Water

Products

Water is commercially available in different qualities. Water for pharmaceutical purposes is available in pharmacies, for example, the purified water (see there). It is produced in pharmacies or ordered from specialized suppliers.

Structure

Pure water (H2O, Mr = 18.015 g/mol) exists as a clear, colorless liquid without odor or taste. It is an inorganic compound which is composed of two atoms of hydrogen and one atom of oxygen. The hydrogen atoms are covalently bonded to the oxygen atom. The internal angle between the hydrogen atoms is 104.45°.

Polarity, solubility and hydrogen bonds

Water is characterized by its polarity, which allows the dissolution of numerous substances, for example, salts and polar compounds. Oxygen (O) has a partial negative (δ-) charge and hydrogen (H) has a partial positive (δ+) charge. The water solubility of most substances increases with increasing temperature. This is why water is heated, for example, for making teas and coffee and as a cleaning agent. However, there are exceptions, for example calcium sulfate (gypsum). Water can form a maximum of four hydrogen bonds. Because it is both a donor and an acceptor for the H-bonds, it has a comparatively high melting point of 0°C and a boiling point of 100°C (standard pressure). At lower pressures, for example in the Alps, the boiling point drops. It occurs on Earth in all three states of aggregation, namely as ice (solid), water (liquid) and as water vapor (gas). In the liquid and solid states, the water molecules are always in contact with each other through the hydrogen bonds.

Density

The density of water at a temperature of 3.98 °C (i.e., about 4 °C) and a pressure of one atmosphere is 1000 kg/m3 or 1 g/cm3. Thus, because the density of water is 1 g/cm3, mass and volume are equivalent. The volume of 1 liter of water is equal to the mass of 1 kg. Unlike many other substances, the density of the solid (ice) is slightly less than that of the liquid water. Therefore, ice floats on water.

Acid-base reactions

Water is amphoteric, meaning that it can react as both an acid (proton donor) and a base (proton acceptor):

  • H2O (water) + H2O (water) H3O+ (oxonium ion) + OH– (hydroxide)

Redox reactions

Reactive metals such as elemental magnesium react with water to form hydrogen. This is a redox reaction. Therefore, burning magnesium cannot be extinguished with water!

  • Mg: (magnesium elemental) + H2O (water) H2 (hydrogen) + MgO (magnesium oxide)

Also known is the violent reaction of elemental sodium with water:

  • 2 Na- (elemental sodium) + 2 H2O (water) 2 Na+ (sodium ion) + 2 OH– − (hydroxide) + H2 (hydrogen)

Oxyhydrogen reaction

The highly exothermic reaction of hydrogen and oxygen, known as the oxyhydrogen reaction, produces water:

  • 2 H2 (hydrogen) + O2 (oxygen) 2 H2O (water)

Water and life

Water is essential for life on earth. For example, the human body is composed of about 60% water. Water is the solvent for biochemical reactions and the main component of cells. For example, it is necessary for the transport of oxygen, carbon dioxide and nutrients in the blood, for digestion and for the excretion of foreign substances. Furthermore, water is also a substrate for photosynthesis in plants:

  • 6 CO2 (carbon dioxide) + 6 H2O (water) C6H12O6 (glucose) + O2 (oxygen)

Water qualities in pharmacy

The pharmacopoeia distinguishes between different qualities of water:

  • This includes purified water (aqua purificata), which is intended for the preparation of medicinal products that do not need to be sterile or pyrogen-free. It is prepared, for example, with the help of distillation. In the process, the water is freed from dissolved substances such as calcium carbonate (lime).
  • Water for injection (Aqua ad iniectabile) is used for the production of drugs intended for parenteral use, for example, infusion and injection preparations.
  • Furthermore, various other types of water are described in the pharmacopoeias, such as highly purified water, water for the preparation of extracts and water as a reagent.

Drinking water (Aqua fontana, Aqua potabile) is not defined by the pharmacopoeia, but by the food book. It is rarely used for the preparation of medicines. An exception is the preparation of antibiotic suspensions for children. Sea water (Auqa marina) is mainly used in moisturizing and cleansing nasal sprays and nasal rinses. It contains various dissolved salts and has a salty taste.

Indications for use

Pharmaceutical indications include (selection):

  • As a pharmaceutical excipient, for the manufacture and preparation of drugs, medical devices and personal care products.
  • As an extracting agent and solvent.
  • In case of dehydration (lack of fluid), in the form of suitable preparations in case of blood loss.
  • By dehydration, plant parts can be preserved (medicinal drugs).
  • As a cleaning agent.
  • For chemical reactions, for example, for the synthesis of active ingredients.
  • For the swallowing of peroral drugs.

Adverse effects

Water does not have a GHS hazard label, but it is still not harmless. For example, depending on its physical state, it can cause frostbite, burns, drowning, accidents and injuries. Water can be contaminated with microorganisms, parasites and pollutants.