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

HInd ⇄ H + + Ind = 5 ± 1= 4 to 6Transition 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 - is Transition Colour [HInd] = [Ind - ]

For this lab you will make an Indicators Chart Indicator Mouat Red Transition PointTransition ColourColour Change as pH Increases orangeRed to yellow

There is one indicator in your lab that is diprotic and can change colour twice. redblue [H 2 Ind] = [HInd - ] blueyellowgreen [HInd - ] = [Ind 2- ] purple H 2 Ind ⇄ H + + HInd - HInd - ⇄ H + +Ind Fake blue

For this lab you will make an Indicators Chart Indicator Mouat Red Transition PointTransition ColourColour Change as pH Increases orangeRed to yellow Fake blue purpleRed to blue Fake blue greenblue 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.

Transition Point= = 11.05

[HInd] Alizarin Yellow HInd ⇄ H + +Ind - Acid form Base form yellowred The solution is yellow when the pH < The solution is red when the pH > 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 - ] Only at the transition pointKa= [H + ] [HInd] = [Ind - ]

Calculate the Ka for methyl orange. Transition point pH= ( )/2 = 3.8 [H + ]=10 -pH = =1.58 x M Ka=[H + ]=2 x 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 ] =6.00 The indicator is chlorophenol red because ( )/2 = 6.0

The formal lab report should contain the following in order 1.Title Page 2.Handout with purpose 3.Introduction 4.Data- coloured sheet 5.Analysis of data- indicator chart in order of increasing pH 6.Questions 7.Conclusion