AWRI Education

Instructor's Manual - Measurement of pH

What is pH?

measuring pHA natural body of water can be acidic, neutral, or basic. Many factors determine this condition including the composition of the material forming the basin holding the water, acidity of rain falling into the water, and the condition of water flowing into the body of water from streams, rivers, or storm runoff. The standard measurement used to indicate acidic or basic conditions is called pH.

Ions are electrically charged atoms or groups of atoms that are capable of conducting an electrical current in a solution. Pure water has a small number of water molecules will break up into positively charged hydrogen atoms (H+) and negatively charged hydroxyl ions (OH-). Since an equal number of negative and positive ions will be formed, the water remains electrically neutral; it is neither acidic or alkaline. Careful measurements show that pure water at 25 C ionizes so that 0.0000001 mole of positive hydrogen ions are liberated per liter of water. This number when written in scientific notation becomes 1 X 10-7. If this number is expressed on a negative logarithmic scale, it becomes 7.

The pH scale is a series of numbers ranging from 0 to 14 which denote various degrees of acidity or alkalinity. Values below 7 and approaching 0 indicate increasing acidity. Values from 7 to 14 indicate increasing alkalinity. Since the scale is logarithmic, the difference between pH 5 and pH 6 is not one but rather ten, that is, pH 5 is ten times more acidic than pH 6 (Table 3).

Table 3.    pH Scale



H+ moles/L


H+ moles/L


H+ moles/L


1 x 100


1 x 10-5


1 x 10-10


1 x 10-1


1 x 10-6


1 x 10-11


1 x 10-2


1 x 10-7


1 x 10-12


1 x 10-3


1 x 10-8


1 x 10-13


1 x 10-4


1 x 10-9


1 x 10-14

How is pH measured?

There are several ways to measure pH, which include pH paper, pH pen, and pH meters. For pH paper, strips of paper are saturated with an indicator that changes color with varying degrees of acidity. The color of the paper is compared to a color scale that is specific to the range and type of paper used. This means of determining pH measures only to about 1 pH unit; however, it is inexpensive. A pH pen is basically a simple electrode similar to that found in a pH meter. Both measure electrical potential associated with the hydrogen ion activity across an electrode immersed in the water sample. Accuracy ranges from 0.1 to 0.01 pH units.

A basic pH meter will have a device to measure voltage, a glass electrode to immerse in the water, a reference electrode that provides a constant electric potential, and a temperature compensation device. The pH readings are temperature dependent. The results are given in either pH units or millivolts (mv).

Many kinds of pH meters have been used on the D. J. Angus and the W. G. Jackson. Specific instructions for the model carried on-board are posted next to the instrument. Before it is used, the Science Instructors calibrate the pH meter. Two standard buffers (pH 7 and pH 10) are used to calibrate the instrument.  Buffers are standard solutions of a known pH value.

What is the significance of pH?

Changes on pH can be associated with wastewater discharges and sources of pollution. However, natural changes in pH occur with variations in levels of carbon dioxide. Carbon dioxide is very soluble in water. It enters the water from the atmosphere and is also generated from animal and plant respiration and decomposition. Dissolved carbon dioxide can combine with water to yield carbonic acid. Plants reduce amounts of carbon dioxide through photosynthesis making surface waters more basic.

Water quality standards generally call for a pH between 6.0 to 9.0. A pH between 6.7 and 8.6 will support a well-balanced fish population. Only a very few species can tolerate pH values less than 5.0 or greater than 9.0 (Table 4).. Lake Michigan water samples typically have a pH range of 7.0 to 8.6.

Table 4        Effects of pH on fish and aquatic life*

Limiting pH values  
Minimum Maximum Effects found in some scientific studies
4.0 10.1 Limits for the most resistant fish species
5.0 9.0 Tolerable range for most fish
4.5 9.0 Trout eggs and larvae develop normally
4.6 9.5 Limits for perch
4.1 9.5 Limits for trout
-- 8.7 Upper limit for good fishing waters
5.4 11.4 Fish avoid waters beyond these limits
6.0 7.2 Optimum (best) range for the fish eggs
7.5 8.4 Best range for the growth of algae
* Water Quality Criteria, California Water Quality Control Board 1963.

Our area certainly has acid rain. An interesting question is why acid rain is not as much of an issue for the Lake Michigan as it is for some lakes in New York and Canada. The actual effect of adding a highly acidic pollutant to a body of water is related to the acid neutralizing or buffering capacity of the water which is reflected in alkalinity measurements. The water of Lake Michigan has a much higher buffering capacity than lakes threatened by acid rain. The limestone (calcium carbonate) in the Lake Michigan basin is a natural buffer that helps to maintain soil and water pH near neutral.

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Page last modified January 31, 2014