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 pH 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 base) – Universal indicator (multi-colored) – Indicators like phenolphthalein – Natural indicators like red cabbage, radishes

Paper testing Paper tests like litmus paper and pH paper 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 paper

pH meter Tests the voltage of the electrolyte Tests the voltage of the electrolyte Converts the voltage to pH Converts the voltage to pH Very cheap, accurate Very cheap, accurate Must be calibrated with a buffer solution 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