Ultra-pure water

The salts contained in the water dissolve into their components and form positively charged cations (including calcium, magnesium, sodium, manganese and iron) and negatively charged anions (chloride, nitrate or sulphate). Ultrapure water, on the other hand, must not contain any impurities or dissolved substances.

Strict requirements for water quality and properties: Pollutant limits, germ concentration and conductivity

Raw water - i.e. untreated water - can come from various sources: Rivers and lakes, as groundwater from wells or rainwater.

The water is treated in various steps (filtration, softening, distillation, desalination or disinfection) until it reaches the desired degree of purity, which is determined by water analyses. Water analysis distinguishes between physical and chemical properties and ingredients to determine the degree of purity. Important physical parameters are the pH value, conductivity or UV adsorption. On the chemical side, the concentration of total organic carbon in the water (TOC value) is an important criterion. The level of this value in turn has an influence on the conductivity of the water. After evaluating all chemical and physical parameters, the purified water product is assigned to a group.

Classification according to degree of purity

The specific conductivity of different water qualities is determined at a water temperature of 25 °C. It is measured in micro-Siemens per centimeter (µS/cm). The lower the conductivity, the higher the purity of the water tested. In addition to strict pollutant limits, the specific conductivity of drinking water according to the German Drinking Water Ordinance (TVO) may be 50 - 5000 µS/cm, for example.

The tap or spring water with its dissolved salts selected as the starting product can be processed into osmosis water using reverse osmosis. In this filtration process, a pressure that is higher than the osmotic pressure is exerted on the more concentrated salt solution and the water is pressed through a semi-permeable membrane. The water molecules migrate through the membrane into a lower concentrated solution. The membrane is only permeable to water molecules, so that impurities and dissolved solids as well as many bacteria and viruses are retained on the side of the higher pressure. As osmosis water is free of harmful substances, it is used as drinking water and in food and beverage production, for example.

Distilled water is produced by distillation. The water evaporates and then condenses as pure water without impurities such as minerals, bacteria, viruses and dissolved gases. Aquadest is suitable for laboratories, medical applications, in battery production and in many appliances where limescale is to be avoided. Distilled water should not be drunk.

Pure water is then obtained through treatment with ion exchangers, reverse osmosis or distillation. This category includes purified water produced in accordance with the German Pharmacopoeia (Deutsches Arzneibuch DAB) with the designation “aqua purificata”, which is used to manufacture many pharmaceutical products. It still contains residues of certain ions, so that its specific conductivity is 1 - 50 µS/cm. Another pure water that is considered even cleaner according to the classification of the DAB is water for injection purposes (aqua ad injectabilia). It is considered sterile and is used to dissolve and dilute all medicines that are administered as injections or infusions.

Fully demineralized water (DM water, or deionized water) is also produced using the reverse osmosis process or via ion exchangers. The ion exchange process demineralizes the water. Cations are exchanged for hydrogen ions (H+) and anions for hydroxide ions (OH-). The result is demineralized water with a specific conductivity of 0.1 - 1 µS/cm. However, organic components as well as viruses and bacteria are not removed by the ion exchange process. This would require filtration. Deionized water is not suitable as drinking water. It is used as a rinsing agent and solvent in chemical and biochemical laboratories.

Ultrapure water or highly pure water is the highest purity level of water. It is produced from DM water through various additional treatment steps, including the use of additional ion exchangers, complete desalination, ultrafiltration with activated carbon as an adsorbent or photo-oxidation to eliminate germs. As a result, only the smallest traces of organic compounds, microorganisms or electrolytes remain and the specific conductivity of ultrapure water is less than 0.1 µS/cm. It is suitable as a solvent for laboratory work, e.g. in clinical diagnostics, in cell cultures, in trace analysis and in sensitive applications in the pharmaceutical industry, biotechnology and the semiconductor industry.

In medicine in particular, ultrapure water must meet strict requirements with regard to nitrate content (< 0.2 mg/l) and organically bound carbon (TOC < 0.5 mg/l) as well as bacterial endotoxins (< 25 ng/l). It is used for rinsing and cleaning medical devices and surgical instruments, which prevents the spread of germs and silicate deposits. Ultrapure water is also an important component in modern hydrogen technology and is used in electrolysis systems such as PEM electrolysis.

Hyper-purified water (aqua valde purificata), which was still listed in the 9th edition of the European Pharmacopoeia (Ph. Eur.), is no longer mentioned.

High-quality materials for ultrapure water applications

By removing all salts, DM water and ultrapure water have a highly aggressive effect on most materials. These water qualities attack conventional brass or steel pipes and dissolve the ions they contain from the metals.

For this reason, valves for this area of application must be made of high-quality stainless steel, but at least AISI 316L (1.4404). Depending on the industry, further requirements may apply: ADI-Free, FDA, UPS Class VI, oil and grease-free, silicone/LABS-free.