The Ph Scale

A pH scale is based how acidic or how basic a solution is. The scale ranges from 0-14 with 7 as the neutral number.

Basically a pH scale measures the concentration of H+ and OH-. The pH of a solution is defined as a negative logarithm of the hydrogen ion concentration and the pH scale measures this and places a value on it ranging from 0 to 14. The pH scale was developed because the concentration of the solution can vary by so many factors they found over time, and a pH scale was the easiest way to express the variation of the solution.

The pH scale ranges from 0 to 14. The 0 end of the scale is where the concentration is increasingly acidic. Moving up around 2 is lemon juice and stomach juices. Then around 3 are vinegar, beer and cola. Next at 4 is tomato juice. Then at 5 is black coffee and rainwater. Followed by urine at 6. Pure water and human blood are at 7. After 7 the concentration starts to become more basic as it heads up the scale. Most biological fluids are between pH 6 and pH8, there are a few exceptions to this like stomach acid. Then between 8 and 9 is seawater. Then at 10 is milk of magnesia. Followed by household ammonia at 11, household bleach at 12. Then between 13 and 14 is oven cleaner.

The internal pH of most living cells is close to 7. When there is even a slight change in the pH, this can be extremely harmful. It is harmful because the chemical processes of the cell are sensitive to the concentration of hydrogen and hydroxide ions. Biological fluids can resist change to their own pH when acids and bases are introduced because of the presence of buffers. Buffers in human blood for example maintain the blood pH very close to 7.4. A person cannot survive if the pH of their blood drops to 7or rises to 7.8. Under normal circumstances the buffering capacity of the blood prevents such swings in the pH level.

An acid adds hydrogen ions to a solution, but it also removes hydroxide ions because of the tendency of H+ to combine wit OH- to form water. The base has the opposite effect with an increasing OH- concentration but also reducing the H+ concentration by the formation of water.

Each pH unit represents a tenfold difference of the H+ and OH- concentration. It is this mathematical feature that makes the pH scale so compact. For example a solution of pH 2 is not twice as acidic as a solution of pH 4, but a thousand times more acidic. So when the pH of a solution changes slightly, it actually changes the concentrations of H+ and OH- substantially.

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