Measurement of pH

What is pH?

A 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.

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 aquatic science instructors calibrate the pH meter. Two standard buffers (pH 7 and pH 10) are used to calibrate the instrument.

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. Lake Michigan water samples typically have a pH range of 7.0 to 8.6.

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.

Instructions for measuring pH:

1. Using the specially marked beakers from the rack at the pH lab station, obtain samples of water from the water sampling devices (Van Dorn Bottles) located on the rear deck. Use the beaker marked pH T to obtain 50 mL of the top water sample from the Van Dorn bottle marked "T". Use the beaker marked pH B to obtain 50 mL of the bottom water sample from the Van Dorn bottle marked "B". Be sure to match the symbols on the beakers with the same symbol on the Van Dorn bottle (the symbol "T" for top and "B" for bottom).
2. Bring the beakers containing the water samples back to the pH lab station and place them in the appropriate places in the rack. Measure the top water sample first then measure the bottom water sample. If a pH pen is used, turn the pen on, place the pen in the sample, and record the reading. Rinse with deionized water and repeat two more times. Turn off the pen when finished.
3. If a pH meter is used, remove the pH probe from the pH probe storage container. Rinse the pH probe with deionized (D.I.) water from the plastic squeeze bottle, catching the rinse water in the large beaker labeled WASTE WATER. Blot away excess D.I. water from the probe before lowering the probe into the top water sample.
4. Tilt the beaker containing the top "T" water sample and slide the white stir bar into the beaker. Turn the stir plate dial on and adjust the numbered stir plate dial until the stir is rotating smoothly and stirring the water sample.
5. Place the pH probe in the beaker with the top water sample, which is being stirred on the magnetic stir plate. Lower the probe into the water sample so that the tip is completely submerged, but not touching either the sides of the beaker or the rotating stir bar.
6. Press the ON/OFF (I/O) key on the pH meter to turn on the meter. When "READY" appears, read and record the numbers. At this point the reading on the meter display is the one that is to be recorded on the data sheet (pH T).
7. Remove the pH probe from the top water sample and position the probe over the WASTE WATER beaker. Rinse the probe with D.I. water from the plastic squeeze bottle and blot away excess D.I. water with paper toweling. Retrieve the stir bar from the top water sample and blot dry with paper toweling. Place the stir bar in the beaker containing the bottom water sample.
8. Repeat step 7 with the bottom "B" sample. Record the reading in pH units for the bottom sample in the appropriate place (pH B) on the data sheet. Round off pH units to one decimal place (>/= .05 would be .1 and <.05 would be .0).
9. Remove the probe by lifting up on the probe arm and rinse the probe with D.I. water over the WASTE WATER beaker. Place the probe in the pH probe storage container.
10. Remove the stir bar from the bottom water sample. Empty both top and bottom water samples into the sink. Rinse the sample beakers and stir bar with D.I. water, wipe dry with paper toweling and store as they were when you started.
11. For GLOBE trips repeat steps 4 through 7 two more times for the top and for the bottom. Calculate an average of the readings for the top sample and the readings for the bottom sample. Record the averages on the data board. Please note that this averaging method is for GLOBE only. The pH readings are logarithmic based and an arithmetic average is not appropriate.

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