It is a well known that pH of pure drinking water is seven. In reality it is possible that the drinking water does not always have the pH seven due to the fact that water pH varies because of different constituents of the water such as minerals and/or different chemicals used to clean the drinking water and to make it bug free. Natural water usually contains a number of microscopic contaminants, along with dissolved minerals such as calcium and iron. One way to remove these elements from water is to boil it until it changes to steam, a process known as distillation. When this steam is allowed to cool down and condense into liquid form again, the result is a purified form called distilled water. After the distillation procedure now water should ideally be nothing but hydrogen and oxygen molecules, with a PH level of 7 as all the additional containment, minerals and impurities have been removed. The pH for normal drinking water should ideally be in the range of 6 to 8.5 . A very low pH indicate it is getting acidic and a higher pH mean water have become basic. pure water (18.2 megohm) is always 7 but water does not stay pure if it comes in contact with the atmosphere. CO2 in the air quickly dissolves into pure water and forms carbonic acid.
Stored pure water has a pH of as low as 5. The longer it sits, the lower the pH. The pH of pure water in open air changes within seconds.
The scientific definition of pH is that it is a measure of the activity of the hydrogen ion (H+) and is measured as the reciprocal of the logarithm of the hydrogen ion activity. Therefore, a water with a pH of 7 has 10-7 moles per liter of hydrogen ions; whereas, a pH of 6 is 10-6 moles per liter. The pH scale ranges from 0 to 14. 
How is pH Measured?
pH is measured on a scale of 0 to 14, with 7 considered neutral, below 7 being acidic, and greater than 7 defined as basic. A lower number represents a greater number of hydrogen ions, and each pH unit represents a 10-fold change in the hydrogen ion concentration and amount of acidity. For instance, a pH of 6 has ten times more free hydrogen ions than a pH of 7, and one hundred times more free hydrogen ions than a pH of 8.
Since pure water has the perfect balance of Hydrogen and hydroxyl, there is no buffer. Anything that touches the water can easily change pH. In order to measure the true pH of pure water you need to add a buffer into pure water. The buffer you need is a concentrated solution of potassium chloride. The potassium chloride solution should be made by dissolving as much potassium chloride as you can into 50-75 mls of pure water. You then add 4 drops of concentrated potassium chloride in a clean empty beaker, add 50-100 ml’s of pure water to that and then measure pH. The potassium chloride does not shift pH and stops the CO2 from forming carbonic acid. If all goes well, the pure water pH should be 7.
Why is pH Important?
pH is highly important and is used to monitor for safe water conditions. Many animals cannot live in a pH level below 5 or above 9. Once the normal pH range for water has been established, a rise or fall in pH can indicate chemical pollution, or acid rain.
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