pH
pH is a measure of the acidity or alkalinity of a solution
pH is defined as the negative logarithm of the Hydrogen ion activity
pH = -log[H+]
More specifically, pH is a measure of the amount of hydrogen ions in a solution
Plants and animals have tolerance ranges for pH, often between 5 and 9
pH concentrations can be affected by many factors
Natural factors
- Natural Acidity of Rain (CO2)
- Carbonate Materials
- Respiration
- Photosynthesis
- Decomposition
Man-made factors
- Acid Rain(SOx, NOx)
- Agricultural Runoff
- Wastewater Discharge
- Industrial Runoff
- Mining Operations
Pollution in the air, soil or directly in the water can all affect pH.
pH levels can fluctuate daily due to photosynthesis and respiration in the water. The degree of change depends on the alkalinity of the water.
A rise in CO2 increases the number of hydrogen ions in the water, reducing pH
CO2 + H2O ⇔ H2CO3
H2CO3 ⇔ (H+) + HCO3-
HCO3– ⇔ (H+) + CO32-
pH can be measured with a glass based combination electrode
A glass electrode is filled with neutral pH solution and a reference electrode includes a Ag-AgCl wire immersed in KCl electrolyte
The reference cell is terminated with a porous junction that allows electrons to pass while resisting the dilution of the KCl solution
Since the H+ concentration in the internal solution is constant, the potential (voltage) difference, measured relative to the Ag-AgCl reference electrode, is proportional to the pH of the solution
H+ ions bind to the glass both inside and outside, setting up a potential gradient across the bulb
Units of measure
Unitless value 0-14
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