Indicators

Investigation at bottom of page

An Indicator is a substance that can tell us if another substance that it is in contact with is acidic or alkaline.

Litmus is a water-soluble mixture of different dyes extracted from lichens, especially Roccella tinctoria.

The mixture has CAS number 1393-92-6. It is often absorbed onto filter paper. The resulting piece of paper or solution with water becomes a pH indicator (one of the oldest), used to test materials for acidity.

Litmus Ranges

ACID pH < 4.3 NEUTRAL pH > 8.3 BASE

pH is a complicated measure of the amount of H+ ions that are in the mix,

A low pH means a lot of H+ ions, a high value indicates that there is a desire for more H+ ions in the solution.

Naturally occurring pH indicators

Many plants or plant parts contain chemicals from the naturally-colored anthocyanin family of compounds. They are red in acidic solutions and blue in basic. Extracting anthocyanins from red cabbage leaves or the skin of a lemon to form a crude acid-base indicator is a popular introductory chemistry demonstration.

Anthocyanins can be extracted from a multitude of colored plants or plant parts, including from leaves (red cabbage); flowers (geranium, poppy, or rose petals); berries (blueberries, blackcurrant); and stems (rhubarb). An exhaustive list would be beyond the scope of this article -

WIKIPEDIA

Project Idea

Why not test some plants to see if they could be used as acid/base indicators

TO MAKE RED CABBAGE INDICATOR

Materials:

Red Cabbage, Beaker(600ml), water, knife, chopping board; Collection bottle; hot plate; plastic bag; test tubes;

Hydrochloric acid(1M); sodium hydroxide(1M); acetic acid(1M); sodium carbonate(1M).

Procedure:

1) Turn on the hot plate to High.

2) Chop up enough red cabbage to fit into ¾ of a 600ml beaker.

3) Add water until it reaches the 300ml mark.

4) Boil the cabbage for 5 minutes.

5) Half fill four test tubes with the red cabbage liquid.

6) Pour the rest of the liquid into the collection bottle for storage.

7) Place red cabbage scraps into a plastic bag.

8) Add Hydrochloric acid(1M) drop-wise to one of the test tube. Record observation.

9) Add sodium hydroxide(1M) drop-wise to one of the other test tubes. Record observation.

10) Add acetic acid(1M) drop-wise to one of the other test tubes. Record observation.

11) Add sodium carbonate(1M) drop-wise to one of the other test tubes. Record observation.

http://scienceweek.ie/features_2007_english_cabbage-juice.asp

We could now find the range of turning pH for the red cabbage indicator and also to view the different colours of Universal Indicator

1) Arrange 7 test tubes in a rack

2) Pour 10ml of hydrochloric acid into the first test tube (pH 1).

3) Pour 1ml from the first test tube into the next test tube and add 9ml of water (pH 2).

4) Pour 1ml from the second test tube into the third test tube and add 9ml of water (pH 3).

5) Continue this procedure until all test tubes have 10ml of solution (pH 4  7).

6) Add 2-3 drops of universal indicator to each test tube.

7) Record observations by colouring the appropriate box in the result table.

8) Clean test tubes and repeat steps 1)-7) using the red cabbage indicator.

9) Clean test tubes and repeat steps 1)-7) starting with sodium hydroxide and using universal indicator (pH 14  8).

10) Clean test tubes and repeat steps 1)-7) starting with sodium hydroxide and using red cabbage indicator.

11) Record observations by colouring the appropriate box in the result table.

So Indicators and their colours,

we see that they change different colours in different solutions

but what makes one change might not effect another indicator.

So do we need to learn them all ?

NO!

well except to know that

Methyl Orange turns Pink at pH 7

A full list of Indicators is here http://sbeccompany.fr/sciences/chimie/indicateurs/liste_indicateurs_pH.pdf

A Listing of most of the commercially available indicators

Indicator pH Range Quantity per 10 ml Acid Base

Thymol Blue 1.2-2.8 1-2 drops 0.1% soln. in aq. red yellow

Pentamethoxy red 1.2-2.3 1 drop 0.1% soln. in 70% alc. red-violet colorless

Tropeolin OO 1.3-3.2 1 drop 1% aq. soln. red yellow

2,4-Dinitrophenol 2.4-4.0 1-2 drops 0.1% soln. in 50% alc. colorless yellow

Methyl yellow 2.9-4.0 1 drop 0.1% soln. in 90% alc. red yellow

Methyl orange 3.1-4.4 1 drop 0.1% aq. soln. red orange

Bromphenol blue 3.0-4.6 1 drop 0.1% aq. soln. yellow blue-violet

Tetrabromphenol blue 3.0-4.6 1 drop 0.1% aq. soln. yellow blue

Alizarin sodium sulfonate 3.7-5.2 1 drop 0.1% aq. soln. yellow violet

a-Naphthyl red 3.7-5.0 1 drop 0.1% soln. in 70% alc. red yellow

p-Ethoxychrysoidine 3.5-5.5 1 drop 0.1% aq. soln. red yellow

Bromcresol green 4.0-5.6 1 drop 0.1% aq. soln. yellow blue

Methyl red 4.4-6.2 1 drop 0.1% aq. soln. red yellow

Bromcresol purple 5.2-6.8 1 drop 0.1% aq. soln. yellow purple

Chlorphenol red 5.4-6.8 1 drop 0.1% aq. soln. yellow red

Bromphenol blue 6.2-7.6 1 drop 0.1% aq. soln. yellow blue

p-Nitrophenol 5.0-7.0 1-5 drops 0.1% aq. soln. colorless yellow

Azolitmin 5.0-8.0 5 drops 0.5% aq. soln. red blue

Phenol red 6.4-8.0 1 drop 0.1% aq. soln. yellow red

Neutral red 6.8-8.0 1 drop 0.1% soln. in 70% alc. red yellow

Rosolic acid 6.8-8.0 1 drop 0.1% soln. in 90% alc. yellow red

Cresol red 7.2-8.8 1 drop 0.1% aq. soln. yellow red

a-Naphtholphthalein 7.3-8.7 1-5 drops 0.1% soln. in 70% alc. rose green

Tropeolin OOO 7.6-8.9 1 drop 0.1% aq. soln. yellow rose-red

Thymol blue 8.0-9.6 1-5 drops 0.1% aq. soln. yellow blue

Phenolphthalein 8.0-10.0 1-5 drops 0.1% soln. in 70% alc. colorless red

a-Naphtholbenzein 9.0-11.0 1-5 drops 0.1% soln. in 90% alc. yellow blue

Thymolphthalein 9.4-10.6 1 drop 0.1% soln. in 90% alc. colorless blue

Nile blue 10.1-11.1 1 drop 0.1% aq. soln. blue red

Alizarin yellow 10.0-12.0 1 drop 0.1% aq. soln. yellow lilac

Salicyl yellow 10.0-12.0 1-5 drops 0.1% soln. in 90% alc. yellow orange-brown

Diazo violet 10.1-12.0 1 drop 0.1% aq. soln. yellow violet

Tropeolin O 11.0-13.0 1 drop 0.1% aq. soln. yellow orange-brown

Nitramine 11.0-13.0 1-2 drops 0.1% soln in 70% alc. colorless orange-brown

Poirrier's blue 11.0-13.0 1 drop 0.1% aq. soln. blue violet-pink

Trinitrobenzoic acid 12.0-13.4 1 drop 0.1% aq. soln. colorless orange-red

Universal indicator is a chemical made by combining other indicators, by doing this we have a way of measuring the pH change of a substance continuously as the reaction takes place.

Using Universal Indicator, random substances can have their acidity checked quite easily because the different indicators are combined in the test papers and we do not need many chemicals in our lab or in the field.

There are more exact pH papers, they are Universal papers but they improve their accuracy by having different coloured compounds at different locations along the strip.

Digital pH Meters

There are many type of digital pH meters that can read the pH of substances, one of the problems with these meters is that they require calibration.

for the pH meters in Hartstown community school the calibration information is here

http://www.hannainst.com/manuals/manHI_98103.pdf