SECTION 6 .3 PG. 245-247

Acid – Base Indicators

Acid – Base Indicators

Substances that change colour when the acidity of the solution changes are known as _________________

A very common indicator used is __________, which is obtained from lichen

___________paper is prepared by soaking absorbent paper with litmus solution and then drying it.

Acid – Base Indicators

Acid-base indicators are unique chemicals because they can exist in ________________, each with a distinctly different colour

The form of the chemical depends on the ___________ of the solution.

They usually have very complicated formulas, so simple __________ are used:

__ – litmus; __ – bromothymol blue; __ – indicator (generic)

The two forms of any indicator depend on whether a particular hydrogen atom is present in the indicator’s molecule.

In general, the lower pH form is designated _________

In general, the higher pH form is designated ________

Acid – Base Indicators

Other acid-base indicators: Because the chemical structure of each indicator is different, the

pH at which the indicator changes from the HIn(aq) form to the In-(aq)

form is different for each indicator. (See inside back cover of textbook)Common name Color of

HIn(aq)pH range of

colour change

Color of In-(aq)

Bromothymol blue

Yellow 6.0-7.6 Blue

Phenolphthalein Colourless

8.2-10.0 Pink

A Close Look at Litmus Paper

The colour changes of litmus are a little more complicated than what you have learned previously.

There is a “fuzzy” region around the neutral point (pH=7) where the colour is not easily distinguished.

Between 6.0-8.0, litmus is in the process of changing colour and you will see mixtures of red and blue.

These intermediate colours are only somewhat useful, with the exception of bromothymol blue, where the green is a distinct colour in the middle of the range

Common name Color of HIn(aq)

pH range of colour change

Color of In-(aq)

Litmus Red 6.0-8.0 Blue

Uses of Acid-Base Indicators

Acid base indicators have two primary uses:1) Indicators are commonly used to mark the end of a

_______________ (you will learn more about this in Chapter 8)

2) Indicators are used to _________________ the pH of a solution by using a number of different indicators. In this method, acid-base indicators are used to replace

the more expensive pH meter for simplicity, although indicators are not as accurate.

Example:Solution After addition to samples

of the solution …pH range implied by each indicator

pH of solution

A

● methyl red was red

● thymol blue turned yellow

● methyl orange turned red

pH must be < 4.8pH > 2.8 but < 8.0pH must be < 3.2

Combining these results, the pH must be between 2.8 and 3.2

Practice

Lab Exercise 6.B pg. 247 Complete the Analysis Portion of the lab below

Solution

After addition to samples of the solution …

pH range implied by each indicator

pH of solution

A

● methyl violet was blue● methyl orange was yellow ● methyl red was red ● phenolphthalein was colourless

B

● indigo carmine was blue● phenol red was yellow● bromocresol green was blue● methyl red was yellow

C

● phenolphthalein was colourless● thymol blue was yellow● bromocresol green was yellow● methyl orange was orange

Practice (continued)

Pg. 247 #1-3 (#4 BONUS)

Quiz Wednesday Properties of Acids/Bases pH, pOH calculations Acid base indicators