Transcript ABC`s of pH

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
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Gastric Juices
Urine
Saliva
Blood
Spinal Fluid
Pancreatic Juice
Bile
1.2 - 3.0
4.6 – 8.0
6.35 - 6.85
7.35 – 7.45
7.4
7.1 – 8.2
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 OHH2O
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
-7
[10 ]
pH = -log
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 acid
HCl  H+ + Cl-
battery acid
sulfuric acid
H2SO4 2H+ + SO42-
vinegar
acetic acid
CH3COOH  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
+
O ]=1
• [H3
x 10-5 M, pH = ?
• 5 (acidic)
• [H3O+]=2.6 x 10-5 M, pH = ?
• 4.59 (acidic)
+
O ]=7.8
• [H3
x
• 2.11 (acidic)
-3
10
M, pH = ?
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 Examples
Milk 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-].
-7
• In basic solutions [OH ] > 1 x 10 M
+
-7
[H3O ] < 1 x 10 M
• pH scale measures acidity without using
exponential numbers.
Base pH Calculation
[H3O+] to ph
• [H3O+]=1 x 10-10 M, pH = ?
• 10 (basic)
+
O ]=6.3
• [H3
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 and Weak Acids and Bases
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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.
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
Stomach acid
+ Mg(OH)2
+ Mylanta
2H20 + MgCl2
 H20 + salt
Hydrochloric 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 giving 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
• Brown
• 65,000 Daltons
• 4 sub units
Hemoglobin
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 Fluids
Mechanism
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- Important regulator of blood
bicarbonate
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 HCO3so 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