pH Formula: Definition, Formula, Equation, Calculation and Application

pH Formula Overview

In the world of chemistry, the pH formula plays a crucial role in determining the acidity or alkalinity of a substance. Whether you're a student, scientist, or simply curious about the science behind pH, this article will provide you with a comprehensive understanding of the pH formula and its significance in maintaining the delicate acid-base balance. Read on to unravel the mysteries of pH and its practical applications.

What is pH?

The pH scale, derived from the German word "Potenz" meaning "Power," is a logarithmic scale used to determine the acidity or alkalinity of a solution. It represents the "power of Hydrogen" and was initially introduced by Soren Sorensen, a Danish chemist.

The pH equation quantifies even minute quantities of H+ ions in a solution. It is defined as the negative logarithm to the base ten of the concentration of H+ or H3O+ in moles per liter.

The pH scale spans from 0 to 14 and serves as a classification system for solutions based on the following criteria:

• pH < 7: Acidic Solution
• pH > 7: Basic Solution
• pH = 7: Neutral Solution

Understanding the pH Scale

The pH scale is logarithmic, meaning that each unit represents a tenfold difference in acidity or alkalinity. For example, a substance with a pH of 4 is ten times more acidic than a substance with a pH of 5. Similarly, a substance with a pH of 9 is ten times more alkaline than a substance with a pH of 8.

pH Formula in Chemistry

The pH scale is utilized in chemistry to express the acidity or alkalinity of a water-based solution. A solution is considered acidic when it contains a higher concentration of H+ ions, resulting in a lower pH compared to alkaline or basic solutions. The pH scale spans from 0 to 14, where a pH value between 0 and 6 indicates acidity, 7 represents neutrality, and a pH value between 8 and 14 signifies alkalinity. An acid, such as hydrochloric acid, serves as a prime example of an acidic substance, while calcium hydroxide exemplifies a base, and sodium chloride is an example of a salt.

The pH value is determined based on the activity of H+ ions within the solution. It is calculated by taking the negative logarithm of the molar concentration of hydronium ions ([H3O+]), which can also be represented as the concentration of H+ ions ([H+]). Thus, the pH equation is expressed as:

pH = -log[H3O+]

pH = -log[H+]

Calculation of Concentration of Ions from pH value

Given the pH value of a solution, it is possible to calculate the concentration of ions present in that solution. The pH equation can be rearranged as follows:

The concentration of H+ ions, [H+], can be determined as [H+] = 10^(-pH).

Similarly, there is another scale called the pOH scale, which is used to measure the concentration of ions in a solution. In the pOH scale, the negative logarithm of [OH-] is denoted as pOH:

pOH = -log [OH-].

The concentration of OH- ions, [OH-], can be calculated using the equation [OH-] = 10^(-pOH).

pH Formula of Water

Pure water is considered neutral, indicating that it has a pH value of 7, which signifies its lack of acidity or basicity. When pure water undergoes auto-ionization of self-ionization, it does so to a negligible extent. The concentration of H+ ions in water is [H+] = 1.0 x 10^-7 moles per litre.

Consequently, the pH of water can be determined as follows:

pH = -log [1.0 x 10^-7]

pH = -(-7)

pH = 7

Ionic Product Constant of Water

The process of self-ionization occurs in water, leading to its dissociation in the following manner:

Based on the Law of Mass Action, the Equilibrium Constant (Kw) can be expressed as Kw = [H3O+] [OH-]/[H2O]. Considering water as the solvent, its concentration can be estimated as 1.

Therefore, Kw = [H3O+] [OH-]. At room temperature, it is observed that Kw equals 1.0 x 10^-14.

From this, it can be inferred that in an aqueous solution:

• [H3O+] > [OH-], resulting in a pH value below 7 (Acidic Solution).
• [H3O+] < [OH-], leading to a pH value above 7 (Basic Solution).
• [H3O+] = [OH-], yielding a pH value of 7 (Neutral Solution).

Relation between pOH, pH & pKw

The interrelationship between pOH, pH, and pKw can be explained as follows:

The negative logarithm of Kw is represented as pKw:

–log Kw = pKw

–log Kw = –log ([H3O+] [OH-])

–log Kw = (–log [H3O+]) + (–log [OH-])

Therefore, we can deduce that pKw = pH + pOH.

Given that Kw equals 1.0×10^-14, it follows that pKw is equal to 14.

Consequently, the equation pH + pOH = 14 holds true.

pH Formula: pH Value Equation

Weak acids undergo partial dissolution in an aqueous solution and establish an equilibrium state. The equilibrium equation for such acids can be represented as:

HA = H+ + A-

The equilibrium constant for the weak acid, Ka, is calculated as the product of the hydronium ion concentration ([H3O+]) and the conjugate base concentration ([A-]):

Ka = [H3O+][A-]

Here,

• [H3O+] represents the hydronium concentration.
• [A-] represents the concentration of the conjugate base.
• HA denotes the concentration of the weak acid.

Similarly, when weak bases are present, they also partially dissolve and reach an equilibrium state. The equilibrium equation for such bases and their partial dissociation can be described as:

BOH = OH- + B+

The equilibrium constant for the weak base, Kb, is determined by the product of the hydroxide ion concentration ([OH-]) and the concentration of the ion formed ([B+]):

Kb = [OH-][B+]

In this context,

• [OH-] represents the concentration of hydroxide ions.
• [B+] represents the concentration of the ion.
• B indicates the concentration of the weak base.

pH Formula in Everyday Life

pH plays a vital role in various aspects of our daily lives, including:

1. Human Body: The human body relies on maintaining a specific pH range for optimal functioning. For example, the pH of blood is tightly regulated at around 7.4. Deviations from this range can lead to health issues.
2. Agriculture: pH affects soil fertility and the availability of nutrients for plant growth. Different plants thrive in different pH ranges, so maintaining an appropriate soil pH is crucial for successful crop cultivation.
3. Water Quality: pH is an essential parameter in assessing the quality of water. Drinking water with an improper pH can have negative effects on human health and can also impact aquatic ecosystems.

Read More About Importance of pH in Everyday Life

pH Formula: Testing and Measurement

To measure pH accurately, various methods and devices are available. The most common instrument used is the pH meter, which measures the electrical potential difference between a reference electrode and a pH-sensitive glass electrode. pH test strips and color-changing indicators are also widely used for quick and convenient pH testing.

pH Formula: Maintaining pH Balance

Maintaining the correct pH balance is vital for many industrial processes and biological systems. Different substances are used to adjust pH levels:

1. Acids and Bases: Adding acids to a solution decreases the pH, while adding bases increases the pH. Substances like hydrochloric acid and sulfuric acid are used to lower pH, while sodium hydroxide and ammonia are commonly used to raise pH levels.
2. Buffers: Buffers are solutions that resist changes in pH when small amounts of acid or base are added. They are widely used in chemical laboratories and biological systems to maintain stable pH conditions.

pH Formula: Application of pH Scale

pH paper serves as a tool to determine the pH level of a solution by immersing it in the solution and comparing the resulting color to a color chart to obtain the corresponding pH value.

The applications of pH include:

1. In industries like pharmacy, the pH value is of utmost importance.
2. Medicines undergo pH checks to ensure their pH matches that of the fluids in the human body.
3. Precise pH measurements of solutions are obtained using a pH meter.
4. A pH meter consists of a connected pH electrode that is dipped into the solution, displaying the pH value.
5. Buffer solutions are formulated to maintain a desired pH level in solutions, allowing them to resist pH changes when acids or bases are introduced.
6. In chemical reactions, maintaining proper pH equilibrium is crucial for optimal reaction conditions.

pH Formula: Things to Remember

1. pH represents the quantitative measure of acidity or alkalinity in an aqueous solution.
2. The pH formula is expressed as pH = -log[H3O+] or pH = -log[H+], where [H3O+] or [H+] denotes the concentration of hydronium ions.
3. The negative logarithm function in the formula helps convert the concentration of hydronium ions into a logarithmic scale.
4. The pH scale ranges from 0 to 14, with 7 being considered neutral.
5. A pH value below 7 indicates acidity, with lower values indicating stronger acidity.
6. A pH value above 7 indicates alkalinity, with higher values indicating stronger alkalinity.
7. For acidic solutions, the concentration of hydronium ions ([H3O+]) is greater than the concentration of hydroxide ions ([OH-]).
8. For neutral solutions, the concentration of hydronium ions ([H3O+]) is equal to the concentration of hydroxide ions ([OH-]).
9. For basic solutions, the concentration of hydronium ions ([H3O+]) is lower than the concentration of hydroxide ions ([OH-]).
10. The pH formula is widely used in various fields, such as chemistry, biology, environmental science, and industry, to measure and control the acidity or alkalinity of solutions.