When we talk about the pH (“potential of hydrogen” or “power of hydrogen”) of a solution, we are discussing the measure of hydrogen ion concentration in that solution. In other words, pH is a scale used to specify the acidity or basicity of an aqueous solution. Generally, acidic solutions that contain higher concentrations of H+ ions have lower pH values than basic or alkaline solutions.

If the temperature is 25 °C and the solution has a pH of less than 7, then it is acidic. Solutions with a pH greater than 7 are basic. If a solution has a pH of 7 at this temperature, then it is neutral (e.g. pure water, which tends to dissociate slightly into equal concentrations of hydrogen and hydroxyl (OH−) ions, with a concentration of 10-7 moles per litre). Solutions are categorized as acidic or basic based on their hydrogen ion (H+) concentration compared to pure water.

Students should remember that the pH of an aqueous solution is based on the pH scale, which typically ranges from 0 to 14 in water.

Acidic solutions have higher hydronium concentrations and lower hydroxide concentrations. The hydrogen ion concentration in acidic solutions is greater than 10-7 moles per litre.

An alkaline (Basic) solution has a high concentration of hydroxide ions and a lower concentration of hydronium ions, with the concentration of H+ ions less than 10-7 moles per litre.

The concentration of hydrogen ions in a solution is expressed in terms of pH.

Additionally, some indicators (e.g. universal indicator paper) can be used to measure pH. This is because the colour of the indicator changes with the pH of the solution. A visual comparison of the colour of the test solution with a defined colour chart is done to determine the pH of the solution accurately to the nearest whole number. pH can also be measured using an electronic pH meter.

Mixture of Two Strong Acids

The strong acids completely dissociate in the given solvent; however, the strength of an acid and the concentration of acid are two distinct terms.

Acid Strength: It is a measure of the degree of ionization of an acid in an aqueous solution. The more cations and anions that are dissociated in the aqueous solution, the stronger the acid is.

Acid concentration: It is a measure of the number of acid ions that are present when the acid is dissolved in a solvent. The concentration is expressed as a ratio of the amount of solute to the amount of solvent in the solution. Therefore, the concentration of hydrogen ions is equal to the concentration of the acid. The concentration of hydrogen ions in the mixture is calculated by adding the concentration of the acid to the total volume of the solution.

Consider a mixture of two strong acids.

N1, V1 is the strength and volume of the first strong acid, and N2, V2 is the strength and volume of the second acid.

The concentration of the hydrogen ion in acid 1 is N1V1 and in acid 2 is N2V2.

Total hydrogen concentration = N1V1 + N2V2

Total volume of solution = V1 + V2

\(\begin{array}{l}[H^+] = \frac{N_1V_1 + N_2V_2}{V_1 + V_2}\end{array}\)

What is the pH of the solution that can be calculated using the below formula?

(\begin{array}{l}-log_{10}\left [ H^{+} \right ] = pH\end{array} )

Mixture of Two Strong Bases

The concentration of the hydroxide ion in the given solution is equal to the sum of the base concentration divided by the total volume, since strong bases are completely ionized.

Consider a mixture of two strong bases.

N1, V1 is the strength and volume of the first strong base, and N2, V2 is the strength and volume of the second base.

The concentration of the hydroxide ion in the first strong base is N1V1 and in the second base is N2V2.

Total hydroxide ion concentration = N1V1 + N2V2

The total volume of the solution = V1 + V2

(\begin{array}{l}[OH^-]=\frac{N_1V_1+N_2V_2}{V_1+V_2}\end{array})

‘(\begin{array}{l}[H^+] = \frac{10^{-14}}{[OH^{-}]}\end{array})’

What is the pH of the solution?

Check Out: Study the pH Change

Mixture of a Strong Acid and a Strong Base

The process of neutralization (pH = 7) occurs when a strong acid and strong base are mixed. The acidity or basicity of the resulting solution depends on the Concentration.

N1, V1 is the strength and volume of the strong acid, and N2, V2 is the strength and volume of the strong base.

If N1V1 > N2V2, the resulting solution will be acidic, with [H$^+$]= $\frac{N1V1-N2V2}{V1+V2}$

If $N_1V_1 < N_2V_2$, the resulting solution will be basic, with $$[OH^-]=\frac{N_2V_2-N_1V_1}{V_1+V_2}$$

Weak Acids

Partial ionization of weak acids can be calculated using Ostwald’s dilution law to determine the pH.

\(\begin{array}{l}H^+ + A^- \rightleftharpoons HA\end{array}\)

Initial concentration (moles/L), C₀: 0

At equilibrium, moles/L of C_1-αC_αC_α

The acid ionization constant, Ka, can be expressed as:

$$Ka = \frac{[H^+][A^-]}{HA} = \frac{(C\alpha +C\alpha)}{c(1-\alpha)} = \frac{c\alpha^{2}}{(1-\alpha)}$$

(i) For very weak electrolytes, since $\alpha \ll 1$, $(1 - \alpha) = 1$

(\therefore \alpha = \sqrt{\frac{Ka}{C}} = \sqrt{KaV})

(ii) Concentration [H+] of ion (=C\alpha = \sqrt{CKa}=\sqrt{\frac{Ka}{V}})

iii) (\begin{array}{l}pH=-\log\sqrt{CKa}=\frac{1}{2}\left(-\log Ka -\log C\right)\end{array} ) ;

Increasing dilution increases the ionization and, as a result, increases the pH according to the equation (\begin{array}{l}pH=\frac{1}{2}(pKa -\log C)\end{array} ).

Mixture of Strong Acid and Weak Monoprotic Acid

The hydrogen ion concentration, [H+], is equal to C1 + C2*α, where C1 and C2 are the concentrations of the strong and weak acids, respectively, and α is the degree of dissociation in the mixture.

The degree of dissociation of the weak acid will be less than the pure acid due to the higher [H+] from the strong acid, known as the levelling effect. If the [H+] is less than 10^-6 mole/l, the hydrogen ion concentration from water also needs to be taken into account.

Mixture of Two Weak Monoprotic Acids

Say the two weak acids HA1 and HA2 have concentrations C1 and C2, respectively, and degree of ionization α1 and α2, respectively.

Initial Concentration (Moles/L)

C₁(1-α₁)C₁α₁ + C₂α₂ = C₁α₁C₂(1-α₂) + C₁α₁C₂α₂

So, $$\begin{array}{l} Ka=\frac{[H^+][A^-]}{[HA]} \ Ka1=\frac{c_1\alpha_1(c_1\alpha_1+c_2\alpha_2)}{C_1(1-\alpha)} \ Ka2=\frac{C_2\alpha_2(1\alpha_1+C_2\alpha_2)}{C_2(1-\alpha_2)} \end{array} $$

Since $\alpha$ is small,

$K_{a1} = (C_1\alpha_1 + C_2\alpha_2)\alpha_1$

$K_{a2} = (C_1\alpha_1 + C_2\alpha_2)\alpha_2$

(\begin{array}{l}\alpha_1 = \frac{K_{a1}}{(C_1\alpha_1 + C_2\alpha_2)};;;;\alpha_2=\frac{K_{a2}}{(C_1\alpha_1 + C_2\alpha_2)}\end{array})

[H^+ = C_1\alpha_1 + C_2\alpha_2 = \sqrt{C_1K_a1 + C_2K_a2}]

Related Topics

Chemical Equilibrium

Ionic Equilibrium – Degree of Ionization and Dissociation

Equilibrium Constant – Characteristics and Applications

Le Chatelier’s Principle on Equilibrium

Solubility and Solubility Product

Acid and Base

pH Scale and Acidity

Hydrolysis, Salts, and Types

Buffer Solutions

Frequently Asked Questions (FAQs)

The pH of an acidic solution is less than 7.

The pH of a basic solution is greater than 7.

The pH of a basic solution is 8-14. Which pH value is considered neutral? 7

The pH value of 7 is considered neutral. What does pH indicate? pH is a measure of the acidity or alkalinity of a solution, with values ranging from 0 to 14. A pH of 7 is considered neutral, with values below 7 being acidic and values above 7 being alkaline.

The pH value is directly correlated to the concentration of hydrogen ions in the solution. As the concentration of hydrogen ions increases, the pH value decreases. Conversely, as the concentration of hydrogen ions decreases, the pH value increases.

The lower the concentration of hydrogen ions, the higher the value of pH, as pH and hydrogen ion concentration are inversely proportional to each other.