Measuring pH

What do you need to know? In this section, we have grouped compounds

as either acidic, neutral or basic. Often that is enough information.

Sometimes a chemist will need to know more specific information, such as the exact pH of the solution.

What do you need to know? Depending upon what we need to know, we

could make a qualitative or quantitative measurement of pH. Qualitative – Use an indicator to determine if the

solution is an acid, a base or a neutral. Quantitative – Use an indicator or meter to

determine the pH of the solution.

Indicators An indicator is a chemical that changes colors

as the pH of the solution changes. There are many different indicators that are

used for a variety of reasons. Many of them are just chemicals that are found in nature.

We will focus on three indicators; litmus, phenolphthalein and universal indicator.

Qualitative Indicator Litmus is a solid that comes from plant

materials. Litmus is typically dissolved in a liquid and

then impregnated on paper. Litmus in an acidic solution is red. Litmus in a basic solution is blue.

More about Litmus Litmus paper comes in two different types:

Red litmus Blue litmus

If an acid is added to blue litmus, it turns red. If a base is added to blue litmus, it stays blue.

Phenolphthalein Phenolphthalein is a solid that is dissolved in

a liquid and typically used as a solution. When phenolphthalein is added to a clear

acidic solution, the solution stays clear. When phenolphthalein is added to a clear,

basic solution, the solution turns pink or red.

PHENOLPHTHALEIN

Quantitative Measurement Sometimes you want to be more exact and

have the pH of a solution. One choice is to use an electronic pH meter!

Very accurate Can be used with solutions that are not clear Easy to use.

pHydrion paper is another choice.

Quantitative Indicator pHydrion paper is paper that changes to a

different color when the pH changes. pHydrion paper is paper that has been

impregnated with a mixture of chemicals. Universal indicator is a solution that works

just like pHydrion paper, it changes to a different color with each pH.

How does pHydrion paper work? Each pH is a different

color. Notice the chart in the picture, pH of 1 is on the left and pH of 14 is on the right.

The colors have the same order as the colors of the rainbow, ROY G. BIV

Remember: When using litmus paper or pHydrion paper,

only use about a centimeter of paper. If a solution isn’t clear, use a pH meter

because you won’t be able to see the color changes.

Wear your goggles whenever you work with acids or bases.

I before E except after C.