INDICATORSin chemistry and everyday life

INDICATORS

SUMMARY

Introduction

Definition of pH

Acids and bases

Bromothymol blue

Natural indicators

pH paper

1.

2.

3.

4.

5.

6.

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A pH indicator is a halochromic chemical

compound that is added in small amounts to a solution

in order to determinate easily the pH (acidity or

alkalinity) of the solution.

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pH is a mathematic tool used in chemistry to measure acidity of a solution

mathematic definition for diluted solutions :

usual pH range : 0-14

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pH indicators exist under severals forms of acide-base couples :

InH in acidic solutions (1st colour)In- in basic solutions (2nd colour)

-> So, the colour of the solution can give us informations about acidity of the solution

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Bromothymol blue is the most famous coloured indicator

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C : pH in transition range of BBT

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A : acid form of BBT

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

350

370

390

410

430

450

470

490

510

530

550

570

590

610

630

650

Wavelenght (nm)

Ab

sorb

ance A

B

C

B : basic form of BBT

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We can find a lot of natural indicators in the nature :

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- Red cabbage (red–pink–violet–blue–green–yellow)- Turmeric / Curry (yellow-orange-brown)- Tea (yellow-brown)- Rose petals (blue-pink)- Artichoke (colorless-yellow)- …

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pH paper (Hydrion Paper) is a paper which contains a Universal Indicator

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"Universal Indicator" means severals indicators mixed in order to emit a different color for each pH value

pH paper is red in acid solutions and purple in basic solutions

acidic basic

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