pH and Acid–Base Indicators
Problem and Purpose:
In this experiment you will learn to use both common indicators and pH paper, assess their
efficacy and test the pH of both common household substances and prepared buffered solutions.
Subsequently, you will investigate the pH behavior of these substances as well as some weak bases.
Introduction:
The properties of acids and bases :
- Acids are substances that form H+ ions when dissolved in water.
- Bases are substances the form OH- ions when dissolved in water.
-
Substances with a low pH (<7) have more H+ ions and are classified
-
Substances with a higher pH (>7) have more OH- ions and are
as acids
classified as bases.
The following is how some paper indicators change colour in the presence of acids and bases:
Blue litmus paper in ACID  turns RED
Blue litmus paper in BASE or WATER  stays BLUE
Red litmus paper in ACID or WATER  stays RED
Red litmus paper in BASE  turns BLUE
In order to determine whether a substance is an acid or base you must use both red and blue
litmus paper. This is mainly to determine if the substance is neutral because the paper will stay the
same colour in both.
Liquid indicators also change colours in similar ways. Here are two liquid indicators and
how they change colour in acids or bases:
Phenolphthalein (colourless) and BASE  turns PINK
Bromothymol blue (blue) and ACID  turns YELLOW
An acid–base indicator is a substance that changes color as the pH of a solution changes.
There are countless numbers of different acid–base indicators, many of which can be extracted from
common plants. Every indicator exhibits a different range of colors at different pH values. For
example, the indicator phenolpthalein is colorless in solutions with a pH less than 8 and pink to deep
red in solutions with a pH greater than 8. Bromothymol blue is yellow in acidic solutions and blue in
basic solutions. Of course, an indicator can be used to judge the pH of a solution only if the
characteristic colors at various pH’s are previously known. Some of the indicators are pictured below:
Indicators work because they are weak acids which, when in solution, exist in equilibrium
with their conjugate base. The acid and its conjugate base each have different colors; as the
equilibrium shifts from one direction to the other, the color of the indicator solution also changes.
Let’s use bromothymol blue as an example; we’ll represent the equilibrium the pertinent equation
establishes as:
Indicators are generally large organic molecules with complicated formulas, so, for
convenience, we’ll abbreviate the indicator as HB.
Consistent with LeChâtelier, if a solution containing the indicator becomes acidic (e.g., with
the addition of H+), the equilibrium will shift to the left, and the solution will become yellow. If, on
the other hand, we make the solution basic (e.g., if OH– ions are added, which would reduce the
hydronium concentration), the equilibrium will shift to the right and the solution will become blue. At
intermediate pH values there will be a mixture of both HB and B–, and the solution will take on a
green color. Some indicators exhibit only two colors and some exhibit a wide range. Each indicator
must be individually studied to determine its behavior as a function of pH.
pH paper consists of strips of filter paper which have been previously soaked in an indicator
and subsequently dried. A drop of an unknown solution can be placed on the pH paper, and the
resulting color compared to a chart. By matching the color of the paper to a color on the chart, the pH
of the solution can be determined.
Materials:
HCl
NaOH
Glucose Solution
Droppers
HNO3
NH3(ammonia)
H2SO4
Spot Plates
Blue & Red Litmus Paper
Phenolphthalein & Bromothymol blue
Procedure:
1.
Using a dropper, collect 3-4 drops of each of the 6 chemical substances on the
side bench and place them in a separate depression on your spot plate as shown in the
diagram.
HCl
Glucose
NaOH
NaCl
H2SO4
NH3
Figure 1. Spot plate with different chemical substances
2.
To each chemical test whether it is an acid or a base by using both blue and
red litmus paper. Record your results in Table 1.
3.
Now add 1-2 drops of phenolphthalein to each chemical. Record and resulting
colour changes in Table 1.
4.
Discard of all the chemicals in the sink and clean the spot plate. Make sure it
is completely dry. Repeat step 1.
5.
Now add 1-2 drops of bromothymol blue to each chemical. Record all
resulting colour changes in Table 1.
6.
Determine whether each chemical is an acid, base or neutral based on your
results.
Results:
Table 1. Results of acid-base testing
Chemical
HCl
NaOH
NaCl
Glucose
H2SO4
NH3
Red
Blue
Litmus
Litmus
Phenolphthalein
Bromothymol
Acid, Base,
Blue
or Neutral?