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What Is pH?
       In science, pH is short for “power of hydrogen.” H is the accepted symbol or abbreviation for hydrogen. This is used to record whether a solution is acidic or basic. This is actually a negative algorithmic scale. The negative aspect means that a lower value is more concentrated and a higher value is more dilute. The fact that it is algorithmic means that a unit change represents a ten-fold change in concentration.
       The actual value of pH measures the moles of hydrogen ions per liter of solution. A pH of “n” means that there are 10-n moles of H+ per liter of solution. To express more simply, a pH of 1 means that there are 0.1 moles of H+ per liter of solution, while a pH of 2 means that there are 0.01 moles of H+ per liter of solution and a pH of 7 means that there are 0.0000001 moles of H+ per liter of solution. The pH scale makes it much simpler to record the concentration of hydrogen ions.
       Water (H2O) is made up of two ions, H+ and OH-. When they are equal, a solution is neutral. That is, it is neither acidic nor basic. This occurs when the pH is 7. The concentration of hydroxide ions (OH-) can be measured as pOH. The relation between pH and pOH is pH + pOH = 14. This means that the pOH of a neutral solution is similarly 7.
       A low pH (or high pOH) is very acidic. For example, a pH of 2 is 100 times as acidic as a pH of 4. If the pH of a solution is slightly below 7, it is a weak acid. Similarly, a pH a bit above 7 is slightly basic and a high pH is a strong base. Strong acids and strong bases are both dangerous, corrosive chemicals. Similarly, acids and bases react readily with one another. This reaction is known as neutralization, since the acid and base neutralize one another.
       In a neutralization reaction, an acid reacts with a base to produce water and a salt. The hydrogen and hydroxide ions join to create a water molecule. A salt is produced by combining the cation from the base with the anion from the acid. If hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the resulting salt is sodium hydroxide (NaCl), commonly known as table salt. Potassium hydroxide (KOH) neutralizes hydroiodic acid (HI) to produce potassium iodide (KI), which is iodized salt.
       Our tongue actually has a limited set of taste sensations. One is the ability to taste sour, which is actually noting the presence of hydrogen ions. This means the lower the pH, the more sour the taste we sense. For example, black coffee has a pH of 5.0 while tomatoes have a pH of 4.5. Vinegar has a pH of 2.8 while lemons have a pH of 2.5. Coca-Cola has a pH of 2.2, due to the fact that it includes phosphoric acid. Battery acid, which is a solution of sulfuric acid, has a pH of 1.0. Stomach acid, also known as gastric juice, is a solution of hydrochloric acid. It normally has a pH between 1.5 and 3.5.
       Acid-base indicators are chemicals that change color with a change in pH. This gives an easy, visible way to measure the acidity of a solution. Bromothymol blue is yellow in an acid and blue in a base. It is green when the pH is between 6.0 and 7.6. Litmus is red in an acid and blue in a base. It changes when the pH is between 4.5 and 8.3. Phenolphthalein is colorless in an acid and red in a base. It is pink when the pH is between 8.2 and 10. When pH paper is used in a lab, it actually contains a combination of acid-base indicators, resulting in a scale based on the range of colors created by combining the colors of the individual acid-base indicators that are present.
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