The pH Scale
- The acidity of an aqueous solution depends on the number of H+ (H3O+) ions in the solution
- pH is defined as:
pH = – log10[H+]
-
- Where [H+] is the concentration of H+ in mol dm–3
- The pH scale is a logarithmic scale with base 10
- This means that each value is 10 times the value below it
- For example, pH 5 is 10 times more acidic than pH 6.
- pH values are usually given to 2 decimal places
- The relationship between concentration is easily seen in the following table:
pH & [H+] Table
| [H+] | Scientific notation | pH |
| 1.0 | 100 | 0 |
| 0.1 | 10–1 | 1 |
| 0.01 | 10–2 | 2 |
| 0.001 | 10–3 | 3 |
| 0.0001 | 10–4 | 4 |
| - | 10–x | x |
Worked example
10.0 cm3 of an aqueous solution of nitric acid of pH = 1.0 is mixed with 990.0 cm3 of distilled water. What is the pH of the final solution?
A. 1
B. 2
C. 3
D. 10
Answer:
- The correct option is C.
- The total volume after dilution is 1000.0 cm3
- So, the concentration of H+ has been reduced by a factor of 100 or 10-2
- This means an increase of 2 pH units
- Therefore, the final solution is pH 3
Examiner Tip
- Make sure you know how to use the antilog (base 10) feature on your calculator
- On most calculators, it is the 10x button
- But on other models, it could be LOG-1, ALOG or even a two-button sequence such as INV + LOG
- The pH scale is a numerical scale that shows how acidic or alkaline a solution is
- The values on the pH scale go from 0 - 14 (extremely acidic substances have values of below 0)
- All acids have pH values below 7, all alkalis have pH values above 7
- The lower the pH then the more acidic the solution is
- The higher the pH then the more alkaline the solution is
The pH scale
The pH scale showing acidity, neutrality and alkalinity
pH of acids
- Acidic solutions (strong or weak) always have more H+ than OH- ions
- Since the concentration of H+ is always greater than the concentration of OH- ions, [H+] is always greater than 10-7 mol dm-3
- Using the pH formula, this means that the pH of acidic solutions is always below 7
- The higher the [H+] of the acid, the lower the pH
pH of bases
- Basic solutions (strong or weak) always have more OH- than H+ ions
- Since the concentration of OH- is always greater than the concentration of H+ ions, [H+] is always smaller than 10-7 mol dm-3
- Using the pH formula, this means that the pH of basic solutions is always above 7
- The higher the [OH-] of the base, the higher the pH
pH of water
- Water at 298K has equal amounts of OH- and H+ ions with concentrations of 10-7 mol dm-3
- To calculate the pH of water, the following formula should be used:
pH = – log10[H+ (aq)]
[H+ (aq)] = concentration of H+/H3O+ ions
pH = -log (10-7) = 7
- Thus, water has a pH of 7 at 298 K
How to measure pH
- The most accurate way to determine the pH is by reading it off a pH meter
- The pH meter is connected to the pH electrode which shows the pH value of the solution
Using a pH meter
The diagram shows a digital pH meter that measures the pH of a solution using a pH electrode
- A less accurate method is to measure the pH using universal indicator paper
- The universal indicator paper is dipped into a solution of acid upon which the paper changes colour
- The colour is then compared to those on a chart which shows the colours corresponding to different pH values
Using universal indicator
The diagram shows the change in colour of the universal indicator paper when dipped in a strong (HCl) and weak (CH3COOH) acid. The colour chart is used to read off the corresponding pH values which are between 1-2 for HCl and 3-4 for CH3COOH
