Medical Biochemistry Department pH and Buffers By All Staff Members of Medical Biochemistry Department

Identify different pH solutions By the end of this topic, the student will be able to: Identify different pH solutions

1-PH Term of pH and its importance pH scale principles of water dissociation and Kw applications in calculating pH 2-Acid and Bases, general properties Definition General properties Classification (strong , weak)------ with part B 3 MEASURING THE pH Indicators PH Meter

Term of pH and its importance pH is a measure of the acidity or alkalinity of an aqueous solution. pH Level - Similar to the body temperature, which has to be maintained; the pH level in the various body fluids are kept at a narrow range for the metabolic reactions to proceed properly.

pH scale ranges from 0 – 14 pH < 7 acid pH = 7 neutral 2 3 4 5 6 7 8 9 10 11 12 acidic (H+) > (OH-) Neutral 25oC (H+) = (OH-) distilled water basic or alkaline (H+) < (OH-) ranges from 0 – 14 pH < 7 acid pH = 7 neutral pH > 7 base

Water has limited capacity to dissociate (ionize) into H+ & OH- Dissociation Of Water H2O H+ + OH- Water has limited capacity to dissociate (ionize) into H+ & OH-

Ka Is The Dissociation Constant Of Water [H2O] [H+] [OH-] = Ka Since water is too little dissociated So, Ka X [H2O] = Kw [H+] [OH-] = Kw 10-7 X 10-7 = Kw = 10-14 mol/L Kw = The Ion Product Of Water

The probability that hydrogen exist in water as ion = 1.8 x 10-9 The number of moles in one litre (1000 g) of H2O is = Weight in g/ Molecular weight So, (1000 g) / (18 g mol–1) = 55.5 mol. The molar concentration of [ H+] = The probability that hydrogen exist in water as ion X molar conc. of water = 1.8 x 10-9 x 55.5 = 99.9x 10-9 ~ 102x 10-9= 10-7 mol/L The probability that hydrogen exist in water as ion = 1.8 x 10-9

Sorensen (1909) suggested that the Since those values involving such –ve powers of 10 which is difficult to use in calculations So Sorensen (1909) suggested that the [H+]concentration of DILUTE aqueous solutions would be better expressed as PH value

What is pH? The pH of a solution is the negative of the logarithm to the base 10 of its hydrogen ion concentration in (mol/L) pH = -log[H+]

pH= -log of H+ Concentration P= -Log H= H+ Concentration pH= -log of H+ Concentration

[OH-] may be represented as pOH value P= -Log OH= [OH- Concentration] pOH= -log of [OH- Concentration]

Since [H+] x [OH-] =10 -14 –log [H+] + -log [OH-] =14 By –log of both sides –log [H+] + -log [OH-] =14 i.e. pH + pOH =14

Q- What is the pH of 0.0001 N HCL sol.? A- [H+] = 0.0001 = 10-4 mol/L pH = - log 10-4 = 4 Q - What is the pH of 0.001 N NaOH sol.? A- [OH-] = 0.001 = 10-3 mol/L pOH = - log 10-3 = 3 Since pH+ pOH=14 So, pH = 14 - pOH= 14 -3= 11

Acids and bases 1-Definition 2-Properties 3-Classification 4-Neuteralization

Acids and bases Acids Bases Taste sour Taste bitter Turn blue litmus paper red form hydrogen ions (H+) in water Bases Taste bitter Turn red litmus paper blue. form hydroxide ions in water

Bases cause OH- to increase Acids cause H+ to increase

Amphoteric Substance They can act as Proton DONORS with bases & Proton ACCEPTORS with acids Eg. Amino acid –H2O

In dilute aqueous solution, as [H+] increases: A. pH decreases B. pOH increases C. [OH–] decreases D. all of the above

Determining pH 1-Indicators 2-pH Meter

Indicators Compounds that change color when in contact with an acid or base They might be in the form of Solutions or litmus paper

How Simple Indicators Work?

Mechanism of action of the litmus paper indicator indicator act as acid and loss protons ionized form Blue color In basic solutions indicator act as bases and accept protons mostly non-ionized form Red color In acidic solutions ionized form+non-ionized form no change in color in litmus paper or Intermediate color in other indicators In intermediate pH

Different Indicators

pH meter Much more precise method

Neutralization HCl + NaOH → H2O + NaCl reaction between an acid & base acid base water salt reaction between an acid & base produces water & a salt an ionic compound

Lab activities 3.understand the concept of neutralization reaction (home activity ) 2.Demonstrate how indicators can be used to test for the presence of acids or bases and the effect of pH on them 1.Identify the pH of different solutions using litmus paper indicator

home activity Observe? and Explain Add a spoon of vinegar Concentrated Baking Soda solution Observe? and Explain In half a glass of water, put a few teaspoons of baking soda and mix in order to obtain a quite concentrated solution. In the same glass, pour a spoon of vinegar.

Extended Modular Program