Water ionization and ionic product
Although water is considered a very weak electrolyte, it still conducts electric current due to a small amount of ions. This process is called autoprotolysis: water molecules exchange protons to form oxonium ions (H₃O⁺) and hydroxide ions (OH⁻). For simplicity, the equation is written in a shortened form: H₂O ⇌ H⁺ + OH⁻.
In pure water at 25 °C, the concentrations of these ions are equal: c(H⁺) = c(OH⁻) = 1 · 10⁻⁷ mol/L. The law of mass action implies a crucial rule for aqueous solutions: the product of the concentrations of hydrogen ions and hydroxide ions is always constant at a given temperature. This value is called the ion product of water (Kw). Under standard conditions, Kw = 1 · 10⁻¹⁴ mol²/L².
It is important to remember that in any aqueous solution, whether acidic or alkaline, both hydrogen ions and hydroxide ions are present simultaneously. When an acid is added, the H⁺ concentration increases, while the OH⁻ concentration drops proportionally so that their product remains equal to Kw.
Definition and pH scale
Since working with small numbers with negative exponents (e.g., 10⁻⁷) is inconvenient, a unified scale was introduced — the hydrogen ion exponent (pH). Mathematically, it is the negative decimal logarithm of the hydrogen ion concentration: pH = -lg c(H⁺).
The pH value determines the nature of the solution medium:
- Neutral medium: pH ≈ 7 (H⁺ concentration equals 1 · 10⁻⁷ mol/L).
- Acidic medium: pH < 7 (H⁺ concentration is high, exceeding 1 · 10⁻⁷ mol/L).
- Alkaline medium: pH > 7 (H⁺ concentration is extremely low, less than 1 · 10⁻⁷ mol/L).
The relationship between pH and hydrogen ion concentration is inverse. The higher the concentration of acid ions (closer to 1 mol/L), the lower the pH value (approaching zero). Conversely, in strongly alkaline solutions, the H⁺ concentration can drop to 10⁻¹⁴ mol/L, at which point the pH approaches 14.
Principles of pH calculation
To calculate the hydrogen ion exponent, it is necessary to know the exact molar concentration of hydrogen ions. For this, the concentration is expressed in exponential form (a number multiplied by 10 to the corresponding power) and then logarithmized.
In the case of strong electrolytes (strong acids, alkalis), dissociation proceeds almost completely to 100%. The ion concentration can be calculated directly from the initial substance concentration, taking into account the stoichiometric coefficients of the dissociation equation. For example, in a HCl solution with a concentration of 1 mol/L, the H⁺ concentration is also 1 mol/L.
For weak electrolytes (e.g., weak acetic acid), the situation is different. They dissociate into ions only partially, so the hydrogen ion concentration will be significantly lower than the concentration of the acid itself. To find c(H⁺), it is necessary to multiply the initial substance concentration (c₀) by the degree of dissociation (α).
Briefly: answers to topical questions
How to calculate the hydrogen ion concentration in a weak electrolyte solution?
To calculate the hydrogen ion concentration in a weak electrolyte solution, a formula relating the total concentration of the substance and its degree of dissociation is used. Since weak electrolytes dissociate only partially, knowing just the initial concentration is not enough. The calculation is performed using the formula: $c(H^+) = c_o \cdot \alpha$, where $c_o$ is the total concentration and $\alpha$ is the degree of dissociation. The dissociation constant ($K_{dis}$) can also be used for calculations. For example, for acetic acid at $c = 0.1$ mol/L and $\alpha = 1.3\%$, the hydrogen ion concentration will be $0.0013$ mol/L.
What are the normal pH values of human blood plasma?
The normal pH value of human blood is maintained at a constant level of 7.40 ± 0.05. This constancy of acid-base homeostasis is ensured by the action of interrelated buffer systems of the body. The blood serum pH is 7.4. Maintaining this parameter of biological fluids is important for the normal activity of enzymes, the regulation of osmotic pressure, and other processes.
Which buffer systems of the body maintain blood pH constancy?
The constancy of blood pH is maintained by four main interrelated chemical buffer systems. They represent the first mobile and effective system for compensating acid-base balance shifts:
- Bicarbonate buffer system — extracellular buffer.
- Phosphate buffer system — intracellular buffer.
- Protein buffer system — extracellular buffer.
- Hemoglobin buffer system — erythrocyte intracellular buffer, accounting for 1/3 of the blood's buffer capacity.
These systems begin to act immediately upon a change in hydrogen ion concentration and are capable of eliminating moderate shifts within 10–40 seconds.
What is the hydroxyl ion exponent (pOH) and by what formula is it related to pH?
The hydroxyl ion exponent (pOH) is the negative decimal logarithm of the activity or concentration of hydroxide ions. It is calculated by the formula: pOH = –lg[OH⁻]. It is related to the hydrogen ion exponent (pH) through the ion product exponent of water (pKw). This relationship is expressed by the following formula: pH + pOH = 14 (or pH + pOH = pKw). In a neutral medium, the values of both exponents are identical and equal to 7.
How does the value of the ion product of water (Kw) change with increasing temperature?
With increasing temperature, the value of the ion product of water (Kw) increases. This happens because the dissociation of water is an endothermic process. Upon heating, chemical equilibrium shifts to the right, resulting in an increase in the degree of dissociation of water molecules. Accordingly, the concentrations of hydrogen ions and hydroxide ions increase, which leads to an increase in Kw. At the same time, Kw depends only on temperature and is a thermodynamic constant.
The MORE hydrogen ions there are, the LOWER the pH value. The relationship is always inverse!
Class and exam questions
Are there hydroxide ions (OH⁻) in a strong acid solution?
Yes, there are. In any aqueous solution, both types of ions are always present. In an acid solution, the OH⁻ concentration is extremely low, but not equal to zero, which is described by the ion product of water.
How to calculate pH if the OH⁻ concentration is known?
First, using the ion product of water (Kw = 1 · 10⁻¹⁴), we find the H⁺ concentration: c(H⁺) = Kw / c(OH⁻). Then we calculate the negative decimal logarithm of the resulting number.
Why is knowing only the molar concentration of the solution insufficient for calculating the pH of weak acids?
Because weak acids do not dissociate completely. To calculate the actual hydrogen ion concentration, it is additionally necessary to take into account the degree of dissociation or the dissociation constant.
What else to study on this topic
- The influence of acid strength on the rate of chemical reactions
- Calculation of ion concentrations in salt solutions of strong electrolytes
- Calculation of pH in mixtures during neutralization reactions
- Amphoteric properties of hydroxides and their behavior in acids and alkalis
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