Concept of pH in Chemistry

Although the pH of a lake or polluted stream is seldom reported in the media, the people who are monitoring the health of the lake water record it daily religiously. But pH is quite an important physical property of water. pH of a stream gives an indication about the salts and other soluble matter present in the water. These soluble salts affect the organisms living in the water. Also changing pH in a stream can be an indicator of increasing pollution or some other environmental factor.

As we know life on our planet is based on the water. Water is a unique solvent as it dissolves a number of chemicals in it. Water molecule is a simple one with H2O as its formula indicating 2 hydrogen atoms joined to 1 oxygen atom. It V shaped geometry imparts it high polarity with which it is able to break down many salt molecules into individual ions called cations and anions. As such water is slightly ionized into equivalent amounts of hydrogen and hydroxyl ions. The value is 10-07 equivalents, a very very tiny value indeed. But this small value is enough to do all the chemistry. When chemicals dissolve in it, the values of hydrogen and hydroxyl ions become unequal. Particularly when any compound containing hydrogen ions is added to water, hydrogen ions become predominant and solution is said to become acidic and such chemicals are called acids. Examples of acids are hydrochloric acid, nitric acid, citric acid and sulphuric acid. On the other hand, when any chemical which have hydroxyl ions is added to water, hydroxyl ions predominate and water becomes basic and such chemicals are called bases.  Examples of bases are sodium hydroxide, washing soda and potassium hydroxide.

pH is a measure of how acidic/basic water is. It is defined as the minus logarithm of hydrogen ions concentration. Since -log[10-07] is 7, the pH of pure water is 7. The range goes from 0-14. Any solution having pH value less than 7 is acidic and above 7 is basic in nature.

The pH of any natural water determines availability of nutrients like phosphorus, nitrogen, and carbon for aquatic life. Many nutrients like phosphorous are very sensitive to pH since it exists in different forms at different pH values. Only a particualr form is assimilated by the organism.

Similarly, pH determines the solubility of heavy metals which are generally toxic in nature. Heavy metals are generally more soluble at lower pH and hence lower pH gives a warning about the possibility of these heavier and poisonous metals.

pH of Battery acid is 1, vinegar is about 3, milk slightly above 6, baking soda between 8 and 9 & ammonia solution about 12.Diagram of pH. pH 1=battery acid, 2=lemon juice, 3-vinegar, 6.5=milk, 8.5=baking soda, sea water, 10.5=Milk of Magnesia, 12=ammonia, 13=lye. ph 3 to 4=Adult fish die. ph 4-5=Fish reproductionaffected. pH 5-6.5=Normal range for precipitat on. pH 6-8=Normal range of stream pH. pH 1-5=Acid rain.

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