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 pH • Blue litmus paper (red = acid) • Red litmus paper (blue = basic) • pH paper (multi-colored) • pH meter (7 is neutral, <7 acid, >7 base) • Universal indicator (multi-colored) • Indicators like phenolphthalein • Natural indicators like red cabbage, radishes
Paper testing • Paper tests like litmus paper and pH paper • Put a stirring rod into the solution and stir. • Take the stirring rod out, and place a drop of the solution from the end of the stirring rod onto a piece of the paper • Read and record the color change. Note what the color indicates. • You should only use a small portion of the paper. You can use one piece of paper for several tests.
pH meter • Tests the voltage of the electrolyte • Converts the voltage to pH • Very cheap, accurate • Must be calibrated with a buffer 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