pH General Information

What is pH measurement ?

pH is the Unit of Measure used to express the degree of acidity of a substance.

The centimeter is a unit measure of length. The gram is a unit measure of weight. So, pH is the unit measure we use to say how much acid is in a substance. The pH scale goes from 0 to 14. A pH of 0 means a very high acid activity. Substances such as lemon juice and vinegar are acidic with pH values of 2 to 3. Nitric acid and hydrochloric acid are very strong with pH values of 0, while stomach acid has a pH of 1. Addition of a strong acid, such as sulfuric acid ( H2SO4 ) to water makes the resulting solution very high in active acid concentration. This is called an acidic solution.

On the other end of the scale are the alkaline substances, which range from 8 to 14. Common alkalis are seawater (pH 8), household ammonia (pH 11), oven cleaners (pH 13), and the very strong alkali, sodium hydroxide (pH 14). The addition of a strong base or alkali material, such as sodium hydroxide ( NaOH ), to water makes the resulting solution very low in active acid concentration. This is called a very basic or alkali solution.

In between these two extremes is a pH of 7. This is the pH of pure water. Water, which is neither very acidic nor very alkali, is said to be neutral.

Technically, the scale actually refers to the concentration of positively-charged hydrogen (H+) ions and negatively-charged hydroxyl (OH-) ions in solution. More hydrogen ions than hydroxyl ions makes an acidic solution, while an alkaline solution contains more hydroxyl ions than hydrogen ions. The pH scale is a logarithmic one, meaning that each pH unit has 10 times as many hydrogen ions as the unit above it. So, at pH 4, there are 10 times more hydrogen ions than at pH 5 and 100 times more hydrogen ions than at pH 6.

The following table will give you an idea of the pH values of common substances:

Substance pH Value
Stomach acid 1.0
Lemon Juice 2.4
Vinegar 2.8
Orange Juice 3.0
Tomato Juice 4.0
Black Coffee 5.0
Detergent 6.5
Milk 6.8
Blood 7.4
Seawater 8.0
Baking soda 9.0
Milk of Magnesia 10.7
Domestic Bleach 11.0
Caustic Soda 14.0
Check out our pH Meters
Check out our pH Meters
From General Chemistry by Brady and Humiston, 1986.

Why pH is Important

The pH or acidity of a solution is important in many different areas:

In Environmental Research and Pollution Control:
The pH of a river or lake is important in maintaining a proper ecological balance. The pH of the water directly affects the physiological functions and nutrient utilization by plant and animal life. Extremes in pH can reduce lake to a lifeless, smelly bog.

Protecting our waterways requires constant monitoring of industrial effluent. Plating and metal finishing plants tend to produce acidic waste water, as do mining operations, Chemical plants often have very alkaline waste water. pH measurements are used as a guide to the proper neutralization of these plant wastes, as well as to monitor the final effluent quality. Occasionally, an acidic stream can be combined with an alkaline stream to produce a final stream which is close to neutral. pH measurements assure the proper management of this cost saving technique.

In Biochemistry:
The pH of our blood is normally controlled to within a few tenths of a pH unit by our body chemistry. If our blood pH changes as much as half a pH unit, serious illness will result. Proper skin pH is essential for a healthy complexion. The pH of one's stomach directly affects the digestive process.

In Agronomy:
The pH of the soil regulates the availability of nutrients for plant growth, as well as the activity of soil bacteria. In alkaline soils ( pH 8 and above ) the amount of nitrogen, phosphorus, iron and other nutrients in solution become so low that special treatment is necessary to insure proper growth.

In Food Science:
The efficient production of food products depends upon careful pH control. The proper curd size, uniformity, and structure of cottage cheese are directly related to the pH at cutting time. Yeast can ferment and leaven dough only within certain pH limits. Jelly will not gel properly unless the pH is in the 3.5 region.

In Chemical Research and Engineering:
Accurate pH measurement is necessary to the study of many chemical processes. The researcher needs to know the pH at which a chemical reaction proceeds at its fastest in order. to understand the reaction. The engineer uses the information to develop practical commercial processes.

From pH Simplified by Robert J. Joyce, Markson Science Inc.

Introduction to pH : General Information

pH in an aqueous solutions is a measure of hydrogen and hydroxide ions. Water molecules dissociate in hydrogen (H+) and hydroxide (OH-) ions,

H2O = H+ + OH-

but the number of ions formed is very small. Water at 25C contains 1 x10-7 mol/l of hydrogen ions and the same concentration of hydroxide ions, where the concentration (mol/l) of hydrogen ions [H+] multiplied by the concentration (mol/l) of hydroxide ions [OH-] is constant:

Kw = [H+] [OH-]

Kw is the dissociation constant for water and it depends on temperature.

Temperature C K w
10 0,2920 * 10 -14
15 0,4505 * 10 -14
20 0,6809* 10 -14
25 1,008 * 10 -14
30 1,469 * 10 -14

Acids in water increase the [H+] and, because the product [H+] [OH-] must be constant, acids decrease the [OH-]. Bases increase [OH-] and decrease [H+]. For example, suppose an acid is added to water at 25C and the acid raises the [H+] to 1.0 x 10-3 mol/l. Because [H+] [OH-] must always equal 1.00 x 10-14, [OH-] will be 1.0 x 10-11 mol/l. pH is the common way of expressing the hydrogen ion concentration [H+]. pH is defined as:

pH = -log [H+]

In the example above, the hydrogen ion concentration is 1.0 x 10-3 mol/l and the pH is 3.00. Alternatively, if adding base changes the [H+] to 1.0 x 10-11 then the pH is 11.0.

In fact equation 2 is valid for highly diluted solutions only. If concentrated solutions of acids or bases or even salts are used, the hydrogen concentration must be replaced by the ion activity a H+ and the hydroxide concentration by a OH- . The relation between concentration and activity of an ion is

aion = fion * [ion]

where f is the activity coefficient for that ion. The reason for the difference of activity and concentration is that in higher concentrated solutions the ions interact with each other and therefore show a different behavior than in diluted solutions. That means in higher concentrated solutions the amount of "real" active ions is lower than expected. This leads to the pH definition

pH = -log aH+
From Electrochemistry: Theory and Practice by Dr. Axel Bier, Product Application Manager
www.HACH.com