Common Indicators

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

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


  • Indicator: A chemical that change colour as the concentration of hydrogen ions in a solution changes.
  • Indicator range: The range of acidity or basicity over which the colour of an indicator changes colour.
  • Indicators can be either natural (extracted from plants) or synthetic.
  • Common indicators include:
    • Litmus: A dye extracted from various species of lichen that changes colour around the neutral range.
    • Phenolphthalein: A synthetic indicator that changes colour in the basic range.
    • Methyl orange: A synthetic indicator that changes colour in the acidic range.
    • Bromothymol blue: A synthetic indicator than changes colour around the neutral range.

Indicator
Highly acidic
Slightly acidic
Neutral
Slightly basic
Highly basic
Litmus

Red
Purple
Blue

Phenolphthalein



Colourless
Red
Methyl orange
Red
Yellow



Bromothymol blue

Yellow
Green
Blue

Colours and approximate ranges of common indicators



Various pH indicators


Indicator Low pH color Transition pH range High pH color
Gentian violet (Methyl violet) yellow 0.0–2.0 blue-violet
Leucomalachite green (first transition) yellow 0.0–2.0 green
Leucomalachite green (second transition) green 11.6–14 colorless
Thymol blue (first transition) red 1.2–2.8 yellow
Thymol blue (second transition) yellow 8.0–9.6 blue
Methyl yellow red 2.9–4.0 yellow
Bromophenol blue yellow 3.0–4.6 purple
Congo red blue-violet 3.0–5.0 red
Methyl orange red 3.1–4.4 orange
Bromocresol green yellow 3.8–5.4 blue-green
Methyl red red 4.4–6.2 yellow
Methyl red / Bromocresol green red 4.5–5.2 green
Azolitmin red 4.5–8.3 blue
Bromocresol purple yellow 5.2–6.8 purple
Bromothymol blue yellow 6.0–7.6 blue
Phenol red yellow 6.8–8.4 red
Neutral red red 6.8–8.0 yellow
Naphtholphthalein colourless to reddish 7.3–8.7 greenish to blue
Cresol Red yellow 7.2–8.8 reddish-purple
Phenolphthalein colorless 8.3–10.0 fuchsia
Thymolphthalein colorless 9.3–10.5 blue
Alizarine Yellow R yellow 10.2–12.0 red
Litmus red 4.5-8.3 blue
The colour changes and ranges of a wide range of indicators
From Wikipedia

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