December 27th, 2018


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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