- Utility Menu
Harvard Natural Sciences Lecture Demonstrations
1 Oxford St Cambridge MA 02138 Science Center B-08A (617) 495-5824
- Key to Catalog
enter search criteria into the search box
Buffer solution, presentation date: .
The ability of a buffer to resist changes in pH is colorfully shown by adding strong acid and base to water and buffer solution with universal indicator.
How it Works
A buffer solution contains both a weak acid and a weak base in equilibrium, which resist changes in pH. Usually, a buffer is composed of a weak acid and its conjugate base, or a weak base and its conjugate acid. Even when you add strong acid or strong base, the pH of a buffer solution changes very little because the pH depends on the log of the H + concentration. The H + concentration in aqueous solution depends on the acid-ionization (equilibrium) constant (Ka) and the ratio of the concentrations of weak acid to weak base as shown in Equation 1 (Tro, 2017):
Equation 2 (Tro, 2017) is the commonly used Henderson-Hasselbalch Equation for buffer solutions. The H-H equation is obtained by taking the negative log of Equation 1:
The buffer solution in this demo is composed of household vinegar (acetic acid) and ammonia (a weak base) from the supermarket with universal indicator, an acid-base pH indicator. Strong acid will react with the ammonia in the buffer, and strong base will react with the acetic acid in the buffer. The acetic acid is about 5% w/v (0.8M), and the ammonia is between 1-5% w/v (0.6 - 3M), depending on its age and brand. So the buffering capacity of the buffer is high enough to easily neutralize single drops of 1M HCl or NaOH, without any visible change in color. A change in color indicates a change in pH because universal indicator ranges from red at pH < 3 to purple at pH > 10, as shown in Figure 1 below.
[Link to image: https://commons.wikimedia.org/wiki/File:Skala_boja_2.JPG]
Pure water, on the other hand, has no weak acid or weak base species in equilibrium to neutralize strong acid or base. So adding one drop of 1M HCl in a petri dish of water causes approximately a 10 3 - 10 4 increase in H + concentration, changing the color of the universal indicator from green to red. A similar increase in hydroxide ion concentration occurs when one drop of 1M NaOH is added to water, changing the color of universal indicator from green to purple.
Setting it Up
Safety: Wear gloves, safety glasses or goggles, and a lab coat. 1M HCl, 1M NaOH, vinegar, and ammonia are harmful to eyes and skin, so wash off immediately if you come into contact.
Materials: Vinegar, household ammonia (make sure it's clear ammonia), universal indicator, 4 petri dishes, 2 - 250 mL beakers, plastic pipets, 1M NaOH, 1M HCl, tray.
Prepping it: In a beaker, mix approximately equal amounts of vinegar and ammonia. Add 3-4 drops of universal indicator. Adjust amounts of vinegar and ammonia as needed to get the green color for pH 7. Fill two petri dishes halfway with the buffer solution. In a different beaker, place some distilled water and add 3-4 drops of universal indicator. The water will probably be slightly acidic from absorbing CO 2 , so add drops of 1M NaOH until you get the same green color as the buffer. Fill two petri dishes halfway with the distilled water solution. Adjust the green color as needed with acid or base to make them appear similar in color to the buffer dishes. Place on white paper or white paper towels on a tray. Label the tops and cover the dishes. Covering the dishes will slow the absorption of CO 2 by the solution, preventing a premature change in color. Provide bottles of 1M HCl and 1M NaOH with plastic pipets.
Demonstrating it: A doc cam is the best way to show this demonstration. Make sure that the dishes are on top of a white surface if using a doc cam. If you don't have a doc cam, use a camera or have students move closer. Remove the covers, and add 1 drop of HCl to one of the water dishes and swirl to mix. Add 1 drop of NaOH to the other water dish and swirl to mix. A dramatic change in color should be seen. Now do the same with the buffer dishes, and there is no visible change in color! However, if you continue to add drops, eventually you will break the buffer. Extend the fun by asking what would happen if you add drops of the acid to the purple colored water, and vice versa. Students will hopefully predict the reversal of the color changes!
Cleanup and Disposal: Pour the solutions down the sink with running water.
Over time, the phenolphthalein in the universal indicator has less effect, so a strong purple color is no longer seen at pH > 10. Adding a drop or two of phenolphthalein to the buffer and water solutions will restore the purple color at high pH.
References and Acknowlegements
Daniel Rosenberg created the buffer in this demonstration when he only had household supplies at hand. Now you can too!
Tro, Nivaldo J. (2017). Chemistry: A molecular approach. Pearson Education, Inc.
Demo Subjects
Newtonian Mechanics Fluid Mechanics Oscillations and Waves Electricity and Magnetism Light and Optics Quantum Physics and Relativity Thermal Physics Condensed Matter Astronomy and Astrophysics Geophysics Chemical Behavior of Matter Mathematical Topics
Key to Catalog Listings
Size : from small [S] (benchtop) to extra large [XL] (most of the hall) Setup Time : <10 min [t], 10-15 min [t+], >15 min [t++] /span> Rating : from good [★] to wow! [★★★★] or not rated [—]
Complete key to listings
IMAGES
VIDEO