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Lab Activity 2: Active Acidity, pH, and Buffer

Lab Activity 2: Active Acidity, pH, and Buffer. IUG, Fall 2012 Dr. Tarek Zaida. Active Acidity. Refers to H + present in a solution due to dissociation of acid. Q: How can active acidity be expressed? An: by pH of a solution.

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Lab Activity 2: Active Acidity, pH, and Buffer

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  1. Lab Activity 2: Active Acidity, pH, and Buffer IUG, Fall 2012 Dr. TarekZaida

  2. Active Acidity • Refers to H+ present in a solution due to dissociation of acid. Q:How can active acidity be expressed? An: by pH of a solution.

  3. pH presents the negative logarithm of the hydrogen ion conc. [H+] • pH = -log [H+] • If [H+] = a x 10-b moles/l • Then pH = b – log (a) • pH of a solution can be measured in 2 different ways: • pH-meter • pH-indicator paper

  4. pH-indicator paper • It contains organic dye whose color is dependent on pH • pH scale is 1 - 14

  5. The pH-meter • Is an instrument equipped with: • A glass electrode • A reference electrode (calomel: Hg, HgCl2) • How does it work? • The instrument measures the potential difference between the glass and the calomel (mercury & mercury chloride) electrodes. • The potential difference is related to [H+] of the solution being tested.

  6. Standardization of pH-meter • All pH-meters should be standardized with buffers of known pH-values before use: • pH4, pH7, pH10 What is a buffer? Solutions made of a mixture of a week acid & it’s conjugate base.

  7. Function of buffers • They are of a vital importance by their ability to maintain the optimal pH in enzyme-catalysed reactions in vitro or in vivo. • How do buffers function in a solution? • They can release H+ in solution if the pH gets basic or • They can bind H+ if the pH gets acidic.

  8. HA H+ + A- Ka , a dissociation constant: Ka = [H+] [A-] [HA] pH = pKa + log [A-] Henderson-Hasselbalch equation [HA] If any 2 parts of the equation are known, the third can be calculated.

  9. If [A-] equal [HA] • Then pH = pKa + log [A-] [HA] Then pH = pKa

  10. Experiment 1: Measurement of pH • Determine the pH of the following solutions using a pH-meter: • 1. Orange or lemon juice • 2. Vinegar • 3. Distilled water • 4. Distilled water boiled • 5. 0.1 N HCl • 6. 0.1 N NaOH

  11. Experiment 2: Preparation of buffers • Prepare the acetate buffer of pH 5.0, keeping in mind that: • pKa of acetic acid is: 4.7 at pH 5.0 • The conjugate base of acetic acid is: CH3COO- Calculations: for using Henderson-Hasselbalch equation: pH is given pKa is also given

  12. pH = pKa + log [A-]/[HA] [A-] = x [HA] = 0.1 – x 5 = 4.7 + log ( x / 0.1- x) 0.3 = log (x/ 0.1 – x) 1.995 = x/0.1 – x And x = 0.1995 – 1.995x 2,995 x = 0.1995 X = 0.1995/2.995 X = 0.0666 mole/l CH3COONa CH3COOH= 0.1 – 0.0666 = 0.0334 mol/l CH3COOH

  13. MW of CH3COONa = 82 MW of CH3COOH = 60 For 0.0666 mol/l of CH3COONa: 0.0666 mol/l x 82 = 5.41 g/l are required For 0.0334 mol/l of CH3COOH: 0.0334 mol/l x 60 = 2.004 g/l Note: Density of CH3COOH = 1.05 2.004 / 1.05 = 1.9 ml /l are required

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