What is pH?
An easier way to measure the strength of an acid or base solution was worked out by the Danish biochemist. S. Sørensen in 1909. He was interested in checking the acidity of beer and introduced a scale known as the 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. Also Read Chemistry Formulas
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, the lower 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
| Concentration of HCl | pH |
| 10–1M | 1 |
| 10–2M | 2 |
| 10–3 M | 3 |
| 10–4 M | 4 |
| 10–5 M | 5 |
| 10–6 M | 6 |
| 10–7 M | 7 (?) |
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 the 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.
pH of Common Acids and Bases
| Substance | Type | Approximate pH | Explanation |
|---|---|---|---|
| Hydrochloric Acid (HCl) | Strong Acid | 0–1 | A highly acidic solution commonly found in laboratories and stomach acid. |
| Sulfuric Acid (H₂SO₄) | Strong Acid | 0–1 | Strong mineral acid used in batteries and industries. |
| Lemon Juice | Weak Acid | 2–3 | Contains citric acid, giving it a sour taste. |
| Vinegar | Weak Acid | 2–3 | Contains acetic acid and is used in cooking and food preservation. |
| Orange Juice | Weak Acid | 3–4 | Contains citric acid and vitamin C. |
| Tomato Juice | Weak Acid | 4–5 | Mildly acidic due to organic acids. |
| Black Coffee | Weak Acid | 5 | Naturally acidic beverage. |
| Milk | Slightly Acidic | 6–6.5 | Contains lactic acid and other dissolved substances. |
| Pure Water | Neutral | 7 | Neither acidic nor basic; standard reference point on the pH scale. |
| Blood | Slightly Basic | 7.35–7.45 | Maintained within a narrow pH range for proper body functions. |
| Baking Soda Solution | Weak Base | 8–9 | Contains sodium bicarbonate and neutralizes acids. |
| Toothpaste | Weak Base | 8–10 | Helps neutralize acids in the mouth. |
| Milk of Magnesia | Weak Base | 10–11 | Used as an antacid to relieve acidity. |
| Ammonia Solution | Moderate Base | 11–12 | Commonly used in cleaning products. |
| Soap Solution | Base | 9–10 | Alkaline in nature and effective for cleaning. |
| Sodium Hydroxide (NaOH) | Strong Base | 13–14 | Highly alkaline substance used in industries. |
pH Scale at a Glance
| pH Range | Nature |
|---|---|
| 0–3 | Strongly Acidic |
| 4–6 | Weakly Acidic |
| 7 | Neutral |
| 8–10 | Weakly Basic |
| 11–14 | Strongly Basic |
FAQs on pH Value
pH is a scale used to measure how acidic or alkaline a substance is. The term pH refers to the concentration of hydrogen ions present in a solution. The pH scale generally ranges from 0 to 14, where a value below 7 indicates acidity, 7 represents neutrality, and values above 7 indicate alkalinity. For example, lemon juice is acidic, pure water is neutral, and soap solutions are alkaline. The pH scale is widely used in chemistry, biology, agriculture, and environmental science to evaluate the properties of liquids and ensure suitable conditions for various chemical and biological processes.
pH is important because it influences many chemical reactions and biological functions. In the human body, maintaining the correct pH level is essential for proper enzyme activity, digestion, and overall health. In agriculture, soil pH affects nutrient availability and plant growth. Water quality is also closely linked to pH, as aquatic organisms require specific pH ranges to survive. Industrial processes, food production, and laboratory experiments often depend on accurate pH control to achieve desired results. Monitoring and adjusting pH helps ensure efficiency, safety, and optimal performance in a wide range of natural and man-made systems.
pH can be measured using several methods, depending on the level of accuracy required. One common method involves using pH indicator papers or strips that change color when dipped into a solution. The resulting color is compared with a reference chart to estimate the pH value. For more precise measurements, a pH meter is used. This electronic instrument measures the hydrogen ion activity in a solution and provides an accurate numerical reading. pH measurement is widely practiced in laboratories, industries, agriculture, and environmental monitoring. Regular testing helps maintain suitable conditions for various scientific and practical applications.
Several factors can influence the pH level of a substance or solution. The addition of acids lowers the pH, making the solution more acidic, while the addition of bases increases the pH and makes it more alkaline. Temperature can also affect pH readings by altering chemical reactions and ion concentrations. In natural environments, factors such as rainfall, pollution, mineral content, and biological activity can change the pH of soil and water. Industrial processes and agricultural practices may also impact pH levels. Understanding these factors is essential for maintaining balanced conditions and ensuring the effectiveness of various chemical and biological systems.