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water and pH

water and pH. Dr. Wesal.A.elHanbli. The human body is composed of over 70% water, and it is a major component of many bodily fluids including blood, urine, and saliva .

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water and pH

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  1. water and pH Dr. Wesal.A.elHanbli

  2. The human body is composed of over 70% water, and it is a major component of many bodily fluids including blood, urine, and saliva. • Water is distributed in the human body between the intracellular compartment and extracellular compartment (intravascular and interstitial compartments) .

  3. Physical and chemical properties of water: • High melting point • High boiling point…. • dipolarity

  4. The total atomic weight of 18 daltons. • The structure of the electrons surrounding water is tetrahedral, resembling a pyramid.

  5. This means that even though the oxygen in water is bound to each of the hydrogens by a covalent bond (sharing a pair of electrons), the oxygen "pulls" the shared electrons closer to itself. • This unequal sharing of the electrons in the O-H bond in water causes the hydrogens to have a partial positive charge (positive dipole), and the oxygen has a partial negative charge (negative dipole). • Water is called dipolar molecule because it has a positive side and a negative side.

  6. Water molecules are attracted to each other, like the positive and negative ends of a magnet making water very dense (hydrogen bonds).

  7. On average, a liquid water molecule will have 3 out of 4 possible hydrogen bonds. • This is because the molecules in liquid are in constant motion. • In ice, water will form all 4 possible hydrogen bonds because the molecules in a solid are essentially locked in place.

  8. Water is a good solvent • The positive side of water (hydrogen) surrounds negatively charged molecules, and the negatively charged side of water (oxygen) surrounds positively charged molecules .

  9. Water cannot dissolve oil. • This is because oily substances are non-polar. Non-polar substances (which lack dipoles) are also called hydrophobic(water fearing).

  10. Ionization of Water and the pH • H2O H+ + OH-

  11. How much H+ and OH- exist in water? Very, very little! • The ratio of either H+ or OH- to H2O in neutral water is 1:1,000,000,000! • Since this is such a small amount of either H+ or OH-, they rarely meet and neutralize each other

  12. The equilibrium constant, Keqdescribes the ionization equilibrium of water: • Keq = [H+][OH-]

  13. For neutral water, the Keq is 1 x 10-14 M and the concentrations of [H+] and [OH-] are each 1 x 10-7 M . • Let's look at that last number • 0.0000001 M

  14. This is obviously a very small number. • A more manageable way to discuss small numbers such as this is to take the negative logarithm. • For thenegative logarithm of [H+], this is called the pH. • In this case: • -log(0.0000001 M) = 7

  15. The pH of a solution is simply the negative logarithm of [H+]. • The pH of a solution describes the acidity of a solution. • Acidic solutions are those with a pH of less than 7 and basic solutions have a pH greater than 7. • A solution, like H2O, with a pH = 7 is neutral.

  16. In the body, the pH of the blood is 7.4 • This corresponds to [H+] of about 40 nM. • This value can only vary from 37nM to 43nM without serious metabolic consequences. (PH 7.35 - 7.45).

  17. what is the pKa ? • In living systems, much of the chemistry involves interactions between acids and bases. • Acids are H+ donors and bases are H+ acceptors. • HA H+ + A-

  18. How readily an acid gives up its H+ is expressed by the acid dissociation constant, or Ka: • Ka = [H+][A-] / [HA]

  19. Equation can be rearranged to: • [H+] = Ka ([HA] / [A-] ) • and by taking the log10 of both sides and multiplying each side by -1, we get: • -log10[H+] = -log10Ka - log10 ([HA] / [A-] )

  20. pH = pKa + log10 ([A-] / [HA] ) • The pKa is the negative logarithm of the Ka. • This equation is known as the Henderson-Hasselbach equation

  21. What is a buffer? • A buffer is a solution that resist changes in pH • It is a molecule that tends to either bind to or release hydrogen ions in order to maintain a particular pH eg in blood It is 7.4. 

  22. Thus, a buffer can either accept or donate hydrogen ions, depending on the solution they are in.  • Since the buffers will accept hydrogen ions in acids and donate hydrogen ions in bases, there must be some in-between-pH where they hit an equilibrium point and do not prefer to either accept or donate hydrogen ions.  • That intermediate point, equilibrium, is the pH that the buffer tends to maintain.   

  23. There are three important buffer systems in our bodies: • bicarbonate buffer system • phosphate buffer system • protein buffer system

  24. All three work similarly-- if they find themselves in a solution with a lot of free hydrogen ions floating around (an acid), they act as bases and suck up the excess hydrogen ions.  • And if they find themselves in a solution lacking free hydrogen ions (a base), they donate their hydrogen ions to the solution.

  25. Acidosis and alkalosis • In a normal subject the pH is maintained between 7.35 - 7.45. • Any increase in pH will result in alkalosis (after excessive vomiting) • Any decrease in pH will result in acidosis ( in complicated type I diabetes diabetic ketoacidosis)

  26. THANKS

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