Unit 5: Indicators & the pH Scale
Measuring acidity and alkalinity using the pH scale and chemical indicators.
5.22 The pH Scale (Acidity, Alkalinity, Neutrality)
The pH scale is a numerical scale used to specify the acidity or basicity (alkalinity) of an aqueous solution. It is a measure of the concentration of hydrogen ions ($H^+$) in a solution. The scale typically ranges from 0 to 14.
- A solution with a pH less than 7 is considered acidic. The lower the pH, the higher the concentration of $H^+$ ions and the stronger the acidity.
- A solution with a pH of exactly 7 is considered neutral. In a neutral solution, the concentration of $H^+$ ions is equal to the concentration of $OH^-$ ions (as in pure water).
- A solution with a pH greater than 7 is considered basic or alkaline. The higher the pH, the lower the concentration of $H^+$ ions and the higher the concentration of $OH^-$ ions.
Examples of common substances:
- Stomach Acid (pH 1-2)
- Lemon Juice (pH 2)
- Vinegar (pH 3)
- Rainwater (pH 5-6)
- Pure Water (pH 7)
- Baking Soda Solution (pH 9)
- Household Bleach (pH 12-13)
- $1M$ NaOH (pH 14)
Solved Examples:
- A solution has a pH of 4. Is it acidic, basic, or neutral?
Solution: Acidic, because the pH is less than 7. - Which is more acidic: a solution with pH 3 or a solution with pH 6?
Solution: The solution with pH 3 is more acidic. It has a higher concentration of $H^+$ ions. - What is the pH of a neutral solution?
Solution: 7. - A sample of soap solution has a pH of 10. Is it acidic or alkaline?
Solution: Alkaline (basic). - Does an acidic solution contain hydroxide ($OH^-$) ions?
Solution: Yes, but the concentration of hydrogen ($H^+$) ions is greater than the concentration of hydroxide ions. - What does a high pH value (e.g., 13) indicate about the concentration of $H^+$ ions?
Solution: It indicates a very low concentration of $H^+$ ions and a very high concentration of $OH^-$ ions. - Arrange the following in order of increasing acidity: pure water, lemon juice, bleach.
Solution: Bleach (most alkaline), pure water (neutral), lemon juice (most acidic). - What is the approximate pH of rainwater? Why is it not neutral?
Solution: Approximately 5-6. It is slightly acidic because atmospheric carbon dioxide dissolves in it to form weak carbonic acid. - A strong acid solution would have a very ___ pH.
Solution: Low (e.g., 0 or 1). - If a solution has a higher concentration of $OH^-$ than $H^+$, what can you say about its pH?
Solution: Its pH will be greater than 7.
5.23 Acid-Base Indicators (Litmus, Phenolphthalein, Methyl Orange)
An acid-base indicator is a substance (usually a weak organic acid or base) that changes colour in response to a change in pH. This colour change allows us to visually determine if a solution is acidic or basic.
The indicator exists in an equilibrium. For a weak acid indicator (HIn), the equilibrium is: $$ \underset{\text{Colour 1}}{\text{HIn(aq)}} \rightleftharpoons H^+(aq) + \underset{\text{Colour 2}}{\text{In}^-(aq)} $$ In an acidic solution (high $[H^+]$), the equilibrium shifts to the left, showing Colour 1. In an alkaline solution (low $[H^+]$), the equilibrium shifts to the right, showing Colour 2.
Different indicators change colour over different pH ranges:
| Indicator | Colour in Acid (Low pH) | Colour in Alkali (High pH) | Approx. pH of Colour Change |
|---|---|---|---|
| Methyl Orange | Red | Yellow | 3.1 - 4.4 |
| Litmus | Red | Blue | 5.0 - 8.0 |
| Phenolphthalein | Colourless | Pink | 8.2 - 10.0 |
Solved Examples:
- What colour is phenolphthalein in a solution with a pH of 4?
Solution: Colourless. Its colour change occurs around pH 8.2-10.0. - A solution turns blue litmus paper red. Is the solution acidic or alkaline?
Solution: Acidic. - What colour would methyl orange be in a solution of sodium hydroxide (pH ~13)?
Solution: Yellow. - A solution is tested with phenolphthalein and remains colourless. What can you conclude about its pH?
Solution: You can conclude that its pH is less than 8.2. It could be acidic or neutral. - Which of the three common indicators would be most suitable for distinguishing between a solution of pH 6 and a solution of pH 9?
Solution: Litmus or phenolphthalein. Litmus would be red/purple at pH 6 and blue at pH 9. Phenolphthalein would be colourless at pH 6 and pink at pH 9. Methyl orange would be yellow in both. - What is the principle behind how an indicator works?
Solution: It is a weak acid or base where the molecule and its conjugate ion have different colours, and the equilibrium between them is sensitive to pH. - A solution turns methyl orange indicator red. What is its approximate pH?
Solution: The pH is approximately 3.1 or lower. - What colour is litmus paper in a neutral solution (pH 7)?
Solution: It would be purple, or it would not change colour (red litmus stays red, blue litmus stays blue). - An unknown solution is pink with phenolphthalein. What would its colour be with methyl orange?
Solution: Yellow. If the pH is high enough to turn phenolphthalein pink (pH > 8.2), it is definitely high enough to be in the yellow range for methyl orange (pH > 4.4). - Why are single indicators limited in determining an exact pH?
Solution: They only show a colour change over a narrow range, so they can only tell you if the pH is above or below that specific range, not the exact value.
5.24 Universal Indicator & pH Colorimetry
To get a better estimate of a solution's pH, a universal indicator is used. This is not a single indicator but a mixture of several different indicators, each of which changes colour at a different pH.
The result is a continuous spectrum of colours across the pH range, allowing for a more precise (though still approximate) measurement of pH. The technique of determining concentration or pH by observing colour is known as colorimetry.
The typical colours of universal indicator are:
- pH 1-3: Red (Strongly Acidic)
- pH 4-5: Orange (Weakly Acidic)
- pH 6: Yellow (Slightly Acidic)
- pH 7: Green (Neutral)
- pH 8: Bluish-Green (Slightly Alkaline)
- pH 9-11: Blue (Alkaline)
- pH 12-14: Violet/Purple (Strongly Alkaline)
Universal indicator is available as a solution or as paper strips (pH paper). By dipping the paper in a solution and comparing the resulting colour to a provided chart, one can estimate the pH value.
Solved Examples:
- What is a universal indicator?
Solution: A mixture of several different indicators designed to show a range of colours over a wide range of pH values. - A solution turns universal indicator green. What is the approximate pH of the solution?
Solution: The pH is approximately 7 (neutral). - What colour would you expect universal indicator to be in lemon juice (pH ~2)?
Solution: Red. - What is the main advantage of using universal indicator over litmus?
Solution: Universal indicator gives an estimate of the actual pH value, whereas litmus only indicates if a solution is acidic or basic. - A soil sample is mixed with water, and the resulting solution turns universal indicator paper yellow. What is the approximate pH of the soil?
Solution: The pH is approximately 6. - What is pH colorimetry?
Solution: The use of colour changes (like with universal indicator) to measure the pH of a substance. - A solution turns universal indicator deep purple. Is it a strong acid or a strong alkali?
Solution: A strong alkali. - How is universal indicator used in practice?
Solution: A few drops of the solution are added to the sample, or a strip of pH paper is dipped into it. The resulting colour is then compared to a standard colour chart to find the corresponding pH. - Is universal indicator more or less precise than a pH meter?
Solution: Less precise. A pH meter gives a direct numerical reading, often to two decimal places. - If a solution is orange with universal indicator, what would its effect be on blue litmus paper?
Solution: An orange colour indicates an acidic pH (around 4-5). Therefore, it would turn blue litmus paper red.