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Acid – Base Indicators

SECTION 6.3 PG. 245-247

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Acid – Base Indicators

  • Substances that change colour when the acidity of the solution changes are known as acid-base indicators

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  • A very common indicator used is litmus, which is obtained from lichen

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

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Acid – Base Indicators

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

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  • The form of the chemical depends on the acidity of the solution.

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  • They usually have very complicated formulas, so simple abbreviations are used:
  • Lt – litmus; Bb – bromothymol blue; In – indicator (generic)

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

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  • In general, the lower pH form is designated HIn(aq)

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  • In general, the higher pH form is designated In-(aq)

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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)

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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

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A Close Look at Litmus Paper

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

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

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

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    • These intermediate colours are generally not useful, with the exception of bromothymol blue, where the green is a distinct colour in the middle of the range

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Common name

Color of HIn(aq)

pH range of colour change

Color of In-(aq)

Litmus

Red

6.0-8.0

Blue

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Uses of Acid-Base Indicators

  • Acid base indicators have two primary uses:
    1. Indicators are commonly used to mark the end of a titration (you will learn more about this in Chapter 8)

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    • Indicators are used to estimate 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, although indicators are not as accurate.

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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 < 4.8

pH > 2.8 but < 8.0

pH must be < 3.2

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

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Practice

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

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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

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B

● indigo carmine was blue

● phenol red was yellow

● bromocresol green was blue

● methyl red was yellow

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C

● phenolphthalein was colourless

● thymol blue was yellow

● bromocresol green was yellow

● methyl orange was orange

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Practice (continued)

  • Pg. 247 #1-3 (#4 BONUS)

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  • Mini Investigation “pH of a solution” pg. 246

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