Weak Acids and Buffers

Why do we as biochemists study Buffers?

The simple answer is that our blood or plasma and that of other mammals has a constant pH!

Regardless of what we consume during the day or the stress conditions we are exposed to our body is capable of maintaining a constant pH.  

The  Buffering capacity in our body depends upon equilibrium between:

1)      Gaseous CO2 (air spaces of the lungs)

2)     Aqueous CO2 (dissolved in the blood)

3)     Carbonic acid

4)     Bicarbonate

To further understand the behavior of buffers in our body ans their role in maintaining the blood pH and thus the acid base balance let us first review how buffers act.

Maintenance of the Acid Base balance of the body will be posted in our next post. 

Acid Dissociation Constants of Weak Acids

Acids and bases that dissociate completely in water, such as hydrochloric acid and sodium hydroxide, are called strong acids and strong base. 

The chloride ion is the base that corresponds to HCl after it has given up its proton. 

Cl^- is called the conjugate base of HCl. 

H3O⁺ is the conjugate acid of H₂O

Weak acids and bases dissociate partially in water.   Acetic acid is the weak acid present in vinegar. The equilibrium constant for the dissociation of a proton from an acid in water is called the acid dissociation constant, Ka. 

         

Henderson–Hasselbach equation (H-H)

It defines the pH of a solution in terms of the pKa of the weak acid form of the acid–base pair and the logarithm of the ratio of concentrations of the dissociated species (conjugate base) to the protonated species (weak acid).

Let us derive the equation

The pH of the solution thus depends upon the ratio of  concentrations of the dissociated species (conjugate base) to the protonated species (weak acid) as the Ka value for each acid is constant.

When the concentrations of a weak acid or the proton donor and its conjugate base or the proton acceptor are exactly the same the pH of the solution is equal to the pKa of the acid.

Titration curve of acetic acid (CH₃COOH)

The pKa values of weak acids are determined by titration with a base.
Monoprotic acids like acetic acid have only one ionizable group, one pKa value.
There is an inflection point (a point of minimum slope) at the midpoint of the titration, when 0.5 equivalent of base has been added This is the point at which [CH3COOH]= [CH3COO- ] and pH pKa.
At the endpoint, all the molecules of acetic acid have been titrated to the conjugate base, acetate.

Weak Acids  Can act as Buffers

Buffered Solutions Resist Changes in pH

•      If the pH of a solution remains nearly constant when small amounts of strong acid or strong base are added the solution is said to be buffered.

•      The ability of a solution to resist changes in pH is known as its buffer capacity.

•      Most effective buffering, indicated by the region of minimum slope on the curve, occurs when concentrations of a weak acid and its conjugate base are when the pH equals the pKa

•      The effective range of buffering is  from one pH unit below to one pH unit above the pKa.

Titration curve for phosphoric acid

Phosphoric acid (H3PO4) is a polyprotic acid. It contains three different hydrogen atoms that can dissociate to form H ions and corresponding conjugate bases with one, two, or three negative charges. So it has three pKa values. 

Why does the titration curve of a week acid look the way it looks?

Using the H-H equation, let’s follow the change of  pH as we increase the ratio of [acid]/[base].

pH = pKa + log  [CH₃COO^-]/[CH₃COOH]

The H-H equation to calculate the pH of solutions or find the ratio of weak acid and conjugate base needed to prepare a buffer solution

Here is an example.

Example: 1.00 mole of phosphoric acid (H3PO4) and 1.75 moles of NaOH are added to 1  L of water. Calculate the pH.

Step 1:      1 mol of H3PO4 + 1.75 mol OH- ----->

                 1 mol H2PO4- + 1 mol H2O + 0.75 mol OH-

Step 2:    1 mol of H2PO4- + 0.75 mol OH- ---->

                 0.25 mol H2PO4- + 0.75 mol HPO42- + 0.75 mol H2O

Step 3:    In the end, we have 0.25 moles of H2PO4- and 0.75 moles of HPO42-, we can calculate the pH using the H-H equation:

Step 4: Look up the pKa of the reaction:

  pKa for H2PO4-  ⇄ HPO42- + H+, is 6.86

Step 5: Calculate [HA] = [H2PO4-]: 0.25 mol/1 L  = 0.25 M

Step 6: calculate [A-] = [HPO42-] : 0.75 mol/1 L  =  0.75 M

Therefore: pH = pKa+ log[A-]/[HA]

         = 6.86 + log((0.75 M)/(0.25 M)) = 7.34

For further information or for few practice exercises visit the following links
https://www.youtube.com/watch?v=jKD06NhAQCI
https://www.youtube.com/watch?v=i1LyWQ_lPik

We will post to you some practice exercises soon

Water and pH

              

           Why is it important to study water and its properties?

The answer is simple.

Water accounts for 60% to 90% of the mass of cells making it the most abundant molecule. Macromolecules of the cell (proteins, polysaccharides, nucleic acids and lipids) assume their characteristic shapes and structures in response to water. 

The physical properties of water allow it to act as a universal solvent for polar and ionic substance. The chemical properties allow it to form weak bonds with other molecules including other water molecules.

Structure of Water Molecule

The Water Molecule is Polar

• A water molecule (H2O) is V-shaped and the angle between the two covalent (O—H) bonds is 104.5° due to the strong repulsion exerted by the lone pairs of electrons on the oxygen atom.
• The uneven distribution of charge that occurs within each O—H bond of the water molecule leads to the appearance of a partial negative charge (δ-)on O and a partial positive charge (δ+) on the hydrogen. This uneven distribution of charge within a bond is known as a dipole and the bond is said to be polar.
• The angled arrangement of the polar O—H bonds of water creates a permanent dipole for the molecule. Thus the polarity of a molecule depends both on the polarity of its covalent bonds and its geometry.

•         Not all molecules are polar.Carbon dioxide contains polar covalent bonds but the bonds are oppositely oriented so the polarities cancel each other and the entire molecule has no net dipole, thus it is nonpolar.

Hydrogen Bonding in Water

Attraction forces between the slightly positive hydrogen atoms of one water molecule and the slightly negative electron pairs of the oxygen atom of another water molecule produce a hydrogen bond. 

In effect, the hydrogen atom is being shared (unequally) between the two oxygen atoms. The distance from the hydrogen atom to the acceptor oxygen atom is about twice the length of the covalent bond.



Water molecules are unusual because they can form four aligned hydrogen bonds with up to four other water molecules.  In liquid water, each H₂O molecule forms, on average, 3.4 H-bonds and thus a high amount of energy is needed to break the H-bonds. This is why water has a high boiling point. In ice, each H₂O forms 4 H-bonds causing ice density to decreases as the number of bonds increases. This is why less dense ice floats in liquid water, and why water expands when frozen. In addition the strength of these 4 H-bonds gives ice a unusually high melting point because a large amount of energy will be needed to disrupt the lattice of ice.

The three states of water and the degree of hydrogen bonding between water molecules are explained in a funny way in the figures below. In the liquid sates, each molecules has some space to move and form h-bond with other molecules interchangeably. The more H-bonds  are formed, the harder it becomes to move as there will be less free space (Ice). The less the attraction and bonds, the more free space to move freely (Gas)

Water Is an Excellent Solvent

Water molecules are polar, have low intrinsic viscosity, are small compared to other solvents, and can associate with solute particles and make them more soluble. Water can interact with and dissolve other polar compounds that ionize. Ionization is associated with the gain or loss of an electron, or an H⁺ ion, giving rise to an atom or a molecule that carries a net charge. Molecules that can dissociate to form ions are called electrolytes. Substances that readily dissolve in water are said to be hydrophilic, polar or water loving. The shell of water molecules that surrounds each ion or molecule is called a solvation sphere. A molecule or ion  surrounded by solvent molecules is said to be solvated. When the solvent is water, such molecules or ions are said to be hydrated.

     

      Nonpolar molecules are said to be hydrophobic, or water fearing and tend to hide from surrounding water molecules. This phenomenon of exclusion of nonpolar substances by water is called the hydrophobic effect. The hydrophobic effect is critical for the folding of proteins and the self-assembly of biological membranes.

  Detergents, sometimes called surfactants, are molecules that are both hydrophilic and hydrophobic. They usually have a hydrophobic chain at least 12 carbon atoms long and an ionic or polar end. Such molecules are said to be amphipathic.

Typical Noncovalent Interactions in Biomolecules

Ionization of Water

• Water has a slight tendency to ionize. Pure water contains a low concentration of hydronium ions (H₃O⁺) and an equal concentration of hydroxide ions (OH^-).  The hydronium and hydroxide ions are formed by a nucleophilic attack of oxygen on one of the protons in an adjacent water molecule. 
• Hydronium (H₃O⁺) ions  act as acids;  are capable of donating a proton to another ion. To simplify chemical equations we often represent the hydronium ion as simply H⁺
•  Hydroxide ions OH^- can accept a proton and be converted back into water molecules and thus act as bases
• The ionization of water can thus be depicted as a simple dissociation of one proton from a single water molecule and can be analyzed quantitatively.

The pH Scale

•      pH is defined as the negative logarithm of the concentration of H+. 

     In pure water [H⁺]  [OH^- ] = 1.0 × 10^–7 M.

•      Pure water is said to be “neutral” with respect to total ionic charge since the concentrations of the positively charged hydrogen ions and the negatively charged hydroxide ions are equal.

•      Neutral solutions have a pH value of 7.0 (the negative value of log 10^–7 is 7.0).

If you want further information go to https://www.youtube.com/watch?v=i1LyWQ_lPik