Chemical Concept

Introduction

Firstly, we introduce the concept of pH, which is the measure of concentration of hydrogen ions present in the aqueous solution. Then, we will introduce pOH, which is the measure of hydroxide ions present in the aqueous solution. Next, we will be elaborating on the tests which are currently being used to test the pH of aqueous solutions. Lastly, we will be elaborating on the differences between alkalinity and basicity.

 

Explanation

1) pH and pOH

With the concept of pH and pOH, we are able to determine the level of acidity and basicity present in the solution. The pH and pOH values, which range between 0 and 14, can be computed based on the following formula:

pH = -log[hydrogen ions]  ,    pOH = -log[hydroxide ions]

where [ ] represents the concentration of solution present in mol/L.

Because pH is a logarithmic function, one unit change in pH (e.g., from 7 to 6) indicates a 10x change in hydrogen ion concentration in that solution. In this case, a substance with pH 6 has 10 times more hydrogen ions than a substance of pH 7.

 

Another useful relationship between pH and pOH would be the concept of ion-product of water (K), which satisfies the following condition:

K = [hydrogen ions] [hydroxide ions] = 1E-14 (at 25 degrees Celsius)

The following relationship is derived consequently:

pH + pOH = 14

With the pH and pOH values, we are able to determine the acidity and basicity of solutions which can be classified as the following:

[hydrogen ions] = [hydroxide ions] (pH = 7 and pOH = 7 => neutral solution)

[hydrogen ions] > [hydroxide ions] (pH < 7 and pOH > 7 => acidic solution)

[hydrogen ions] < [hydroxide ions] (pH > 7 and pOH < 7 => basic solution)

2) Tests used to measure the pH values

  • Litmus Paper
    • Comes in 2 colours: red and blue
       Red litmus paper  Blue litmus paper
       Acidic Solution  Remains red Turns red
       Neutral Solution  Remains red Remains blue
       Basic Solution  Turns blue Remains blue

  • Methyl Orange and Phenolphthalein
    • Used to test the colour change of acids and alkaline at different pH
      Colour in acidic solution pH at colour change Colour in basic solutions
       Methyl Orange  Red colour 3 – 5 Yellow colour
       Phenolphthalein Colourless 8 – 10 Pink colour
  • Universal Indicator
    • Used to provide a more accurate pH value of the solution. Each different colour, ranging from red to purple, defines a pH value.
    • For example, red => pH 1, green => pH 7 and purple => pH 14

    • Range of colours to determine the pH of solutions using Universal Indicator

       

  • pH Strip
    • A more accurate indicator of the pH than using litmus paper
    • Different colours represent the different pH level in the solution and can be compared on a scale (as shown in the picture below)

    • pH strip indicator

  • pH Meter
    • A faster and more accurate way, compared to the universal indicator, in measuring the pH of any solution
    • It is an electronic instrument with a special sensor which is able to detect the different acids and bases. Such a pH meter can be seen below

    • pH meter

      pH meter

 

3) Differences between alkalinity and basicity

Technically, alkalinity and basicity are two different terms that people usually have misconceptions about.

Alkalinity usually refers to the parameter used to measure how much acid is required to lower the pH in a given environment and also to measure the how much acids and bases can be neutralised. For example, for aquatic bodies such as lakes, reservoirs and oceans, it is important for the water bodies to maintain the required pH to ensure the survival of aquatic life. So bicarbonates and carbonate ions act as buffers to maintain the required pH in different water bodies. Therefore, the hydrogen ions from water can react with buffers to increase the pH if it is too acidic.

On the other hand, basicity is a “property of bases, measured on the pH scale”. The way we define basicity has been mentioned above.

 

References

  1. Acids, bases and the pH value. (n.d.). Retrieved March 29, 2017, from http://www.mstworkbooks.co.za/natural-sciences/gr9/gr9-mm-05.html
  2. BBC – KS3 Bitesize Science – Acids, bases and metals : Revision, Page 4. (n.d.). Retrieved March 08, 2017, from http://www.bbc.co.uk/bitesize/ks3/science/chemical_material_behaviour/acids_bases_metals/revision/4/
  3. Bettinger, K. (2012). Books Worth Reading. Retrieved March 29, 2017, from https://www.pinterest.com/pin/83738874292057429/
  4. CM8001 Lecture slides (for chemical formulas)
  5. Differences Between Alkalinity and Basicity. (2015, November 19). Retrieved March 29, 2017, from http://www.differencebetween.com/difference-between-alkalinity-and-vs-basicity/