Chemistry

Acid-base and pH indicators


Acid-base indicators are organic substances that, when they come in contact with an acid, become one color, and when they come in contact with a base, they get another color.

Thus, to know if a substance is acid or base, we can use an organic indicator to identify chemical function.

Examples of acid-base indicators are phenolphthalein, methyl orange, litmus paper, bromothymol blue.

Some natural indicators can also be used, such as red cabbage and the hortensis and hibiscus flower. See the coloration that key indicators can acquire when they come in contact with an acid or base.

INDICATOR

ACID

BASE

NEUTRAL

Phenolphthalein

COLORLESS

ROSE

COLORLESS

LITMUS

ROSE

BLUE

-

For the other indicators:

- Red cabbage, in aqueous medium, turns red in contact with acid, green in contact with base and red when neutral.
- Methyl orange turns red in contact with acid, yellow-orange on base and when neutral;
- Bromothymol blue turns yellow in acid, and blue in base and when neutral;
- The hydrangea flower turns blue in acid medium and pink in base;
- The hibiscus or Venus mime, which is pink in color, turns orange-red in contact with acid and green in basic medium.

Some acid-base indicators are so efficient that they even indicate the degree of acidity or alkalinity (basicity) of substances. This degree is called pH (hydrogen product), which measures the amount of H + cation in solutions.

There is a scale of acidity and alkalinity ranging from zero to fourteen. The largest number indicates basic (alkaline) solution and the smallest number indicates an acidic solution. If the pH value is seven, ie half, then the solution is neither acidic nor basic, it is neutral.

The closer the solution gets to zero, the more acidic it is. The closer the solution gets to fourteen, the more basic it is.

PH range

|_________|_________|
0                 7                14
neutral acid base

In practice, pH can be measured with acid-base indicators and also by means of devices that measure the electrical conductivity of solutions.

The indicators change color at different pH values. For this color change we call it turning and for the pH value we give the name of turning point.

Here are some daily examples of pH values:

ALKALINE CHARACTER

PRODUCT

14

Caustic Soda Solution (NaOH)

13

12

Lime water

11

10

Alkaline Toothpaste

9

8

NaHCO3 aqueous solution

NEUTRAL CHARACTER

7

Pure water

ACID CHARACTER

6

Tap water, rainwater

5

Soft drinks

4

Acid rain

3

Vinegar

2

Lemon juice

1

Gastric Juice (HCl)

0

HCl aqueous solution