SECTION 6.3 PG Acid – Base Indicators

Substances that change colour when the acidity of the solution changes are known as acid-base indicators A very common indicator used is litmus, which is obtained from lichen Litmus 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 two forms, each with a distinctly different colour The form of the chemical depends on the acidity of the solution. They usually have very complicated formulas, so simple abbreviations are used: Lt – litmus; Bb – bromothymol blue; In – 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 HIn (aq) In general, the higher pH form is designated In - (aq)

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 nameColor of HIn(aq) pH range of colour change Color of In - (aq) Bromothymol blueYellow Blue PhenolphthaleinColourless 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 , litmus is in the process of changing colour and you will see mixtures of red and blue.  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 bromothymol blue Common nameColor of HIn(aq) pH range of colour change Color of In - (aq) LitmusRed Blue

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) 2) 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. Example: SolutionAfter 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

Practice Lab Exercise 6.B pg. 247  Complete the Analysis Portion of the lab below SolutionAfter 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 1.6 or higher 4.4 or higher 4.8 or lower 8.2 or lower between 4.4 & or lower 6.6 or lower 5.4 or higher 6.0 or higher between 6.0 & or lower between 2.8 & or lower between 3.2 & 4.4 between 3.2 & 3.8

Practice (continued) Pg. 247 #1-3