What is the pH Scale?

pH Scale TestingThe pH scale was designed to measure the acidity or alkalinity of a liquid solution, in the case of a swimming pool, water, or H20.

The actual meaning of the letters p and H in the name of the scale is debatable. The most popular explanation is that pH is stands for potential hydrogen. Others say the P stands for Power, the word power being potenz in German. Others argue the P stands for puissance, power in French, or the Latin pondus.

Whatever the letters stand for, it is agreed that the scale measures the concentration of hydrogen ions in a liquid. Water molecules are made up of one oxygen atom and two hydrogen atoms. An ion is an atom with a positive or negative electrical charge.

In water, it is possible for hydrogen atoms to break away from the molecule; this forms one positively charged hydrogen ion and one negatively charged hydroxide ion. In a given amount of water, this will only happen naturally in about two molecules out of one billion.

pH Imbalances

Pure freshly distilled water has a neutral pH of 7.0, exactly in the middle of the scale, which runs from 0 to 14. Zero is the extreme acidic end of the scale, representing a greater proportion of hydrogen ions, and a pH of fourteen is the extreme alkaline end of the scale, representing a greater proportion of hydroxide ions.

In a swimming pool, it is important to monitor the pH balance, and aim for a pH of between 7.2 and 7.6. Water in a pool with pH that is too low can corrode tile grout, plaster, cement, metal railing, equipment and pipes. It is also irritating to the skin and eyes of swimmers. Water in a pool of too high a pH can cause chemical deposits to form on the pools walls, pumps, pipes, filtration and other equipment, wreaking havoc with water flow and filtration.

Technical Definition of pH

pH is defined as minus the decimal logarithm of the hydrogen ion activity in a solution. Because of its logarithmic nature, pH is a quantity with no dimension. The theoretical definition, by itself, has no practical application, because the hydrogen ion activity is the product of the concentration and an activity coefficient.

The single-ion activity coefficient of the hydrogen ion is a quantity which cannot be measured experimentally. To get around this difficulty, the electrode is calibrated in terms of solutions of known activity.

The applied definition of pH is officially defined by International Standard ISO 31-8 Annex C. Because pH is a logarithmic scale, a difference of one pH unit is equivalent to a ten-fold difference in hydrogen ion concentration.

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