Indicators

Perform a first-hand investigation to prepare and test a natural indicator

Various fruits, vegetables, and other items can be used to prepare indicators which are quite effective. This study guide will use red cabbage as an example, outlining the procedure you would take to prepare the indicator, and results you should expect.

Procedure:

  1. Cut or grate a portion of a red cabbage, placing the small pieces into a large beaker and add enough boiling water to the beaker to cover the red cabbage (or use room-temperature water and heat with a Bunsen burner).
  2. Allow the red cabbage pieces to boil for several minutes, not stopping until the water has become a dark, reddish or purplish
  3. Decant or filter the mixture into another Record the colour of the indicator obtained.
  4. Test various substances with the indicator and record any changes in Some substances you may wish to test include baking soda, vinegar and house salt (NaCl).

Expected results:

The colour of the starting indicator will vary according to several factors such as water pH, but it will generally be a dark reddish or purplish colour.

When an acidic substance is added to the indicator, the resultant colour should be pink. When a basic substance is added to the indicator, the resultant colour should be green. Between these two colours, the colour should be purple.

An accurate range will not be expected of you. Simply know the basic colours at both ends (Pink- Green).

Remember- Boiling red cabbage yields a purple indicator which turns pink in acid, and green in base.

 

Identify data and choose resources to gather information about the colour changes of a range of indicators

The four indicators litmus, phenolphthalein, methyl orange, and bromothymol blue will generally be enough for testing you may require, but to satisfy this dotpoint are a few more indicators and their respective colours and ranges.

 

Indicator Name pH Range Lower-range colour Upper-range colour
Alizarine Yellow 10.2-12.0 Yellow Red
Bromocresol Green 3.8-5.4 Yellow Cyan
Methyl Red 4.4-6.2 Red Yellow
Phenol Red 6.8-8.4 Yellow Red
Thymol Blue 1.2-2.8 Red Yellow

 

With multiple indicators, you can narrow the possible pH range quite easily. For instance, if a substance turns blue when bromothymol blue is added, and is colourless when phenolphthalein is added, then the pH must be between 7.6 and 8.3.

 

Identify that indicators such as litmus, phenolphthalein, methyl orange and bromothymol blue can be used to determine the acidic or basic nature of a material over a range, and that the range is identified by change in indicator colour

I don’t recommend remembering all the indicators listed below. Remembering phenolphthalein and bromothymol blue should suffice.

An indicator is a substance which, in solution, changes colour depending on the pH of a substance. However, indicators do not quantitatively measure a substance’s pH by itself, as a change in colour only gives a loose guide as to the nature of the substance. Generally, an indicator will only show you if a substance is an acid or a base, but will not tell you how strongly acidic and basic the substance is. An exception to this is universal indicator, which can be used to give a relatively accurate indication of a substance’s nature.

Below is a table of the indicators litmus, phenolphthalein, methyl orange, and bromothymol blue, showing their ranges and the different colour transitions each one is subject to.

Remember- Indicators are substances which change colour depending on whether they are in an acidic or basic environment.An effective indicator is one that allows you to easily distinguish between an acid and a base. One that gradually changes (Large range) is not as useful as one with a noticeable point at which it changes (Small range).

Classify common substances as acidic, basic or neutral

Broadly, acids are substances which generally taste sour, and will corrode metal. Stronger concen- trations of acid will also give a burning feeling upon contact. Acids can conduct electricity.

Examples of common substances which are acidic in nature include: Lemon juice, vinegar, formic acid from ant and bee stings, as well as common acids within the classroom such as carbonic, hydrochloric, and sulfuric acid.

Bases are often bitter tasting, and slippery to the touch. Like acid, stronger concentrations will often give a burning sensation if it comes into contact with skin. Bases also conduct electricity.

Examples of common substances which are basic in nature include: Ammonia, baking soda, soap, toothpaste, antacids, and various household detergents and cleaning solutions, as well as common bases within the classroom such as sodium hydroxide, potassium hydroxide, and calcium carbonate.

Examples of neutral substances include common table salt (NaCl) and pure water.

Remember- Acidic substances include lemon juice and vinegar. Neutral substances include table salt and water. Basic substances include baking soda and detergents.

Solve problems by applying information about the colour changes of indicators to classify some household substances as acidic, neutral or basic

This is simply a practical version of dotpoint 2.2.3 on page 46. Take the time to understand how indicators work- particularly when multiple indicators are used to narrow the possible pH range of a substance down- and this dotpoint should prove no problem.

Possible results

Adding Phenol Red to lemon juice turned the indicator yellow, indicating those substances were acidic. When Phenol Red was added to baking soda, it turned red, indicating it was basic.

Similarly, when Alizarine Yellow was added to vinegar, it turned yellow. However, since the lower range of Alizarine is a pH of 10.2, this only indicated vinegar was not strongly basic. However, since vinegar also turned red when Methyl Red was added, it must therefore be acidic.

Identify and describe some everyday uses of indicators including the testing of soil acidity/basicity

Focusing on the soil point below should be enough to answer any dotpoints. However, keeping fish tanks and swimming pools in your head may prove useful just to pull it out if necessary. Another possibility which may or may not be considered everyday use is managing certain chemical wastes from labs, workshops (e.g. disposing of acids in the school labs, or disposing of photographic waste from studios).

Indicators find a wide variety of uses everyday. Below is a list of some uses:

  • Soil- Different plants, flowers, fruits and vegetables require different levels of pH in order to thrive. In addition environmentalists often monitor and record soil pH in different areas for various For example, hydrangeas grow purple flowers in acidic conditions and pink flowers in basic conditions. Monitoring pH is thus important if a specific colour is desired.
  • Fish tanks- Different fish require different levels of pH (Although this range is quite narrow) in order to
  • Swimming pools- Swimming pools must be maintained at a pH a little higher than 7 for safety reasons.