Slide 1
A scale for acidity
pH describes hydrogen-ion activity in a solution. Its logarithmic form compresses changes that would otherwise span very large numerical ranges.
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Activity is an effective measure that accounts for interactions in solution. In sufficiently dilute introductory examples it is often approximated by concentration, but precision measurements must keep the distinction clear.
Slide 2
One step means a factor of ten
A decrease of one pH unit corresponds to ten times the hydrogen-ion activity. A decrease of two units corresponds to one hundred times.
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Comparing pH 5 with pH 7 therefore gives a hundredfold activity ratio. It does not mean one sample contains one hundred times as much total acid, because acid strength and buffering also matter.
Slide 3
Neutral depends on conditions
Neutral water has equal hydrogen-ion and hydroxide-ion activities. Its pH is about seven at room temperature, but the neutral value changes with temperature.
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Heating changes the equilibrium by which water forms these ions. A pH shift caused by temperature need not mean that an outside acid was added, which is important when comparing measurements made under different conditions.
Slide 4
Buffers resist a change
A buffer contains chemical partners that can take up added acid or base. It can limit pH change over a finite range.
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Two liquids starting at the same pH can respond very differently to an equal addition of acid. Buffer capacity describes resistance to that addition, whereas the initial pH describes the starting condition.
Slide 5
Measure with a reference
A pH meter needs calibration with appropriate reference solutions. Temperature and electrode condition affect the reliability of the result.
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Indicator dyes provide colour-based estimates over limited ranges. An instrument’s extra decimal places do not remove sampling, calibration or temperature errors, so reported precision should match the quality of the procedure.
Sources
- pH and Water
US Geological Survey
- Measurement of pH
US Geological Survey