The pH scale — that staple of introductory chemistry courses — is the measuring stick that scientists use to gauge acidity in the Chesapeake Bay.

The scale normally runs from 0 to 14; the lower the number, the greater the acidity. A pH of 7, the middle point, is regarded as neutral pH. That is the reading for distilled water, or pure H₂O with nothing dissolved in it.

The pH scale indicates the concentration of hydrogen ions in a solution. Strong acids like sulfuric acid contain lots of unattached hydrogen ions floating in them; strong alkalines, like bleach, contain few.

The scale is logarithmic, which means it measures a very big range. A difference of one pH unit is equivalent to a 10-fold difference in hydrogen ions. A reading of "4" is 10 times as acidic as a 5 and 100 times as acidic as a 6.

The "H" in pH refers to hydrogen, but the "p" has several meanings (for example, "power"), depending on which science historian you ask.

The pH in the Bay varies widely over time and along its length, influenced by salinity and temperature. The open ocean's different chemistry makes its pH range tighter and, on average, less acidic than the Bay's.

Neither the Bay's waters nor surface waters of the ocean typically register an average pH in the acidic range, below 7. Their waters are not acidic. Scientists, however, speak of ocean water as "acidifying" or "acidified" because its pH has dropped over time, making it more acidic than a few decades ago. On average, the ocean's pH has dropped from 8.2 to 8.1 since the industrial age began. That seemingly small change translates to a 30-percent increase in relative acidity. Scientists forecast a further drop of about 0.3 pH units by the year 2100, lowering pH to 7.8. That level would translate to a further increase of 150 percent in relative acidity.

Parts of the Bay are already at least as acidic as pH 7.8. As for the trend of pH in the Chesapeake, that is less extensively studied.