Acids, Bases and Salts – III (pH)

Acids, Bases and Salts – III

pH scale

pH scale 

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When an acid is dissolved in water, it dissociates hydrogen. Because of this dissociation of hydrogen ion in aqueous solution, number of hydrogen ions(H+) increases and number of hydroxide ions (OH-) decreases.

Example:

•    Hydrochloric acid (HCl) gives hydrogen ion (H+) and chloride ion (Cl-) when it is dissolved in water.

• Nitric acid (HNO₃) gives hydrogen ion (H+) and nitrate ion (NO₃-) in water.

•    Acetic acid (CH₃COOH) gives acetate ion (CH₃COO-) and hydrogen ion (H+).

 

Similarly a base dissociates hydroxide ion in water. Because of this dissociation of hydroxide ion (OH-) in aqueous solution, number of hydroxide ions (OH-) increases and number of hydrogen ions (H+) decreases.

Example:

•         When sodium hydroxide is dissolved in water, it dissociates hydroxide ion and sodium ion.

•      When potassium hydroxide is dissolved in water, it dissociates hydroxide ion and potassium ion.

Nature of solution

•        If the concentration of hydrogen ion (H+) is greater than hydroxide ion (OH-), the solution is called acidic.
•       If the concentration of hydrogen ion (H+) is smaller than the hydroxide ion (OH-), the solution is called basic.
•       If the concentration of hydrogen ion (H+) is equal to the concentration of hydroxide ion (OH-), the solution is called neutral solution.
•       If the concentration of hydrogen ion (H+) is much greater than hydroxide ion (OH-), the solution is called highly acidic.
•       If the concentration of hydrogen ion (H+) is much less than the hydroxide ion (OH-), the solution is called highly basic.

pH is a scale which quantifies the concentration of hydrogen ion in a solution. The range of pH scale is between 0 to 14 and the pH value increases with decrease in hydrogen ion concentration.

•          If the value of pH is 0, this shows maximum hydrogen ion concentration.
•         pH value equal to 14 shows lowest hydrogen ion concentration.
•            pH value equal to 7 shows that hydrogen ion concentration is equal to hydroxide ion concentration.
•        A neutral solution, such as distilled water has value of hydrogen ion concentration equal to 7 on pH scale.
•          The acidic solution has value of hydrogen ion concentration less than 7 on pH scale.
•         The basic solution has value of hydrogen ion concentration greater than 7 on pH scale.

Thus we can say that pH is inversely proportional to concentration of H+ ion.

pH value of different substances

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Universal Indicator –

Using a litmus paper, phenolphthalein, methyl orange, etc. only the acidic or basic character of a solution can be determined, but use of these indicators does not give the idea about the strength (pH value) of acid or base. So, to get the strength as well as acidic and basic nature of a given solution, universal indicator is used.

      ·         Universal indicator shows different colour over the range of pH value from 1 to 14 for a given solution.

       ·         Universal indicator is available both in the form of strips and solution.

      ·         The colour matching chart is supplied with universal indicator which shows the different colours for different values of pH.

Colour of Universal Indicator for different values of pH scale

Importance of pH in Everyday Life

       ·         pH in Digestive System -

When we start eating, our body produces strong dilute hydrochloric acid (HCl) to begin the process of digestion. Normally, your body produces just the amount of HCl and other chemicals it needs for digestion.

But, under certain conditions, our stomach starts producing more HCl acid. Too much stomach acid can cause pain and irritation. This problem is called acidity. At that point, we may have to take medicines called antacids that decrease the amount of active HCl in your stomach.

Antacids are actually anti acids or we can say basic in nature which neutralize acids in stomach and we feels good. For example milk of magnesia Mg(OH)₂.

·      Tooth Decay -

Our mouth is full of bacteria. Some bacteria are helpful. But some can be harmful such as those that play a role in the tooth decay process.

Tooth decay is the result of an infection with certain types of bacteria. These bacteria use sugars in food to make acids. Over time, these acids can make a cavity in the tooth.

Toothpaste is used to protect our teeth from those acids. Toothpaste is basic in nature which neutralizes the acids and thus protects our teeth.

     ·         Effects of pH on Animals and Plants -

pH of acid rain is around 5 – 6. This is highly acidic. Due to acid rain, acid reaches into oceans, lakes and affects the life of aquatic animals and plants. Some aquatic animals and plants are died due to acid rain and some plants lose lots of their leaves.

To neutralize this acidic behaviour of lakes and oceans, calcium carbonate (CaCO₃) is added into it.

Acids and Bases in absence of Water -

A dry acid does not dissociate in hydrogen ions. When we dissolve it with water, then it shows the acidic behaviour.

For example, dry hydrochloric acid (gas) does not change the colour of blue litmus paper to red because a dry acid does not dissociate hydrogen ion. This is the cause that a moist litmus paper is used to check the acidic or basic character of a gas.

Activity –

      ·         Take about 1g solid NaCl in a clean and dry test tube.

      ·         Add some concentrated sulphuric acid to the test tube.

Observation – A gas is released out, test the evolved gas with dry and wet blue litmus paper. Wet litmus paper changes its colour and there is no change in colour of dry litmus paper.

Reaction involved – 2 NaCl   +  H₂SO₄    →    Na₂SO₄    +   HCl

Conclusion – The change is colour in case of wet litmus paper shows that HCl gas releases its hydrogen ion in the presence of water.

Similarly at room temperature, sodium hydroxide is in the form of solid. So as a solid, NaOH does not dissociate into hydroxide ion. When we dissolve it with water, then it shows its basic behaviour.