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Alkalinity is a term used to describe the buffering or acid-neutralizing ability of a sample from a body of water (such as a lake or stream). It is an aggregate property of water and is reported in units of milligrams of calcium carbonate per Liter of sample (mg CaCO3/L) or as parts per million (ppm).

As an aggregate property, one cannot interpret specific properties of the water's action unless its components are exactly known. In other words, there are many species dissolved in the water than can neutralize acid, but unless you know the exact identity and percentage of each species, you cannot make assumptions about individual species present. However, the majority components of Pennsylvania's waters are bicarbonate (HCO3-) and carbonate (CO32-) ions from limestone.

Method for Determining Alkalinity

  • A 50-mL aliquot of sample water is placed in a 150-mL beaker.
  • A calibrated pH probe is placed in the sample, along with a stir bar.
  • The sample is titrated with 0.02 N HCl[1] until the pH 4.5±0.1.

Alkalinity Calculation

Alkalinity is reported as the equivalent to milligrams of calcium carbonate per Liter (mg CaCO3 / L) or ppm, and is calculated as:

ppm alkalinity =

where is the volume of HCl (in milliliters) used to reach a pH 4.5; is the Normality of the HCl; and is the sample volume (in milliliters).

Comments and Interpretation of Results

A few of the sites sampled at Beaver Run are contaminated by abandoned (acid) mine drainage. As a result, the pH values of these sites is often < 4.5 to start, and so alkalinity values cannot be measured.

The normal or range of alkalinity in Pennsylvania streams is between 20-200 parts per million (ppm). Because of geology and other factors, there is no set standard for alkalinity in streams. In general, higher alkalinity relates to a) better acid-neutralizing ability (offsetting effects of acid rain, for example) and b) more nutrients for fish and other animals in a stream or lake.

Values below 20 ppm indicate a potential problem with the stream, usually due to abandoned mine drainage or other man-made phenomena.

See also: Acidity


  1. N = Normality; HCl is hydrochloric acid.

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