certain vividly coloured substances change colour depending on whether they are in an Acid 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 methyl orange 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 bromothymol blue: 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 ] 4 .