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Key Concepts

  • pH as a measure of acidity was first defined and used by the Danish biochemist S.P.L. Sørensen in 1909*.

  • 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 25oC:
    An acid has a pH less than 7.
    A base has a pH greater than 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.

    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 bases#.

pH of Some Common Solutions

pHExamples (the pH, at 25oC, 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 or methanoic acid, HCOOH)
3lemon juice (contains citric acid), vinegar (contains acetic, or ethanoic, acid), coca cola (contains carbonic acid)
4wine, 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)

Examples

1. Which of the following substances is an acid at 25oC?
  • 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:
    1. acid : pH < 7
    2. neutral : pH = 7
    3. 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.

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

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

  1. Estimate the pH of each substance at 25oC:
    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

3. The pH of an unknown solution was measured at 25oC 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 25oC:
    solution pH
    white wine 4
    white viengar 3
    dissolved table salt (NaCl) 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


What would you like to do now?

*Sørensen defined pH as the negative logarithm (to base 10) of the hydrogen ion concentration in solution.
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.

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

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