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.