Lactic acid is an organic acid with the molecular formula CH3CH(OH)COOH. It appears white in its solid-state and becomes colorless, odorless in its dissolved state. Carl Wilhelm Scheele, a Swedish scientist was the first person who isolated lactic acid from sour milk in 1780.
Lactic acid has different uses in food, pharmaceuticals, textile dyeing, and leather tanning. It plays a significant role in biochemical processes like the release of norepinephrine in the brain, healing of traumatic brain injuries.
In this article, we will discuss the most searched questions related to Lactic acid as below:
- Is lactic acid a strong or weak acid?
- the pH of lactic acid
- How lactic acid is produced?
- Chemical structure of Lactic acid
So, what is the pH of lactic acid? The pH of 1mM lactic acid is 3.51 indicating it to be a weak acid. Unlike Strong acids, It partially dissociates in its aqueous solution or water resulting in the release of few H+ ions and lactate ions. The pKa value for lactic acid is 3.86 and acids with pKa values of more than 2 are weak acids.
Lactic acid is a bi-functional molecule containing a hydroxyl group and carboxyl acid group.
pH is a logarithmic concentration of free hydrogen ions present in gram-equivalents per liter in aqueous or liquid solutions.
It may vary from 0 to 14 where pH 7 is neutral and pHs below and above 7 indicate acidity and basicity respectively. Strong acids have a pH value lower than 3.
What are Acids?
There are different theories of defining an acid and base in chemistry including Arrhenius theory and Bronsted-Lowry theory.
Arrhenius theory for acid and base:
This theory defines an acid as a substance that dissociates in water and donates free hydrogen ions. Whereas, a base is a substance that dissociates in water and donates free hydroxide ions.
For example, according to Arrhenius theory, Lactic acid is an acid because it dissociates into lactate ions and hydrogen ions in water.
CH3CH(OH)COOH (aq) ⇌ H+(aq) + CH3CH(OH)COO-(aq)
Bronsted-Lowry theory for acid and base:
This theory defines an acid as a substance that can act as a proton donor and a base as a proton acceptor to form a respective conjugate base and conjugate acid.
For example, when lactic acid is dissolved in water, it donates its one proton to hydrogen molecule to form hydronium ion (H30+) and itself becomes a conjugate base, lactate ion (CH3CH(OH)COO-).
Merits and Demerits of Acid Theories
|Merit: It explains the neutralization reaction
|Merit: It explains the acidic or basic character of both neutral and ionic species in both aqueous and non-aqueous medium
|Demerit: It fails to explain the basic nature of compounds like HNO2 that don’t contain hydroxide ions.
|Demerit: it fails to explain the acidic character of compounds like aluminum trichloride and boron trifluoride that don’t qualify as acids because of lack of hydrogen
Is Lactic Acid a Strong or Weak Acid?
The acidic strength of an acid depends on its tendency to dissociate as well as the concentration of released hydrogen ions in water.
Let’s discuss the characteristics of the strong and weak acids first.
Characteristics of Strong Acid
• Ionizes completely in an aqueous solution
• Releases good amount of hydrogen ions after dissociation in water
• Strong tendency to donate a proton to other species and itself becomes a weak conjugate base
• Higher degrees of dissociation (α)
• High value for acid dissociation constant (Ka)
• Low value for logarithmic constant (pKa)
• pH value ranges between 0 and 3
• Examples: Hydrochloric acid, Nitric acid, Sulphuric acid
Related Topic: Is HNO3 a Strong Acid
Characteristics of Weak Acid
• Ionizes partially in an aqueous solution
• Releases low amount of hydrogen ions after dissociation in water
• Form a strong conjugate base and quickly takes up the donated proton
• Lower degrees of dissociation (α)
• Low value for acid dissociation constant (Ka)
• High value for logarithmic constant (pKa)
• pH value ranges between 3 and 7
• Examples: acetic acid, lactic acid, phosphoric acid
Related topic: is Acetic acid a strong acid
Now is lactic acid a strong or weak acid?
Let’s understand it with the pKa value of lactic acid.
pKa stands for acid dissociation constant. It determines the strength of an acid. Stronger acid exhibits smaller pKa values and vice versa.
The formula for calculating pKa is
pKa = -log10Ka
where Ka = [A-] [H+] / [AH][A-] is concentration of conjugate base [H+] is concentration of hydrogen ion [AH] is concentration of acid
Lactic acid is an alpha-hydroxy acid with two functional groups – hydroxyl group and carboxylic group. The dissociation of lactic acid is as follows:
CH3CH(OH)COOH (aq) ⇌ H+(aq) + CH3CH(OH)COO-(aq)
Ka for Lactic acid = [CH3CH(OH)COO-] [H+] / [CH3CH(OH)COOH]
Ka for Lactic acid at 20 degree Celcius is calculated to be 1.38 × 10-4
Therefore, pKa value for lactic acid = -log10(1.38 × 10-4)
pKa value for lactic acid is 3.86
Acids with pKa values of more than 2 are weak acids hence, lactic acid is also a weak acid.
pH of Lactic Acid
pH stands for the power of hydrogen. It is a negative algorithm (with base 10) of hydrogen ion concentration in an aqueous solution.
It translates H+ ion concentration in an aqueous solution into numbers ranging from 0 to 14. It has no units.
Strongly acidic solutions have pH values equal to zero while strongly basic solutions have pH values equal to 14. Solutions with pH values equal to 7 are neutral.
Effect of temperature on pH
• Temperature shows no effect on pH of strong acids and strong bases
• Increase in temperature causes a decrease in pH of weak acids due to an increase in ionization
• Increase in temperature causes an increase in pH of weak bases due to an increase in the concentration of hydroxide ions
I have written a specific article on it. Check out the article on Does temperature affect pH
The formula for calculating pH:
pH = -log [H+]
Although the above pH equation remains the same for both strong and weak acids, the acid dissociation constant is required to find H+ concentration for weak acids because, unlike strong acids, weak acids partially dissociate in an aqueous solution.
Let’s understand it by taking an example of 0.100M solution strong acid (HCl) and weak acid (lactic acid).
pH of strong acid, HCl
HCl(aq) → H+(aq) + Cl-(aq)
HCl is a strong acid so it dissociates completely into H+ and Cl- ions. Therefore, [HCl]=[H+]= 0.100M
pH = -log10 (0.100) = 1
pH of weak acid, Lactic acid
CH3CH(OH)COOH(aq) ⇌ H+(aq) + CH3CH(OH)COO-(aq)
Lactic acid is a weak acid so it partially dissociates into H+ and lactate ions. So, [H+] cannot be equal to [CH3CH(OH)COOH].
Here, [H+] can be calculated using the Ka value of lactic acid.
Ka for Lactic acid = [CH3CH(OH)COO-] [H+] / [CH3CH(OH)COOH] = 1.38 × 10-4
|0.100 – x
Ka = (x) (x) / (0.100 – x) = 1.38 × 10-4
Assuming 0.100 – x = 0.100
x2/0.100 = 1.38 × 10-4
x = 3.7 × 10-3 mole/Liter
pH = -log (3.7 × 10-3) = 2.43
pH of 0.100 M lactic acid is 2.43
How is Lactic Acid Produced?
Lactic acid can be prepared industrially by bacterial fermentation of carbohydrates and nutrients.
It is an anaerobic reaction in which Glucose or 6-Carbon sugars get converted into lactate in the presence of microorganisms of the genus Lactobacillus.
Fermentation can be of two types- homolactic fermentation and heterolactic fermentation.
• Homolactic fermentation: In this fermentation, one molecule of glucose gets converted to two molecules of lactic acid
• Heterolactic fermentation: In this fermentation, one molecule of glucose gets converted to one molecule of lactic acid along with Carbon dioxide and ethanol
Lactic acid fermentation also occurs in some bacteria and animal cells (muscle cells). In absence of oxygen, pyruvate, a product of the breakdown of glucose or 6-carbon sugar, undergo lactic acid fermentation.
Chemical Structure of Lactic Acid
Lactic acid has two functional groups – α-hydroxyl and the carboxylic group. It is a chiral molecule because a chiral atom (C-2) is attached to four unique groups.
The central carbon atom (chiral atom) is directly attached to a hydroxyl group, hydrogen atom, and two terminal carbon atoms which are part of the carboxylic group and methyl group respectively.
As a result, lactic acid forms two optically active isomers – Levo (L-lactic acid) or (S)-lactic acid and Dextro (D-lactic acid) or (R)-lactic acid.
These two isomers are imposable mirror representations of one another.
Does Milk have Lactic Acid?
Yes, the acidity of milk is due to the presence of lactic acid. Lactobacillus, lactic bacteria ferment milk sugar (lactose) into lactic acid or lactate which lowers the pH of milk and subsequently makes the environment unsuitable for other bacteria.
You must also read out article pH of Milk – Acidic or Alkaline
However, when the concentration of lactic acid in milk gets higher or the pH of milk gets lower, lactic acid denatures milk protein, casein leading to coagulation of protein and formation of sour milk.
Related Topic: Is Milk Homogeneous or Heterogeneous Mixture
Frequently Asked Questions (FAQs)
What does Lactic acid do to your body?
When muscle cells do not get a sufficient amount of oxygen during intense physical activities, they undergo anaerobic fermentation in which pyruvate gets converted into lactic acid causing nausea, weakness, and cramps.
Is the pH of lactic acid neutral?
No, the pH of lactic acid is not neutral. It releases hydrogen ions when dissolved in water which leads to an acidic pH value of 2.43.
Does lactic acid form any geometrical isomer?
Lactic acid has no C=C therefore it does not form any geometrical isomers. It forms only two optical isomers due to the presence of one chiral carbon atom.
Lactic acid is an alpha-hydroxy organic acid with the chemical formula C3H603. It is present in a wide range of foods like dairy products, traditional pickles, soy products, and more.
Lactic acid has one chiral carbon atom which leads to two spatial isomers – Levo (L-lactic acid) and Dextro (D-lactic acid).
Lactic acid is a weak acid with an acid dissociation constant (pKa) value of 3.86. Unlike strong acids like HCl, it partially dissociates into its constituent ions in its aqueous solution.
The production process of lactic acid includes the fermentation of carbohydrates and nutrients by the action of lactobacillus. Sometimes, the fermentation leads to the formation of carbon dioxide and ethanol along with lactic acid