FUNDAMENTALS OF ACID – BASE

(pH and Buffers)

Herbert M. San PedroLecturer

Acid - Base

An important factor in the proper behavior of many biochemical phenomena (processes)

Any deviation from the expected acid-base balance may lead to some disorders, most common are the alkalosis and acidosis.

Can be measured by knowing the Hydronium ion concentration [ ] (established by Sorensen) - pH

DISSOCIATION PRINCIPLE

Ability to form ions in solution. Water, plays a vital role in the

determination of the degree of acidity (alkalinity) of many inorganic substances, likewise in the biochemical processes.

Water being a naïve substance, can act as both an acid or base ( amphoterism )

Cont.

Water molecules themselves may react with each other and create ions (auto-ionization of water – autoprotolysis)

Eqn: H2O + H2O ↔ H3O+ + - OH

Or Simplified by: H2O ↔ H+ + - OH

Cont.

The presence of such ions contribute significantly to the over-all properties of water.

Going further analytically: the auto-ionization of water is expressed in terms of equilibrium expression:

K = [H+ ][- OH] / [H2O}K = dissociation constant

Cont. To further simplify the equation: it is

considered that the concentration of pure water being experimentally to high (55.56 mol/L) is doubtedly affected by dissociation (making it constant).

Eqn becomes: K[H2O] = Kw (water const.)Kw = [H+ ][- OH ] = ion product of water

Simplifying things out, it is from the above eqn.

wherein the concept pH was introduced and also applied to acidic and basic solutions.

Cont.

pH – degree of acidity, value is affected by the amount of hydronium ion present in solution.

Low pH – High concentration of hydronium ion. (below 7)

High pH – Low concentration of hydronium ion. (above 7)

The formula: pH = - log [H+ ] : UE

Sample Calculations:

1. What is the pH of a solution whose hydrogen ion concentration is 3.2 x 10-4 mol/L.

Solution: pH = - log [H+ ]

= - log (3,2 x 10-4 ) = - (-3.5)

pH = 3.5

Cont:

2. What is the pH of a solution whose hydroxide ion concentration is 4,0 x 10-4 mol/L: (in similar fashion pOH = - log [- OH])

Considering analytically the ion product of water Kw = [H+ ][- OH] = 1 x 10-14 taking the log of both sides this becomes:

pH + pOH = 14 : UE

Cont.

To solve the problem:[- OH] = 4.0 x 10-4

pOH = - log [- OH] = - log (4.0 x 10-4 )

= - (-3.4) pOH = 3.4 pH + pOH = 14 pH = 14 – pOH

= 14 – 3.4 pH = 10.4

Cont.

Considering the vice-versa:[H+ ] = 10-pH : UE

1. Given the pH of a substance to be equal to 5. Calculate its hydrogen ion concentration.

[H+ ] = antilog (-pH) = antilog ( - 5)

[H+ ] = 1 x 10-5 mol/L

The Weak Acid / Weak Base Many biochemicals possess functional

groups that are weak acids or bases. Thus knowledge of the dissociation of WA/ WB is basic to understanding the influence of intracellular pH on structure and biologic activity.

And these therefore require a different approach in calculating for their acidity and basicity (acidity / basicity dissociation constant; Ka or Kb) :

HA ↔ H+ + - A (weak acid)Ka = [H+ ][- A] / [HA]

HENDERSON-HASSELBACH EQUATION

A very useful equation (UE) in calculating for pH involving weak acids/weak base and their conjugates.

Derivation: HA ↔ H+ + - A

Ka = [H+ ][- A]/[HA]Ka[HA] = [H+ ][- A][H+ ] = Ka[HA]/[- A]

Cont.

Log [H+ ] = Log (Ka[HA]/[- A])= Log Ka + Log [HA]/[- A]

Multiplying both sides by -1-Log [H+ ] = - Log Ka – Log [HA]/[- A]

pH = pKa – Log [HA]/[- A]pH = pKa + log [- A]/[HA] : UE

BUFFERS

Solution of Weak Acid/Base and their conjugate. (salts)

Has the ability to resist change in pH after the addition of strong acids/bases.

To measure the pH changed that occurred after the addition of either strong acid/base the HHE is being used.

Cont.

There are so many buffers in the human body and all of these are responsible to maintain the proper pH the body requires (physiologic buffers)

a. Bicarbonate, Phosphate, Protein Buffers

Any deviation from the desired pH may lead to disorders like acidosis or alkalosis.

Acid-Base Balance

Blood place a crucial role In the over-all acid-base balance, It must retain (through the help of buffers) its normal pH (even after the addition of strong acid/bases) so as not jeopardize the biochemical processes.

The Kidney and the Lungs are the two major organs that help the blood to regulate its pH

Cont.

The exhaling of CO2 by the lungs and regulation of HCO3

- by the kidney, are the responsible processes how the blood is capable of maintaining its pH.

> Any trouble acquired by either of the organs or conditions/situations again acquired by any persons may suggest possible imbalance to the equilibrium

Cont.CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

-

LUNGS

Respiratory Acidosis

- Decreased by the plasma pH, increase in partial pressure of CO2 and variable increase in bicarbonate concentration:

-- impaired ventilation, severe damage in the lungs and there is no gas exchange that takes place across lungs and blood capillaries. – Metabolic Alkalosis (kidney)

-- headache, blurred vision, fatigue and weakness

Cont.

Respiratory Alkalosis

-Increase in plasma pH, drop in partial pressure of CO2 and a variable decrease in bicarbonate concentration. – Metabolic acidosis

-Pulmonary decrease, aspirin overdose, excessive exercise and cirrhosis. Overstimulation of respiratory control center (need for more Oxygen) – Congestive Heart Failure, Living at High altitudes

--Hyperventilation, possible convulsions, lightheadedness, (return back the exhaled CO2 ).

Cont.KIDNEYS

Metabolic Acidosis

-Drop of plasma pH as a result of a decrease in the bicarbonate concentration – Respiratory Alkalosis

-- uncontrolled DM with ketosis, renal failure, poisoning with acid substance, severe diarrhea with loss of bicarbonates, lactic acidosis and severe dehydration.

-- Intravenous admin of bicarbonate, fluids, insulin therapy.

Cont.Metabolic Alkalosis

-Increase in plasma pH, increase in bicarbonate concentration – Respiratory Acidosis

-- loss of hydrogen or a retention of bicarbonate

-- prolonged vomiting, gastric suction, bicarbonate overdose, loss of potassium along with chlorine ions in severe exercise. Diuretic therapy, blood transfusion. Renal failure (persistent)

--slow respiration (hypoventilation), numbness, convulsions, weakness and muscle cramps.

THANK YOU!!!

Long Examination on Thursday (July 15, 2010)

from the Beginning –up to Last Topic

--No Meeting on Saturday

(July 10, 2010)