Products
Acids are found in numerous drugs as active ingredients or as excipients. As pure substances, they are available in pharmacies and drugstores. In the household, they are found, for example, in lemon juice, fruit juice, vinegar and cleaning agents.
Definition
Acids (HA), with the exception of Lewis acids, are chemical compounds that contain a proton (H
+
) to a base. They are therefore also called proton donors. With water, they form the hydronium ion H
3
O
+
:
- HA + H
2
O ⇄ H
3
O
+
+ A
–
Reaction with a base (B):
- HA + B ⇄ HB
+
+ A
–
This reaction is reversible due to a deprotonation of the base. Because it proceeds in both directions, an equilibrium is established. H
+
is formally a hydrogen atom without an electron. Because hydrogen consists of only one proton and one electron, only the single proton remains when the electron is given up. By the way, a free proton does not occur; there is always a base to take it up. In the acid-base reaction, the conjugate base is formed from the acid and the conjugate acid is formed from the base.
Multi-proton acids
Acids differ in the number of protons available. In hydrochloric acid (HCl), there is only one proton, while in phosphoric acid (H
3
PO
4
) there are three. We speak of mono-, di- and triprotonic acids. Three-step deprotonation of phosphoric acid:
- H
3
PO
4
H
2
PO
4
–
+ H
+
HPO
4
2-
+ H
+
PO
4
3-
+ H
+
Acid strength (pKa)
Furthermore, acids differ in strength, that is, in the tendency to release the proton. For example, sulfuric acid, nitric acid, and hydrochloric acid are strong acids, while acetic acid is a weak acid. The strong acids dissociate completely to protons and the conjugate base, while the weak acids dissociate only to a small extent. The pKa value (or pKs) is the measure of the strength of the acid. The lower the value the stronger the acid. The pKa is derived from the acid dissociation constant (acid constant) Ka (Ks). It is the negative decadic logarithm.
- PKa = -log Ka
It must be noted that this is a logarithmic scale. A difference of 1 on this scale corresponds to 10
1
, a difference of 5 therefore 10
5
! Between ethanol (pKa: 15.9) and acetic acid (pKa: 4.75) there is an enormous difference of 10
11
.
Salts
The corresponding salts of acids, for example nitrates, acetates, sulfates, carbonates or phosphates, are also of great importance in pharmacy. Salts are formed when an acid is neutralized with a base:
- HCl (hydrochloric acid) + NaOH (sodium hydroxide) NaCl (sodium chloride) + H
2
O (water)
PH value
Aqueous solutions of acids have a pH below 7. The pH is the negative decadic logarithm of the hydronium ion concentration:
- pH = -log C(H
3
O
+
)
The pH scale ranges from 0 (acidic) to 14 (basic). Again, a difference of 1 means the value 10
1
.
Effects
Acids have corrosive, irritant, antiseptic and antimicrobial properties. They have an acidic taste and can attack many materials, for example base metals and limestone.
Fields of application
Pharmaceutical and medical applications include:
- As a caustic against warts and corns.
- For the production of active ingredient salts.
- For the prevention and treatment of infectious diseases, as a disinfectant.
- Acids are used in sufficient dilution with decreased gastric acid production to improve digestion.
- Raising gastric pH, such as with proton pump inhibitors, can decrease the bioavailability of other medications. Acids, for example in the form of a cola drink, can be taken to temporarily acidify the stomach.
- As pharmaceutical excipients, for example, as preservatives, in effervescent tablets and to adjust the pH. For the preparation of buffer solutions.
- As cleaning agents.
- For chemical syntheses, as reagents.
- As food additives.
Representative (selection)
Inorganic acids, mineral acids:
- Boric acid
- Carbonic acid
- Phosphoric acid
- Nitric acid
- Hydrochloric acid
- Sulfuric acid
- Sulfurous acid
Organic acids:
- Malic acid
- Formic acid
- Ascorbic acid
- Benzoic acid
- Chloroacetic acid
- Acetic acid
- Fatty acids
- Lactic acid
- Nucleic acids
- Oxalic acid
- Picric acid
- Salicylic acid
- Trichloroacetic acid
- Tartaric acid
- Citric acid
Many active pharmaceutical ingredients are acids.
Adverse effects
Acids have corrosive and irritant properties and can cause burns to the skin, mucous membranes, and eyes. Ingestion of strong acids may be life-threatening. The appropriate precautions in the safety data sheet must be observed (e.g., protective gloves, fume hood, safety glasses, laboratory coat, respiratory protection).
