 certain vividly coloured substances change colour depending on whether they are in an Base Indicators acidic or basic conditions  litmus is an example of this • litmus is red in an acidic environment • litmus is blue in a basic environment

 An acid-base indicator is any substance that changes colour in acidic or basic solution  there are several different indicators  an indicator’s colour change is linked with a + specific [H3O ]  Some common indicators are listed on page 230 of your textbook

1 Red cabbage juice is a useful Example indicator pH range Colour Change Methyl violet 0.0-1.6 Yellow to blue

 If methyl violet is added to a solution with a pH of  < 0 then the indicator is yellow  between 0 and 1.6 then the indicator is green (an intermediate colour)  >1.6 then the indicator is blue

 What colour would be in a  It is possible to estimate the pH of a solution + -4 using indicators solution of [H3O ] = 2.5 x 10 mol/L?  pH = -log (2.5 x 10-4 mol/L)  A solution was tested with different indicators with the following results:  pH = 3.62  Indicator would be orange  methyl orange : yellow  : yellow  bromocrescol green: blue

2 Try this using page 230 of your textbook  methyl orange : yellow  When different indicators were added to a  pH> 4.4 solution of unknown pH, the following results  bromothymol blue: yellow were noted. What is the pH of the following  pH<6.0 solution?  bromocrescol green: blue  pH>5.4  methyl violet: blue  methyl orange: yellow  Therefore the pH is between 5.4 and 6.0  methyl red: red  phenothalein: colourless

 When different indicators were added to a solution of unknown pH, the following results were noted. What is the pH of the following  A universal indicator solution? is made by mixing several indicators  methyl violet: blue pH>1.6 together  methyl orange: yellow pH>4.4  Universal indicator  methyl red: red pH<4.8 has different  phenothalein: colourless pH < 8.2 charateristic colours  Therefore the pH of the solution is between 4.4 over a range of pH and 4.8 values

3 Assignment

 pH meters are the  Page 242 Questions 2-11 most precise way to  Hand in for marks on Friday measure the pH of a solution  pH meters use electrodes to + measure [H3O ]

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