How to Purify Pure Water Further for Lab and Industrial Use

What defines the chemical composition of distilled water?

Distilled water consists of H2O molecules that have been vaporized and condensed to leave behind non-volatile impurities. It typically contains no dissolved minerals, resulting in a conductivity often ranging from 0.5 to 5.0 microsiemens per centimeter, depending on the number of distillation cycles performed.

How does the process of reverse osmosis differ from deionization?

Reverse osmosis uses physical pressure to force water through a semipermeable membrane, while deionization relies on chemical exchange to remove charged ions.

• 95-99% removal: Reverse osmosis membranes effectively reject the vast majority of dissolved salts and organic molecules larger than 200 Daltons.
• Ion exchange resins: Deionization targets specific charged particles, swapping hydrogen and hydroxyl ions for dissolved minerals to achieve high resistivity.
• Energy requirements: Reverse osmosis systems demand high-pressure pumps to overcome osmotic pressure, whereas deionization systems operate primarily based on the surface area and chemical capacity of the resin beads.
• Selectivity differences: Reverse osmosis is superior for removing non-ionic contaminants, while deionization excels at reaching the theoretical resistivity limit of 18.2 megohm-cm.

What are the primary industrial applications for ultra-pure water?

Ultra-pure water is essential for semiconductor manufacturing, where it cleans silicon wafers, and for high-precision analytical chemistry, such as mass spectrometry. Many people assume that ultra-pure water is the safest choice for human consumption, but it is actually aggressive and can leach minerals from pipes, containers, and even the human body if ingested in large quantities. It lacks the essential electrolytes found in tap or mineral water, making it unsuitable for hydration. Its high affinity for carbon dioxide means it becomes acidic almost immediately upon exposure to air, which can cause significant corrosion in standard plumbing systems.

How can laboratories measure the conductivity and resistivity of aqueous solutions?

Laboratories measure these properties using a conductivity cell consisting of two platinum electrodes submerged in the water sample. The meter applies an alternating current between the electrodes to prevent polarization, which would otherwise skew the readings by creating a layer of ions at the electrode surface. Resistivity is simply the reciprocal of conductivity, providing a more sensitive scale for high-purity water where ion concentrations are extremely low.

For example, a reading of 18.2 megohm-cm indicates an exceptionally low ion content, characteristic of high-quality laboratory water. One nuance is that temperature significantly affects these measurements, as ion mobility increases with heat. Most modern meters include automatic temperature compensation to normalize readings to 25 degrees Celsius, though this correction becomes less accurate as the water purity decreases and the ionic strength of the solution rises.

What contaminants remain in water after standard filtration methods?

Standard filtration methods often fail to capture dissolved gases, volatile organic compounds, and microscopic viruses that pass through typical pore sizes.

• 0.22 microns: Standard membrane filters are excellent for bacteria but allow smaller viruses and dissolved gases like oxygen or nitrogen to remain in the liquid.
• Dissolved gases: Components like carbon dioxide or ammonia are not physically trapped by standard filters and remain dissolved, altering the pH of the processed water.
• Organic molecules: Small, uncharged organic compounds often bypass mechanical filters, requiring carbon adsorption or advanced oxidation to be effectively removed from the stream.
• Ionic species: Dissolved salts, such as sodium or chloride, pass through standard mechanical filters without resistance, necessitating ion-exchange or membrane-based separation techniques for removal.

Why is it necessary to prevent re-contamination of treated water during storage?

Preventing re-contamination is necessary because high-purity water acts as a universal solvent, actively seeking to regain the ionic balance it lost during purification. Once water reaches a state of high resistivity, it becomes chemically hungry. It will aggressively pull ions from any surface it touches, including storage tank walls, tubing, or even the atmosphere. If the storage environment is not perfectly inert, the water quality will degrade rapidly, rendering the purification efforts useless for sensitive analytical procedures.

A common example of this is the rapid drop in resistivity caused by the absorption of atmospheric carbon dioxide, which forms carbonic acid and introduces ions into the water. An exception exists in closed-loop systems where the water is continuously recirculated through a polishing loop. In these setups, the constant flow and repeated exposure to ion-exchange resins compensate for minor leaching, allowing the water to maintain its high-purity status despite the presence of storage vessels.

How do ultraviolet light systems contribute to the sterilization of water?

Ultraviolet light systems sterilize water by emitting radiation at a wavelength of 254 nanometers, which disrupts the DNA of microorganisms. This prevents bacteria and viruses from replicating, effectively neutralizing them without adding chemicals. Most systems achieve a log-reduction of biological contaminants when the water receives an adequate dose of energy.

What are the health implications of consuming demineralized water over long periods?

Consuming demineralized water over long periods is generally safe for healthy adults, though it provides no dietary minerals and may cause mild electrolyte imbalances if the diet is also deficient.

Use these values to assess the total mineral intake from your daily water consumption compared to recommended daily allowances.