difference between distilled water and distilled water containg sodium hydroxide
Answers
Answer: Deionizing systems use a mixture of cation and anion exchange resins (usually in a mixed bed). These resins exchange cations and anions in the source water for H+ and OH-, respectively. The H+ and OH- combine to form H2O, leaving only the residual H+ and OH- produced by autodissociation (autoprotolysis), H2O = H+ + OH-; the equilibrium constant of this reaction = 1 x 10^-14 at 25 °C. So the pH of deionized (DI) water is close to 7 **at delivery** and the electrolytic conductivity is about 0.055 µS/cm (corresponds to resistivity ~18 MΩ cm, hence the - technically incorrect, as the units are incorrect - term "18 megohm" water). However, this is true only if the DI water **has not** been in contact with atmospheric CO2. If it is in equilibrium with atmospheric CO2, the conductivity is on the order of 1 µS/cm (resistivity ~1 MΩ cm) and the pH is ~5,6 , both owing to dissociation of the dissolved CO2 to H+ and HCO3-. DI (or any other CO2-free-) water avidly takes up atmospheric CO2 and rapidly approaches the equilibrated values of pH and conductivity. For this reason, conductivity sensors in DI systems are located immediately following the last resin bed (column) and the water is continuously circulated, so that the conductivity value is obtained before the water has a chance to take up any CO2.
Distillation relies on phase separation to eliminate the dissolved ions (which remain in the pot). However, the steam is in contact with the atmosphere, and distilled water generally has electrolytic conductivity and pH values simliar to those of DI water that has equilibrated with the atmosphere (i.e., pH 5.6 and conductivity 1 µS/cm). Distillation systems can only remove CO2 if elaborate measures are taken to avoid contact with the atmosphere (e.g., flushing with pure N2 and blanketing the product H2O with N2).
DI systems do not remove molecular species (e.g. sugar, most other organics) from the product water. However, many commercial systems include an activated charcoal (or similar) column that removes many organics. Distillation will remove nonvolatile molecular species, but not volatile ones (they distill over with the steam).
Note that DI systems do not remove dissolved O2, as it is a molecular species.
The same dissociation reactions occur in both distilled and DI water. DI resins can "bleed" organics, although many of these will be taken up by the activated charcoal.
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Explanation:
Distillation of water
Distillation is one of the oldest water purification procedures. The procedure is pretty much the same as distillation of a hydrosol or an essential oil. The water is boiled and the resulting water vapour is re-condensed to water at the other end.
This is one of the most expensive and energy consuming water treatment procedures. In this process, all suspended material (if present), mineral and microbiological contaminants are left behind. However, organic and volatile material with a boiling point less than or near to that of water would be carried away with water vapour through the condenser.
For certain laboratory applications, a second distillation is carried out and this water is called bidest or double distilled water. This method is only confined to certain laboratory procedures however.
Deionization of water
Deionization is as a well-known old method for water purification. In this method, special ion-exchange resins remove anions and cations present in water with hydronium and hydroxide ions (H3O+, OH-). The result is a deionized water. Most dissolved water impurities are salts such as Magnesium sulphate, Sodium Chloride etc. Cation exchangers retain cations (Magnesium++, Calcium++, Na+, etc.) and release hydronium ion (H3O+) and anion exchangers retain anions (Sulphate, Chloride, etc.) and release hydroxide ion (OH-).
This method does not purify microorganisms, volatile material, organic impurities or suspended material. After a period of time (depending on the original quality of the water) the ion-exchange resins are saturated and are re-generated for further use.