Liquid pH indicators

pH indicators are substances, usually a weak acid or base, that changes color depending on the pH of the solution it is mixed with. Indicators are often made from plant extracts.

Color change interval

Indicators don't change color sharply at one particular pH level. They change over a narrow range of pH. And this color change interval is different for each indicator. For optimal accuracy, the narrower the pH range of the color change the better.


Example of common indicators of pH

Litmus

litmus pH indicator

Color in Acid: Red
Color in Base: Blue
Approximate pH range for color change: 5.0 - 8.0
(The color you see will be a mixture of the two.)

A commonly used acid-base indicator is litmus. Litmus is a natural indicator obtained from certain lichens. Even the litmus color change happens over an unusually wide range, it is useful for detecting acids and bases because it changes color around pH 7.


Phenolphthalein

Phenolphthalein pH indicator

Color in Acid: Colorless
Color in Base: Pinkish violet color
Approximate pH range for color change: 8.3 -10.0
(The color you see will be a mixture of the two.)

Phenolpthalein is used to test whether something is highly alkaline (> pH 10).


Methyl Orange

Methyl Orange pH indicator

Color in Acid: Red
Color in Base: Yellow
Approximate pH range for color change: 3.1-4.4
(The color you see will be a mixture of the two.)

Methyl orange is used to test whether something is highly acidic (< pH 3).


and more . . .

There are many more different indicators of pH. The three indicators above are just examples, showing how an indicator works.


Universal indicator

A universal indicator is actually a mixture of several indicators. The advantage of the universal indicator is that it displays a variety of colors over the whole pH scale. This makes it possible to determine an approximate pH of a solution over a range from 1-14.


How to use liquid pH indicators

• Add the number of drops of the indicator solution to the sample according to the instructions.

• Match the reaction color that emerges with the corresponding color of the color chart that is included with the indicator.

• Read off the pH value.


Advantages of using liquid pH indicators

+ Liquid indicators are mainly used for titration.

+ Easy to use. Even a more convenient way to check pH is to use a pH test strip.

+ Cheap compared to a pH meter and electrode.

+ Well suited for measurements in drinking water, wastewater, surface water, aquarium water, swimming-pool water, etc.


and disadvantages . . .

- The measured solution should be colorless and clear. In the event, the sample has any slight intrinsic color, check the accompanying instructions of the pH indicator for information.

- By the nature of this test method, the accuracy is lower compared to the use of a pH electrode.

- It's a manual process, impossible to automate or to collect measurement values via a computer.


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