Composition of a Penny: Skill Builidng Lab

Acid-Base Chemistry
Cardiac Arrest and Buffer Restoration:
Application Lab
 2003, Sharmaine S. Cady
East Stroudsburg University
Skills to build:
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Using a burette
Using a pH meter
Doing acid-base calculations
Correcting an acid-base blood imbalance
Buffering of Blood
The buffering of the blood is important to maintaining good health. The normal
pH range for blood is very narrow: 7.35-7.45. Slight disturbances to the acid-base
balance in the blood can significantly impair the proper function of the blood and the
tissues and organs it supplies. If the pH of the blood drops below 6.9 or increases
above 7.8, a potential fatal condition exists.
The major buffer in the bloodstream is carbonic acid/hydrogen carbonate, or
H2CO3/HCO3-. Since the amount of H2CO3 is very low in blood, dissolved CO2 from
tissues aids in maintaining the buffering capacity. The important equilibria are:
CO2(aq) + H2O(l) S H2CO3(aq)
H2CO3(aq) + H2O(l) S H3O+(aq) + HCO3-(aq)
From the first equilibrium, the amount of acid is proportional to pCO2, and the
Henderson-Hasselbalch equation is expressed using pCO2 as a measure of the acid
content of the buffer:
pH = 6.1 + log
[HCO -3 ]
0.03 x pCO 2
The constant 0.03 relates pCO2 to the amount of dissolved CO2 in plasma. Since CO2 is
an important component of the buffer, the respiratory center in the brain is sensitive to
changes in pCO2 and pH in the blood.
Cardiac Arrest!
The ratio of hydrogen carbonate to carbonic acid is 20:1 at a blood pH of 7.40.
Hence, blood has a higher capacity to buffer excess acids than base. When this ratio
deviates from normal, acidosis or alkalosis results. As the ratio decreases, the blood pH
drops and acidosis occurs. As the ratio increases, the blood pH increases and alkalosis
occurs. Changes to the 20:1 ratio occur as a result of metabolic or respiratory changes
in the body. Table 1 shows conditions that lead to these acid-base imbalances.
Table 1. Causes of Blood Acid-Base Imbalances
Type
Causes
respiratory acidosis
anesthesia, crushed thorax, bronchitis,
emphysema, pneumonia, asthma,
pulmonary edema
respiratory alkalosis
anxiety, fever, hepatic cirrhosis,
hypoxemia, overventilation of patients on
respirators
metabolic acidosis
diabetes, chronic alcoholism, kidney
failure, diarrhea, salicylate poisoning
metabolic alkalosis
vomiting, diuretics, laxative abuse, gastric
suctioning
Changes in pCO2 and HCO3- concentration along with pH point to a particular
type of acid-base disorder, and compensatory mechanisms attempt to restore the
normal ratio. Table 2 shows the specifics for each acid-base disorder.
Table 2. Blood Acid-Base Disorders
Acid-Base
Disorder
pH
Primary Cause
Compensation
respiratory acidosis
decreases
PCO2 increases
kidneys increase
[HCO3-]
respiratory alkalosis
increases
PCO2 decreases
kidneys decrease
[HCO3-]
metabolic acidosis
decreases
[HCO3-] decreases
hyperventilation,
pCO2 decreases
metabolic alkalosis
increases
[HCO3-] increases
hypoventilation,
pCO2 increases
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Cardiac Arrest!
Cardiac Arrest
Clinically, cardiac arrest is an absent or inadequate ventricular contraction that
results in systemic circulatory failure. Eighty percent of cardiac arrests are the result of
electrical dysfunction that often presents as ventricular fibrillation prior to onset. The
remaining twenty percent is mechanical failure. Additional causes include circulatory
shock or abnormalities in ventilation leading to significant respiratory acidosis
(cardiopulmonary arrest). Although either the heart or lungs may fail first, both events
usually are closely related.
In a cardiac arrest, the blood stops circulating and CO2 builds up in the
bloodstream. This shifts the equilibria to the acidic side and blood pH falls. Normal
metabolism ceases since the supply of oxygen to tissues is no longer available. As a
result, glucose partially breaks down into lactic acid, which is released by the tissues into
the bloodstream, contributing to an increase in blood acidity and a further lowering of the
pH. The 20:1 ratio falls and acidosis develops.
If the patient is resuscitated, the damage to the blood buffer system may be so
severe that the patient is still in grave danger. The administration of bicarbonate by
medical personnel is used to prevent further complications and provide immediate
restoration of the blood buffer. Since emergency personnel do not have time to "titrate"
the patient, they must add a reasonable amount and then check to see if the treatment is
working. Hence, the understanding of blood pH and buffers is essential to proper
methods in the emergency room.
In this experiment, the carbonic acid/hydrogen carbonate buffer is difficult to
study because CO2 is easily lost to the atmosphere. Instead, the dihydrogen
phosphate/hydrogen phosphate buffer is used to simulate the body's blood buffering
system. This buffer behaves similarly to the H2CO3/HCO3- buffer when acid or base is
added. You will be given a blood sample form a patient suffering acute acidosis after
cardiac arrest and resuscitation. The pH is determined and the amount of HPO4necessary to restore normal pH is calculated. This amount is administered to the
"patient." The instructor measures the new pH to see if the patient has been saved.
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Cardiac Arrest!
Experimental Methods and Materials
Safety considerations
Wear suitable protective clothing, gloves, and eye/face protection!
SODIUM DIHYDROGEN PHOSPHATE
caution! may cause irritation to skin, eyes, and respiratory tract
may be harmful if swallowed or inhaled
SODIUM HYDROGEN PHOSPHATE
caution! may cause irritation to skin, eyes, and respiratory tract
may be harmful if swallowed or inhaled
Restoring a blood buffer
Obtain 25.00 mL of an acidified blood sample, that is actually a H2PO4-/HPO42buffer. Record the pH. Titrate the buffer with 0.50 mL increments of HPO42- to
determine the changes in pH with added HPO42-. Determine the number of moles of
HPO42- needed to restore the blood buffer to its normal range of 7.35-7.45.
Restoration chart
Using the information from the titration, prepare a chart that gives the number of
moles needed to restore the blood to normal pH. List pH values in 0.05 increments
from 7.00 to 7.35. Determine the number of moles of needed to restore the blood to
a value in the normal range for each pH.
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Cardiac Arrest!
Patient sample
Record your patient letter. Place 50.00 mL of the patient sample into a 250-mL
beaker. Record the pH. Determine the number of moles Na2HPO4 needed to
restore the simulated blood sample to a pH in the normal range. Note that you are
using twice the volume for your patient than you used in the titration. Determine the
number of mL Na2HPO4 that contains that number of moles. Fill a burette with
Na2HPO4. Add the calculated mL of Na2HPO4 to the "patient." Take the blood
sample to the instructor to determine the new pH.
Laboratory Report
Answer the following questions in your lab notebook and your report:
1. Discuss the importance of understanding acid-base chemistry, buffers, and the
calculations involved to providing proper care to patients.
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