information about pH, indicators etc.

1. Acid-Base Indicators
By Dr Rahul Saxena
SAHS, Sharda University

2.  pH: (Sorenson , 1909)
• pH = - log [H+] or = log10 1/ [H+]
• Expressed in moles/ lit
• Ranges from 0-14
• pH 0 i.e. 1 mole H+/lit
• pH 14 i.e. 10-14moles H+/lit
2

3. 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.

4. 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,
radishes

5. 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.

6. pH paper

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

8. 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

9. 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

10. 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