Light-duty detegents are weakly alkaline, while heavy-duty detergents are moderately alkaline. The differing effects these have is investigated using standard washing conditions.

Class practical

There are essentially two types of detergent for washing clotheslight-duty for finer fabrics, and heavy-duty for more robust fabrics subject to heavier soiling. One important difference between these is the pH they produce in the washing water – light-duty detergents are weakly alkaline, while heavy-duty detergents are moderately alkaline. The differing effects these have on soiled fabrics is investigated using standardised washing conditions.

Finer fabrics may use dyes which are stable when washed with light-duty detergents, but may be badly affected by the use of heavy-duty detergents. This can be investigated using samples of dyed woollen cloth.

Lesson organisation

This set of experiments may be organised as an enquiry into why detergent manufacturers insist that you should use different types of detergent for different kinds of fabric. The series of experiments involved may need from 30 to 60 minutes, depending on the range of experiments carried out by each working group. Since the method will be essentially the same, no matter what the group is setting out to investigate, it is reasonable to allocate different tasks to different working groups as a way to economise on time.

Apparatus Chemicals

Each working group will need:

Eye protection

Beakers (1 dm3), 2

Stirrers (Note 2)

Tongs or a glass stirring rod

Bunsen burner

Tripod and gauze

Thermometer, 0 - 110°C (Note 3)

Heat resistant mat or Stirrer-hotplate

Squares of standardised soiled cloth, 2 (Note 4)

Full-range indicator paper strips, 2

For extension work on dyes:

Dyed wool samples (Notes 5 and 6)

Detergent solution, light-duty, 250 cm(Note 1)

Detergent solution, heavy-duty, 250 cm3 (Note 1)

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection. Do not sit down when heating and stirring hot detergent solutions. 

1 Detergent solution - Modern detergents come in a bewildering variety of formulations. For this experiment, the suggested light-duty detergent is Dreft powder, and the suggested heavy-duty detergent is Daz powder. Liquid or tablet versions, and other makes, can be used but some research may be needed to establish which category they fit into and whether the different formulations makes them appropriate for this experiment. The two solutions should be made up in bulk from powder at a concentration of 0.25% w/v (25 g per 10 dm3).

2 The washing trials in the beakers need controlled agitation, and should be the same for each beaker. If resources allow, this can be done using stirrer hotplates to heat and stir using a magnetic follower. More likely, this will have to be done using two glass stirring rods, one for each beaker, for each group. 

3 The thermometer may be liquid-in-glass or electronic, but must be capable of reading safely up to 110 °C.

4 Standardised soiled cloth samples may be prepared by taking a large piece of clean white cotton cloth (for example, an old sheet or shirt) and using a roller to apply a thin, even coating of a mixture of lampblack (fine carbon powder) and olive oil. Squares of this soiled cloth, 5 x 5 cm, may then be cut for the experiment.

5 Wool samples dyed with non-fast dyes are needed. Old woollen clothes, already washed repeatedly, are unlikely to show much difference on washing in the two detergents. New woollen cloth in a strong colour may be available, though again may not show much change if dyed with a fast dye. Trials may need to be done to find samples that will fade noticeably on washing in a heavy-duty detergent. One possible source may be remnants of balls of strongly coloured knitting wool. Trial runs will be necessary for dye-fastness experiments to ensure a reasonable difference in outcome can be seen. A longer wash time may be needed.

6 It may be advisable to carry out trial runs before the lesson to establish whether a four-minute wash is sufficient to show a significant difference between the two detergents. If it is not, try extending the wash times by 1 minute intervals until the difference is clearly seen. This will depend on how heavily soiled the cloth was made initially.

Procedure

a Pour about 250 cm3 of each detergent solution into separate 1 dm3 beakers. Using the strips of indicator paper, measure and record the pH of each solution.

b Warm one solution to 50° - 60°C over a low Bunsen flame, or on stirrer-hotplate.

c Drop a square of soiled cloth into the beaker, noting the time.

d Stir the contents gently and steadily, establishing a technique that can be repeated similarly with the second beaker. If using a glass rod, use a to-and-fro motion.

e After 4 minutes, remove the cloth from the beaker using tongs or a glass rod and allow to drain.

f When both samples have been washed, compare the results.

g If testing the colour-fastness of wool dyes, repeat a – f. with fresh samples of detergent solution in clean beakers.

Teaching notes

The differences between light-duty and heavy-duty detergents depend on the presence in the latter of alkali metal phosphates, which raise the pH of the washing water to a value around 10 (only pH 8 for light-duty). They may also contain oxidising agents.

Preliminary trials may be needed before the lesson to establish the best conditions, including timing, to produce clear differences between the washed soiled samples, and, if appropriate, the dyed wool samples.

It used to be possible to obtain dyed wool samples in the past from the dye manufacturers. Since most of these dye manufacturers have changed their owners or even gone out of business, it is not possible to recommend a supplier or a particular dye. If samples of lissamine green (also used as a food colorant, E142) or lissamine blue can be obtained, home-dyed wool samples can be produced that will show clear differences.

For a fuller coverage of the action of detergents and their compositions, see the website links below.

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

There is some background information on the website of Procter & Gamble.

Page last updated October 2015