PH SCALE 151The pH is a measure of the acidityof a solution. A low pH corresponds to a high
value of [H 3 O] or [H(aq)], showing the solution to be strongly acidic (Table 9.1).
The values of [H(aq)], and thus pH, can vary depending on the acidity or basicity
of the solution. The pH scale below shows the range of common pH values.
(^0123456) | (^7) | 891011121314
increasingly acidic increasingly basic
Acidic solutions have pH values below 7. Solutions with pH values above 7 are
basic. A pH of 7 at 25C (the pH of pure water) is neutral. A pH metercan be used to
measure the pH of a solution. It consists of an electrode which is dipped into the
solution and connected to a device which gives a digital readout. Alternatively,
universal indicator papermay be used. It contains a mixture of dyes that changes to
different colours depending on the pH of the solution (see page 301).
Table 9.1Approximate
pH values of common
substances
Substance pH
Gastric juice 1.2–3.0
Vinegar 3.0
Lemonade 3.0–3.5
Tomato juice 4.2
Coffee 5.0
Saliva 6.6
Milk 6.7
Pure water 7.0
Blood 7.4
Eggs 7.8
The pH scale
Exercise 9N
Example 9.11
What is the pH of a solution of 0.020 mol dm^3 nitric acid,
HNO 3 at 25C?
Answer
As the acid may be assumed to be completely dissociated (the concentration of
the acid equals the concentration of hydrogen ions),
[H(aq)]0.020
pHlog [H(aq)]log [0.020] 1.70
Since pH is a logarithmic quantity, it has no units. (For an explanation of
significant figures see Unit 1, page 10.)
Classify the following as strongly acidic,
weakly acidic, strongly basic or weakly basic:
(i) lemon juice (pH 4)
(ii) rain water (pH 6 because it contains
dissolved carbon dioxide)
(iii) bicarbonate of soda (or sodium
hydrogencarbonate, pH 12)
(iv)sodium hydroxide of concentration
0.1 mol dm^3
(v) soap solution (pH 9)
(vi)wine (pH 3.5)
(vii)household ammonia (pH 12).
Example 9.12
The pH of blood is 7.4. What is the hydrogen ion concentration
in blood?
Answer
pHlog [H(aq)] 7.4
[H(aq)] 10 7.4
[H(aq)] 4 10 ^8 mol dm^3