What is it?
pH is defined as:
pH = - log10 [H+]
where [H+] = concentration of H+
The pH scale ranges from 0 to 14, that denotes various degrees of acidity or alkalinity. A value of 7 is neutral; values below 7 and approaching 0 indicate increasingly acid conditions (higher H+ concentrations), while values above 7 approaching 14 indicate increasing alkalinity. Since these are logarithmic values, each integer represents a H+ concentration ten times greater than the next higher number. The average of pH values is not a meaningful statistic because of the above logarithmic relationship. Rather, a pH value corresponding to the average H+ concentration should be calculated (from antilogarithms of pH).
How is it measured?
The measurement of pH (hydrogen ion concentration) employs an electrode consisting of a proton selective glass reservoir filled with a pH 7 reference solution. Protons (H+ ions) interact with the glass, setting up a voltage potential across the glass. Since the H+ concentration of the reference solution does change, the difference between the voltage potentials are proportional to the observed pH.
The pH electrode (as part of the YSI 6600 sonde), is calibrated prior to each deployment. A two-point procedure utilizing reference (buffer) solutions with known pHs is used.
Why is it important?
A wide range of pH values is encountered in different lakes and reservoirs associated primarily with the different ionic chemistries of the respective watersheds/tributaries. Inorganic carbon constituents the major pH buffering system in most fresh waters. pH is an important regulator of chemical reactions and an important influence on aquatic biota (including composition). The temporal and vertical patterns of pH in lakes and reservoirs are mediated through the dynamics of photosynthetic consumption and respiratory/decomposition production of carbon dioxide (CO2). Photosynthetic uptake of CO2 tends to increase pH (e.g., during phytoplankton blooms) while decomposition/respiration tends to decrease pH.
What to Look for in Our Systems?
Central New York lakes, the Seneca River and Onondaga Creek are generally considered to be alkaline, hardwater lakes, where pH is well-buffered against major changes. Values of pH between 7 and 8.5 usually prevail. Higher values within this range generally occur in the upper productive layers where photosynthetic rates are the highest. Lower values prevail in the hypolimnion of more productive lakes due to respiration and decay of organic matter that has settled out of the productive overlaying epilimnion layer.
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