AcidsLesson 11Indicators

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

HInd ⇄ H+ + Ind-

0 2 4 5 6 7 8 10 12 14

= 5 ± 1 = 4 to 6 Transition Point

We use HInd to represent any indicator

One drop of buffer of each pH is placed on an acetate

One drop of the each indicator is added to each buffer

Experiment

Indicator: Mouat Red

Hind is Ind- isTransition Colour

[HInd] = [Ind-]

For this lab you will make an Indicators Chart

Indicator

Mouat Red

Transition Point Transition Colour Colour Change as pH Increases

4 - 6 orange Red to yellow

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.

red blue

[H2Ind] = [HInd-]

blue yellowgreen

[HInd-] = [Ind2-]

purple

H2Ind ⇄ H+ + HInd-

HInd- ⇄ H+ + Ind2-

0 2 4 6 7 8 10 12 14

Fake blue

For this lab you will make an Indicators Chart

Indicator

Mouat Red

Transition Point Transition Colour Colour Change as pH Increases

4 - 6 orange Red to yellow

Fake blue 1 - 3 purple Red to blue

0 2 4 6 7 8 10 12 14

Fake blue 7 - 9 green blue to yellow

HInd ⇄ H+ + Ind-

[HInd] = [Ind-]

orange

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

YellowRed

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. 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 + 12.02

= 11.05

[HInd]

Alizarin YellowHInd ⇄ H+ + Ind-

Acid form Base formyellow redThe solution is yellow when the pH < 11.05The solution is red when the pH > 11.05At pH = 11.05, the transition point, the: [HInd] = [Ind-] The solution looks orange, which is a blend of yellow and red.

Ka = [H+][Ind-]

Only at the transition pointKa = [H+]

[HInd] = [Ind-]

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 MKa = [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

1. Title Page2. Handout with purpose3. Introduction4. Data- coloured sheet5. Analysis of data- indicator chart in order of increasing pH6. Questions7. Conclusion