De-Ionised Water (DI) or sometimes called Demineralised Water (DM) is water that has had almost all of its minerals, Cations (sodium, calcium, metals) and Anions (chloride, sulphate, salts) removed.

Ions with a positive charge are called Cations and those with a negative charge Anions. The process used De-Ionisation or Demineralisation is a chemical process using specially manufactured resins which exchange Cations to Hydrogen ions (H⁺⁺) and Anions to Hydroxide ions (OH^—) for dissolved minerals to recombine (H) + (OH) to form water (H₂O) which is not an ion. Because most water impurities are dissolved salts de-ionised water produces highly purified water that is similar to distilled water without the disadvantages of not having a slow hot process but a quicker through resin cold process.

Separate bed vs Mixed bed.

Demineralisation therefore requires using a least two types of ion exchange resins to produce De-Ionised water. One resin will remove +ve charged ions the other will remove -ve charged ions.

In a dual bed system the cation resin always is first inline, all the +ve charged cations are attracted to the cation resin beads and exchanged for hydrogen, the anion  -ve charged ions are not attracted to the cation resin and pass through. The anions will be attracted to the anion resin beads to form hydroxide ions leaving the hydrogen ions to react with these hydroxide -ve ions to form purified water (H₂0). It is possible to have a slight sodium leakage which raises the conductivity higher than the original level of the initial raw water. Sodium ions have a similar attraction to that of hydrogen and does not exchange fully to that ion.

In a mixed bed system the strong acid cation and the strong base anion are intermixed usually 50/50. This makes the vessel 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 the quality of finished water in usually of the highest quality. This method is very well used with a Reverse Osmosis unit in series with the mixed bed and resultant RO/DI water a best method and volume for process to use.

Quality is measured using electrical conductivity. The ionic concentration is measured by this electrical conductance, the higher the ions in the water the higher the reading and less quality of the grade of water produced. A lower conductivity value is considered more “Deionised” than a water with a high conductivity value.