 # pH Scale

## 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 AcidpH < 7 neutralpH = 7 BasepH > 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 neutralpH = 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:

pHExamples (the pH, at 25°C, is only an approximation in most of these examples)
01 mol L-1 HCl(aq)
10.1 mol L-1 HCl(aq)
2Gastric juice (0.01 mol L-1 HCl(aq))
ant venom (contains formic acid or methanoic acid, HCOOH)
3lemon juice (contains citric acid)
vinegar (contains acetic acid, or ethanoic acid, CH3COOH)
coca cola (contains carbonic acid)
4wine or beer
aspirin dissolved in a glass of water
5coffee
tomato juice
orange juice
6tap water
saliva
cow's milk
urine
7pure liquid water
aqueous sodium chloride solution (NaCl(aq))
8sea water
9some soaps
baking soda (sodium bicarbonate, bicarbonate of soda, or sodium hydrogen carbonate, NaHCO3)
10toothpaste
some detergents
11washing soda
some detergents
12cleaning products
13caustic oven cleaners
0.1 mol L-1 NaOH(aq)
141 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

1. Estimate the pH of each solution using the table above:

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

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

• 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

1. Estimate the pH of each substance at 25°C:

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

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

3. 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.

1. 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

2. 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

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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).