Hard water

Water hardness is a term system of applied chemistry, which has evolved from the needs of the use of natural water with its dissolved substances. Specifically, with water hardness, the equivalent concentration of dissolved ions in the water of the alkaline earth metals, in specific contexts but also referred to the anionic partner. The " hardness minerals " include mainly calcium and magnesium ions and normally contained only traces of strontium and barium ions. The dissolved mineral deposits can form insoluble compounds, especially lime and calcium soaps. This tendency to form insoluble compounds is the reason for the observance of the dissolved alkaline earths, which has led to the emergence of the concept and theory system about the water hardness.

• Soft water is better for all applications where the water is heated, however, for washing, for casting of indoor plants etc. A disadvantage is the strong foam formation in detergents and poor removability of soap, for example, when washing your hands. Soft water is in crystalline regions with granite, gneiss and basalt rocks available. Even rain water is soft.
• Hard water leads to calcification of household appliances, increases the consumption of detergents and soaps, impaired or promotes, depending on the degree of hardness, the taste and the look of delicate foods and beverages (eg, tea). Hard water comes from regions where prevailing sand and limestone.

• 6.1 revision of hardness ranges

Formation

The water hardness is produced during the passage of water through soil and / or groundwater aquifers ( aquifers ). Therefore, it depends strongly on the geological substratum, which and how much hardeners can go into solution. This corresponds to the geographical distribution of water hardness.

Magnesium and calcium ions can be most easily pass through the solution process in the water, such as by resolution of gypsum (CaSO4 × 2 H2O). Groundwaters from gypsum-bearing layers can reach in extreme cases, the saturation concentration for gypsum, which corresponds to a hardness of 78.5 ° f and 44 ° dH.

The vast majority of water hardness but normally arises as a carbonate by dissolution of limestone (CaCO3 ) or dolomite (Ca -Mg mixed carbonate ) by carbonic acid to form soluble bicarbonates (HCO3-). The carbon dioxide comes mainly from the respiration of organisms in the soil, where, in particular, the microbial degradation of organic matter provides increased CO2 concentrations.

For example, following the dissolution of pure limestone with the following empirical formula:

In addition to the acids present in acid rain, which became known by the term acid rain also lead to an increase in hardness. Above all they are sulfuric acid ( H2SO4), which arises on sulfur dioxide and the formation of sulfurous acid in the combustion of fuels containing sulfur and nitric acid ( HNO3), which is formed via the intermediate stage of the nitrogen oxides in particularly hot burns. By reducing air pollution ( eg flue gas desulphurisation and exhaust catalysts) these loads have been reduced dramatically in recent decades.

During decomposition of plant matter ( dead roots, leaf litter, under plowed straws ) in soil or during discharge agricultural fertilizer, the nitrogen contained in it is first released as ammonium (NH4 ). This is followed by a bacterial oxidation process joins, called nitrification. The ammonium is first oxidized to nitrous acid ( HNO 2 ), and finally to form nitric acid (HNO3). This nitric acid dissolves limestone - to mineral deposits - and in the absence of lime from clay minerals. Therefore threaten agricultural soils to acidify without lime. In these cases, a liming is required. Also, this may be partly responsible for the increase in hardness in ground water.

In groundwaters, which are influenced by agricultural activities, the hardness above 30 ° f and 17 ° dH, in some cases even above 40 ° f and 23 ° dH, increase. This is due to both increased formation of carbonic acid as well as to increased nitrification.

Rainwater can only exceptionally be absorb hardeners, when the atmosphere contains calcareous dust particles. Usually, therefore, the hardness of rainwater near zero. Also drinking water reservoirs and lakes often contain even in calcareous areas of water of low hardness when her catchment area covers a relatively small geographical area and the rainwater will flow mainly superficial.

Classification

What is subsumed under the term, is a different system of coupled chemical equilibria. These are firstly, the solubility equilibria between the various alkaline earth metal ion and the corresponding carbonate and sulfate precipitates ( calcite, dolomite, barite, gypsum etc.). Inextricably linked with it is on the other hand the solution and dissociation of carbon dioxide and carbonic acid carbonate system. The illustrated below common classification of hardness in different sub-terms picks out individual aspects of this complex equilibrium system and each appoint their quantitative proportion.

Total hardness

Total hardness is the sum of the concentrations of the cations of alkaline earth metals in water. These cations have a large, positive physiological significance, however, interfere with some uses of the water. To form fed to the water insoluble calcium soaps soaps with these cations, which have no more cleaning action. When washing fabrics in water with high total water hardness, calcium soaps lead to contamination of the textiles. Soaps are among the anionic surfactants and are particularly sensitive to hard water. The wash performance of other modern surfactants in detergents other hand, is hardly affected by water hardness. Nevertheless, the detergent contains about 30 % substances that soften hard water. (see also component detergent and water hardness and washing)

Carbonate hardness

In terms of water hardness is the concentration of the anion bicarbonate (HCO3-) of special importance. It refers to the concentration of bicarbonate ions or the equivalent thereto of the alkaline earth metal ions as carbonate, temporary or temporary hardness. A water is located in the so-called Kalkkohlensäure - equilibrium if it contains exactly as much carbon dioxide that it separates just any lime, but can also solve any lime. If such a water carbon dioxide withdrawn himself sparingly soluble compounds such as calcite and dolomite form as particularly poorly soluble mixed carbonate ( scale, marl ). That depends on the complex, temperature-dependent calcium - carbonate - carbon dioxide balance ( often simply referred to as lime and carbonic acid ) from. Due to the temperature dependence of this equilibrium system also deposits in the preparation of hot water ( hot water systems, coffee makers, pots ) form. In the waters of a corresponding process as biogenic decalcification takes place as a result of the damage caused by the photosynthesis of aquatic plants and phytoplankton loss of carbon dioxide.

Nichtcarbonat hardness

As Nichtcarbonat - permanent or permanent hardness is that portion of the total water hardness, which is not bound to bicarbonate or carbonate, and therefore in principle can not be removed as calcium or magnesium from the water. This proportion is not removable ( "bound" ) of anions such as chlorides, nitrates and sulfates balanced. In which different concentrations of these anions are present exactly in terms of water hardness does not matter, but gives information about the origin of these shares. In fact, this permanent hardness influenced decisively the precipitation behavior of the carbonate - share because the thus increased levels of calcium and magnesium in the calculation of the ion products go with the carbonate and thus the thresholds such as the " associated carbonic acid " for the occurrence of increase hardness typical precipitation reactions.

Often the concentrations of magnesium and calcium ions are determined separately and then referred to as " magnesium hardness " or " calcium hardness ". Their sum is a good approximation of total water hardness.

Methods for determining

• The best known practical method for determining the total hardness is the complexometric titration with an aqueous solution of the disodium salt of ethylenediaminetetraacetic acid ( EDTA, trade name: Titriplex III ) of known concentration. EDTA, together with the hardness minerals Ca2 and Mg2 -soluble, stable chelate. 100 ml of the water sample to be tested are 25 % ammonia solution, 2 ml of a pH 11 buffer (ammonia - ammonium acetate ) and the indicator Eriochrome black- T. Typically, the indicator, together with the buffer as a so -called " tracer buffer tablets " available. The indicator when masked with a yellow dye, together with the Ca2 and Mg2 a red colored complex. Are these ions bound by EDTA at the end of the titration, the Eriochrome black - T is available free and is colored green. When unmasked an indicator of color change from blue to magenta is. The total hardness is calculated from the number of ml EDTA solution. At a water sample of 100 ml is equivalent to 1 ml of consumed EDTA solution (c = 0.1 mol / l) 5.6 ° dH ( German hardness ), which corresponds to 1 mmol / l alkaline earth metal ions. To the calcium and magnesium concentration is demonstrated separately at a lower pH of about 8 titrated only against Ca2 with EDTA, because at this pH the Mg -EDTA complex is not yet stable. At the transition point of calcium, the pH is then adjusted to 11 and titrated with EDTA against Mg2 .
• An older method is the hydrolytic precipitation titration with alcoholic Kaliumpalmitatlösung react the palmitic acid in the Palmitationen with calcium and magnesium ions to form the corresponding insoluble salt ( calcium soap ). At the equivalence point is reached Palmitationen react hydrolytically hydroxide ions which are detected using phenolphthalein as an indicator. 1 ml of a Kaliumpalmitatlösung with the concentration of 0.1 mol / l corresponds to a total hardness of 1 meq / l

• The carbonate hardness is determined by the acid - binding capacity (ABC ). For this purpose, 100 ml of water with hydrochloric acid (c = 0.1 mol / l ) to pH 4.3 titrated ( pH meter or envelope of methyl orange indicator). This is ( almost ) everything carbonate and bicarbonate is converted to " free carbonic acid ." Therefore, the acid consumption in ml corresponding to the hydrogen carbonate concentration in meq / l Multiplying by 2.8 gives German degrees of hardness (° dH ) if the result of the calculation does not exceed the total hardness. Results in the analysis of natural water for carbonate hardness, a higher value than for the total hardness, so this water also contains sodium bicarbonate. The carbonate hardness is the same as the total hardness in this case, since this can not be greater than the total hardness.
• In analytical laboratories, the alkaline earth metal ions as well as the anions of the acid residues with the aid of ion chromatography or capillary electrophoresis can be determined. Calcium can also be determined spectroscopically using the flame atomic emission spectrometry.

Units and Conversion

After the SI system of units ( l mmol / ) is the content of the alkaline earth metal ions, ie, the total hardness in moles per liter, and given the low concentrations in millimoles per liter indicated.

The water hardness was given in Germany and Austria early in degrees German hardness (° dH). In this case, 1 ° dH was formally than 10 mg CaO per liter of water defined. The other hardeners such as magnesium were defined as equivalent amount of this (7.19 mg MgO per liter). Later, the specification of the water hardness in the practical molar equivalent unit milliequivalents per liter ( mEq / L) was used. Today, the above molar details are legally required, regardless of the practical requirements.

In Switzerland, the French degrees of hardness ° fH shall prevail.

In other countries were or are other units in use that are only partially comparable, however. Comparable they are assuming a standard ion ratio. This is possible because the majority of natural waters having a relatively similar cation distribution, regardless of the total salt content. Only under this condition is the following table for conversion apply:

The unit 1 ppm is used contrary to the very meaning of the word in the sense of 1 mg / l CaCO3 here.

Are the values ​​for magnesium (Mg) and calcium (Ca ) is known, the hardness of the water can (for example, mineral water) can be calculated as:

Physiological significance

Magnesium and calcium are essential for the organism. The human body contains 0.47 g / kg of magnesium and 15 g / kg of calcium. For the body's supply of these elements, however, the drinking water plays a subordinate role. Strontium is also such as calcium contained in the bone, but has no special physiological significance.

Barium is toxic in dissolved form. In sulfate- containing waters of toxicological concern concentrations are not achieved because the extremely low solubility of barium sulfate is formed. Barium sulfate is the main component of orally administered medical X-ray contrast agent.

Suitability for washing in Germany

On packages of laundry and cleaning products that contain phosphates and other curing binding substances, had according to § 7 para 1 sentence 1 No. 5 WRMG ( Washing and Cleansing Agents Act of 1987, Federal Law Gazette I, p 875 ) since 1988, graduated dosing recommendations are given in milliliters for the hardness ranges from 1 to 4. Prescribed by law while the details were regarding millimoles per liter total hardness. There the following hardness ranges were defined:

Revision of hardness ranges

On 1 February 2007, the revised Washing and Cleaning Agents Act (WRMG ) was adopted by the German Bundestag, which entered into force on 5 May 2007. This, inter alia, the hardness ranges were adjusted to European standards and the indication millimoles per liter total hardness is replaced by the ( nonsensical from a chemical point of view ) Indication millimoles of calcium per liter. Water utilities will probably continue to publish the total hardness, this is the law but not required. According to opinions of the BMU over the DVGW millimoles per liter of calcium carbonate to be unchanged construed as millimoles per liter total hardness. The new hardness ranges are hardly different from the previous, only the regions 3 and 4 are "hard" merged to form the hardness and the digits 1,2,3 and 4 are represented by the - already in use - features " soft", " medium" and " replaced hard ". The new hardness ranges are defined as follows:

On the packaging of detergents must be specified according to § 8 paragraph 1 sentence 1 WRMG recommended quantities and / or dosage instructions expressed in milliliters or grams for a standard washing machine load for for soft, medium and hard and making provision for one or two washes. In order to save detergent, one must know the local water hardness and then reads on the package, the associated amount of detergent from. For harder water (hardness range 3 - "hard" ) you should add a separate, non-phosphate water softener at temperatures from 60 ° C. The water utilities notify the customer of the local water hardness or send stickers, which you conveniently adhered to the washing machine.

Exist for drinking water regulations on the water hardness, see there.

Scale in Switzerland

According to the food law is divided the water in Switzerland to six firmness levels, which per liter ( calcium and magnesium particles number per liter of water ) or in French degrees of hardness ° fH are given in millimoles.

While the water in the foothills of the Alps, in the Alps and in the Southern Alps is usually very soft or soft, it is medium hard in the Jura and Plateau hard to very hard.

Enthärtungsmethoden

Decarbonization: This measure only the carbonate hardness is reduced. The water calcium hydroxide is added as a " lime ", which triggers the following reaction:

A decarbonization is carried out in some German water works very hard waters.

Softening by ion exchange: ion exchangers are regenerated with sodium chloride, are able to replace calcium and magnesium ions with sodium ions. This principle is used, for example in dishwashers to protect the heating elements and to avoid " lime scale " on the dishes. Water softeners for non- professional use for softening of drinking water using this principle. Occasionally it is also used for the preparation of small quantities of water, such as for watering plants or tea making facilities.

Demineralization: A demineralisation not only eliminates mineral deposits, but all ions. This is achieved by a combination of cation and anion exchangers. Demineralized water is used in all areas where water is needed in pure form. The largest quantities are used as boiler feed water used. Achieve a similar result the reverse osmosis and distillation, in which nonionic dissolved solids are removed.

Other methods: The complex formation with polyphosphates reduces the hardness, however, leads to eutrophication of surface waters. Detergents often contain small amounts of complexing agents, softening occurs but today mainly by cation exchangers such as zeolite A. This prevents the formation of calcium soaps to prevent increases the stability necessary for the wash emulsion and protects the heating elements of the washing machine.

With steam locomotives, the inside feed water treatment is being used.

Equipment with electric or magnetic fields do not lead to the elimination of hardness, and its effect is more than controversial. At best, it is conceivable that during the crystallization of the excess calcium carbonate under the influence of these fields, the unstable aragonite form is formed, which consists of fine needle shaped crystals and remains suspended. The normal crystallization to the more stable calcite, however, is the well-known encrustations ( scale ). The effect of this type of water treatment is limited in time and therefore goes to a certain flow path behind the appliance again lost. A prerequisite for the described effect seems to be that alternating fields are used or that water is swirled in a static field. Therefore patch magnetic boots remain eg on the water line of no effect whatever.

Alternatively prevent sacrificial anode systems based on zinc crystallization of calcite on surfaces. Escaping zinc ions react with the carbonate ions dissolved in the water and form nuclei, from which only weakly crystalline and non-adherent minerals occur directly in the water phase. These are then transported with the water flow and reduce by their abrasive action of existing deposits. This is not a real removal of water hardness, but a prevention of limescale.