pH Theory

What exactly is pH?  The pH definition is based on the amount of hydrogen ions available in a solution.  The Danish scientist S.P.L. Sørensen first proposed the term pH as an abbreviation of "pondus hydrogenii" in 1909, to express very small concentrations of hydrogen ions.

Since this point an enormous amount of development work, covering both the theory and practice of pH, has taken place making pH an important factor determining the majority of chemical practices. on the theory and practical side 

The original definition was the negative base logarithm of the hydrogen ion concentration, which is:- pH = - log10[H+]

Since the most chemical and biological reactions are governed by the hydrogen ion activity, the definition is:-  pH = - log10aH+

pH Scale

The pH scale was established to provide a convenient and standardised method of quantifying the acidity or basicity of a particular solution. The range of the pH scale is based on the dissociation constant of water, Kw where Kw = [H+] [OH-] = 10-14

The extremes of the pH scale are established at pH 0 to pH 14.  With strong acids or bases, pH values below pH 0 and above pH 14 are possible, but such samples are rarely measured.  The pH value of some common solutions can be found below:-

  • 0 pHpH of battery acid

    Lead acid batteries use Sulphuric Acid acid as an electrolyte - rather unsuprisingly lead acid batteries have a low pH falling close to ~ 0pH

  • 1 pHpH of stomach acid

    Acid in the stomach aids digestion and is suprisingly strong and contains Hydrochloric acid - with quoted figures as low as ~ 1.5pH

  • 2 pHpH of lemons

    Lemon juice is an excellent source of citric acid which results in lemon juice having a low pH of approximately 2.2pH

  • 3 pHpH of orange juice

    Containing about 1/5th the citric acid content as lemons, orange juice can have a pH as low as 2.9pH up to approximately 4.00pH depending on the type.

  • 4 pHpH of red wine

    A glass of red wine is also fairly acidic - depending upon the grape variety and how the wine was conditioned - wine pH can vary from ~ 3.9 up to 4.5pH.

  • 5 pHpH of black coffee

    A freshly brewed mug of black coffee can have a pH value varying from 4.3pH up to around 5.2pH - depending on how long the coffee was brewed for and the origin of the beans

  • 6 pHpH of milk

    Fresh milk from cows contains trace amount of lactic acid, which is probably the main reason for a slightly acidic pH reading of between 6.4pH - 6.7pH.

  • 7 pHpH of demin water

    Pure water should have a pH of 7.0pH - when buffering a pH electrode we'd recommend using a 7.00pH buffer solution though

  • 8 pHpH of sea water

    Surface Sea water normally has a pH between 8.1pH - 8.3pH. As such a huge system there can be geographic fluctuations

  • 9 pHpH of tooth paste

    Even though toothpaste contain fluoride, which is slightly acidic (~4pH) toothpaste is generally more base with a pH of between 8.8pH and 9.1pH

  • 10 pHpH of indigestion tablets

    Indigestion tablets are used to neutralise naturally occuring acids which cause pain and discomfort. As expected these have a higher pH between 9.8pH - 10.2pH

  • 11 pHpH of ammonia

    Ammonia solution has a variety of uses both domestically and in industial environments - it's a good degreaser and cleaning agent and has a pH of around 11pH

  • 12 pHpH of bleach

    Domestic bleach products normally contain sodium hypochlorite at a strength of around 5% - which is an excellent oxidising agent and also has a high pH of around 12pH

  • 13 pHpH of sodium hydroxide

    Domestic oven cleaners can contain Sodium Hydroxide which is an excellent degreaser. Depending upon the strength of Sodium Hydroxide solution - the pH can typically be beteeen 12.8pH up to 13.5pH

  • 14 pHpH of drain cleaner

    Drain cleaner which is based on a caustic soda formulation hss a very high pH from 12.8pH right up to ~ 14pH - it's normally very good at unblocking drains too.

Logarithmic Nature of the pH Scale

When acids and bases are dissolved in water, they alter the relative amounts of [H+] and [OH-] ions in solution.  If an acid is dissolved in water, it increases the hydrogen [H+] ion concentration.

Because the product hydrogen [H+] and hydroxide [OH-] must remain constant, the hydroxide ion [OH-concentration must decrease. If a base is dissolved into water then the converse occurs.

As the hydrogen [H+and hydroxide [OH-] are in equilibrium the pH can also be viewed as a simultaneous measurement of both acidity and basicity, since by knowing the concentration of either the hydrogen [H+or hydroxide [OH-ion, one can determine the other.

pH is a logarithmic function. A change in one pH unit is equal to a ten-fold change in H+ ion concentration.  The table below illustrates the relationship between the H+ and OH- ions at different pH values:

0 pH (100) 1 (10-14) 0.00000000000001
1 pH (10-1) 0.1 (10-13) 0.0000000000001
2 pH (10-2) 0.01 (10-12) 0.000000000001
3 pH (10-3) 0.001 (10-11) 0.00000000001
4 pH (10-4) 0.0001 (10-10) 0.0000000001
5 pH (10-5) 0.00001 (10-9) 0.000000001
6 pH (10-6) 0.000001 (10-8) 0.00000001
7 pH (10-7) 0.0000001 (10-7) 0.0000001
8 pH (10-8) 0.00000001 (10-6) 0.000001
9 pH (10-9) 0.000000001 (10-5) 0.00001
10 pH (10-10) 0.0000000001 (10-4) 0.0001
11 pH (10-11) 0.00000000001 (10-3) 0.001
12 pH (10-12) 0.000000000001 (10-2) 0.01
13 pH (10-13) 0.0000000000001 (10-1) 0.1
14 pH (10-14) 0.00000000000001 (100) 1

Hydrogen Ions and Aqueous Solutions

In any collection of water molecules a small number of water molecules will disassociate to form [H+] and [OH-] ions:-

H2O =  H+ + OH- 

At 25°C, pure water contains 1 x 10-7 moles per litre of hydrogen ions [H+and 1 x 10-7 moles per litre of hydroxide ions [OH-].

In any aqueous solution, the concentration of hydrogen ions multiplied by the concentration of hydroxide ions is constant, allowing us to define the dissociation constant for water, Kw where the brackets indicate molar concentrations.

Kw = [H+] [OH-] = 10-14

The disassociation constant is temperature dependant.  This means that the pH measurement is also dependent upon temperature.  There's more detailed information on the temperature affects of pH here.

pH of non-aqueous solutions

While It is possible to measure the pH of non-aqueous solutions, the resulting measurement is often not an absolute measurement but an indicative measurement.  There are many variables to be aware of when measuring non-aqueous solutions and there is some more information in our measuring pH of non-aqueous solutions section.

Why measure pH?

pH measurement is critical to many industries including manufacturing, food & beverage, biotechnology, pulp & paper and naturally, the fine chemical manufacturing process.

pH is also heavily relied upon in water quality analysis, both on the supply or water treatment side as well as the waste water or effluent treatment side too.  Exceeding defined pH limits when discharging effluent or waste water into a water course is a very serious offence.

pH is hugely important to water quality analysis and is often a primary measurement in the process control and manufacturing sectors.

Within our Knowledge Base we have some more information on pH measurements and pH Theory which are listed below:-

How to measure pH?

How to measure pH?

The theory behind how we measure pH and some of the practical considerations to measuring pH in a process critical or industrial environment.

Find out more

How does temperature affect pH?

How does temperature affect pH?

Temperature is a core part of the Nernst equation has affects the way in which pH is indicated. 

Find our more how temperature affects the pH measurement and when and how to use automatic temperature compensation.

Find out more

Can we measure the pH of non-aqueous solutions?

Can we measure the pH of non-aqueous solutions?

Measuring the pH of non-aqueous solutions involves complications and compromises but can form an important part of some processes - particularly where proprietary chemical blends are involved.

Find out more

How to buffer a pH electrode?

How to buffer a pH electrode?

Something that every engineer or professional who works with pH will be aware of is the need to buffer a pH electrode and calibrate it to an instrument.

Find out how to buffer a pH electrode here.

Find out more

Choosing a pH sensor connector

Choosing a pH sensor connector

Before ordered a replacement pH electrode - it's important to eliminate the first barrier to operation - how do you connect a pH electrode to a pH instrument?.

In the choosing a pH sensor connector guide we've listed the most common pH sensor connectors.

Find out more