Commercial acid-base indicators

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Commercial acid-base indicators When indicators prepared from plant materials are stored in a laboratory for future use, they keep changing in composition and properties as time goes on. These changes give inconsistent results. For this reason, commercial indicators are prepared because they give more consistent results. Examples of commercial indicators include: methyl orange, litmus, phenolphthalein and universal indicator as shown in Figure 1.8.

The universal indicator gives precisely more accurate results.

\n Activity 1.1(d), you will find out how litmus paper behaves as an indicator.

;=\ Activity 1.1 (d): Investigating the effect of different substances on litmus paper

In this activity, you will work in groups. What you need

a measuring cylinder wood ash solution vinegar lemon juice

• litmus paper (red and     •   diluted liquid soap
  blue)                                    »  tea leaves
• a beaker                              •   distilled water

•   a dropper                           * soda ash solution
«  carbonated water             •  diluted bleach

What to do

1. Using a measuring cylinder, measure 2 cm³ of lemon juice and pour it into a
   beaker.
2. Dip blue and red litmus papers into the lemon juice in the beaker, one at a
   time.
3. Record your observations in Table 1.3.
4. Repeat your activity with vinegar, diluted bleach, diluted liquid soap, tea leaves,
   wood ash solution, soda ash solution, carbonated water, and distilled water.
5. Individually, copy and complete Table 13.

Table 1.3: Effect of different substances on litmus paper

Substance	Colour change when  blue litmus paper is dipped	Colour change I when red litmus ; paper is dipped	Conclusion (acidic, basic or neutral)
Lemon juice
Vinegar
Diluted bleach
Diluted liquid soap
Tea leaves
Solution of wood ash
Soda solution
Carbonated water
Distilled water

6.  Present your findings to the class.

Safety precaution:

To avoid the irritating vapour of ammonia, carry out this experiment in an open space with open windows. Keep a distance of at least 2 metres away from the container of ammonia solution

Assignment 1.1

1.  In groups, mention the colour changes that occur when
phenolphthalein and methyl orange indicators are separately added to;

a)   lemon juice                                      b) soda ash solution

2. What general conclusion can you derive from your observations?

You will observe that different indicators exhibit different colours in acidic and basic media, as shown in Table 1.4.

Table 1.4: colours exhibited by different indicators in acidic and alkaline media

Name of indicator	colour in acidic solution	colour in alkali
Litmus	Red	Blue
Methyl orange	Red	Yellow
Phenolphthalein	Colourles	Pink