PH

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Basic Theory

The concept of acids and bases is introduced in many KS3/KS4 Science textbooks and those who have followed higher level studies in Science will be more familiar with the theory which underpins the subject. While this theory is useful from the point of view of the work that technicians are involved in, this article does not try to reproduce this. Instead, the reader is referred to the many texts that are available and only the basic concepts are given below.

  • pH (written as small 'p' capital 'H'), is the amount of hydrogen ions (H+) in a solution. It is measured on a logarithmic scale, [to the base 10] so it is important to remember that each pH scale has ten times more, or fewer, amount of hydrogen ions as the previous. So pH 2 has 10 times less H+ ions than pH1; pH3 has 100 times less H+ ions than pH1 [but only 10 times that pH2] and so on, [or visa versa.]
  • The pH scale measures how acidic (or how alkaline) a substance is. At 25OC;
    • An acid is a liquid that acts as a proton H+ donor. A liquid with a pH 1 to 6 is classed as an acid.
    • A Base is a liquid that acts as a proton acceptor H+. A liquid with a pH 8 to 14 is classed as an alkali.
    • Water commonly has a pH of 7.0 and is classed as neutral. However, distilled water may show a slight acidity due to the fact that Carbon dioxide from the air can dissolve in the liquid.

There are several different methods to measure the pH. One of these is using a piece of test pH indicator paper. When the paper is pushed into a solution it will change color. Each different color indicates a different pH-value. This general method is not very accurate and it is not suitable to determine more exact pH

Making pH Indicator paper
One method used in schools for making pH indicator paper is as follows;
Make your own "litmus paper"

  1. Chop a red cabbage up with a sharp knife.
  2. Crush it with a "pestle and mortar" OR put it in a food mixer and grind it.
  3. Add a very small amount of water.
  4. Decant the blue liquid (pour the liquid into a clean glass).
  5. Soak some blotting paper in the blue liquid.
  6. Let the blotting paper dry out.
  7. Cut the blue papers into small squares.



Other methods of demostrating pH

  • Use a strong tea solution [black tea works better than green] this also changes colour and clarity at different pHs'- although not as spectacular as the red cabbage, it does have the similar benefit of relating it to the world and substances that students know.
  • pH Indicators Solutions



Testph1.jpg

Of course Technicians do not need to make pH paper for use in the classroom environment. Instead this is generally bought in as indicator paper (20 strips in a booklet) that turns color corresponding to a pH on a color key.

  • Universal Indicator paper - generally covering pH 1 to 11. Each book is supplied with eleven color matches in steps of 1 pH (chart included inside cover of book).
  • Red Litmus paper - The red litmus will turn blue when it comes into contact with an alkaline solution.
  • Blue Litmus paper - The blue litmus will turn red or pink when it comes into contact with an acidic solution
Testph2.jpg

Another method of measuring the pH of a solution is to use a pH electrode which is coupled to a pH meter.



Other pH indicators

Note: Gray blocks indicate approximate pH range where the indicator is changing colour. You are also strongly encouraged to check CLEAPSS recipe cards for additional help and support for creating pH indicators

PH indicators.png


This article is not yet complete. You are invited to continue by adding more information about pH electrodes & meters



See also;



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