English version   Catalan version

The acid rain monitoring 

RainBasic Ideas



Acids and Bases

From the beginning of science, chemists classified inorganic compounds in three great groups, based on their properties: acids, bases and salts.

At the beginning of XIX th century acids and bases were described by their properties:

The main properties of acids were: typical sour flavour called acid; production of punctures in contact with skin; their aqueous dilutions change the colour of many vegetable colouring for example, they produce red colour with tornasol (blue); they content hydrogen that can be liberated in form of gas when an active metal is added to their solutions; they lose all their properties when they react with metallic hydroxyls.

The properties of bases were described as: a characteristic bitter flavour; their aqueous dilutions present a soft sensation on touch; they also change the colour of many vegetable colouring; they precipitate a lot of components that are soluble in acids and finally they lose all their properties when react with acids.

This last quality of neutralising their characteristic properties when they react between them was called neutralisation.

Chemists wanted to explain the behaviour of acids and bases better, so then different theories appeared.

Arrhenius (1887) concluded that characteristic properties of acid dissolution were due to hydrogen ions (H+). In the case of basic dissolution, the responsible were hydroxyl anions (OH-).

Br÷nsted and Lowry (1923) proposed the next explanation: an acid is a substance capable of giving up a proton to a base, and a base can accept this proton.

Lewis (1938) developed another theory. An acid is a substance than can accept the sharing of two electrons, whereas a base is a substance than can give up, to share, a pair of electrons.


To get further information about this subject you can visit a Web page on acids, bases and pH at the University Thomas Jefferson, in the USA.

to the top


What's pH? pH mesurment

The pH is a measure of a substance acidity. The more acidity a substance has the more H+ ions it has. (in fact, in an aqueous solution, these ions are found in the form H3O+)

The tecnical definition of pH uses the logarithmic function as follows:

pH = -log [H+]

[H+] means the concentration of H+

For example, if [H+]= 10-3 mols/dm3, we have pH=3

For example, if si [H+] = 10-8 mols/dm3, we have pH=8, etc.

Pure water is neutral. Its pH is 7.

Acids have pH under 7, and bases have pH higer than 7.

The usual scale of pH is from 0 (strong acid) to 14 (strong base). In the steps from 0 to 7 we have some solutions like folowing:

	solution of HCl 1 M ..........pH=0.0
	Orange juice..................pH=2.8
	Rainfall water................pH=6.5
	Pure water at 25║C............pH=7.0
	Solution of NaOH 1 M..........pH=14.0


There are many methods to measure the pH: The use of a pHmeter device (this is the most modern), The use of universal indicator paper, organic colouring indicators (their colour changes depending on the pH) like red roses petals...


We explain how we measure pH at the methodology section of this work.

You can visit a Web page with an explanation on how to use a pHmeter.

To get further information about this subject you can visit a Web page on acids, bases and pH at the University Thomas Jefferson, in the USA.

to the top


Meteorology: Acid rain

The rain water can present more acidity than normal. This can cause some problems in the places where this kind of rain is falling.

Non contamined rain water presents pH values beetween 5.5 and 7.5. It has a little acidity by nature, because it has some gases in solution, like CO2. But pollution can increase the rain water acidity. We talk about acid rain when the rain water pH is between 3 and 5.

The acid rain has an effect on living beings. The river and lakes pH descends, and fish, insects, plancton, etc, can die. The forests can be destroyed by the action of acid rain.

Another effect of the acid rain is over metals and marble. This way their descomposition is accelerated.

How acid rain is produced?

there are some chemical reaction in the air that produce acidity. The volcanoes eruptions, the descomposition of plants, and above all, the human activities (use of combustible fossils in power stations, engine powered vehicles...) are the causes of the sulphur and nitrogen oxides detected in the air. They can transform into acids by the reaction with the water in the air. The reaction products are sulphuric acid and nitric acid. Both acids, in solution with the rain, are the responsible ones for the corrosive effects of the acid rain.

You can get more information about meteorology at the Catalan Meteorological Service Web.

For further information about acid rain, you can visit Marc Vinyes, Bertran Biayna and Eduard Pou Web where you can see a nice chapter on atmospheric pollution.

to the top