Ph Curves And Indicators Flashcards
What are indicators ?
- indicators are weak acids where HA and A- are different colours
What happens to the indicators at low pH ?
HA is the main species present and so changes colour to the colour of the HA
What happens to the indicators at high pH ?
OH- is the main species present and so changes colour to the colour of the A-
What do indicators vary in ?
The pH at which the colour changes
What are 2 examples of indicators?
- methyl orange
- phenolphthalein
What is the colour of methyl orange when an acid is present ?
Red
What is the colour of methyl orange when a base is present ?
Yellow
What is the pH range of the colour change in methyl orange ?
3.2 to 4.4
What colour is phenolphthalein when an acid is present ?
Colourless
What colour is phenolphthalein when a base is present ?
Pink
What is the pH range of colour change of phenolphthalein?
8.2 to 10
What do pH curves show ?
- these show how the pH changes as an alkali is added to an acid or vice versa
What is meant by the equivalence point ?
- the point when the moles of an alkali added equals the moles of acid present
- but the pH is not always 7 at this point
What does the pH curve show in most acid-alkali reactions ?
They show a rapid change in pH around the equivalence point
What is the end point of a titration ?
- when the indicator changes colour, and if a suitable indicator is used then the end point should coincide with the equivalence point
