3, making it significantly more acidic than pH 4.
Question 2
a. Methyl orange is an indicator. What does this mean?
Answer:
- Methyl Orange as an Indicator: It means that methyl orange changes colour when added to acidic or alkaline solutions. Specifically, it turns red in acidic solutions and yellow in alkaline solutions, allowing for the detection of pH changes.
b. Is a solution acidic, alkaline, or neutral if its pH is:
i. 7
- Answer: Neutral. A pH of 7 indicates a neutral solution, such as pure water.
ii. 8
- Answer: Alkaline. A pH of 8 is above 7, indicating a slightly alkaline (basic) solution.
iii. 11
- Answer: Alkaline. A pH of 11 is strongly alkaline, indicating a highly basic solution.
iv. 3
- Answer: Acidic. A pH of 3 is below 7, indicating a strongly acidic solution.
Question 3
Which solution is more acidic: an acid with a pH of 4 or an acid with a pH of 1?
Answer:
An acid with a pH of 1 is more acidic than an acid with a pH of 4. On the pH scale, lower values indicate higher acidity. Specifically, each unit decrease represents a tenfold increase in hydrogen ion concentration. Therefore, pH 1 is ten times more acidic than pH 2 and one hundred times more.
Quizzes
Quiz 1