Is RO water acidic

Deionized and demineralized water

About deionized, demineralized and distilled water and measurement of the purity of such water

It is quite difficult to find a clear definition of distilled, demineralized and deionized water. The production of distilled water is probably the easiest entry into the field of the production of (highly) pure water.

The distillation is one of the oldest and best known methods of producing pure water. The raw water is evaporated in a still and condensed back into liquid water in a cooling unit (condenser). Dissolved ingredients such as salts remain in the evaporation vessel while the water leaves it in the form of steam. In addition to the salts, however, the raw water can contain volatile substances that, like water, evaporate and condense. These include, above all, alcohols. Higher quality stills can selectively distill water alone alongside other ingredients present. However, the transfer of volatile substances and small amounts of non-volatile ingredients through bursting water bubbles cannot be completely avoided. The highest degrees of purity that can be achieved with such stills are 1.0 MΩ.cm. The pH of the distilled water produced becomes slightly acidic (4.5 - 5.0) if carbon dioxide from the ambient air can dissolve in it.

Deionization: Process using special Ion exchangerswhich remove the ions of the salts from the water. Theoretically, 100% of the salts contained can be removed. Deionization does not reduce any organic components in the water, viruses or bacteria. It happens, however, that these remain in the ion exchange resins by chance, especially strongly basic anion exchange resins can remove gram-negative bacteria. [4]

Demineralization: Any process used to remove minerals from water. In general, this term is used for Ion exchange processes used. [1]

Highly pure water, ultrapure water: Is intensely treated water, which has a high specific resistance and no organic components. It is mainly required in the semiconductor and pharmaceutical industries. [4]

Deionization takes place through the binding of dissolved, electrically charged substances in water to cation or anion exchange resins. The Resins are located in containers and the water to be treated flows through them. The whole process is called Ion exchangeand can be used in various applications for the production of deionized water.

  • Strongly acidic cations + strongly basic anion exchangers

    These systems consist of two containers - one filled with cation exchange resin from H.+-Form and another with anion exchange resin of the OH--Shape. First, the strongly acidic cation exchanger is charged with raw water, so that an exchange of cations present in the water for hydrogen ions (H.+) he follows. The water to be treated then flows through the strongly basic anion exchanger in order to convert negatively charged ions against hydroxide ions (OH-) to exchange. The registered hydrogen (H+) and hydroxdiones (OH-) combine to form water (H2O). [2]
    This arrangement of ion exchangers can remove almost all ions in the water. In many cases it is advisable to carry out a CO before charging the anion exchanger2-Degassing upstream to reduce the ion concentration that is fed to the anion exchanger. The degassing achieves a reduction in CO2-Content around a few mg / l, which means that a smaller amount of the strongly basic anion exchange resin is required. In addition, these measures can save regeneration chemicals.
    In general, the use of a strongly acidic cation and a strongly basic anion exchanger is the basic arrangement for producing deionized water for a variety of applications. [3]

  • Strongly acidic cations + weakly basic anions + strongly basic anion exchanger

    The combination of ion exchangers described in the title represents an extension of the variant described in the previous paragraph. With this arrangement, high quality deionized water can be produced, while at the same time utilizing economic advantages in the treatment of raw water with high concentrations of strong anions (chlorites and sulfates) become. As the title shows, the system consists of an additional weakly basic anion exchanger upstream of the strongly basic exchanger. The optional carbon dioxide gas extraction can be installed either after the cation or between the two anion exchangers. (see figure below)
    The anion exchangers are regenerated with sodium hydroxide solution (NaOH), which is first passed over the strongly basic anion exchange resin and then over the weakly basic resin. This procedure effectively consumes less sodium hydroxide solution (NaOH), since the regeneration chemical can still be used effectively for the weakly basic anion exchange resin after passing through the cation exchanger. In addition, the weakly basic resin assumes a protective function for the strongly basic anion exchange resin when raw water with a high proportion of organic matter is treated. [3]

  • Mixed bed ion exchanger

    In mixed-bed deionization, the cation and anion exchange resins are mixed in a pressure vessel. The two resins are mixed by the action of compressed air so that the entire bed can be viewed as an unlimited number of cation and anion exchangers in series. [2.3]

    In order to be able to carry out the regeneration of the mixed-bed ion exchanger, the two resins have to be separated hydraulically. Since the anion exchange resin is lighter than the cation exchange resin, it rises to the surface while the cation exchange resin collects at the bottom. After the separation, regeneration takes place with caustic soda and a strong acid. Excess regeneration chemicals must be removed by means of a separate rinse for each bed.
    Mixed bed ion exchangers have the following advantages:

    - the water produced has a very high purity
    - The quality of the pure water remains constant during the manufacturing phase
    - the pH value is almost neutral
    - The flushing water requirement is very low

    The low exchange capacity and the complex operation, with separation and remixing for regeneration, can be described as disadvantageous. [3]

Next to the Ion exchange there is the possibility of deionized water through Reverse osmosis systems to manufacture. The Reverse osmosis is the finest known filtration to date. The process enables the removal of tiny particles from solutions. Reverse osmosis is used to treat water, remove salts and other contaminants in order to improve the color, taste and other properties of a fluid. The process is also suitable for retaining bacteria, salts, sugars, proteins, particles, dyes and other components with a molecular weight of more than 150-250 Dalton.
As a so-called “single-pass” system, the process can produce very high levels of purity. Even higher degrees of purity are achieved with the "double-pass" system with retention levels of 99.9% for viruses and bacteria. Pressures in the range from 3.4 to 69 bar are the driving force for the reverse osmosis process. As a result, it is significantly more energy-intensive than phase change processes such as distillation, but at the same time more effective than ion exchange processes, which also have a high need for regeneration chemicals. In reverse osmosis, the retention of ions is supported by charged particles. As a result, charged ions such as salts can be retained better by reverse osmosis membranes than uncharged particles such as organics. It can be said that the higher the charge and the larger a particle, the better it is removed from the raw water during reverse osmosis. [4]

Measurement of the purity of deionized water

The purity of water can be determined using various methods. This includes recording the weight of all dissolved substances in the water. In addition to determining the weight of foreign substances, their amount can be determined by using their influence on the increase in the boiling point or the decrease in the freezing point of water. The refractive index is also influenced by solids in the water. Alternatively, the purity can be determined on the basis of electrical conductivity or resistance. Deionized water hardly contains any ions, so that it has a low electrical conductivity and a high resistance.

Ultra-pure waterPure waterPurified water
resistance10-18 MΩ.cm1-10 MΩ.cm1-0.02 MΩ.cm
conductivity0.1-0.0555 µS / cm1.0-0.1 µS / cm1-50 µS / cm
Manufacturing processMixed bed ion exchangerStrongly basic mixed bed systemWeakly basic mixed bed system

PH value

Pure water is usually slightly acidic PH value on and distilled water can reach pH levels of around 5.8. The reason for this is to be seen in the fact that distilled water dissolves carbon dioxide from the ambient air. The carbon dioxide is absorbed until a dynamic equilibrium is established between the water and the ambient air. The amount of carbon dioxide ultimately dissolved in the water is determined by the carbon dioxide concentration in the surrounding atmosphere. The dissolved carbon dioxide reacts with the water and ultimately forms carbonic acid.

2 H2O + CO2 -> H2O + H2CO3 (Carbonic acid) -> (H30+) + (HCO3-) (hydrogen carbonate)

Only freshly made distilled water has one PH value from about 7 to, however, as soon as it is influenced by the presence of carbon dioxide, a slightly acidic pH value sets in in the course of a few hours. It should also be pointed out that the pH value of ultrapure water is difficult to determine. Due to its purity, ultrapure water not only quickly absorbs impurities that affect the pH value (e.g. carbon dioxide), but also makes pH value measurement difficult due to its low conductivity.
The absorption of only a few ppm of carbon dioxide can lead to a pH of 4.5 in highly pure water, although the water is still of high quality.

An exact PH value Determination for Ultrapure water enables the determination of the electrical resistance. For a given resistance value, the pH value must be within certain limits. For example, a water sample with an electrical resistance of 10 MWcm would have to have a pH value in the range of 6.6 and 7.6. This relationship between electrical resistance and pH value of highly pure water is shown in the adjacent figure. [2]

The pH value is shown on the abscissa and the electrical resistance in MΩ / cm on the ordinate.

Small comparison

Compared to beverages, however, the pH of distilled water is only slightly acidic.

beveragesPH value
distilled water5,8
orange juice3,5
Soft drinks2,0-4,0
(Stomach acid)1,0-2,0
(Battery acid)1,0