WCLN - Indicators - How They Work - Chemistry

Introduction to Acid-Base Indicators

0:00acid-base indicators are added to
0:04solutions and show specific color
0:06changes as the ph of the solution
0:08changes will start with an example
0:10called methyl violet indicator when it's
0:14added to a solution with the ph of zero
0:16its color is yellow as we gradually
0:19increase the pH from 0 to 1.6 the color
0:23gradually changes from yellow to green
0:25to blue
0:28when we gradually increase the pH from
0:301.6 all the way up to 14 the color
0:34remains blue
0:37so now we have a line down here join
0:40that methyl violet changes gradually
0:41from yellow to green to blue between 0
0:45and 1.6 and stays blue at ph is above
0:481.6
0:51so now we'll start with the new
0:52indicator orange for it is read at a ph
0:55of 0 as we gradually increase the pH
0:58from 0 to 1 point for the color stays
1:02red
1:05as we increase the pH from 1.4 2.8 the
1:10color gradually changes from red to
1:12orange to yellow
1:15now we'll continue raising the ph we see
1:18it remains yellow all the way up to 14
1:23we now have a line down here showing the
1:25colors of orange for from ph 0 up to 14
1:31now we'll start with a new solution to
1:33which we've added the indicator thymol
1:35blue at ph 0 we see thymol blue as red
1:39as we increase the pH up to 1.2 it
1:43remains red
1:45as we gradually increase the pH from 1.2
1:4822.8 the color changes from red orange
1:53yellow
1:54now will keep on increasing the pH from
1:572.82 a it stays yellow
2:01going from a 29.6 it changes from yellow
2:05to green to blue
2:07and going up from 9.6 214 it stays blue
2:14so now we have a record of the many
2:16color changes that final blue undergoes
2:18as we change the pH from zero up to 14
2:23as well as records for orange for and
2:25methyl violet
2:27the acid-base indicator table you are
2:29given contains the indicators we looked
2:32at and quite a few more note how the
2:34information on the table corresponds to
2:37the colors on the diagrams
2:42notice the colors of thymol blue at
2:44lower ph's are near the top and those at
2:48higher ph's are further down the table
2:52let's look a little closer at a
2:54particular indicator will use
2:56bromothymol blue
2:58the first color name is yellow this
3:01means the color of brown family blue is
3:03yellow if the ph is less than or equal
3:06to 6
3:07the second color name is blue this means
3:10the color of brown primal blue is blue
3:12if the ph is greater than or equal to
3:157.6
3:17the two numbers here give us the PH
3:20range in which the color gradually
3:22changes
3:23so between six and seven point six the
3:26color gradually changes from yellow at
3:28six to blue at 7.6 halfway between six
3:33and seven point six is 6.8 it is green
3:36at 6.8
3:38most indicators are organic compounds
3:41with complex structures here's the
3:44structure of one form of the indicator
3:46Rome some all blue
3:48like all indicators bromothymol blue has
3:51two different forms and acid form and a
3:53base form
3:55the acid form has one more proton and
3:57the base form the difference is enough
4:00to alter the structure of the molecule
4:02enough to change its color
4:04the two different forms are always
4:06different colors here is the structure
4:08of the acid form of braun tamil blue its
4:11color is yellow
4:13here's the structure for the base former
4:15brawn thymol blue its color is blue
4:19just save us from writing complex
4:21formulas we can represent the formula of
4:24the acid form of any indicator by the
4:26simple generic formula hiin
4:31and the formula of the base by the
4:33simple generic formula i am-
4:37and all indicators there is an
4:38equilibrium involving a weak acid and a
4:41weak base
4:43the acid form of an indicator which will
4:46call hiin is a weak acid the acid former
4:50bromothymol blue is yellow but this is
4:52not true for all indicators each
4:54indicator has its own specific colors
4:59the base form of an indicator is the
5:01conjugate base of the acid form the base
5:04form of the indicator broth I'm all blue
5:06is blue