What is pH?

What is pH?

An easier way to measure the strength of an acid or base solution was worked out by the Danish biochemist. S. Sorensen in 1909.He was interested in checking the acidity of beer and introduced a scale known as pH scale (In German ‘p’ stands for ‘potenz’ meaning power).

Formula of pH

Many properties of aqueous solutions depend on the concentration of H+ ions in the solutions and therefore there is a need to express these concentrations in simple terms. For this purpose, we introduce the concept of pH.

pH = – log aH⁺ (where aH+ is the activity of H⁺ ions).

The activity of H⁺ ions is the concentration of free H+ ions in a solution. By free, we mean those that are at a large distance from the other ion so as not to experience its pull. We can infer from this that in dilute solutions, the activity of an ion is the same as its molar concentration since more solvent molecules would separate the two ions. For concentrated solutions, the activity would be much less than the concentration itself.

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Characteristic of pH and pH Scale

The scale runs from 0 to 14 and the characteristics of the scale are:

Acids have pH less than 7

The more acidic is a solution, lesser will be its pH

Neutral solutions (e.g., water) have pH of 7

Alkalies have pH more than 7

The more alkaline is a solution, higher will be its pH.

The pH scale may be shown as follows:

The pH of a solution is inversely proportional to the concentration of hydrogen ions in it. that is, a solution having a high concentration of hydrogen ions has a low pH value. On the other hand, a solution having low concentration of hydrogen ions has a high pH value.

The relation between them can also be expressed as:

pH = – log [H⁺]

Determination of pH of acids or bases

For strong acid

HCl (aq.) → H^+(aq.) + Cl^–(aq.)

pH = –log [H⁺]

If conc. of HCl is less than 10^–6M then we take conc. of H⁺ from water into accounts i.e. 10^–7. Otherwise we would neglect the conc. of H⁺ from water.

pH calculation for Strong Acids

Let us now see how to calculate the pH of a solution of a strong acid in water (it should be noted that pH calculations are only made for aqueous solutions). Let the strong acid be HCl. If we take 10–1M HCl, the [H+] would be 10–1 M, as HCl is a strong acid and would dissociate completely. Therefore the pH would be,

pH = –log 10^–1 = 1

We can see that for 10^–7M of HCl we have some hesitation in talking about the pH. This is because if we use our expression of pH, it works out to be 7 which is somehow associated with neutrality. We shall now explain how to calculate the pH of 10–7M HCl. Before we do this we shall discuss the dissociation of water.

When we add 10^–7 M HCl in water, the [H⁺] from HCl would be10^–7 M. But pH is the negative logarithm of the total H⁺ ion concentration of a solution and not that part which comes from only HCl ( you might wonder why we have changed our stand. This will become clear in a short while). Therefore, we need to add the concentration of H⁺ ions coming from water also. This can be done as

In presence of HCl, the dissociation of water will be suppressed & hence the concentration of H+ would be less than 10^-7 M.

Importance of pH to Aquatic Life

The human body works efficiently within a pH range of 7 and 7.8. Some living organisms can survive only within a narrow range of pH. In industrial areas, the pH of rainwater is less than 5.6, on account of the dissolution of acidic gases, such as sulphur dioxide and nitrogen dioxide. Such rain with a pH of less than 5.6 is called acid rain.

When the acid rainwater flows into rivers, it lowers the pH of river water, i.e., river water gets acidic. This in turn kills most of the aquatic life.

Importance of pH in Soil

Some crops, such as citrus fruit plants grow better in alkaline soil. Sugarcane grows better in neutral soil. Acidic soil is generally unfit for growing any crops. The soils get acidic when plant roots and plant residues are not removed from the soil. They rot in the soil to produce acids. Soils also get acidic when an excess of artificial fertilisers is used. The acidic soil can be brought back to normal health by spraying slaked~ lime in the fields. Conversely, alkaline soils can be cured by spraying gypsum powder. As stated earlier, different plants grow better in soil with specific pH values. Thus, the farmers need to get the pH value of their soil tested, before growing a particular crop. Otherwise, they are going to suffer huge financial losses due to crop failure.

pH in our Digestive System

Many people have a frequent complaint, commonly called acidity of the stomach. Our stomach produces gastric juice which contains a large amount of hydrochloric acid. The acid so produced does not harm the stomach walls, but kills germs and bacteria which enter our digestive system along with food. Thus, in a way, it protects us from diseases and helps digestion.

However, when we overeat, eat spicy foods or have worries or tension, the stomach produces a large amount of hydrochloric acid. Sometimes, this excess acid damages the walls of the stomach and causes stomach ulcers.

The acidity of the stomach can be reduced by taking antacid tablets. These tablets generally consist of magnesium hydroxide and aluminium hydroxide, which are mild bases. They react chemically with the hydrochloric acid and neutralise it.