Acid-Base Indicators

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Transcript Acid-Base Indicators

Acid-Base Indicators
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.
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, <7 acid, >7 base)
– Universal indicator (multi-colored)
– Indicators like phenolphthalein
– Natural indicators like red cabbage,
Paper testing
• Paper tests like litmus paper
and pH paper
– Put a stirring rod into the solution and
– Take the stirring rod out, and place a
drop of the solution from the end of
the stirring rod onto a piece of the
– 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.
pH paper
pH meter
• Tests the voltage of the
• Converts the voltage to pH
• Very cheap, accurate
• Must be calibrated with a
buffer solution
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
• Once the drops are added, the sample is
• Some dyes are natural, like radish skin or
red cabbage
Choosing the Correct Indicator
• Not all indicators shift colors at pH 7.
• Many change colors over a wide range of pH
• 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
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