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Acids Lesson 11 Indicators Theory
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Ishihara Test for Colour Blindness – if you can read all of the numbers you have good colour vision
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Objectives 1. What are indicators? 2. Applications of indicators? 3. Formulae of indicator 4. Transition colour (lab) 5. Polyprotic Indicators 6. Find Ka of indicators (Lab) 7. pH range of indicators
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pH SCALE pH is the concentration of ____ expressed as a logarithm. Every change in the pH scale of one unit is a change in ______ times the concentration of H+. Example: pH 4 is 10X more acidic than pH 5. pH 3 is 1000X more acidic than pH 5. [ H+] 10
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Why is it Important to Maintain pH? Hydrangea in acidic soil
Hydrangea in basic soil
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pH INDICATORS
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pH INDICATORS
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pH INDICATORS
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pH INDICATORS Cabbage Juice paper in baking soda (BASE)
Cabbage Juice paper in lemon juice (ACID)
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pH INDICATORS
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pH INDICATORS
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Indicators as complex organic molecules
1. Indicators as complex organic molecules
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Indicators Indicators are organic chemicals with large complex formulas. They are weak acids where the acid and base forms have different colours.
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Methyl Red
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Methyl Red HMeth Meth-
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Methyl Red HIn In-
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Acid Colour Base Colour
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Make 14 different solutions
from pH 1-14 using 0.32 M HCl and 0.1 M NaOH 50 mL Each, use 100 mL beaker Label the beaker Check pH using pH paper
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Experiment different pH solution Indicator: Mouat Red One drop of the each indicator is added to each buffer We use HInd to represent any indicator HInd + H2O ⇄ H3O Ind- Hind is Transition Colour Ind- is Transition Point = 5 ± 1 = 4 to 6 [HInd] = [Ind-]
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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
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Find Transition point from range
2. Find Transition point from range
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There is one indicator in your lab that is diprotic and can change colour twice.
Fake blue H2Ind +H2O ⇄ H3O HInd- red purple blue [H2Ind] = [HInd-] HInd ⇄ H+ + Ind2- blue green yellow [HInd-] = [Ind2-]
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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
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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 = 2 =
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Sample Question Find Transition point of Methyl Orange
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Predict Acid Indicator and Base Indicator Colour
3. Predict Acid Indicator and Base Indicator Colour
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HInd + H2O ⇄ H3O+ + 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
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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.
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What colour will Bromocresol green give in 0.001M HCl?
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What colour will Orange IV give in 0.001M NaOH?
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What colour will Neutral Red give in 0.000321M HI?
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Calculate Ka of the Indicator
3. Calculate Ka of the Indicator
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Alizarin Yellow HInd +H2O ⇄ H3O+ + 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
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Calculate the Ka for methyl orange.
Transition point pH = ( )/2 = 3.8 [H+] = 10-pH = = 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 M pH = -log[H+] = -Log[1.0 x 10-6] = 6.00 The indicator is chlorophenol red because ( )/2 = 6.0
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pH INDICATORS
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pH INDICATORS
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Finding Ka of Red Cabbage
Lab. Finding Ka of Red Cabbage
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Finding Ka of Red Cabbage
Lab. Finding Ka of Red Cabbage Discuss steps —
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Finding pH range using different indicators
4. Finding pH range using different indicators
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5. What is the pH of a solution that is amber in neutral red and
5. What is the pH of a solution that is amber in neutral red and yellow in thymol blue? Look at page 7 amber in neutral red pH is 8 or higher yellow in thymol blue pH is 8 or less pH = 8
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6. What is the pH of a solution that is blue in Indigo carmine
6. What is the pH of a solution that is blue in Indigo carmine and thymolphthalein? Look at page 7 blue in indigo carmine pH is 11.4 or less blue in thymolphthalein pH is 10.6 or greater pH = 11
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Find pH of a solution Indicator Colour Range Final pH
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Page 163 Q 117, 118, 120
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