Acid-Base Indicators

1
Acid-Base Indicators
2
Measuring pH

• Now that we know how to calculate pH of a
  solution, how do we measure it in the lab?
• The following slides will provide numerous
  examples of ways to measure pH in the laboratory.

3
pH testing

• There are several ways to test pH
• Blue litmus paper (red acid)
• Red litmus paper (blue basic)
• pH paper (multi-colored)
• pH meter (7 is neutral, lt7 acid, gt7 base)
• Universal indicator (multi-colored)
• Indicators like phenolphthalein
• Natural indicators like red cabbage, radishes

4
Paper testing

• Paper tests like litmus paper and pH paper
• Put a stirring rod into the solution and stir.
• Take the stirring rod out, and place a drop of
  the solution from the end of the stirring rod
  onto a piece of the paper
• Read and record the color change. Note what the
  color indicates.
• You should only use a small portion of the paper.
  You can use one piece of paper for several tests.

5
pH paper
6
pH meter

• Tests the voltage of the electrolyte
• Converts the voltage to pH
• Very cheap, accurate
• Must be calibrated with a buffer solution

7
pH indicators

• Indicators are dyes that can be added that will
  change color in the presence of an acid or base.
• Most are large molecules that act as weak acids.
  The color changes occur when the molecules is
  ionized (acting as an acid) or un-ionized (acting
  as a base).
• Some indicators only work in a specific range of
  pH. Choosing the right one is important!
• Once the drops are added, the sample is ruined
• Some dyes are natural, like radish skin or red
  cabbage

8
Choosing the Correct Indicator

• Not all indicators shift colors at pH 7.
• Many change colors over a wide range of pH
  values.
• When testing a sample for acidity or basicity it
  is best to choose an indicator that has a color
  change close to pH 7
• Or use pH paper
• Or use a pH meter to get a digital reading

9
Sample Indicator pH Ranges

• Crystal Violet yellow (pH 1) blue
• Cresol Red red (pH 2) yellow (pH 8) red
• Thymol Blue red (pH 3) yellow (pH 8) green/blue
• Bromophenol blue yellow (pH 4) blue
• Methyl orange red (pH 4.5) orange/yellow
• Bromcresol green yellow (pH 4.5) green/blue
• Methyl red pink (pH 5.5) yellow
• Bromcresol purple yellow (pH 6) purple
• Alizarin yellow (pH 6.5) red (pH 12) purple
• Bromthymol blue yellow (pH 7 ) green/blue
• Phenol red yellow (pH 7.2) red
• Phenolpthalein clear (pH 8.3) pink
• Thymolphtalien clear (pH 10) blue
• Alizarin yellow GG orange (pH 10.5) red
• Universal Indicator red (pH 4) orange (pH 6)
  yellow (pH 7) green (pH 8) blue (pH 9) purple