pH Scale Chemistry Tutorial

Key Concepts

pH as a measure of acidity was first defined and used by the Danish biochemist S.P.L. Sørensen in 1909¹.

Sørensen's definition of pH results in a convenient scale for describing the acidity of solutions:

pH=0	pH=1	pH=2	pH=3	pH=4	pH=5	pH=6	pH=7	pH=8	pH=9	pH=10	pH=11	pH=12	pH=13	pH=14
Most Acidic		← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ←											Least Acidic

The lower the value of the pH of a solution, the more acidic the solution is.

The pH scale can also be used to describe how basic (or alkaline) a solution is:

pH=0	pH=1	pH=2	pH=3	pH=4	pH=5	pH=6	pH=7	pH=8	pH=9	pH=10	pH=11	pH=12	pH=13	pH=14
Least Basic		→ → → → → → → → → → → → → → → → → → → → → → → → → → → →											Most Basic

The greater the value of the pH of the solution, the more basic (or alkaline) the solution is.

For dilute aqueous solutions at 25°C:²

An acid has a pH less than 7
acid: pH < 7.
A base has a pH greater than 7
base: pH > 7

If the solution has a pH of 7 it is said to be neutral, that is, the solution is neither an acid nor a base.
neutral: pH = 7

pH=0	pH=1	pH=2	pH=3	pH=4	pH=5	pH=6	pH=7	pH=8	pH=9	pH=10	pH=11	pH=12	pH=13	pH=14
Acid pH < 7							neutral pH = 7	Base pH > 7

Sometimes we might want to describe the relative acidity, or basicity (alkalinity), of a solution. For this we can use the following terms:

pH=0	pH=1	pH=2	pH=3	pH=4	pH=5	pH=6	pH=7	pH=8	pH=9	pH=10	pH=11	pH=12	pH=13	pH=14
Very Acidic		Acidic			Slightly Acidic		neutral pH = 7	Slightly Basic		Basic			Very Basic

DO NOT use the terms "strong" or "weak" to describe the relative pH of acids or bases³.

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pH of Some Common Solutions

The table below gives an approximate pH for some common aqueous solutions you might encounter at home or in the school laboratory:

pH	Examples (the pH, at 25°C, is only an approximation in most of these examples)
0	1 mol L^-1 HCl_(aq)
1	0.1 mol L^-1 HCl_(aq)
2	Gastric juice (0.01 mol L^-1 HCl_(aq)) ant venom (contains formic acid or methanoic acid, HCOOH)
3	lemon juice (contains citric acid) vinegar (contains acetic acid, or ethanoic acid, CH₃COOH) coca cola (contains carbonic acid)
4	wine or beer aspirin dissolved in a glass of water
5	coffee tomato juice orange juice
6	tap water saliva cow's milk urine
7	pure liquid water aqueous sodium chloride solution (NaCl_(aq))
8	sea water
9	some soaps baking soda (sodium bicarbonate, bicarbonate of soda, or sodium hydrogen carbonate, NaHCO₃)
10	toothpaste some detergents
11	washing soda some detergents
12	cleaning products
13	caustic oven cleaners 0.1 mol L^-1 NaOH_(aq)
14	1 mol L^-1 NaOH_(aq)

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Worked Examples

Question 1. Which of the following aqueous solutions is an acid at 25°C?

sea water

soapy water

vinegar found in the kitchen

oven cleaner

Estimate the pH of each solution using the table above:

solution pH

sea water 8

soapy water 9

vinegar 3

oven cleaner 13

solution	pH
sea water	8
soapy water	9
vinegar	3
oven cleaner	13

Use the pH to determine whether each solution is an acid or a base at 25^oC:

acid : pH < 7

neutral : pH = 7

base : pH > 7

solution	pH
sea water	8	pH > 7	base
soapy water	9	pH > 7	base
vinegar	3	pH < 7	acid
oven cleaner	13	pH > 7	base

Only vinegar has a pH less than 7 so it is an acid.

Question 2. Place the following substances, at 25°C, in order from least acidic to most acidic:

toothpaste

lemon juice

cow's milk

ant venom

Estimate the pH of each substance at 25°C:

solution pH

toothpaste 10

lemon juice 3

cow's milk 6

ant venom 2

solution	pH
toothpaste	10
lemon juice	3
cow's milk	6
ant venom	2

Place the substances in order of pH from high pH to low pH

solution	pH
toothpaste	10	highest pH
cow's milk	6	↓
lemon juice	3	↓
ant venom	2	lowest pH

Use the pH values to determine the relative acidity of the solutions:

low pH is more acidic than high pH

solution	pH
toothpaste	10	highest pH	least acidic
cow's milk	6	↓	↓
lemon juice	3	↓	↓
ant venom	2	lowest pH	most acidic

The substances in order from least acidic to most acidic are:

toothpaste, cow's milk, lemon juice, ant venom

Question 3. The pH of an unknown solution was measured at 25°C and found to be 7.
The solution is known to be either white wine, white vinegar or table salt (sodium chloride) dissolved in water.
Identify the unknown solution.

Estimate the pH of each the named solutions at 25°C:

solution pH

white wine 4

white viengar 3

dissolved table salt (NaCl_(aq)) 7

Identify the solution with a pH = 7 : dissolved table salt
The unknown solution is most likely to be table salt dissolved in water because both wine and vinegar are acids with pH < 7

solution	pH
white wine	4
white viengar	3
dissolved table salt (NaCl_(aq))	7

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1. Sørensen defined pH as the negative logarithm (to base 10) of the hydrogen ion concentration in solution.
pH = -log₁₀[H⁺]
The definition was later revised to one based on the activity of hydrogen ions in solution.
However, if the solution is sufficiently dilute so that the activity of hydrogen ions can be approximated by their concentration, we can still use Sørensen's definition.

2. pH of a solution is dependent on the nature of the solute and the solvent. The solvent you will most commonly encounter in the school laboratory is water. When a solute dissolves in water it forms an aqueous solution.
The pH of a solution, including an aqueous solution, is dependent on temperature, so, we MUST specify the temperature of the solution. You can see how the pH of water varies with temperature here
However, if you are given a problem in which the type of solution and/or temperature is NOT specified, you should probably ASSUME:
(i) it is an aqueous solution
(ii) temperature is 25°C

3. "Strong" and "weak" are used to describe the strength of an acid or a base.
The strength of an acid or strength of a base is related to its ability to donate or accept protons NOT to the concentration of hydrogen ions in the solution (pH).