What is pH definition formula and how to calculate pH
pH is a measure of how acidic or alkaline a substance is in comparison to water. It is one of the very common parameters we talk about in Chemistry, especially in analytical chemistry. The reason is that the chemical equilibrium of the majority of chemical reactions is affected by the pH of the aqueous medium. This could be extended to other media too. At several conditions, pH also affects the kinetics of the reactions, their rates and catalyst action. Let's have other information related to pH such as what is pH value, pH value of acids and other substances etc.
Concept of pH
The pH is defined mathematically as the negative log (base 10) of the H+ ion concentration in mol/Litre. Since the numbers are less than 1 and also vary over a wide range, a negative logarithmic scale is chosen to represent the pH. Also, as the scale is in negative logarithm, lower pH means a higher hydrogen ion concentration (acidic nature) and similarly a higher pH means lower hydrogen ion concentration (basic nature). Also, the base 10 logarithm represents a 10 fold change of hydrogen ion concentration as a unit change on the pH scale.
The pH of human tissues normally falls between 7.36 and 7.44. Although the neutral pH at 20°C has a value of 7.4, the dissociation of water molecules is relatively higher at physiological temperatures. Hence, at 37°C, the neutral pH is represented by the value of 6.8. This is the reason why body fluids are slightly alkaline.
A pH value of 7.7 represents a Hydrogen ion concentration of 0.00000002 mol/L. Let’s try to understand this mathematically:
pH = 7.7
H+ = 10-7.7 mol/L
= 10-8 x 100.3 mol/L
=0.00000001 x 2 mol/L
= 0.00000002 mol/L
= 0.00002 mmol/L
(1 mol/L = 1000 mmol/L)
What is pH Value?
The pH range goes from 0 upto 14, where 7 represents neutral (pH value of water). If the pH is less than 7, it shows the acidic nature of the substance. If it is greater than 7, it shows the alkaline or basic nature of the substance. pH is actually a measurement of relative amounts of hydrogen and hydroxyl ions (H+ and OH-). Solutions having more free hydrogen ions are acidic and solutions having more hydroxyl ions are basic. The addition of chemicals alters the pH of water. Hence, pH is an important factor to consider in order to know what is changing chemically.
What is The pH Scale?
The pH scale is a scale that quantifies the acidic or alkaline behaviour of a substance. The range of the pH scale is mapped from 0 to 14 for highly acidic to highly alkaline. pH 7 denotes a neutral (neither acidic nor basic) substance.
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Every unit decrement of the pH scale reading signifies the increase in the concentration of hydrogen ions by a factor of 10 times. For instance, pH 3 denotes ten times stronger acidic than pH 4. A similar relationship holds for the pH values above 7 and the alkaline nature of the compound. A unit increment in pH above 7 denotes a ten times stronger base. For instance, a pH of 9 denotes a ten times stronger base than the pH of 8.
pH Value of Acids and Bases
Whether a solution is an acid or a base is decided by the hydrogen ions present in the solution. A very small fraction of pure water molecules also dissociates at room temperature, forming hydrogen and hydroxide ion pairs. Some of the hydrogen ions get combined with water molecules to form hydronium ions (H3O+). For simplicity, if we consider hydronium ions as hydrogen ions, there will always be equal numbers of hydrogen and hydroxide ions present in water. Hence, it is neither acidic nor alkaline.
In an acidic solution, a hydrogen ion donating substance is present. This shifts the balanced condition or equilibrium between the hydrogen and hydroxide ions. The solution now contains hydrogen ions more than hydroxide ions and hence behaves as an acid.
In a basic solution, just the opposite happens. A hydrogen ion accepting substance is present in such a solution. Or in another way, we can say that the number of hydroxide ions becomes more than that of hydrogen ions. Thus, the equilibrium is shifted in a direction just opposite to that of acids. Hence, the solution behaves as a basic solution.
pH of Food
Being a direct measure of acidic content, ph becomes one of the most common analytical measurements in industrial food processing. The pH of the food influences the amounts and types of preservatives to be used, and it becomes the most decisive factor in the selection of the container in which they will be packed.
Among various reasons for measuring pH in food processing, some are listed below:
To keep the production properties consistent between different batches
To optimise the efficiency of production and keep the cost-optimal
To keep the food safe to consume without causing any health problems to consumers
To meet the standards and regulation conditions set by regulatory bodies
As the pH scale is logarithmic, even a small change in its reading cannot be avoided. A change of 0.3 on the pH scale reflects the double acidic concentration. Hence, even a small variation of pH is significant and has impacts on the flavour, consistency and shelf life of packed foods.
The departments of health and other regulatory bodies suggest a certain allowed value range of pH of the sanitation solutions be used. For instance, a pH between 8 and 10 is suggested from chlorine concentration. Similarly, the iodine solution is supposed to fall below five on the pH scale.
The pH of some common things that we see in our daily lives are shown below:
The pH of Some Common Substances (pH Table)
Did You Know?
Only aqueous solutions can be classified into acids, bases or neutral solutions. Other non-aqueous liquids such as oils cannot be acid or base.
Acids and bases have the property of reacting with each other in a process called neutralisation. The reaction ends up producing salt, water and leaving the pH between those of the reacting components and closer to neutral.
As they split into ions which behave as charge carriers in aqueous solutions, acids and bases both conduct electricity.
Conclusion
pH scale information is the measure of the acidic or alkaline nature of aqueous solutions. The range of pH scales varies from 0 up to 14. The midpoint of the scale, i.e. 7 denotes a neutral solution (equivalent to pure water). pH less than 7 reflects the acidic nature,, and greater than 7 reflects the basic nature. pH basically is the measure of hydrogen and hydroxyl ions present in a solution.
FAQs on Understanding pH Scale in Chemistry
1. What is pH in chemistry?
The pH is a numerical scale that measures the acidity or basicity of a solution based on its hydrogen ion concentration. It is defined as pH = −log10[H+], where [H+] is the molar concentration of hydrogen ions in mol/L.
- pH < 7 → acidic solution
- pH = 7 → neutral solution (pure water at 25°C)
- pH > 7 → basic (alkaline) solution
2. How do you calculate pH from hydrogen ion concentration?
To calculate pH, use the formula pH = −log10[H+].
- Step 1: Determine the hydrogen ion concentration in mol/L.
- Step 2: Take the base-10 logarithm of [H+].
- Step 3: Add a negative sign.
3. What is the pH of pure water?
The pH of pure water at 25°C is 7, which means it is neutral. Pure water undergoes self-ionization: H2O(l) ⇌ H+(aq) + OH−(aq). At 25°C:
- [H+] = 1.0 × 10−7 mol/L
- [OH−] = 1.0 × 10−7 mol/L
4. What is the relationship between pH and pOH?
The relationship between pH and pOH at 25°C is given by pH + pOH = 14. The pOH is defined as pOH = −log10[OH−]. This relationship comes from the ionic product of water: Kw = [H+][OH−] = 1.0 × 10−14 at 25°C. If you know one value, subtract it from 14 to find the other.
5. How do you calculate pH of a strong acid?
The pH of a strong acid is calculated directly from its concentration because it completely ionizes in water. For example, hydrochloric acid dissociates as HCl(aq) → H+(aq) + Cl−(aq).
- Step 1: Determine the molarity of the acid.
- Step 2: Since it fully dissociates, [H+] equals the acid concentration.
- Step 3: Apply pH = −log[H+].
6. How do you calculate pH of a strong base?
To calculate the pH of a strong base, first find pOH using hydroxide ion concentration, then use pH = 14 − pOH. For example, sodium hydroxide dissociates as NaOH(aq) → Na+(aq) + OH−(aq).
- Step 1: Determine [OH−].
- Step 2: Calculate pOH = −log[OH−].
- Step 3: Subtract from 14 to find pH.
7. Why does the pH scale usually range from 0 to 14?
The pH scale ranges from 0 to 14 because the ionic product of water at 25°C is Kw = 1.0 × 10−14. Since pH + pOH = 14, the typical limits in aqueous solutions fall between 0 and 14.
- Very strong acids can have pH < 0.
- Very strong bases can have pH > 14.
8. What is the difference between acidic, basic, and neutral solutions in terms of pH?
An acidic solution has pH < 7, a neutral solution has pH = 7, and a basic solution has pH > 7 at 25°C. This classification depends on the relative concentrations of hydrogen and hydroxide ions.
- Acidic: [H+] > [OH−]
- Neutral: [H+] = [OH−]
- Basic (alkaline): [H+] < [OH−]
9. How is pH measured in the laboratory?
pH is measured using a pH meter or acid–base indicators.
- pH meter: Uses a glass electrode to measure hydrogen ion activity accurately.
- Indicators: Substances like litmus or phenolphthalein that change color over a specific pH range.
- Universal indicator: Provides a color scale for approximate pH values.
10. What is the pH of a weak acid and how is it calculated?
The pH of a weak acid is calculated using its acid dissociation constant Ka because it only partially ionizes in water. For example, acetic acid ionizes as CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq).
- Step 1: Write the expression Ka = [H+][A−]/[HA].
- Step 2: Use an ICE table to find [H+].
- Step 3: Apply pH = −log[H+].
