Ch' 13 pH and Titratable Acidity

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Ch. 13 pH and Titratable Acidity
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The confusing concentration units

• Percentage
• (w/w)
• 20 g sucrose 80 g water ? 20 (w/w)
• (w/ v)
• 20 g sucrose in 100 mL solution ? 20 (w/v)
• (v/v)
• 50 mL alcohol in 100 mL solution ? 50 (v/v)
• 50 mL alcohol 50 mL water ? 100 mL

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Dimensionless quantity

• parts per million (ppm)
• parts per billion (ppb)
• Pure numbers without associated units of
  measurement, are the result of unit cancellation
  !
• 100 mg/1 kg 100 mg/1000,000 mg 100 ppm
• In diluted water solution, the density is about 1
  g/mL
• ppm µg solute/ ml of solution µg solute/ g of
  solution
• Not applicable to other non-aqueous solution

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Gold standard with less confusion

• International System of Units (SI)
• http//physics.nist.gov/cuu/Units/index.html

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Molarity and normality

• Molarity moles/L (SI unit)
• Normality equivalents/L
• Useful in titration
• Not a SI unit
• Equivalent of a compound depends on a specific
  reaction
• Learn more on your problem solving part III

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Strong acid and alkaline

• 1 M NaOH ___ N NaOH (Sodium hydroxide)
• 1 M HCl ___ N HCl (hydrochloric acid)
• 1 M H2SO4 ___ N H2SO4 (Sulfuric acid)
• 1 M H3PO4 ___ N H3PO4 (phosphoric acid)

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Weak acids in food
Citric acid 3 COOH 1 M 3 N
Tartaric acid 2 COOH 1 M 2 N
Malic acid 2 COOH 1 M N
Lactic acid 1 COOH 1 M N
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The Seesaw in water
HOH 10-14 Kw (25oC)
Acidic H gt OH Neutral H
OH Alkaline H lt OH
OH-
H
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The power of Hydrogen (pH)

• pH -log of H OR log of 1/ H. (Table 13-3)
• H3O 2.24 10-3 M. What is the pH? (2.65)
• pH 5.50. What is the H3O ? (3.16 10-6 M)
• What is the magnitude of difference between
• pH 4 and pH 6 ?
• pH 2 and pH 7?
• pH 1 and pH 9?

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100,000
100,000,000
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Acid
Neutral
Alkaline
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Ch. 13 pH and Titratable Acidity (TA)

• Food Chemistry 101 What is the difference
  between pH and TA?

High or low
Decrease or increase

• As pH increases the TA ________.
• As pH decreases the TA ________.
• As H increase the pH ________.
• As H decrease the pH ________.
• High acid foods have a ____ pH?
• Low acid foods have a ____ pH?

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pH meter pH measurement for dummies

• A landmark invention in chemistry
• 1936 by Dr. Arnold O. Beckman
• Comes in all shape, size, and price
• Calibrate periodically with commercial pH
  calibration solutions

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Antique pH meters

• Mechanism Potentiometry
• (Figure 13-1 pp. 212-215)
• Reference electrode
• Indicator (Glass) electrode

Beckman's Model G pH meter
Beckman's first pH meter
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Modern pH meters

• Combine reference, indicator, and temperature
  sensing in one electrode
• Cover pH range 0-11 and auto-correction for
  temperature up to 50C
• How does temperature affect pH?

What is the pH of pure water at 25C?
The pH of pure water at 50C is 6.6.
The pH of pure water at 0C is 7.5
Kw increases with temperature Kw increases, H
pH
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Dissociation of weak acid in water
HA? H A

• HA is the undissociated acid
• A- is the conjugate base

Acid dissociation constant
pKa -log Ka pH - log H
pH pKa log (A/HA) Eq. 13, page
216
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Henderson Hasselbach Eq.13

• Describes dissociation properties of weak acids
• What is the difference between a weak acid and a
  strong acid?
• Which are used in the food industry? Why?
• pH pKa log A- / HA
• When pH pKa, than A- and HA are equal, or
  half of the acid dissociate to generate H
• Work on practice problems 1-25 at the end of Ch.
  13

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pKa is an indicator of acid strength
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Acids in foods

• Natural composition
• Apple
• Orange
• Tomato
• (Table 13-6)

• Added
• Benzoic acid
• Acetic acid
• Citric acid
• Ascorbic acid
• Phosphoric acid

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Acids generated in gut

• Fermentable carbohydrates (fibers,
  Oligosaccharides)
• Gut microflora (10 of your body cells)
• Acetic acid, propionic acid, butyric acid
• Many other simple phenolic acids