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The Concept and importance of pH Scale

24-FEB-2016 15:01

    S.P.L Sorenson, a Danish biochemist in 1909 devised a scale known as pH to represents the H+ ion concentration of an aqueous solution. The pH value of any solution is a number which simply represents the acidity and basicity of that solution. The pH value of any solution is numerically equal to the logarithm of the inverse of the hydrogen ion (H+) concentration. Hence, the pH of solution is referred to as the negative logarithm of hydrogen ion.

    pH = -log [H+]

          = log 1/ [H+]

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    Basic concept of pH value

    • pH of Neutral Solution (Pure Water): pH of water is 7. Whenever the pH of a solution is 7, it will be a neutral solution. Such a solution will have no effect on any litmus solution or any other indicator.
    • pH of an Acidic Solution: All the acidic solutions have a pH less than 7. So, whenever a solution has a pH less than 7, it will be acidic in nature and it will turn blue litmus into red as well as methyl orange pink and phenolphthalein colourless.
    • pH of a basic solution: All the alkaline solution have a pH of more than 7. So, whenever a solution has more than 7 values then it will be basic in nature and it will turn red litmus to blue, methyl orange to yellow and phenolphthalein to pink.

    pH values of the common substance from our daily life

    Solution

    pH

    Solution

    pH

    Conic HCl

    Saliva (before meal)

    7.4

    Oil HCl

    1.0

    Saliva (after meal)

    5.8

    Gastric Juice

    1.4

    Blood

    7.4

    Lemon Juice

    2.5

    Eggs

    7.8

    Vinegar

    4.0

    Toothpaste

    8.0

    Tomato Juice

    4.1

    Baking Soda Solution

    8.5

    Coffee

    5.0

    Washing Soda Solution

    9.0

    Soft Drink

    6.0

    Milk of Magnesia

    10.5

    Milk

    6.5

    Household Ammonia

    11.6

    Pure Water

    7.0

    Dilute Sodium Hydroxide

    13.0

    Concentrated Sodium Hydroxide

    14

     Things to remember about pH Value and concentration

    • An acid solution having low pH is stronger than another solution having higher pH values. A solution having pH of 2 is a stronger than a solution having pH of 5.
    • An alkali solution having higher pH value is a stronger than a solution of pH 10.
    •  Very Strong acids solution can have pH values less than zero and very strong basic solutions can pH greater than 14.

    Universal Indicator of colour at pH scale

    It is a mixture of different indicators (or dyes) which gives different colours at different pH values of the entire pH scale. The colour produced by universal indicators at various pH values are given in the table below:

    Colour

    pH

    Colour

    pH

    Dark Red

    Greenish Yellow

    8

    Red

    1

    Blue

    9

    Red

    2

    Navy Blue

    10

    Orange Red

    3

    Purple

    11

    Orange

    4

    Dark Purple

    12

    Orange Yellow

    5

    Violet

    13

    Greenish Yellow

    6

    Violet

    14

    Green

    7

     

    Importance of pH

    • In Agriculture: By determining the pH of the soil. We can find whether it is acidic or alkaline. This helps in deciding the type of fertilizer to be used and the types of crops to sown.
    • In Biological process: By knowing pH we can adjust the medium of biological processes like fermentation, enzyme hydrolysis, sterilization etc.
    • In corrosion research: By measuring the pH of sea-water, the effect of alkaline sea-water on the material used for building ships and submarines is studied.

    Buffer Solution

    A Buffer Solution or a buffer is defined as a solution whose pH does not change when small amount of an acid or base is added in it. It is used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. Many life forms thrive only in a relatively small pH range so they utilize a buffer solution to maintain a constant pH. In nature, the bicarbonate buffering system is used to regulate the pH of blood. The solution of Sodium Acetete and Acetic acid is an example of an effective buffer solution. The solution in which buffer solution is accomodated that works like an extremely slow acid.

    Image Courtesy: www.kangensui.us

     

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