Acid-Base Equilibria (7) Control of Blood pH

Edexcel A level Chemistry (2017)

Topic 12: Acid–Base Equilibria:

Here are the final learning objectives in this topic:

12/23.  To be able to understand why there is a difference in enthalpy changes of neutralization values for strong and weak acids

12/24. To be able to understand the roles of carbonic acid molecules and hydrogen carbonate ions in controlling the pH of blood

Maintaining Blood pH levels:

Blood maintains a roughly constant pH of between 7.35 and 7.45.  It is a very narrow range of pH made possible because blood is buffered against the effect of excess acid or alkali.

If the pH falls below 7.35 a condition called acidosis occurs.  And if the pH rises above 7.45 a condition called alkalosis occurs. 

There are chemical reactions in the body whose products affect blood pH.  Respiration produces Carbon Dioxide.  Diffusing into the blood stream, carbon dioxide forms carbonic acid.  This increase in acidity has to be controlled.

Blood buffers based on CO₂, hydrogen carbonate and water ensure that the pH of blood remains roughly 7.4.

Inside the red blood cells the pH is 7.25.   

Here the buffer is also aqueous CO₂ or carbonic acid and hydrogen carbonate ions but haemoglobin molecules are acidic and lower the pH.

The buffer in blood depends on aqueous carbon dioxide CO₂(aq) sometimes called carbonic acid H₂CO₃

Aqueous CO₂ (written as H₂CO₃) acts as a weak acid.

Hydrogen carbonate ions HCO₃^— act as its conjugate base.

If acid is added to the blood, the equilibrium shifts to the left as the hydrogen carbonate ions take up the excess H+

If alkali is added, the equilibrium shifts to the right as the OH— ions react with the H+ ions and more H₂CO₃ dissociates restoring the H+ level.

There are two organs that help control the levels of carbonic acid and hydrogen carbonate in the blood stream.  

The lungs remove excess carbon dioxide and the kidneys remove excess hydrogen carbonate ions.

Patients who have survived being badly burned or been exposed to heavy smoke or fumes are treated with an intravenous drip.  

This helps return the blood to a normal pH range. 

It increases flow into the kidneys and so removes hydrogen carbonate from the blood which in turn leads to the conversion of excess levels of CO₂ into hydrogen carbonate.

In this way the blood is brought back to a healthy pH level.

If you are a runner you will know that if you run and then run faster your breathing rate increases and becomes deeper utilising more of your lung capacity.

What you are doing is vigorously expelling CO₂ from the lungs and consequently from the blood stream to ensure your blood pH level remains within the normal range.

Problem:

In a sample of blood, the concentration of hydrogen carbonate ion is  3.0 × 10^—2 mol.dm^—3 and the concentration of carbon dioxide is 1.5 × 10^—3 mol.dm^—3 and K_a  = 4.5  × 10^—7 mol.dm^—3 .

What is the hydrogen ion concentration and the pH of this blood sample?