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What is a weak acid?

A weak acid is one that in aqueous solution has little ability to dissociate into hydrogen (H + ) and its corresponding conjugate base. Unlike strong acids, whose dissociation is complete (100%) and practically irreversible, weak acids hardly dissociate and in a reversible way.

Therefore, a weak acid generates fewer H + ions in water than would be expected. Among the most representative weak acids we have acetic acid, the main component of vinegar, which ionizes approximately 0.5% under normal conditions.

This means that the largest proportion of a solution of acetic acid in water, such as that of 5% kitchen vinegar, is in the undissociated form (CH 3 COOH); while a small portion is found as the conjugate base (CH 3 COO  ), accompanied by H + ions , or more correctly, H 3 O + ions .

Then, since the amount of hydrogen that is released by a weak acid in aqueous solution is low, the pH they produce is higher (basic) than that produced by a strong acid. For example, vinegar has a pH of 2.5, orange juice has a pH of 3.3 – 4.2, and lemon juice has a pH of 2.

These pH’s, although acidic, are higher than the pH’s that produce a solution of a strong acid (HCl, H 2 SO 4 , etc.) that can reach the value of 1 or less (negative pH values).

Characteristics of weak acids

pH of your solutions

Weak acids usually have a pH between 2.0 and less than 7.0, so they turn the color of litmus paper red. These pH values ​​are much higher than the pH values ​​produced by strong acids, although there are exceptions.

For example, a 1 mM solution of hydrochloric acid (a strong acid), has a pH of 3.0, while a 1 mM solution of hydrofluoric acid (a “weak” acid), has a pH of 3.2, which is close to that of strong acid.

Partial ionization

Weak acids in solution ionize little or partially in water, dissociating into the conjugate base of the acid and the hydrogen ion. Thus, a weak acid in aqueous solution is mostly undissociated, with a small proportion dissociated.

As the amount of hydrogen released by a weak acid in water is low, the pH produced by this acid is higher than that produced by a strong acid dissolved in water, since these are usually 100% ionized.

Reversible ionization

The ionization of a weak acid is usually outlined in the following way:

HA ⇌ A      + H 3 O 

Where HA is the weak acid, A  its conjugate base, and H 3 O + the hydronium cation, which is the form in which H + exists or manifests itself in water. The concentration (or activity) of H 3 O + is used instead of H + to determine the pH of the solution.

Degree of ionization

Some acids are weaker than others, which is to say, they are less ionized. Therefore, the degree of ionization allows a glimpse of how acidic an acid solution is under certain conditions.

To express the degree of ionization, several parameters were introduced, such as the ionization or acidity constant (Ka) and the pKa: a way of expressing Ka in a simple number that omits the use of a number expressed in negative powers of ten (10) , as it exists in the Ka.

The constants Ka and pKa are used mainly for weak acids.

Ka = [A  ] [H + ] / [HA]

The brackets in the formula represent concentrations.

pKa = – log Ka

The pKa value of a weak acid usually ranges from 2 to 12. But there are exceptions. For example: iodic acid (HIO 3 ) has a pKa of 0.77, which is considered a relatively small value, but not negative, for a weak acid in a strict sense.

Conjugate base stability

Weak acids have a strong bond with the acidic hydrogen atom (H δ + -X δ- ), thus they have a low degree of hydrogen dissociation. This results in that the conjugate base of the weak acid has a great avidity for hydrogen, being a strong conjugate base.

That is, the conjugate base of a weak acid is very unstable, because it accepts the returned H + very easily. The more unstable the conjugate base is, the weaker the acid in question.

Low ionic conductivity

Being little ionized, weak acids are poor conductors of electricity, unlike strong acids that are good conductors of electricity.

Presence in nature

Weak acids are widely distributed in nature, for example, they are in citrus fruits (lemon, grapefruit, orange, etc).

They are also present in numerous processes that occur in all living things. Some of these weak biological acids are pyruvic, lactic, citric, fumaric, succinic, glutamic, etc., and even oxygenated hemoglobin releases hydrogen ions, so it behaves like a weak acid.

Then, it can be concluded that man lives in permanent contact with weak acids.

How to calculate the pH of a weak acid?

The pH of a weak acid can be calculated by using the acid ionization constant (Ka), or by using the Henderson-Hasselbalch equation. In the first example we will use Ka.

With constant acidity

Calculate the pH of a 0.5 M solution of benzoic acid (C 6 H 5 COOH), which has an ionization constant (Ka) = 1.8 x 10 -5

6 H 5 COOH ⇌ C 6 H 5 COO     + H +  (1)

If the amount of acid that dissociates is called X, then 0.5 MX will be the concentration of the weak undissociated acid at equilibrium, where X is the concentration of the conjugate base of benzoic acid and the concentration of the hydrogen ion.

Equation (1) can be represented as follows:

0.5 MX ⇌ X + X

Weak acids ionize in a very low proportion, so that the concentration of benzoic acid in the equilibrium of (0.5 MX), can be assumed as 0.5 M, since X <<< 0.5. Substituting in the expression for Ka we solve for X:

Ka = [A  ] [H + ] / [HA]

1.8 x 10 -5 = (X) (X) / 0.5 M

2 = (1.8 x 10 -5 ) (0.5 M)

X = 3 x 10 -3 M

Therefore, [H + ] is equal to 3 x 10 -3 M. We then solve by calculating the pH:

pH = – log [H + ]

= – log [3 x 10 -3 ]

pH = 2.52

Note that the pH is relatively acidic, since it is below 3 or 4.

With Henderson-Hasselbalch equation

The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution:

pH = pKa + log [conjugate base] / [acid]

Example:

Sodium hydroxide (NaOH) is added to a solution of acetic acid, producing a final concentration of acetic acid (CH 3 COOH) of 1.11 x 10 -2 mol / L and a concentration of the conjugate base (CH 3 COO  ) of 0.44 x 10 -2 mol / L. If the pKa of acetic acid is 4.75 calculate the pH of the solution.

pH = 4.75 + log [A  ] / [HA]

pH = 4.75 + log (0.44 x 10 -2 / 1.11 x 10 -2 )

= 4.75 + log 0.396

= 4,348

Examples of weak acids

Listed below are several weak acids accompanied by their pKa values:

  • CH 3 COOH (acetic acid), pKa 4.75
  • HCOOH (formic acid), pKa 3.74
  • 6 H 5 COOH (benzoic acid), pKa 5.59
  • CH 2 ClCOOH (chloroacetic acid), pKa 2.85
  • HCN (hydrocyanic acid), pKa 9.31
  • HF (hydrofluoric acid), pKa 3.17
  • 2 H 5 COOH (propionic acid), pKa 4.89
  • HNO 2 (nitrous acid), pKa 3.35
  • 2 S (hydrogen sulfide), pKa 1 6.0 and pKa 2 12.92
  • CCl 3 COOH (trichloroacetic acid), pKa 0.52
  • 2 CO 3 (carbonic acid), pKa 1 6.37 and pKa 2 10.22
  • 3 PO 4 (phosphoric acid), pKa 1 2.12, pKa 2 4.21 and pKa 3 11.0
  • 6 H 5 SO 3 H (benzenesulfonic acid), pKa 0.70
  • HIO 3 (iodic acid), pKa 0.77
  • HClO 2 (chlorous acid), pKa 2.0
  • CH 3 CH 2 (OH) COOH (lactic acid), pKa 3.08

Weak acids with therapeutic uses

  • Salicylic acid, pKa 3.00
  • Acetylsalicylic acid, pKa 3.49
  • Furosemide, pKa 3.90
  • Ibuprofen, pKa 4.40
  • Levodopa, pKa 2.30
  • Phenobarbital, pKa 7.40
  • Warfarin, pKa 5.0
  • Theophylline, pKa 8.80

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