The Basics of Lab Water Purification
Ultra-Pure Water, containing nothing but hydrogen and oxygen, has a specific resistance of 18.2 megohm-cm at 25 °C. Since conductance or conductivity is the reciprocal of resistance or resistivity, a cubic centimeter of pure water has a specific conductance of about 0.055 micromhos (i.e. microseimens) per cm at 25 °C. The conductance arises from the partial dissociation of pure water into hydrogen (H+) and hydroxyl (OH-) ions. Since conductivity is an increasing function of increasing water temperature, the temperature at which conductivity is measured MUST be taken into account. Most conductivity measuring devices simultaneously measure the water temperature, and compensate the conductivity reading as if it were taken at 25 °C.
But, type I Ultra-Pure Water does not like to stay pure! In its "ultra-pure state", water is defined as the world's strongest solvent! It aggressively attacks its surroundings, and begins to dissolve them! And, it doesn't care what they are made of! It’s just as happy dissolving solids as it is dissolving gasses or other liquids. It likes to dissolve both metallic and nonmetallic solids. It likes to dissolve both organic and inorganic compounds. In the process, it also likes to acquire suspended solids, colloids, microorganisms, pyrogens, endotoxins, viruses and enzymes, such as DNase and RNase!
At the beginning of a lab water purification train is a source of tap water to be purified. But what is tap water? Can it be defined? Not in general. The local tap water quality is the result of many variables having to do with where it came from, where and how long and under what conditions it was stored, and what has the human race done to contaminate it?
Tap water comes with a lot of particles, free of charge. Particles are described as having a diameter of so many microns. A micron is 1/1000 of a meter. For comparison, a human hair is about 70-100 microns in diameter, a pinpoint is about 60 microns, and a red blood cell is about 7 microns. Your local water treatment plant operates on the theory that if you can't see it, you won't mind drinking it! Therefore, they typically filter tap water down to about 25 microns, which means that they remove most particles larger than 25 microns in diameter. Just coincidentally, the smallest particles visible to the naked eye are in the range of 30-40 microns in diameter! The particulate matter in tap water comes from its surroundings, and can include pieces of rock, iron, sand, sediment, pieces that fall off the piping and container system (including leaded solder), organic and inorganic colloids, organic matter from decaying plants, remains from dead animals, live microscopic animals, bacteria (dead or alive), pyrogen and endotoxin (fragments of bacterial cell walls), viruses, and enzymes.
Dissolved inorganic solids, such as silicates, sulfates, chlorides, fluorides, bicarbonates, phosphates, nitrates and ferrous (metallic) compounds, are present in the tap water as cations (positively charged ions) and anions (negatively charged ions). Dissolved inorganic solids (and gases) are what gives water its conductivity. Each one contributes to conductivity at its own rate, that is standardized to a CaCO3 or NaCl equivalent. These contaminants
dissolve into the water from its surroundings, such as aquifers, lakes, streams, and rivers. In some cases, humans introduce them into the water supply through the use of fertilizers, pesticides, and plumbing. The longer water sits in its surroundings, the more chemicals it dissolves. Calcium and magnesium compounds come from the natural limestone caves that water collects in. They are what causes "hardness" in water. They have limited solubility in water, and tend to chemically react with soaps and detergents by coming out of solution and forming "soap scum".
Dissolved ionized gasses include CO2, which purified water will actually absorb right from the air! CO2 is also one of the by-products resulting from ultraviolet oxidation. CO2 contributes to the water's conductivity.
Dissolved organic compounds come from animal and plant decay, and from human pollution. They can include natural proteins, alcohol resulting from plant decay, chloramines resulting from chlorination of tap water, and compounds left over from the use of fertilizers, pesticides, detergents, and gas & oil spills. Dissolved non-ionized gasses, of which oxygen is the most common, do not contribute to conductivity, but can corrode metallic surfaces that the water is contained in.
In order to purify water to levels acceptable to laboratories, several technologies must be combined in a logical sequence to achieve the desired results. Note that, although distillation is a water purification technology, we don't include it herein, because deionization has essentially replaced distillation in the modern world. In addition, distillation has very high energy and maintenance costs. While a few researchers still swear by (and at) their stills, most have switched to a DI system.