15.3
pH Scale
Hydronium and hydroxide ions are present both in pure water and in all aqueous solutions, and their concentrations are inversely proportional as deter…
The concentration of hydronium ions in an aqueous solution is usually written with negative exponents and can be as small as 1 × 10−14 M. The pH scale was developed by chemist Soren Sorenson in 1909 as a more convenient way to quickly compare the acidity of different solutions.
The pH of a solution is the negative logarithm of its hydronium ion concentration. For example, a solution with 1 × 10−5 M hydronium concentration has a pH of 5. The greater the hydronium ion concentration of a solution, the lower its pH.
As pH is expressed on a logarithmic scale, a single-unit change corresponds to a 10-fold increase or decrease of the hydronium ion concentration. A solution with a pH of 3 will have ten times more hydronium ions than a solution with a pH 4 and one hundred times more than a solution with a pH of 5.
An acidic solution has a higher concentration of hydronium ions than hydroxide ions and a pH less than 7, whereas a basic solution has a lower concentration of hydronium ions than hydroxide ions and a pH greater than 7.
A neutral solution with an equal concentration of hydronium and hydroxide ions has a pH of 7.
The concentration of hydroxide ions can also be expressed as pOH. pOH is the negative logarithm of the hydroxide ion concentration. The higher the hydroxide ion concentration, the lower its pOH value.
A pH or pOH value of an aqueous solution ranges from 0 – 14. This is because KW, the equilibrium constant for the autoionization of water, is equal to 1 × 10−14.
Taking the negative log of both sides of the equation results in an equation where pKW is equal to the sum of pH and pOH. Since the negative log of 1 × 10−14 is 14, the sum of the pH and pOH of an aqueous solution will always be 14. This can be used to determine the pH value when the pOH value is known and vice versa.
For example, a solution with a pOH of 10 has a pH of 4.
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Q1: Why was the pH scale developed?
Chemist Soren Sorenson developed the pH scale in 1909 to provide a more convenient way to quickly compare the acidity of different solutions. Since hydronium ion concentrations can range from 1 M to 1 × 10−14 M, expressing these values directly is impractical. The pH scale uses logarithms to compress this enormous range into a simple 0-14 scale.
Q2: How does a single pH unit change affect hydronium ion concentration?
Because pH is expressed on a logarithmic scale, a single-unit change corresponds to a 10-fold increase or decrease in hydronium ion concentration. For example, a solution with pH 3 contains ten times more hydronium ions than a solution with pH 4, and one hundred times more than a solution with pH 5. This logarithmic relationship makes pH a convenient way to express vast concentration differences.
Q3: What is the relationship between pH and pOH in aqueous solutions?
The pH and pOH of any aqueous solution always sum to 14 at 25°C because the ion product of water (Kw) equals 1 × 10−14. Since pH is the negative logarithm of hydronium ion concentration and pOH is the negative logarithm of hydroxide ion concentration, this relationship allows you to calculate one value when the other is known. For instance, a solution with pOH of 10 has a pH of 4.
Q4: How do acidic, basic, and neutral solutions differ in ion concentration?
An acidic solution has a higher concentration of hydronium ions than hydroxide ions and a pH less than 7. A basic solution has a lower concentration of hydronium ions than hydroxide ions and a pH greater than 7. A neutral solution has equal concentrations of hydronium and hydroxide ions with a pH of 7. These distinctions apply at 25°C, where Kw equals 1 × 10−14.
Q5: What determines the pH range of 0 to 14?
The pH range of 0 to 14 is determined by the ion product of water (Kw), which equals 1 × 10−14 at 25°C. Taking the negative logarithm of this value yields 14, establishing the upper limit. The relationship pH + pOH = 14 ensures that aqueous solutions at standard temperature fall within this range, with pH values below 7 indicating acidity and above 7 indicating basicity.
Q6: How does temperature affect pH classifications of solutions?
Temperature changes alter the ion product of water (Kw), which shifts the pH values that define acidic, neutral, and basic solutions. At 80°C, pure water has a hydronium ion concentration of 4.9 × 10−7 M, giving pH and pOH values of 6.31, making neutral solutions have pH = 6.31 rather than 7. This distinction is important when studying processes like enzyme reactions in warm-blooded organisms at 36-40°C.
Q7: How is pH mathematically calculated from hydronium ion concentration?
pH is calculated as the negative logarithm of the hydronium ion concentration: pH = −log[H3O+]. For example, a solution with 1 × 10−5 M hydronium concentration has a pH of 5. This formula can be rearranged to find hydronium concentration from pH, allowing chemists to understand weak acid solutions and dissociation constant calculations when analyzing solution properties.