Acid Base Indicators and Titrations

Goal Chemistry Unit 10 Acids and Bases C5.7C Describe tests that can be used to distinguish an acid from a base.

This demo/lab introduces acid / base indicators and simple titrations Indicators are used to distinguish acids from bases and to indicate the end point of a titration during neutralization reactions. This demo/lab has three parts that can be used separately or in conjunction during an acid / base unit.

Background

Different indicators are very specific for a pH value. This should be demonstrated early skipping Bogens or the universal indicator for the last section of the lab. How deep you go into the working of indicators is dependent on the level of your class. The simple basics are they turn color at a specific pH range.

Vocabulary

Acid, Base, Indicator, Titration, End Point

Materials

15 ml square dropping bottles ( or something similar ) The number will be dependent upon the number of solutions you want them to test and the number of lab stations you want to set up ( partners or groups ). Litmus paper, red and blue, a variety of indicators, spot or well plates. Acid solution Ex: HCl 1M, .75M, .5M, .25M, .1M and base solutions Ex: NaOH in similar concentrations.

Procedure 1

Set a variety of dropper bottles up with low concentration acids and bases. Lable them as unknowns ( a,b,c or 1,2,3 ) at least 3 to 5 of each. Using spot plates or cavity plates have the students test all the unknowns with both Red and Blue litmus paper. This will separate them into the two groups, acids and bases. Now have them test each with a variety of indicators : phenolthaline, bromothymol blue, methyl orange etc. They will need a data chart to record all results.

Procedure 2

This step will be a basic titration to identify the varying strengths of the acids and bases from procedure 1. Demo adding a drop of phenolthaline to an acid and then a drop to a base as a review that phenol turns pink in a base. Now add 10 ml ( they will using drops but you want more volume so they can see it ) of an acid to a small flask and two drops of phenol. Now slowly titrate some of the NaOH into the flask until it turns. Swirl the flask watching it turn clear. Continue until it remains pink explaining what is happening and why. Now add some acid until it remains clear repeating this back and forth process so they can see the reversibility of the reaction.

Now it is their turn, Lable the .5M NaOH and then have them titrate the acids placing them in order of strengths bases upon the amount of .5M NaOH it took to titrate them. Once they have the acid in order they can now pick one to titrate the remaining bases. You may want to ask them how many drops of bases it will take to turn and acid pink if you do not put the two drops of phenol in the acid ?

Procedure 3

This is commonly called a back titration and the kids really like it. Start with a demo of Bogens in an acid, a base and distilled water. In the distilled water ( green) now add a drop of acid ( yellow) then a drop of base ( green ) another drop of base ( blue ) then a drop of acid ( green ) and so on until they see how neutral can be obtained. For your demo use the same strength of acid and base and then use different strengths so they see the relationship of strength to volume.

Now using the acids from procedure 2 have them neutralize ( turn ) green each of them using bogens and a chosen base. Have them start with 10 drops of acid and two drops of Bogens. Because the strengths are different they will pass the end point and have to add a drop or more of acid to get back. This process will go back and forth until the correct ratio is obtained. Have them make a data table to keep track of the number of total drops of acid and base it took. When they are done with them all, have them add the total number of drops of acid and base for each trial and then have them repeat by adding that total together without the indicator and bringing it to you where you can add the indicator to get the green for an A.