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The strength of an acid depends on the concentration of the hydronium ions (H3O+) present in a solution. We know that greater the number of hydronium ions present, greater is the strength of acid. However, some acids do not dissociate to any appreciable extent in water such as carbonic acid. Therefore, these acids will have a low concentration of hydronium ions.
An acid, which dissociates completely or almost completely in water, is classified as a strong acid. It must be noted that in these acids all the hydrogen ions (H+) combine with water molecule and exist as hydronium ions (H3O+).
Examples of strong acids are: hydrochloric acid, sulphuric acid, nitric acid etc.
An acid that dissociates only partially when dissolved in water, is classified as a weak acid. Most of the molecules remain in solution in molecular form itself in such acid.
Examples are: acetic acid, formic acid, carbonic acid etc.
OR
The strength of a base depends on the concentration of the hydroxyl ions when it is dissolved in water. A base that dissociates completely or almost completely in water is classified as a strong base. The greater the number of hydroxyl ions the base produces, the stronger is the base.
Example:
Weak Base
A base that dissociates in water only partially is known as a weak base.
Example:
Examples of Bases
| Sodium hydroxide: NaOH
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Magnesium hydroxide: Mg(OH)2
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| Potassium hydroxide: KOH
Calcium hydroxide: Ca(OH)2
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Ammonium hydroxide: NH4OH
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It is quite clear that the strength of acids and bases can be expressed in terms of the concentration of hydrogen ions, i.e., hydronium ions (H3O+) or hydroxyl ions (OH-) in a solution. The concentration of hydronium ions in a solution can be expressed as moles of hydronium ions per litre of solution. This is expressed by a term called 'pH' which represents the 'power of hydrogen ion concentration' in solution.
The pH value is defined as: 'the negative logarithm (to the base 10) of the hydrogen ion concentration, expressed in moles per liter'; i.e.,
pH = - log10[H+]
Where, [H+] is the hydrogen ion concentration in molecule/Liter.
The strength of various acids and bases is expressed on the pH scale. The pH scale is a 14 point scale, which expresses the strength of an acid or a base in terms of the hydronium ion (H+) concentration. The range from 0 to 14 gives a measurement of comparative strength of acid and base solutions.
Pure water and other neutral solutions have a pH value of 7. A pH value less than 7 indicates that the solution is acidic, and a pH value greater than 7 indicates that the solution is basic.
How does one monitor these pH changes? By using substances called indicators which show change in colour over some pH range, one can measure the pH of a solution.
| Cresol blue
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1.2 - 1.8
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Red
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Yellow
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| Thymol blue
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1.2 - 2.8
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Red
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Yellow
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| Methyl blue
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2.9 - 4.0
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Red
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Yellow
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| Methyl orange
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3.1 - 4.4
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Pink
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Yellow
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| Methyl red
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4.2 - 6.3
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Red
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Yellow
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| Litmus
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5.0 - 8.0
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Red
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Blue
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| Bromothymol blue
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6.0 - 7.6
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Yellow
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Blue
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| Phenol red
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6.4 - 8.2
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Yellow
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Red
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| Thymol blue (base)
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8.1 - 9.6
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Yellow
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Blue
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| Phenolphthalein
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8.3 - 10.0
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Colourless
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Blue
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| Thymolphthalein
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8.3 - 10.5
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Colourless
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Blue
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| Alizarin yellow R
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10.1 - 12.0
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Blue
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Yellow
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| Nitramine
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10.8 - 13.0
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Colourless
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Orange/brown
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The colour change varies considerably from one indicator to another over a pH range, and so a universal indicator is used. It is a mixed indicator, which not only shows whether the solution is acidic or basic, but also shows the approximate pH values by giving a wider range of colours for different values of pH.
| Hydrochloric acid
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1.0
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Bread
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5.5
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| Sulphuric acid
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1.2
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Potatoes
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5.8
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| Gastric juice
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2.0
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Rain water
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6.2
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| Lemon
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2.3
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Milk
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6.5
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| Vinegar (Acetic acid)
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2.8
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Pure water
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7.0
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| Soft drink
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3.0
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Egg
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7.8
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| Apple
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3.1
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Sea water
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8.5
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| Grape
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3.1
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Ammonium hydroxide
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11.1
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| Tomato
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4.2
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Sodium hydroxide
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13.0
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| Banana
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4.6
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Biochemical Systems
pH plays a very significant role in biochemical reactions. For example, the blood in our bodies is maintained at a pH value of 7.36 - 7.42 due to bicarbonate - carbonic acid mixture. A mere change of 0.2 pH units can cause death.
Certain enzymes in the body get activated only at certain definite pH values.
The stomach produces hydrochloric acid that helps in the digestion of food. Under normal conditions this acid does not harm the stomach. During indigestion the stomach produces too much acid causing pain and irritation. The pain due to excess acid can be got over by neutralizing it with a base. A base like magnesium hydroxide (Milk of magnesia), a mild base, is often used for this purpose. These are also called antacids - substances neutralizing the excess acid.
Similarly bee-sting or ant sting and the stinging hair of nettle leaves produce an acid called methanoic acid which causes burning pain and irritation. Use of a mild base like baking soda on the stung area gives relief.
Tooth enamel is made up of calcium phosphate the hardest substance in the body and does not dissolve in water. However it begins to corrode when the pH in the mouth is below 5.5. How does the pH in the mouth get lowered?
The bacteria present in the mouth act on sugar and food particles remaining in the mouth after eating to produce acids. The best way to prevent this is to clean the mouth after eating food or use toothpastes, which are generally basic, for cleaning the teeth so as to prevent tooth decay.
Agriculture
The pH of the soil is very important for a proper crop yield. Salts such as carbonates, bicarbonates, phosphates, and organic acids render the soils acidic or basic. Farmers and gardeners need to know how acidic or basic the soil is so that they know which plants to grow. The choice of fertilizers also depends upon pH of the soil. Most vegetables like carrot and cabbage like a neutral soil (pH 7.0).
Industry
Practically all industries use acids or bases in one process or the other. Major industries, which employ these are, Paper, Dyes, Ink, Paints and Drugs industries.
The control of pH is very important in the field of food preservation too.
Environment
Normally while rain travels through the air, it dissolves floating chemicals and washes down particles that are suspended in air. At the start of its journey raindrops are neutral (pH = 7). In clean air, rain picks up materials that occur naturally such as dust, pollen, some CO2 and other chemicals produced by lightening or volcanic activities. These substances make the rain only slightly acidic (pH 6), which is not dangerous.
However, when rain falls through polluted air, it comes across chemicals such as gaseous oxides of sulphur (SOx), oxides of nitrogen (NOx), mists of acids such as hydrochloric and phosphoric acid, released from automobile exhausts industrial plants, electric power plants etc. These substances dissolve in falling rain making it more acidic than normal with pH range between 5.6 -3.5. In some cases the pH
even gets lower to 2. This leads to acid rain. The term acid rain is used here to describe all types of precipitation, namely; rain, snow, fog and dew more acidic than normal.
Damage to Plants
Acidic water is dangerous for plants. Sulphuric and nitric acid rain damages the bark and leaves of trees and harms the fine root hairs of many plants which are needed to absorb water. It washes nutrients out of the soil. Leaf pigments are decolourised because acid affects green pigment (chlorophyll) of plants. Agricultural productivity is also decreased. Several non-woody plants, such as barley, cotton and fruit trees like apple, pear, etc., are severely affected by acid rain. Since the acid concentration increases near the base of clouds by density, high altitude trees and vegetation may be exposed to pH levels as low as 3. Unique areas such as the Black Forest in Germany and sugar maples in Vermont (USA) are particularly threatened.
Damage to Animals
Acid rain chemically strips waterways of necessary nutrients and lowers the pH to where plants and animals cannot live. Most of the aquatic animals cannot survive when the pH is less than 4. Some species of fish, such as salmon, die even when the pH is less than 5.5. Certain species of algae and zooplankton are eliminated at pH less than 6. A reduction in the zooplankton and bottom fauna ultimately affects the food availability for the fish population. The problem is most severe downwind of industrial areas where fishing and tourism are major sources of income.
Material Damage
Metallic surfaces exposed to acid rain are readily corroded. Textile fabrics, paper and leather products lose their material strength or disintegrate by the acid rain. Building materials such as limestone, marble, dolomite, mortar and slate are weakened on reaction with acid rains because of the formation of soluble compounds.
CaCO3 + H2SO4 CaSO4 + H2O + CO2
These soluble compounds like limestone gradually dissolve so that the building material gradually crumbles away. Historical monuments are particularly prone to acid rain.
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