Di Water (deionized water): The Pure Way of the Aquatic Lifestyle

By Hannah HenegarPurifying-Process-Flow

What is Deionized Water?

Deionization (“DI Water” or “Demineralization”) simply means the removal of ions. Ions are electrically charged atoms or molecules found in water that have either a net negative or positive charge. For many applications that use water as a rinse or ingredient, these ions are considered impurities and must be removed from the water.  Ion exchange resins are used to exchange non desirable cations and anions with hydrogen and hydroxyl, respectively, forming pure water (H20), which is not an ion.

How does ion exchange work?       

Ion exchange resins are used to produce deionized water. These resins are small plastic beads that are composed of organic polymer chains that have charged functional groups built into the resin bead. Each functional group has either a fixed positive or negative charge. Different ions are attracted to a resin bead with different strengths. For example, calcium is more strongly attracted to a cation resin bead than sodium is. The hydrogen on the cation resin bead and the hydroxyl on the anion resin bead do not have a strong attraction to the bead. This is what allows ion exchange to take place. As positively charged cations flow across cation resin beads, the cations are exchanged for hydrogen (H+). Likewise, as negatively charged anions flow across anion resin beads, the anions are exchanged for hydroxyl (OH-). When you combine hydrogen (H+) and hydroxyl (OH-) you form pure H20. Eventually all of the exchange sites on the cation and anion resin beads are used up and the tank no longer produces deionized water. At this point, the resin beads require regeneration to prepare them for use again. Demineralization therefore requires using at least two types of ion exchange resins to produce deionized water. One resin will remove positively charged ions and the other will remove negatively charged ions. In a dual bed system, the cation resin is always first in line. As city water enters the tank filled with cation resin, all positively charged cations are attracted to the cation resin bead and exchanged for hydrogen. The negatively charged anions are not attracted to the cation resin bead and pass through. For example, let’s examine calcium chloride in the feed water. In solution, the calcium ion is positively charged and will attach itself to the cation bead and will release a hydrogen ion. The chloride has a negative charge and therefore will not attach itself to the cation resin bead. The hydrogen, which has a positive charge, will attach itself to the chloride ion, forming hydrochloric acid (HCl). The resulting effluent from a SAC exchanger will have a very low pH and a much higher conductivity than the incoming feed water. The effluent from the cation resin will consist of strong and weak acids. This acidic water will then enter a tank filled with anion resin. The anion resin will attract negatively charged anions such as chloride and exchange them for hydroxyl. The result is hydrogen (H+) and hydroxyl (OH-), which forms H20. In reality, a dual bed system does not produce true H20 due to “sodium leakage”. If sodium leaks past the cation exchange tank, then it combines with hydroxyl to form sodium hydroxide which has a high conductivity. Sodium leakage occurs because sodium and hydrogen have a very similar attraction to the cation resin bead and sometimes the sodium ion does not exchange itself for a hydrogen ion. In a mixed bed system, the strong acid cation and strong base anion resin are intermixed. This effectively makes the mixed bed tank act like thousands of dual bed units in one tank. The cation/anion exchange is taking place over and over within the resin bed. Sodium leakage is addressed because of the sheer number of repeated cation/anion exchanges taking place. By using a mixed bed you can produce the highest quality of deionized water possible.

You can compare the deionized water vs distilled water, but to say that the distilled water is the same as deionized water is a mistake. The terms “distilled water” and “deionized water” are often misunderstood.The oldest method for production of pure water is the thermal method or distillation – water evaporation from the surface and condensation. The basis of the process is the transfer of water in the vapour phase with its subsequent condensation. The main drawback of this method is the very high maintenance costs of the electricity needed to convert the water into the steam. In addition, in the process of steam formation along with water molecules other solutes can enter the steam according to their volatility. Evaporation is achieved in various ways: the vacuum above the water, heating, etc.

Let’s consider the distillation. What’s happening in the process of distillation? The water molecules have the boiling point of 100°C or 212° F. Other substances have different boiling points. The substance that boils at a lower temperature evaporates first. The boiling point of various impurities is higher, and, theoretically, they will begin to evaporate, when the water has already boiled out. The substance that boils at a lower temperature evaporates first. Due to this difference the water is separated.

As a result, theoretically after the distillation the absolutely pure water is obtained. Actually, organic substances, which have similar boiling point than that of water can slip in the distilled water. For example, if the water contains the oil drops they can be found also in the distillate. There are practically no salts in the distilled water, because the salt boils at a much higher temperature. To eliminate the problem of organic substances, the water distillers have pre- and post water filters.The absolute advantage of the distilled water is the complete absence of harmful substances.

 

Deionized water is deeply demineralized, ultrapure water with the resistivity close to 18 megohm-cm. It is used in microelectronics, printed circuit boards, instrument manufacture, pharmacy, washing liquids, etc.

In order to obtain the high quality pure deionized water, a multi-stage water purification process can be used. After pre-cleaning, the water is supplied to the reverse osmosis membrane, and then the water is filtered through a special deionization medium, which removes the rest of the ions in the water. The purity of deionized water can exceed the purity of distilled water. The pH of a solution is a measure of its ratio of hydrogen atoms to hydroxide radicals, which are molecules composed of one oxygen and one hydrogen atom. If the ratio is one-to-one, the solution is neutral, and its pH is 7. A low-pH solution is acidic and a high-pH solution is basic. Ideally, distilled water is neutral, with a pH of 7.

 

Sources:

http://www.waterandmorehub.com/deionized-water-vs-distilled-water/

http://puretecwater.com/what-is-deionized-water.html

http://www.ehow.com/about_4623914_ph-distilled-water.html