Distilled,deionised and demineralised water and measuring of the purity
It is quite difficult to find clear definitions and standards for distilled, demineralised and deionised water. Probably the easiest way to familiarise in the topic of producing (ultra) pure water is to start with the oldest and best-know method: distilling.
Distilled water is water that has been boiled in an apparatus called a "still" and then recondensed in a cooling unit ("condenser") to return the water to the liquid state. Distilling is used to purify water. Dissolved contaminants like salts are left behind in the boiling pot as the water vapour rises away. It might not work if the contaminants are volatile so that they also boil and recondense, such as having some dissolved alcohol. Very elegant stills can selectively condense (liquefy) water from other volatile substances, but most distillation processes allow carry-over of at least some volatile substances, and a very little of the non-volatile material that was carried into the water vapour stream as bubbles burst at the surface of the boiling water. Maximum purity from such stills is usually 1.0 MWcm; and since there is no protection from carbon dioxide (CO2) dissolving into the distillate the pH is generally 4.5-5.0. Additionally, you have to be careful not to re-contaminate the water after distilling it.
Deionisation: Process utilizing special-manufactured ion exchange resins which remove ionised salts from water. Can theoretically remove 100 % of salts. Deionisation typically does not remove organics, virus or bacteria except through “accidental” trapping in the resin and specially made strong base anion resins which will remove gram-negative bacteria. 
Demineralisation: Any process used to remove minerals from water, however, commonly the term is restricted to ion exchange processes. 
Ultra pure water: Highly-treated water of high resistivity and no organics; usually used in the semiconductor and pharmaceutical industries 
Deionisation entails removal of electrically charged (ionised) 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. This process is called ion exchange and can be used in different ways to produce deionised water of various qualities.
Strong acid cation + Strong base anion resin systems These systems consist of two vessels - one containing a cation-exchange resin in the hydrogen (H+) form and the other containing an anion resin in the hydroxyl (OH-) form (see picture below). Water flows through the cation column, whereupon all the cations are exchanged for hydrogen ions. The decationised water then flows through the anion column. This time, all the negatively charged ions are exchanged for hydroxide ions which then combine with the hydrogen ions to form water (H2O).  These systems remove all ions, including silica. In the majority of cases it is advisable to reduce the flux of ions passed to the anion exchanger by installing a CO2 removal unit between the ion exchange vessels. This reduces the CO2 content to a few mg/l and brings about a reduction of the following strong base anion resin volume and in the regeneration reagent requirements. In general the strong acid cation and strong base anion resin system is the simplest arrangement and a deionised water that may be used in a wide variety of applications can be obtained with it. 
Strong acid cation + weak base anion + Strong base anion resin systems This combination is a variation of the previous one. It provides the same quality of deionised water, while offering economic advantages when treating water which contains high loads of strong anions (chlorides and sulphates). The subtitle shows that the system is equipped with an extra weak base anion exchanger before the final strong base anion exchanger. The optional CO2 removal unit may be installed either after the cation exchanger, or between the two anion exchangers (see picture below). The regeneration of the anion exchangers takes place with caustic soda (NaOH) solution first passing through the strong base resin and then through the weak base resin. This method requires less caustic soda than the method described before because the remaining regeneration solution after the strong base anion exchanger is usually sufficient to regenerate the weak base resin completely. Moreover, when raw water contains a high proportion of organic matter, the weak base resin protects the strong base resin. 
Mixed-bed Deionisation In mixed-bed deionisers the cation-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The two resins are mixed by agitation with compressed air, so that the hole bed can be regard as an infinite number of anion and cation exchangers in series (mixed bed resin). [2,3]
To carry out regeneration, the two resins are separated hydraulically during the loosening phase. As the anion resin is lighter than the cation resin it rises to the top, while the cation resin falls to the bottom. After the separation step the regeneration is carried out with caustic soda and a strong acid. Any excess regenerant is removed by rinsing each bed separately. The advantages of mixed bed systems are as follows:
- the water obtained is of very high purity and its quality remains constant throughout the cycle, - pH is almost neutral, - rinse water requirements are very low.
The disadvantages of mixed bed systems are a lower exchange capacity and a more complicated operating procedure because of separation and remixing steps which have to be carried out. 
Next to the ion exchange systems deionised water can be produced with reverse osmosis plants. Reverse osmosis is the finest filtration known. This process will allow the removal of particles as small as ions from a solution. Reverse osmosis is used to purify water and remove salts and other impurities in order to improve the color, taste or properties of the fluid. Reverse osmosis is capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, and other constituents that have a molecular weight of greater than 150-250 Daltons. RO can meet most water standards with a single-pass system and the highest standards with a double-pass system. This process achieves rejections of 99.9+% of viruses, bacteria and pyrogens. Pressure in the range of 50 to 1000 psig (3.4 to 69 bar) is the driving force of the RO purification process. It is much more energy-efficient compared to phase change processes (distillation) and more efficient than the strong chemicals required for ion exchange regeneration. The separation of ions with reverse osmosis is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected. 
Measuring of the purity
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.
Pure water by definition is slightly acidic and distilled water will test out around pH 5.8. The reason is that distilled water dissolves carbon dioxide from the air. It dissolves carbon dioxide until it is in dynamic equilibrium with the atmosphere. That means that the amount being dissolved balances the amount coming out of solution. The total amount in the water is determined by the concentration in the atmosphere. The dissolved carbon dioxide reacts with the water and finally forms carbonic acid.
Only recently been produced distilled water has a pH-value of approximately 7, but affected by the presence of carbon dioxide it will reach a slightly acidic pH-value within a couple of hours. Additional, it is important to mention that the pH of ultra-pure water is difficult to measure. Not only does high-purity water rapidly pick up contaminants - such as carbon dioxide (CO2) - that affect its pH, but it also has a low conductivity that can affect the accuracy of pH meters. For instance, absorption of just a few ppm of CO2 can cause the pH of ultra-pure water to drop to 4.5, although the water is still of essentially high quality.
The most accurate estimation of the pH of ultra-pure water is obtained by measuring its resistance; for a given resistance, the pH must lie between certain limits. For example, if the resistance is 10.0 MWcm, the pH must lie between 6.6 and 7.6. The relationship between the resistance and pH of high-purity water is shown in the figure below. 
Electrical resistivity versus pH of deionised water 
Compared with other beverages deionised water has apparently a slightly acidic pH-value.
According to the Merck Manual the human body uses buffers to balance the pH. If a person consumes something acid, the blood will produce more bicarbonate and less carbon dioxide to neutralize the acidity. Likewise the blood will produce more carbon dioxide and less bicarbonate if a alkaline substance is consumed. So drinking distilled water, will not put a human body in an acidic state.
Sources:  F. N. Kemmer; The Nalco water handbook; 2. Edition; 1988  www.purite.com  Degremont; Water treatment handbook; sixth edition; 1991  Osmonics Pure Water Handbook; 2. Edition; 1997
Should you know of any other interesting or more recent book, report, article or publication, concerning deionised and demineralised water. Please let us know, so that we can include some more facts in our above text.