pH Collection Protocol

Background    The pH scale measures the relative acidity or alkalinity of any substance. The scale ranges from very strong acid, at pH 0, to very strong base, at pH 14 (Figure 1). Pure water has a neutral pH of 7, with an equal concentration of H+ (hydrogen) and OH- (hydroxyl) ions. Acidic water has a high concentration of hydrogen ions and a low concentration of hydroxyl ions. The reverse occurs in alkaline water. The scale is logarithmic, so a one unit difference in pH reflects a tenfold change in acid or alkaline concentration. For example, vinegar (pH 3) is ten thousand times more acidic than distilled water (pH 7).

Most aquatic organisms are sensitive to small pH changes and prefer a pH of 6.0 to 8.5. Waters with a pH beyond this range usually do not have enough species to maintain a food web. There are some specially adapted communities that inhabit water outside this pH range. They inhabit naturally acidic streams that drain hot springs or have high levels of tannic acid, or very productive alkaline lakes and streams.

Stream pH level depends on the geology of the surrounding area, and usually falls between 6.5 and 8.0. Streams that drain soils with high mineral content usually are alkaline, whereas streams that drain coniferous forests usually are acidic. Algal photosynthesis during a bloom can cause increased pH.

Air pollution from automobile and industrial emissions creates "acid rain" when nitrous oxide and sulphur dioxide dissolve in rainwater. Rain is normally acidic (pH 5.0 to 5.6), because water absorbs carbon dioxide from the air and transforms it into a weak acid. Heavy rainfall, snow melt, and road runoff can affect pH.

Procedure: Determine the pH when you take the late afternoon temperature measurement. (if you are taking both morning and afternoon temperature readings.) Otherwise, take the pH when you take the temperature measurement and record the time of day and location at which the measurement is made. The pH should be determined using the Hach pH test kit, following the procedure supplied with the kit.

pH reading (units)	_________
Time of Day:	_________	am/pm
Location:	_________

If you use pH paper to determine the pH, dip the test paper in the stream and compare the colour with the colour chart supplied with the pH papers. If you use a pH meter make sure it has been properly maintained, has fresh batteries, and has been calibrated against a sample of known pH. Enter the data from your pH test in the appropriate space on the Data Entry Form, depending on the method(s) used. It is a good idea to make several tests at the same site and average the results.

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