What is pH

pH is a quantitative measure of the acidity or basicity of an aqueous solution. It refers to the negative logarithm of H⁺ ion concentration. The term pH is derived from the German word "potenz" which means "power," and H is derived from the chemical symbol of hydrogen. So, pH stands for 'power of hydrogen' or the meaning of pH is also justified as the power of hydrogen.

pH Equation

The equation which is used to calculate pH was introduced by Danish biochemist Soren Peter Lauritz Sorensen. The pH equation is shown below;

pH= -log₁₀ (H⁺) (hydronium ion)

log = base- 10 logarithm

(H⁺) = hydrogen ion concentration (moles per litre)

pH Values of Common Chemicals

We come across various acids with low pH and bases with high pH in our daily life, some of which are described below with their pH values.

• Hydrochloric acid (HCl): 0
• Lemon juice : 2
• Vinegar : 2.2
• Wine : 4
• Pure water : 7.0 (neutral)
• Human blood: 7.4
• Soapy water : 12
• Bleach : 13
• Sodium hydroxide: 14

IUPAC definition of pH

The pH scale used by International Union of Pure and Applied Chemistry (IUPAC) is slightly different. It is based on electrochemical measurements of a standard buffer solution as shown below;

pH = -log aH+

aH+ shows the activity of hydrogen (effective concentration of hydrogen ions in a solution, which can be different from the true concentration of hydrogen).

This scale also uses thermodynamics factors that may affect pH. However, the standard pH definition is sufficient in most cases.

How pH is measured?

To measure the approximate or rough value of pH, we can use litmus paper or similar papers that tend to change their colour to indicate a certain pH value. Some indicators and pH papers only tell whether the solution is acidic or basic or tell pH value within a certain range.

Uses of pH

• It is not only used in science and industry but also in everyday life. It is used in cooking, preparing cocktails, in cleaners and in food preservation. pH value is also required in the maintenance of swimming pools and in water purification, agriculture, medicine, chemistry, engineering, etc.
• The value of pH tells the strengths of acids and bases quantitatively. It is a universal indicator that shows different colours for different concentrations of hydrogen ions in a solution.
• An optimum or balanced value of pH shows good health. The pH or level of hydrogen in our body is based on the type of food and drink we consume.

What is a pH scale?

It is a tool used to measure the acidity or alkalinity of a solution. The pH paper or pH test strips are used to measure pH. For example, litmus paper is a commonly used pH test paper or scale that ranges from 0 to 14. The colour that appears on the litmus paper while checking the pH matches with one of the numbers calibrated on the pH scale to indicate the pH value or nature of that liquid. For example, the pH value of vinegar on the pH scale is 2.4, which indicates vinegar is an acid.

Similarly, an aqueous solution whose pH is less than 7 at 25 degree centigrade is considered acidic, whereas, a solution with pH more than 7 are basic or alkaline in nature. The pH value 7 at 25 degree centigrade indicates a neutral solution for example; pure water due to the equal concentration of H⁺ and OH^- ions in pure water. Besides, the pH value can be negative (less than 0) in the case of a very strong acid and can be greater than 14 in the case of a strong base.

So, the pH scale is developed to measure the hydrogen ion concentration. Its values range from 0 to 14, which are known as pH values. So, we can say that the pH value is a number that indicates the acidic and basic nature of a solution.

Limitations of pH scale

There are some limitations of the pH scale, which are as follows;

• The pH value of two different solutions does not give an idea about their comparative strengths. For example, a solution with pH value 1 may have 100 times more hydrogen ions than that of a solution with pH value 3. Similarly, A 4x10^-1 N HCl has twice the concentration of 2x10^-1 N HCl, but there is a small difference in their pH values, which are 4.40 and 4.70 respectively.
• Acid cannot have a pH value more than 7, but the solution of acid having a low concentration (10^-8 N) has a pH value more than 7.
• The pH value of a strong acid is 0. But, if the concentration of the solution is 2N, 3N or 10N the pH value will be negative.
• Instead of pH Hammett acidity functions are used when the concentration is high.

Although pH means potent of hydrogen ion, which refers to potent or power of hydrogen ion or tells hydrogen ion concentration, it does not mean that it can only tell the strength of acids. It also tells the strength of bases. For example, a solution with a strong acidic character will have a poor basic character. Similar, a solution, which is a weak acid, will have a strong basic character and thus it can determine the basicity as well along with the acidity of a solution.

Furthermore, the acids and bases are opposite to each other. So, if hydrogen ions are more in a solution, it will be more acidic but due to the low concentration of hydrogen ions, it will be considered more basic. So, pH tells you both, the strength of an acid and the strength of a base.

In salts, acidic strength is equal to basic strength. So, their solutions are considered neutral solutions.

How to calculate pH

pH= -log₁₀ (H⁺) (hydronium ion)

The pH equation shown above shows that pH is calculated based on the hydrogen ion concentration, if you do not know this concentration, pH cannot be calculated. Let us understand hydrogen ion concentration first.

(H+) = concentration of hydrogen ions (H⁺), it refers to a mole of hydrogen ion in one litre of the acidic or basic solution.

In a solution of HCl in water or aqueous solution of HCl, the concentration of hydrogen ions (h⁺) is 10^-4. So, the pH of this solution can be calculated as follows;

pH = -log₁₀^(H+)

= -log₁₀ ^(10-4)

= - (- 4). log₁₀¹⁰

pH = 4

Similarly, we can calculate the pH of water as shown below;

The water is neutral as it releases both hydrogen ions and hydroxyl ions in equal amounts when it dissociates. The acidic property (release of hydrogen ions) is neutralized by the basic property (releases of hydroxyl ions), so it is actually neutral. However, the release of hydrogen ions occurs, so there will be some pH value, which may indicate a neutral solution, which is neither acidic nor basic.

At 298 K the hydrogen ion concentration (H⁺) in water = 10^-7, which means 10^-7 moles per litre.

So, pH of water can be calculate by using the pH equation as follows;

pH = -log₁₀^(H+)

= -log₁₀^(10-7)

= - (- 7) . log₁₀¹⁰

pH = 7

Like water, all neutral compounds have a pH value of 7.

The more acidic a solution, the less will be the pH value. So, we can say that pH value will be inversely proportional to acidity. However, it is directly proportional to the basicity. The more basic a solution, the more will be the pH value.

As pH is linked with H⁺, we can say that the higher the hydronium ion concentration (acidic character) in a solution, the lower will be the pH value of that solution. So, in simple words, pH value is a number that indicates the acidic and basic nature of a solution.

If pH > 7, solution is basic

If pH = 7, solution is neutral

If pH < 7, solution is acidic

Milk is slightly acidic as its pH value is 6. Whereas, the pH of blood ranges from 7.35 to 7.45.

Importance of pH in life

i) Effect on life:

The life of humans and all other living organisms like animals, plants, aquatic organisms etc., is dependent on the pH as they are sensitive to pH. The living organisms can survive within a narrow range of pH. For example, our body needs pH between 7.0 to 7.8 to work normally. It means our body adapt to a pH that occurs within this range such as 7.2, 7.5, etc., but cannot adapt to pH less than 7 or more than 7.8. Similarly, when the rain water that is acidic in nature enters the large water bodies like rivers, ponds alters their pH that may affect the life of aquatic animals.

ii) Effect on soil and cultivation:

The pH of the soil is checked before sowing the seed or before cultivating a crop. If the soil pH is not compatible with the crop, the crop may not grow and may cause a huge loss.

The pH of soil also tells us which type of fertilizers can be used, if we use fertilizers whose pH is not good for the soil, it may reduce the fertility of the soil or make it infertile. So, pH helps in the selection of fertilizers for a crop based on the pH of soil. For example, wild grass grows well in soil with pH 5, whereas at pH 6 green grass grows well.

Furthermore, some plants have acids such as nettle leaves have spikes that inject methanoic acid into the skin when someone comes in contact with them. The skin becomes red due to this acid. The cure of this skin redness is also present in the plant called dock plant that is often found beside the nettle plant. The dock plant leaves when rubbed over the affected area, it neutralizes the action of acid and thus alleviates the redness.

iii) Effect on digestive system:

HCl is produced in the stomach. It provides the required pH for the digestion of food. Its pH is around 5.5. However, if HCl is produced in large quantity, it may cause pain and irritation. In this case, antacid like milk of magnesia is used to maintain the required pH.

pH also affects tooth enamel, which is made of calcium phosphate. The pH in the mouth if goes below 5.5, may cause the decay of tooth enamel. The toothpaste contains the basic salt that helps maintain the pH in the mouth to prevent tooth enamel decay.

Points to remember

• pH of strong acid or base is dependent upon temperature.
• pH of weak acids tends to decrease with an increase in temperature as ionization increases with temperature.
• All liquids do not show pH values. Only aqueous solutions (something dissolved in water) show a pH value (acidic, neutral or basic). If water is not present, there is no pH. For example, there are various chemicals and liquids that do not have pH values such as vegetable oils, gasoline, pure alcohol do not have any pH values.