pH is an indication of the H+ (hydrogen ion) concentration of a substance. Each measured substance is given a pH value on a scale that ranges from 0 to 14.
pH Scale & Acidity of Common Substances (Source: US Fish & Wildlife Service)
The pH scale is logarithmic (increases by an order of 10 for each grade on the scale and is based on the relation between H^(+) and OH^(-) (hydroxide ion). Pure or deionized water contains equal concentrations of H^(+) and OH^(-) and is therefore considered neutral (pH=7). If the H^(+) concentration is greater than that of OH^(-), the substance is said to be acidic and if the OH^(-) exceeds the H^(+) level, the substance is alkaline.
pH and the Relationship Between H+ & OH-
Many organisms have a specific tolerance range when it comes to acidity. Although bacteria can tolerate an extreme range of acidity levels, organisms such as trout, mayflies and stoneflies have a short-range acidity tolerance.
pH Ranges That Support Life (Source: Water Quality Monitoring Handbook, 1994)
Keep in mind that a pH change from 8 to 7 means that the stream is 10 times more acidic. The pH of pristine freshwater ecosystems in Manitoba is around 8.3 making them slightly alkaline. Soil and substrate alkalinity is the major contributor to this pH level. Organic acids, released into the freshwater ecosystem from plant and animal tissues and processes affect freshwater pH and cause it to become less alkaline.
Respiration in the freshwater ecosystem produces CO2 as a by-product which when dissolved in water reduces water alkalinity as well. This is because the water-carbon dioxide reaction results in the production of the weak acid, carbonic acid.
Freshwater pH can be measured by a variety of simple methods. Low range indicator papers and pH meters all give a quick, accurate measure of acidity levels. Complete the following table by listing several indicators available to you.
Simple pH Indicators
| INDICATOR | COLOUR IN ACIDIC CONDITIONS | COLOUR IN NEUTRAL CONDITIONS | COLOUR IN ALKALINE CONDITIONS |
Can you suggest some pros and cons of using indicators such as these?
Use the indicators or pH meters available to determine the pH value of the freshwater at your sites.
Freshwater pH Analysis
| SITE A | SITE B | SITE C | SITE D | |
| pH |
Consider the sources of water and the results obtained. What inferences can be made on the basis of these results?
© 2004 Manitoba Waterways Project
Site Developed by: J. Cook
Questions or comments: Contact the Manitoba Waterways Project
Many files on this site are best viewed with the Adobe Reader.
To download the Adobe reader, visit the Adobe website.