Acid / base chemistry Flashcards
What is an acidulant, and what are some common examples?
Acidulants lower the pH of foods and impart a tart taste
-citric acid (most common)
-phosphoric acid (used in cola drinks)
what is the equation to calculate pH from [H+]?
pH = -log [H+]
How do strong and weak acids differ in dissociation?
Strong Acids: Fully dissociate in water (e.g., HCl)
Weak Acids: Partially dissociate, forming an equilibrium (e.g., acetic acid)
how is pK calculated? what does the magnitude of numbers indicate?
pK = -log Ka
large Ka = small pk = strong acid
What does the Henderson-Hasselbalch equation describe?
pH= pKa + log [HA] / [A-]
It relates the pH of a solution to the pKa of an acid and the ratio of the conjugate base to the acid
what does the final [hydrogen] of a solution depend on?
the acids tendency to ionize
How do titration curves differ for strong acids, weak acids, and polyprotic acids? what equation can be used and when?
Strong Acids: Show a sharp pH rise near equivalence
Weak Acids: Show buffering regions near pKa
Polyprotic Acids: Exhibit multiple equivalence points due to successive dissociations
The henderson Hasselbach equation can then be used for weak acid-base systems to calculate pH at any point along the curve
what species reflect what pH for phoshphoric acid?
H3PO4 -> H2PO4- : pH = 0-4.7
H2PO4- -> HPO4 2- : pH = 4.7 - 9.7
HPO4 2- -> PO4 3- : pH = 9.7 -14
What makes a buffer solution effective?
A buffer is effective when it consists of a weak acid and its conjugate base (salt), resisting pH changes within a specific range
-acetic acid and sodium acetate
Name examples of food buffers and their pH ranges
Citric acid-sodium citrate: pH 2.1–4.7
Acetic acid-sodium acetate: pH 3.6–5.6
Why is phosphoric acid used in food and beverages?
It acts as an acidulant, pH adjuster, and chelating agent
-In cola drinks, it provides a harsh, biting taste
Which beverages have the lowest pH values, and what is the impact? what is the most common acid added to these drinks?
Lemon juice (pH 2.25) and cola drinks (pH ~2.37) are highly acidic, contributing to tooth enamel erosion
-citric acid, phosphoric then malic acid
What are some common food acidulants and their applications?
Citric acid: #1 acidulant, flavor, pH control, chelation
Phosphoric acid: #2 acidulant, soft drinks, pH control, chelation
Lactic acid: Acidulant, preservative
Sorbic acid: Antifungal additive
How is citric acid produced?
by the fermentation of molasses by “Aspergillus niger” fungus
What determines the acidity of carboxylic acids?
The acidity of carboxylic acids is influenced by the stability of the conjugate base (RCO₂⁻). Greater stabilization increases acidity
How does resonance affect the acidity of carboxylic acids?
The conjugate base (RCO₂⁻) is stabilized by resonance, as the negative charge is delocalized between the two oxygen atoms, making the acid stronger
which acid would you expect to be the strongest? why?
lactic acid
-it has more OH groups which:
1) pulls e- density away from COOH making the H+ on the COOH more positive
2) acidity of the H+ is increase making it easier to dissociate
3) delocalization and reduced e- density stabilizes the conjugate base after proton dissociation
How does the inductive effect of -OH groups increase acid strength?
The electronegative oxygen in -OH groups pulls electron density away from the acidic proton, increasing its partial positive charge and making it easier to dissociate
What are the acid-base properties of ascorbic acid? explain why?
Ascorbic acid has two dissociable protons, making it a weak diprotic acid
The first dissociation has a pKa of 4.1, and the second dissociation occurs at a much higher pKa
-the more acidic proton is more easily lost due to the ability to delocalize and stabolize the negative charge
What is ascorbic acid, and why is it important?
Ascorbic acid (Vitamin C) is a water-soluble vitamin (lactone) that acts as a reducing agent and antioxidant in food and biological systems
What are the pKa values of ascorbic acid, and what do they represent?
Ascorbic acid has two pKa values:
pKa₁ = 4.1: First proton dissociation
pKa₂ = 11.8: Second proton dissociation
What does the Brix measurement represent in food chemistry?
Brix measures the percentage of soluble solids (mainly sugars) in a solution, indicating the sweetness of fruit juices and syrups
what does the magnitude of values indicate for the brix number?
higher values indicate a [higher] of sugar
-flavourful, ripe fruits
lower values indicate [lower] of sugars
-less ripe fruits / diluted solutions
How do amino acids behave at low pH? why?
At low pH, amino acids are positively charged, with the amino group protonated to form NH3+
-the pka is ~9-11 so a pH below that will mean the group is protonated
