07. the pH scale

The smaller the pH value, the more H⁺ ions are in the solution. Each value in the pH scale represents a tenfold increase in the number of hydrogen ions. Clearly, the more concentrated the solution of acid, the greater the number of hydrogen ions. Secondly, the extent of dissociation of a molecule into ions depends on the temperature and therefore the higher the temperature, the greater the number of hydrogen ions. Thirdly, the type of acid makes a difference; at the same concentration, a 'strong acid' will ionise much more than a 'weak acid'. In summary, the pH of a solution depends on several variables; the acid, the concentration of the acid and temperature.

An alkaline solution is one which contains a water-soluble base. The hydroxide ions of the base react with the hydrogen ions of water, and this means the concentration of hydrogen ions in alkaline solutions is lower than in pure water. Alkaline solutions have a pH value greater than 7. The greater the pH value, the lower the concentration of hydrogen ions, and the higher the corresponding concentration of hydroxide ions. As with acids, the alkalinity of a solution depends on several variables, including the base, the concentration of the base and the temperature.

ACTIVITY: pH

ATL

Critical-thinking skills: Revise understanding based on new information and evidence.
1. Identify whether a solution is acidic, alkaline or neutral, if its pH is: 9, 4, 7, 1, 10, 2.
2. Compare two solutions with pH 8 and pH 10. In the more alkaline solution, by what factor is the number of hydrogen ions decreased?
3. Imagine 10²⁰ molecules of each of the following pairs of acids were dissolved in 0.1 dm³ water (see acids table in lesson 05.)
  1. a Deduce which of these pairs has the most hydrogen ions, and why:
  2. i ethanoic acid and hydrochloric acid
  3. ii sulfuric acid and citric acid
  4. b For each pair of solutions, identify which is the better conductor of electricity.
4. a Compare the expressions:

- 1. i a concentrated solution of an acid
  2. ii a solution of a strong acid.
  3. b Sketch a diagram representing the particles in a concentrated solution and a dilute solution.

Assessment opportunities

In this activity you have practiced skills that can be assessed with Criterion A: Knowing and understanding.

Serial dilutions

A serial dilution is one in which each successive sample is a further dilution of the previous solution. To make a serial dilution in which each sample is diluted by a factor of 10, add 0.01 dm³ (1 cm³) of the stock to 0.09 dm³ (9 cm³) of water. A fresh pipette should be used for each of these steps.

After mixing, repeat the process, using the diluted solution instead of the stock.

Sample (test tube number)

Source of the 0.01 dm³

amount

Original stock

Test tube 1

Test tube 2

Test tube 3

test tube 4

Water added

+ 0.09 dm³

+0.09 dm³

Dilution factor

(of the original)

1/10

1/100

1/1 000

1/10 000

Because the dilution factor is carefully controlled serial dilutions are useful for investigating properties that are difficult to interpret at high concentrations for example bacterial counts.

Activity:

Using the information above as a guide, create serial dilutions of 1.0 mol dm^-3, 0.1 mol dm^-3, 0.01 mol dm^-3, and 0.001 mol dm^-3 for the following acids and bases:

hydrochloric acid,

ethanoic acid,

sodium hydroxide,

ammonia,

To each solution that you make, add ~5 drops of universal indicator solution and record the colour.

Use a pH probe to measure the pH of each dilution of each solution.

Analysis:

Present your quantitative and qualitative results in a table.

How does the pH indicated by the colour of the universal indicator solution compare to the pH value from the pH probe?

Based on this conclusion, which indicator (Universal indicator solution or pH probe) would you use to monitor the neutralisation reaction between an acid and a base?