Acid – Base Indicators SECTION 6.3 PG. 245-247 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 name Color of pH range of Color of In- HIn(aq) colour change (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 generally not useful, with the exception of bromothymol blue, where the green is a distinct colour in the middle of the range Common name Color of pH range of Color of In- HIn(aq) colour change (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 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: Solution After addition to samples of pH range implied by pH of solution the solution … each indicator ● methyl red was red pH must be < 4.8 Combining these A ● thymol blue turned yellow pH > 2.8 but < 8.0 results, the pH must be ● methyl orange turned red pH must be < 3.2 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 pH range implied by pH of solution solution … each indicator ● methyl violet was blue ● methyl orange was yellow A ● methyl red was red ● phenolphthalein was colourless ● indigo carmine was blue ● phenol red was yellow B ● bromocresol green was blue ● methyl red was yellow ● phenolphthalein was colourless ● thymol blue was yellow C ● bromocresol green was yellow ● methyl orange was orange Practice (continued) Pg. 247 #1-3 (#4 BONUS) Mini Investigation “pH of a solution” pg. 246 .