Michel Lotito (6/15/50-6/25/07)

Monsieur Mangetout“Mr. Eat-Everything”

Homeostatic Connections To pH• Carbon dioxide sensors

in carotid artery measure blood pH and tell brain when to signal lungs to breathe

• Absorption and digestion require specific pH

• pH may prohibit parasitic infections

• Changes in pH may alter or destroy enzymes

How Is pH Maintained?

• Body is slightly alkaline, but ranges from pH 2 to pH 11 in matter of centimeters

• Excess hydronium and hydroxide ions must be neutralized

• BUFFERS resist changes in pH

What Are Buffers And How Do They Work?

• Substances that resist changes in pH

• Buffers are themselves weak acids and/or bases

• These weak acids and/or bases disassociate in solution, neutralizing the excess hydronium (H+)or hydroxide (OH-) ions

• As the excess H+ and OH- ions are no longer “free”, their concentration does not effectively increase; pH stays the same

PURPOSE

• To evaluate and compare the relative buffering capacity of seven organic and inorganic substances through calculation of ΔpH (change in pH)

Seven Potential BuffersAlbumin

Casein

Magnesium Hydroxide

Sodium Bicarbonate

Sodium Chloride

Sodium Stearate

Water

Albumin

• Protein found in egg whites

• Comprised of amino acids which have a positive and negative functional groups

Casein

• Protein found in mammalian milk

• Contains a water repelling core and polar/charged tails

Magnesium Hydroxide

• Active ingredient in Milk of Magnesia

• An strong electrolyte, meaning rapid disassociation into ions

Sodium Bicarbonate

• Baking soda

• Disassociates into two charged ions, sodium (+) and bicarbonate (-)

Sodium Chloride

• Table Salt

• Inorganic substance with weak ionic bonding

Sodium Stearate

• Key ingredient of soap

• Found in non-polar fats and oils

Water

• Amphoteric = may serve as an acid or a base when in solution

• Neutral pH

Buffering Capacity Protocol1. Place 25mL of your

first buffer into each of two beakers

2. Remove storage bottles from pH probes and immerse tip of probe in beaker. Record the initial pH in table 2

3. To the left beaker, add 5 drops 0.1M HCl

4. To the right beaker, 5 drops 0.1M NaOH

5. Record the pH for each probe in table 2

6. Repeat steps 3-5 until a total of 30 drops of acid and base have been added to their respective beakers

Protocol Part 2• Discard the water-acid and water-base

mixtures from the first experiment and rinse the beakers

• Obtain 25mL of the next assigned buffer for each of the two beakers

• Repeat the first protocol up to 30 drops of acid & base respectively, recording the pH values in table 1

• Calculate the ΔpHacid, ΔpHbase and ΔpHtotal

• Post your data on the board in the front of the room

Calculating Buffering Capacity• To calculate ΔpHacid and ΔpHbase for a buffer, find

the absolute value of the difference between the initial (0 drops) and final (30 drops) values for both probes. Record these values in table 1.

• To determine and then rank the buffering capacities, calculate the ΔpHtotal by adding the ΔpHacid and ΔpHbase . Record these values in table 1.

• For water’s buffering capacity, use the data you collected in the Acids, Bases & Indicators lab.