MadSci Network: Chemistry
Query:

Re: Which is more pure distilled or deionized water?

Date: Thu Jul 8 16:11:52 1999
Posted By: Dean Cliver, Faculty, Food Safety Unit, Uiversity of California, Davis
Area of science: Chemistry
ID: 930856549.Ch
Message:

Water purity may be measured in various ways.  One can attempt to determine 
the weight of all of the dissolved material ("solute"); this is most easily 
done for dissolved solids, as opposed to dissolved liquids or gases.  In 
addition to actually weighing the impurities, one can estimate their level 
by the degree to which they increase the boiling point or lower the 
freezing point of water.  The refractive index (a measure of how 
transparent materials bend light waves) is also affected by solutes in 
water.  Alternately, water purity can be quickly estimated on the basis of 
electrical conductivity or resistance  very pure water conducts 
electricity poorly, so its resistance is high.  

Distillation entails converting water from the liquid state to the gaseous 
state and back to liquid again in an apparatus called a "still," comprising 
a boiling vessel to vaporize the water and a cooling unit ("condenser") to 
return the water to the liquid state.  Most dissolved solids are left 
behind in an increasingly concentrated solution, so that the boiling point 
of the liquid water increases.  These are said to be "nonvolatile."  
Substances that vaporize with the water are "volatile."  Very elegant 
stills can selectively condense (liquefy) water from among other volatile 
substances, but most distillation allows carry-over of at least some 
volatile substances, and a very little of the nonvolatile material that was 
carried into the water vapor stream as bubbles burst at the surface of the 
boiling water.

Deionization entails removal of electrically charged (ionized) dissolved 
substances by binding them to positively or negatively charged sites on a 
resin as the water passes through a column packed with this resin.  Because 
the resin also collects other dissolved substances that can feed bacteria, 
it is not unusual to find bacterial growth in a deionization column.  
However, the water that comes out of one of these is usually very low in 
conductivity  the solutes that enable electricity to pass have been 
removed.

Water used in my laboratory is first distilled and then deionized.   It has 
very low electrical conductivity, but it may contain a few by-products of 
the bacteria that grew on the deionizer resin.  These can largely be 
removed with activated carbon, but we have not needed to go this far in 
recent years.   Long ago, we had a commercial ultrapurification apparatus 
that took distilled water and removed bacteria with a special filter, ions 
with a resin, and other impurities with activated carbon.  The manual that 
came with the apparatus said (approximately), "The best thing we can tell 
you about storage of ultrapure water is  don't!"  As you supposed, 
ultrapure water in contact with perfectly clean glass will manage to 
dissolve substances from the glass surface.  The stored water is generally 
still purer than what comes out of our present treatment system, but not 
nearly as pure as it was when it emerged from the ultrapurification 
apparatus.  

So, that's the story.  Distillation and deionization may take different 
things out of water and yet leave some impurities behind.  On the other 
hand, the purer the water, the more difficult it becomes to store without 
losing purity.  Water we drink need not be as pure as water used for some 
laboratory purposes, but each laboratory has to adjust its purification 
system to the intended use of the water  and then devise a monitoring 
system to ensure that the required purification is being attained.  


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