Group Work 9

Equilibria: Acids and Bases: Buffers

Group Work 8

[note: this groupwrok was created by Dr. Samantha Glazier, a serious rower of crew who competed nationally. She was a post-doctoral teaching Fellow at BMC several years agao.]

There is no Zen Moment. Instead find a space against the wall….

It is time to start training for the 2006 Olympics in Torino, Italy!

The Exercise.

Find a place on the wall, and position yourself as if you were sitting on a chair with your thighs parallel to the floor. Hold this position for 60 seconds. Assign a rating to your comfort level (1= "I could hold a 50 pound weight comfortably in my lap" and 10 = "All I see is a white light")

The Background Physiochemical Process.

During exercise your muscles get the energy needed to contract and relax from the conversion of adenosine triphosphate (ATP) to adenosine diphosphate (ADP):

ATP --> ADP + Energy

The ATP is made by as part of the series of reactions where glucose is “burned” (oxidized) to ultimately yield carbon dioxide and water. An intermediate step is used to convert glucose and ADP into ATP and lactic acid (a waste product). The role of oxygen in the overall process is in the reaction with lactic acid:

Glucose + ADP --> lactic acid + ATP

Lactic acid + O₂ --> CO₂+ H₂O

1. What physiological response removes the products during exercise?

Breathing harder to take in more oxygen and to remove CO₂

2. Write down the lactic acid (CH₃CHOCOO-H) equilibrium in water:

CH₃CHOCOO-H + H₂O <---> CH₃CHOCOO^- + H₃O⁺

3. What causes the "burn" you (may have) felt in your legs during the exercise above. As you continued to put an energy demand on your muscles, energy conversion went from aerobic (with oxygen) to anaerobic (without oxygen)).

From your knowledge about acids and bases, describe what happened to the pH in your muscles in the final seconds of the wall sit when you were in pain.

The lack of oxygen shifts the equilibrium of the reaction towards the reactants (lactic acid), which then produces more acid (H₃O⁺) in the equilibrium you wrote in #1 — ouch! So it is not exactly the lactic acid that hurts but the production of H₃O⁺ that causes the pain.

The attached Graph 18 shows how the concentration of lactic acid in the blood changes with how hard three different people are working. Look at the middle curve for the amateur. There is a region where the concentration of lactic acid in the blood increases slowly then jumps dramatically. If you reached about a 7 during the wall sit you probably hit this region of the graph.

more Background.

The lactic acid is transported to other areas in your body where oxygen is available in your blood stream. During this transport of lactic acid, dramatic changes in the pH of your blood must be buffered (compensated for) or you will die.

The buffer system in the blood is based on the carbon-containing product of respiration.

4. What is the carbon product of respiration? CO₂

5. Write all of the relevant equilibria of the buffer system in blood.

CO₂ + H₂O <==> H₂CO₃

H₂CO₃ + H₂O <==> HCO₃^-

HCO₃^- + H₂O <==> CO₃^2- (Buffer)

6.The titration plot is for Na₂CO₃.titrated with HCl. Identify what carbonate species predominate in the regions of the titration plot and label all the significant regions in the plot.

You should identify the tow equivalience points, between CO₃^2- and HCO₃^- , and between HCO₃^- and H₂CO3 and the two buffer regions around the two pKa= pH points.

7. Write an equation to show how the blood buffer system is able to neutralize the acid produced during exercise.

HCO₃^- + H₃O⁺<==> H₂CO₃ ₃ + H₂O

HCO₃^- + CH₃CHOCOO-H <==> CH₃CHOCOO^- + H₂CO₃

8. There is a very useful formula (biochemists use it a lot) to easily calculate the pH of a buffer solution.

pH = pKa + log( [A-] / [HA] )

where HA is the acid form ( i.e., protonated) of the buffer

and A- is the conjugate base form (i.e., deprotonated).

In blood, [HCO₃^-] = 0.015M. The [H₂CO₃] depends on the amount of CO₂ dissolved in the blood: To calculate this amount you need to know the following information:

CO₂(aq) + H₂O <=> H₂CO₃ for which Keq = 0.0031.

K_a for H₂CO₃ is 0.000148.

(a) Calculate the [H₂CO₃] if the concentration of dissolved gas, [CO₂(aq)] = 0.0012M

K = [H₂CO₃]/ [CO₂]

0.0031 = [H₂CO₃₃]/0.0012

[H₂CO₃] = 3.7x10^-6 M

(b) Now, what is the pH of the blood buffer system?

pH = pKa + log([HCO₃^-] / [H₂CO₃] )

pH = (-log 0.000148) + log(0.015/3.7x10^-6)

pH = 7.4 (slightly basic)

Beyond Groupwork 8 - 18 points total

1. (6 points) Look again at Graph 18, the plot of lactate concentration vs workload for untrained, amateur and professional athletes. Find the horizontal line marking the conversion from aerobic to anaerobic processes, the “anaerobic threshod”. It shows what concentration of lactic acid (incorrectly called “lactate” on graph) occurs at the onset of the anaerobic threshold. Using this information and the pKa = 3.86 for lactic acid, what pH does this correspond to?

NOTE that this equation: pH = pKa + log( [A-] / [HA] ) canNOT be used to solve this problem since you are given no information about the [A-] concentration. The info you have is a pKa and a concentration of lactic acid ( from the Graph 18), or [HA] = 0.004 M = 4 mM.

You must use the "ice box" method to solve this question since BOTH [A-] and [H+] are unknowns.

Then you can write: Ka = [A-] [H+] / [HA] or 10^-3.86 = [lactate][H+]/[lactic acid]

and so: Ka = 1.38 x 10^-4 = (x)(x)/ (0.004 -x) and making the assumption that x is small compared to 0.004 M then

1.38 x 10^-4 = (x)(x)/ (0.004)

x = 7.43 x 10^-4 = [H+]

pH = - log (7.43 x 10^-4 ) = 3.13

(note that if you solve the quadratic function you obtain a very slightly differnet answer of 3.17 for pH)

2. (6 points) Now what about the sensation of lactic acid build up in your muscles? Two consequences of an acidic environment are (1) the decreased ability of hemoglobin (a protein) to transport oxygen and (2) stimulating neurotransmitters that communicate important information to the brain - in this case, pain. Hemoglobin and neurotransmitters are made up of amino acids. Two specific examples of neurotransmitters are the amino acids glycine and glutamine:

a) What if blood did not have a buffer system? Suggest how a lower pH affects the neurotransmitter amino acids glycine and glutamine. Identify what regions of these structure will change as the lactic acid concentration builds up under anaerobic workout.

The decrease in pH means that there is an increase of hydronium (or more simply, H+) ion. Therefore , an increase in protons will affect any basic sites (note that already protonated -COOH groups will not be affected). Both neurotransmitters have -NH2 groups. THese wil become protonated in acidic solution to make charged -NH3+ groups.

b) Speculate how this change of the neurotransmitters might affect their function.

Based on your answer in (a) you may speculate that the change in CHARGE and the STRUCTURE of the amino acid will affect their function in signaling pain.

3. (6 points) If the bicarbonate concentration in blood fell 33% to 0.010 M, while the concentration of [CO2(aq)] remained at equilibrium of 0.0012M, what would the pH of blood be?

For this problem you can use the buffer equation pH = pKa + log([HCO₃^-] / [H₂CO₃] )

You are given the value for [HCO₃^-] = 0.010 M, but you need to find [H₂CO₃]. You are given the CO2 concentration, and so looking back into the last question (#8) on the groupwork you can find this:

K = [H₂CO₃]/ [CO₂] and so 0.0031 = [H₂CO₃₃]/0.0012

then [H₂CO₃] = 3.7x10^-6 M

Now you can proceed:

pH = (-log 0.000148) + log(0.010/3.7x10^-6)

pH = 7.26 - only a very small decrease