Long ago there was little concern to thoroughly purify water because pure water had extremely few applications. In the modern times, however, there is an increasing demand of water in its purest possible form. Water is never absolutely pure. It always contains impurities such as minerals and bacteria in varying amounts. Water being a general solvent allows dissolution of many substances which contribute to its impurity.
The presence of water impurities has become a problem since the dawn of man on this planet. Diseases like cholera have been blamed to contaminated drinking water and until now it remains a problem. But centuries ago when water purification was not as sophisticated as today's, these problems were fatal. Water is a favorable environment for microorganisms like bacteria. Aside from microscopic organisms, other impurities may be present. Water always comes with dissolved minerals like calcium and magnesium. These minerals may be beneficial at times. At other times, they are just not needed. Water that contains high amounts of these minerals is called hard water. Hard water forms scum with soap. This scum adheres to sinks and tiles forming yellowish deposits. The same thing happens to clothes, which sustain yellowish discoloration upon washing in hard water.
INVERSE OSMOSIS
Purification of water is essential in many industries. Manufacturers prefer water at its best purified form. There are lots ways to remove water impurities and one is through de-ionization.
As mentioned earlier, calcium, magnesium, sodium and chloride are some of the ions present in water. The removal of these ions requires a process called ion exchange. The positive and negative ions (cations and anions respectively) present in the water are exchanged with hydrogen (H+) and hydroxyl ions (OH-). Notice that H+ and OH- are actually ions of water itself - H2O. This mechanism involves the use of resin, which has an affinity for ions. The resin beads used for this purpose contain binding sites for ion exchange to occur. After a time, the resin beads become depleted of their ion exchange capacity and they lose ion affinity. When this happens, a concentrated acid and an alkali are introduced into the resin bed. This removes accumulated ions through a process called displacement and at the same time restores H+ and OH- ions in place.
The effectiveness of this process is determined by the resulting water quality itself. Such outcome depends on a few factors. The kind of resins employed affects the process. Some resins are used for softening. Others are for demineralization. The capacity of resin to undergo regeneration is also another factor.
There are a few ways to test the quality of deionized water. This type of water contains much lower amounts of ions as the name suggests. Thus a good way of testing is through the resistivity and conductivity of water that results upon analysis. Water that is pure is never a conductor of electricity. It is the presence of ion impurities that water becomes a conductor. Thus water with extremely low amounts of ions, almost negligible, has an extremely low electrical conductivity. To make this more apparent, the amount of dissolved ions in water determines its conductivity and hence resistivity. Note that resistivity is the inverse, the opposite property of conductivity. The two properties are generally used to determine ion concentration and assess water quality.
A major problem in this process has something to do with temperature. Physics has it that temperature affects water conductivity. Thus the temperature for measuring water conductivity is standardized internationally at 25 degrees Celsius. This allows validity and reliability of data anywhere in the world. Another concern springs from the precision of temperature measurement. Ordinary water has a conductivity that changes by 2% in every degree Celsius rise in temperature. This change in conductivity, however, changes dramatically as the amount of ions decrease. Hence, deionized water samples can have conductivities that change by as much as 10% per °C change. This means that precise temperature is vital.
The Different Purification Processes Involved in Deionized Water INVERSE OSMOSIS
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