ABC’s of pH

Acids and Bases

Revised 10-03-2006

Homeostasis

• Chemical reaction in the body are very sensitive to changes in pH.

pH values in Body

• Gastric Juices 1.2 - 3.0• Urine 4.6 – 8.0• Saliva 6.35 - 6.85• Blood 7.35 – 7.45• Spinal Fluid 7.4• Pancreatic Juice 7.1 – 8.2• Bile 7.6 – 8.6

pH

• pH is based on the relative concentration of hydrogen ions (H+ or H3O+) in a solution.

Water ionizes into H+ and OH-

H2O H3O+ + OH-

Scientists measure the pH of solutions using a pH meter

The meter reads the H+ ion concentration in solution indicating the pH of the solution.

Mathematically it is

• pH is based on the relative concentration of hydrogen ions in a solution.

pH = -log [H+]• Water at 25oC contains 1/10,000,000 mole of H+ ions = 10 -7 moles/liter

pH = -log [10-7]pH = 7

pH

logarithmic• a pH of 6 is ten times more acidic than

distilled water.

• Also, a solution that has a pH of 8 is ten times more basic (alkaline) than distilled water.

• Likewise, a solution that has a pH of 4 is 100 times more acidic than one which has a pH of 6.

• The pH scale is logarithmic (that's what the "p" stands for; the "H" is short for hydrogen);

• therefore, a solution that has a pH of 6 is ten times more acidic than distilled water.

• Also, a solution that has a pH of 8 is ten times more basic (alkaline) than distilled water.

• Likewise, a solution that has a pH of 4 is 100 times more acidic than one which has a pH of 6.

Properties of Acids

• Substances with a pH below 7 are acids. • An acid is any substance that ionizes (release charged

particles when dissolved) in water to produce H+ hydrogen ions or free protons in water.

• Acids tend to taste sour. • Change blue litmus paper to red.• React with metals to release hydrogen gas.• Neutralize bases, forming water and salts

• Acidic solutions have concentrations of H3O+ above 1 X 10-7

Acid Examples

stomach juices hydrochloric acidHCl H+ + Cl-

battery acid sulfuric acidH2SO4 2H+ + SO4

2-

vinegar acetic acidCH3COOH H+ + CH3COO-

Measuring Acidity

• Added acids increase [H3O+]

• In acid solutions [H3O+] > 1 x 10-7 M

• pH scale measures acidity without using exponential numbers.

Acid pH Calculations

• [H3O+]=1 x 10-5 M, pH = ?

• 5 (acidic)

• [H3O+]=2.6 x 10-5 M, pH = ?

• 4.59 (acidic)

• [H3O+]=7.8 x 10-3 M, pH = ?

• 2.11 (acidic)

Properties of Bases

• have a pH greater than 7. • A base is a substance that ionizes in water solution to

produce OH- hydroxide ions. • Bases tend to feel slippery to the skin.• Bases accept protons• Bitter taste• Bases turn red litmus paper blue.• Neutralize acids, forming water and salts.

• Basic solutions have H3O+ concentrations less than 1 X 10-7

Base ExamplesMilk of Magnesia

Mg (OH)2 2OH- + Mg2+

Mylanta

Mg (OH)2 + Al(OH)3 5OH- + Mg2+ + Al3+

Maalox

Mg (OH)2 + Al(OH)3 5OH- + Mg2+ + Al3+

Measuring Acidity

• Added bases increase [OH-].

• In basic solutions [OH-] > 1 x 10-7 M

[H3O+] < 1 x 10-7 M

• pH scale measures acidity without using exponential numbers.

Base pH Calculation [H3O+] to ph

• [H3O+]=1 x 10-10 M, pH = ?

• 10 (basic)

• [H3O+]=6.3 x 10-9 M, pH = ?

• 8.20 (basic)

• What if the number is not 1?

pH Calculations

• pH to [H3O+]?

– inverse log of negative pH– orange juice, pH 3.5 [H3O+]=?

• [H3O+] = 10-3.5 = 3.2 x 10-4 M

– saliva, pH 6.2 [H3O+]=?

• [H3O+] = 10-6.2 = 6.3 x 10-7 M

• Strong acids ionize completely in water

• Weak acids ionize only partially in water

• Strong bases ionize completely in water.

• Weak bases ionize only partially in water.

Strong and Weak Acids and Bases

Neutralization

• Acids and bases can neutralize each other by forming water and salt.

• Salt - a crystalline compound composed of the negative ion of an acid and the positive ion of a base.

H+A- + B+OH- H20 + A-B+ (salt)

Examples

Stomach acid + Milk of Magnesia H20 + salt

Hydrochloric Acid + Magnesium Hydroxide H20 + salt

2HCl + Mg(OH)2 2H20 + MgCl2

Stomach acid + Mylanta H20 + saltHydrochloric Acid + Magnesium Hydroxide and Aluminum Hydroxide H20 + salt

5HCl + Mg(OH)2 and Al(OH)3 5H20 + MgCl2 + AlCl3

Buffer

• Buffers act as a reservoir for hydrogen ions, donating or removing them from solution as necessary.

• a solution that can receive moderate amounts of either acid or base without the significant change in its pH.

Buffer

• A buffer is a mixture of a weak acid with the salt of the acid.

• If small amounts of acid is added, hydronium ions are neutralized by reacting with the salt in solution

• If small amounts of base is added, hydroxide ions are neutralized by reacting with the acid.

Protein Buffer System

• Intracellular Fluid

• Blood Plasma

• Red Blood Cells– Hemoglobin

• Blood Plasma– Albumin

Protein Buffer Systems

• carboxyl group (—COOH)

• amino group (—NH2);

Protein Buffer Systems

• carboxyl group (—COOH)

–Releases H+ ↑pH

Protein Buffer Systems

• amino group (—NH2);

–Forms NH3+ ↓pH

20 Amino Acids

Hemoglobin

• The protein hemoglobin is an important buffer of H+ in red blood cells. As blood flows through the systemic capillaries, carbon dioxide (CO2) passes from tissue cells into red blood cells, where it combines with water (H2O) to form carbonic acid (H2CO3). Once formed, H2CO3 dissociates into H+ and HCO3-.

• At the same time that CO2 is entering red blood cells, oxyhemoglobin (Hb-O2) is giv ing up its oxygen to tissue cells. Reduced hemoglobin (deoxyhe-moglobin) is an excellent buffer of H+, so it picks up most of the H+. For this reason, reduced hemoglobin usually is written as Hb-H. The following reactions summarize these relations

Hemoglobin• Brown

• 65,000 Daltons

• 4 sub units

Hemoglobin

Carbonic Acid- Bicarbonate

• Carbonic acid – H2CO3

– weak acid

• Bicarbonate ion – HCO3

-

– weak base

Carbonic acid-bicarbonate buffer system

carbonic acid-bicarbonate buffer system

Carbonic Acid- Bicarbonate

Carbonic Acid- Bicarbonate

carbonic acid-bicarbonate buffer system

Phosphate Buffering System

Phosphate Buffering System

Phosphate Buffering System

• Intracellular Fluid

• Cytosol

• Kidneys

Exhalation of CO2

Breathing and pH levels

Negative Feedback Loop

Kidney Excretion of H+

Summary of Mechanisms that Maintain pH of Body FluidsMechanism Comments

Buffer Systems Most consist of a weak acid and the salt of that acid, which functions as a weak base. They prevent drastic changes in body fluid pH.

Proteins The most abundant buffers in body cells and blood. Histidine and cysteine are the two amino acids that contribute most of the buffering capacity of proteins. Hemoglobin inside red blood cells is a good buffer.

Summary of Mechanisms that Maintain pH of Body Fluids

Mechanism Comments

Carbonic acid-bicarbonate

Important regulator of blood pH. The most abundant buffers in extracellular fluid (ECF).

Phosphates Important buffers in intracellular fluid and in urine.

Summary of Mechanisms that Maintain pH of Body Fluids

Mechanism Comments

Exhalation of CO2

With increased exhalation of C02, pH rises (fewer H+).

With decreased exhalation of CO2, pH falls (more H+).

Kidneys Renal tubules secrete H+ into the urine and reabsorb HCO3

-

so it is not lost in the urine.

pH Summary

• Acids dissociate in water to increase the concentration of H+.– pH values lower than 7

• Bases combine with H+ ions when dissolved in water, thus decreasing H+ concentration.– pH values above 7

• Buffers act as a reservoir for hydrogen ions, donating or removing them from solution as necessary.

Calibration of pH Meter

• Use pH standards

–pH 7

–pH 10

Calibrate pH Meter

• Steps:• Turn the pH meter on by pressing the

“on/off” button.• Remove the pH electrode from the green

pH 7 solution and rinse the electrode well with distilled water into the “waste” 600ml beaker.

• .

• Place the electrode into the orange pH 4 solution, and press “cal”. Observe the digital numbers in the readout window, when the numbers stop changing press the “read” button. The display should now read 4.00

Rinse Electrode each time

Calibrate pH Meter

24 well container

Read pH of Solutions

Ammonia

• Predict pH

Apple Juice

• Predict pH

Baking Soda

• Predict pH

Berry Juice

• Predict pH

Bleach

• Predict pH

Grapefruit Juice

• Predict pH

Mylanta

• Predict pH

Phillips’ Milk of Magnesia

• Predict pH

Tomato Juice

• Predict pH

Vinegar

• Predict pH