Acids Lesson 11 Indicators Theory

Ishihara Test for Colour Blindness – if you can read all of the numbers you have good colour vision

Indicators Indicators are organic chemicals with large complex formulas. They are weak acids where the acid and base forms have different colours.

Methyl Red

Acid Colour Base Colour

Experiment One drop of buffer of each pH is placed on an acetate Indicator: Mouat Red One drop of the each indicator is added to each buffer 0 2 4 5 6 7 8 10 12 14 We use HInd to represent any indicator HInd ⇄ H+ + Ind- HInd is Transition Colour Ind- is Transition Point = 5 ± 1 = 4 to 6 [HInd] = [Ind-]

For this lab you will make an Indicators Chart Transition Point Transition Colour Colour Change as pH Increases orange Red to yellow Mouat Red 4 - 6 0 2 4 5 6 7 8 10 12 14

There is one indicator in your lab that is diprotic and can change colour twice. Fake blue H2Ind ⇄ H+ + HInd- red purple blue [H2Ind] = [HInd-] HInd- ⇄ H+ + Ind2- blue green yellow [HInd-] = [Ind2-] 0 2 4 6 7 8 10 12 14

For this lab you will make an Indicators Chart Transition Point Transition Colour Colour Change as pH Increases purple Red to blue Fake blue 1 - 3 orange Red to yellow Mouat Red 4 - 6 green blue to yellow Fake blue 7 - 9 0 2 4 6 7 8 10 12 14

HInd ⇄ H+ + Ind- Red Yellow Le Chatelier’s Principle can be used to explain the colours of Mouat Red. In acid [H+] is high and it shifts left and turns red. In base [H+] is low and it shifts right and turns yellow. At pH = 5 [HInd] = [Ind-] orange

Look up Alizarin yellow on page 7 of your data package Look up Alizarin yellow on page 7 of your data package. It tells you that it changes from yellow to red as pH increases. This means that the acid form is yellow and the base form is red.

Look up Alizarin yellow on page 7 of your data package Look up Alizarin yellow on page 7 of your data package. It tells you that it changes from yellow to red as pH increases. This means that the acid form is yellow and the base form is red. Transition Point = 10.1 + 12.0 2 = 11.05

Alizarin Yellow HInd ⇄ H+ + Ind- Acid form Base form yellow red The solution is yellow when the pH < 11.05 The solution is red when the pH > 11.05 At pH = 11.05, the transition point, the: [HInd] = [Ind-] The solution looks orange, which is a blend of yellow and red. Ka = [H+][Ind-] [HInd] = [Ind-] [HInd] Ka = [H+] Only at the transition point

Calculate the Ka for methyl orange. Transition point pH = (3.2 + 4.4)/2 = 3.8 [H+] = 10-pH = 10-3.8 = 1.58 x 10-4 M Ka = [H+] = 2 x 10-4 An indicator has a Ka = 1.0 x 10-6, calculate the pH of the transition point. Ka = [H+] = 1.0 x 10-6 M pH = -log[H+] = -Log[1.0 x 10-6] = 6.00 The indicator is chlorophenol red because (5.2 + 6.8)/2 = 6.0

The formal lab report should contain the following in order Title Page Handout with purpose Introduction Data- coloured sheet Analysis of data- indicator chart in order of increasing pH Questions Conclusion