Acid-Base Indicators

Measuring pH

• Now that we know how to calculate pH of a solution, how do we measure it in the lab?– The following slides will provide numerous

examples of ways to measure pH in the laboratory.

pH testing• There are several ways to test pHThere are several ways to test pH– Blue litmus paper (red = acid)Blue litmus paper (red = acid)– Red litmus paper (blue = basic)Red litmus paper (blue = basic)– pH paper (multi-colored)pH paper (multi-colored)– pH meter (7 is neutral, <7 acid, >7 base)pH meter (7 is neutral, <7 acid, >7 base)– Universal indicator (multi-colored)Universal indicator (multi-colored)– Indicators like phenolphthaleinIndicators like phenolphthalein– Natural indicators like red cabbage, Natural indicators like red cabbage,

radishesradishes

Paper testing• Paper tests like litmus paper Paper tests like litmus paper

and pH paperand pH paper– Put a stirring rod into the solution and Put a stirring rod into the solution and

stir.stir.– Take the stirring rod out, and place a Take the stirring rod out, and place a

drop of the solution from the end of drop of the solution from the end of the stirring rod onto a piece of the the stirring rod onto a piece of the paperpaper

– Read and record the color change. Read and record the color change. Note what the color indicates.Note what the color indicates.

– You should only use a small portion of You should only use a small portion of the paper. You can use one piece of the paper. You can use one piece of paper for several tests.paper for several tests.

pH paperpH paper

pH meter

• Tests the voltage of the Tests the voltage of the electrolyteelectrolyte

• Converts the voltage to pHConverts the voltage to pH• Very cheap, accurateVery cheap, accurate• Must be calibrated with a Must be calibrated with a

buffer solutionbuffer solution

pH indicators• Indicators are dyes that can be added that

will change color in the presence of an acid or base.

• Most are large molecules that act as weak acids. The color changes occur when the molecules is ionized (acting as an acid) or un-ionized (acting as a base).

• Some indicators only work in a specific range of pH. Choosing the right one is important!

• Once the drops are added, the sample is ruined

• Some dyes are natural, like radish skin or red cabbage

Choosing the Correct Indicator

• Not all indicators shift colors at pH 7. • Many change colors over a wide range of pH

values.• When testing a sample for acidity or basicity it

is best to choose an indicator that has a color change close to pH 7 – Or use pH paper– Or use a pH meter to get a digital reading

Sample Indicator pH Ranges• Crystal Violet = yellow (pH 1) blue• Cresol Red = red (pH 2) yellow (pH 8) red• Thymol Blue = red (pH 3) yellow (pH 8) green/blue• Bromophenol blue = yellow (pH 4) blue• Methyl orange = red (pH 4.5) orange/yellow• Bromcresol green = yellow (pH 4.5) green/blue• Methyl red = pink (pH 5.5) yellow• Bromcresol purple = yellow (pH 6) purple• Alizarin = yellow (pH 6.5) red (pH 12) purple• Bromthymol blue = yellow (pH 7 ) green/blue• Phenol red = yellow (pH 7.2) red• Phenolpthalein = clear (pH 8.3) pink• Thymolphtalien = clear (pH 10) blue• Alizarin yellow GG = orange (pH 10.5) red• Universal Indicator = red (pH 4) orange (pH 6) yellow (pH 7) green (pH 8) blue (pH

9) purple