Water Chemistry: pH

pH

• pH is the measure of hydrogen ions (H+)– Negative logarithm of the H+ concentration

• Higher the pH, the lower the H+ concentration (Base or Alkaline)

• Lower the pH, the Higher the H+ concentration (Acid)

• Logarithmic Scale: 10 times each step– Example: There are 10 times as many hydrogen ions

available at pH 7 then pH 8• Range of 0-14

pH

• The pH of water determines solubility – Effects how much nutrients can be utilized

• How much and what form• And if aquatic life can use it

– Heavy metals affected as well• Degree of solubility determines toxicity• Generally more toxic at lower pH (acidic) because

they are more soluble

pH

• Natural Variation– Photosynthesis uses up dissolved carbon

dioxide • Results in an increase of pH (more basic or

alkaline)

– Respiration produces Carbon Dioxide• Results in a decrease of pH (more acidic)

pH

• So, would pH go up or down at night?

• Would pH go up or down with an increased depth?

pH

• Thankfully, aquatic environments can resist these changes– Buffering Capacity keeps the pH of Natural

waters between 6.5 and 8.5

pH

• Impact of pollution– Increased Temperature or excess nutrients

result in an increased pH (more acidic)

pH

• Remember:– More Acidic = lower pH value– More Basic/Alkaline = higher pH value