Water hardness 2 9. Water hardness measurement

Water hardness is a traditional measure of the ability of water to react with soap: hard water requires a significant amount of soap to form foam. Scale deposits in hot water pipes, boilers, and other household appliances are caused by hard water. Water hardness is caused by dissolved polyvalent metal ions. In fresh water, the main ions that cause stiffness are calcium and magnesium; ions of strontium, iron, barium and manganese are also important. Water hardness is usually determined by the reaction of polyvalent metal ions present in water with chelating agents, such as EDTA, and is expressed as the equivalent concentration of calcium carbonate. Rigidity can also be estimated by determining the individual concentrations of its constituent components, the sum of which is expressed in terms of an equivalent amount of calcium carbonate. The degree of hardness of drinking water is classified based on equivalent concentrations of CaCO 3 in it, as follows:

Soft - 0-60 mg / l

Medium hardness - 60-120 mg / l

Hard - 120-180 mg / l

Very hard - 180 mg / l and higher.

Hardness is also classified based on equivalent concentrations of CaO or Ca (OH) 2. In the SI system, it is also recommended to express rigidity in moles of Ca 2+ per m 3.

Despite the fact that the stiffness is due to cations, it can also be considered as carbonate (disposable) and non-carbonate (constant) stiffness. Carbonate hardness indicates the amount of carbonates and bicarbonates in solution that can be removed or precipitated by boiling. This type of stiffness is responsible for scale deposits in hot water pipelines and boilers. Non-carbonate hardness is caused by a combination of hardness ions with sulfates, chlorides and nitrates and is defined as “permanent hardness” because it cannot be removed by boiling.

Alkalinity, as an indicator of water buffering, is closely related to hardness. Alkalinity is mostly caused by anions or molecular forms of weak acids, mainly hydroxides, bicarbonates and carbonates; in the presence of other forms in water, such as borates, phosphates, silicates and organic acids, they also make a small contribution to the alkalinity of water. Regardless of which dissolved forms provide the alkalinity of water, it is always expressed as an equivalent amount of calcium carbonate.

In cases where the alkalinity of surface water is determined by the presence of carbonates and / or bicarbonates, its value is usually close to the value of hardness.

Hard water distribution

The main natural sources of water hardness are sedimentary rocks, filtration and runoff from the soil. Hard water usually forms in areas with a dense arable layer and calcareous formations. Groundwater is usually characterized by greater rigidity than surface water. Groundwater rich in carboxylic acids usually has high solubility in soils and rocks containing measurable amounts of minerals such as calcite, gypsum and dolomite, resulting in hardness levels of up to several thousand mg / l.

The main industrial sources of stiffness are the effluents of enterprises producing inorganic chemicals and the mining industry. Calcium oxide is used in the construction industry in mortar, plaster and other materials. It is also used in the production of pulp and paper, sugar refining, oil refining, tanning, water and wastewater treatment. Magnesium is also used in various processes in the textile, tanning and paper industries. Magnesium alloys are widely used in foundry and stamping, portable machines, baggage equipment and household products of wide application. Magnesium salts are also used in the production of magnesium metal, fertilizers, ceramics, explosives and medicines.

Health Effects of Hard Water

As noted in the article on calcium and magnesium, the main factors determining the hardness of water are calcium and magnesium ions. There is no evidence of adverse health effects specifically associated with high levels of calcium or magnesium in drinking water.

In addition to household inconveniences due to the use of water with a high degree of hardness, another possible inconvenience may occur when magnesium is bound to a sulfate ion, as a result of which water acquires laxative properties.

The taste threshold for calcium ion in drinking water varies from the anions present; for a magnesium ion, the taste threshold is less. Further details regarding the relationship between water hardness and cardiovascular disease can be found in Part III, which addresses health aspects for the inorganic components of water. Recommended water values \u200b\u200bfor calcium and magnesium are not offered, since such a value is offered for general hardness based on aesthetic considerations.

Other aspects

Soft water is more likely to cause pipe corrosion, and as a result some heavy metals, such as copper, zinc, lead and cadmium, may be present in the drinking water in the distribution system. The degree of such corrosion and dissolution of metals is also a function of pH, alkalinity and the concentration of dissolved oxygen. In some communities, corrosion is so severe that special precautions have to be taken in the water supply system.

In areas with very hard water, house pipes may become clogged with scale; hard water also deposits on kitchen utensils and increases soap consumption. Thus, hard water can be not only unpleasant, but also economically burdensome for the consumer. The perception of water hardness by the population is not the same in different localities, it is often associated with the hardness that the consumer has been accustomed to for several years, and water with hardness of more than 500 mg / l does not cause objection in many settlements. Although an acceptable balance between corrosion and scale problems provides a hardness level of approximately 100 mg CaCO 3 / L.



As you know from the school chemistry course, ordinary water contains calcium and magnesium ions. The increased content of Ca 2+ and Mg 2+ ions gives water a negative quality, called rigidity.

CaCO 3 + CO 2 + H 2 O \u003d Ca (NSO 3) 2

MgCO 3 + CO 2 + H 2 O \u003d Mg (HCO 3) 2

This process is widely carried out under natural conditions, leading to the erosion of eroded limestones into surface waters, and then into the seas and oceans.

Non-carbonate (permanent) stiffness due to the presence in the water of sulfates, magnesium and calcium chlorides, as well as other salts (MgSO 4, MgCl 2, CaCl 2).

Total hardness \u003d carbonate (temporary) hardness + non-carbonate (permanent) hardness.

In everyday life, anyone can face the task of measuring the hardness of water at home. Rigidity negatively affects the quality of various processes in which water with increased rigidity is used. The lower the percentage of dissolved salts in water, the softer and healthier the water. The work of the dishwasher, the amount of washing powder, the quality of water in the aquarium, the need to install a water softener, etc. In general, there are many goals.

In Russia, stiffness is measured in "degrees of stiffness" (1 ° W \u003d 1 mEq / L \u003d 1/2 mol / m3). Other units of water hardness are accepted abroad.

Stiffness Units

1 ° W \u003d 20.04 mg Ca 2+ or 12.15 Mg 2+ in 1 dm 3 of water;
1 ° DH \u003d 10 mg CaO in 1 dm 3 water;
1 ° Clark \u003d 10 mg CaCO 3 in 0.7 dm 3 water;
1 ° F \u003d 10 mg CaCO 3 in 1 dm 3 water;
1 ppm \u003d 1 mg CaCO 3 in 1 dm 3 water.

According to the intensity of scale formation in the teapot, certain conclusions can be drawn: the more plaque, the harder the water.

Comparative qualitative   The conclusion about water hardness can be made as follows. Apply a drop of rain, boiled and unboiled tap water onto a glass slide. After drying according to the intensity of precipitation formed, you can conclude that your water is hard. Rain water is the softest, because it has practically no salts of calcium and magnesium. The precipitate after evaporation of unboiled water will allow us to conclude about the general hardness, and boiled - about temporary hardness.

But at home, you can fairly accurately and quantitatively assess the hardness of the water. From the course of organic chemistry, you know that laundry soap, like any other, is difficult to soap in hard water. This method is based on the fact that as soon as the soap binds the excess of calcium and magnesium salts, a soap foam appears. To determine the hardness of water, you need to weigh one gram of laundry soap, grind it and carefully, so as not to form a foam, dissolve in a small amount of hot distilled water. Distilled water can be bought at pharmacies or car dealerships. It is used to add to the battery with increasing electrolyte concentration.

Next, pour the soap solution into a cylindrical glass and add distilled water to the level of 6 centimeters if the soap is 60% or to the level of 7 centimeters if the soap is 72%. The percentage of soap is indicated on the bar. Now, every centimeter of the level of soapy solution contains a quantity of soap that can bind hardness salts, the amount of which corresponds to 1 ° dH in 1 liter of water. Next, pour half a liter of test water into a liter jar. And continuously stirring, gradually add the prepared soap solution from a glass to a jar of test water. At first, only gray flakes will be on the surface. Then there will be multi-colored soap bubbles. The appearance of a stable white soapy foam suggests that all hardness salts in the test water are bound. Now we look at our glass and determine how many centimeters of the solution we had to pour from the glass into the test water. Each centimeter bound in half a liter of water the amount of salts corresponding to 2 ° dH. Thus, if you had to pour 4 centimeters of soap solution into the water before the appearance of the foam, then the hardness of the test water is 8 ° dH.

If you poured the entire soap solution into the water, but the foam did not appear, this means that the hardness of the test water is greater than 12 ° dH. In this case, the test water is diluted with distilled water twice. And we are doing the analysis again. Now the result of stiffness will need to be multiplied by two. The resulting value will correspond to the hardness of the investigated water.

From the table you can determine the quality of the studied water:

It is not possible to determine the rigidity with this method to the nearest thousandths of a degree, but it is quite possible to assess the sharp departure of the total hardness from the norm with an accuracy of 1-2 ° dH. A spread of readings of 1-2 degrees is quite acceptable. Given the simplicity and accessibility of the method, it certainly can be successfully applied.

Using this method, it is possible in the field to assess the hardness of water from various water sources and perform interesting design and research work.

Sources:

1Rudzitis G.E. Chemistry. Inorganic chemistry. Organic chemistry. Grade 9: textbook. for general education. organizations with adj to electron. media (DVD): basic level / G.E. Rudzitis, F.G. Feldman. - M .: Education, 2013 .-- 224 p.: Ill.

People in different countries have long come to the need to normalize it, because high rigidity is bad: both pipes are clogged and it’s impossible to wash properly. But they began to do this in each country in their own way, who, based on traditional units of measurement and methods for determining calcium and magnesium ions, because there were no uniform standardized international units then.

It is known that there is nothing worse than bad habits - it is very difficult to get rid of them! In the coffee literature (although stiffness is not inherently a coffee term!) In different countries, stiffness is still measured in degrees, with each country having its own, different from all others. Only Russian and German degrees of rigidity are identical, though, long ago canceled in both of these countries, but persistently existing in the definition of concepts.

In the USSR, until 1952, degrees of rigidity were used, which coincided with the German ones. In Russia, the normal concentration of calcium and magnesium ions, expressed in milligrams of equivalent per liter (mEq / l), is used to measure stiffness. One mEq / L corresponds to a content of 20.04 milligrams of Ca2 + or 12.16 milligrams of Mg2 + in a liter of water (atomic mass divided by valency).

In other countries it is customary to denote stiffness in CONDITIONAL degrees:

German degrees (dGH)

1 ° \u003d 1 part calcium oxide — CaO in 100,000 parts of water, or 0.719 parts of magnesium oxide — MgO in 100,000 parts of water, or 10 mg of CaO in 1 liter of water, or 7,194 mg of MgO in 1 liter of water. dGH (dH) and dKH are currently most commonly used in aquariums as a unit of stiffness, the designation dGH - refers to the total hardness, dKH - to carbonate;

French degrees (fh)

1 ° \u003d 1 part CaCO 3 in 100,000 parts of water, or 10 mg CaCO 3 in 1 liter of water;

American degrees (usH)

1 ° \u003d 1 gran (0.0648 g) CaCO 3 in 1 gallon (American! 3.785 l) of water. Dividing the grams into liters we get: 17.12 mg / l CaCO 3. However, there is another definition of the American degree: 1 part CaCO 3 in 1,000,000 parts of water (in English literature the expression for concentration, as 1 part per 1,000,000 parts is called ppm - part per million), and is often used. it is identical to 1 mg / l). Thus, this 1 American degree \u003d 1 mg CaCO 3 in 1 liter of water. It is this value of the American degree that is accepted in all tables with transition coefficients for the conversion of some units of stiffness to others.

English degrees (Clark)

1 ° \u003d 1 gran (0.0648 g) in 1 gallon (English! 4.546 l) of water \u003d 14.254 mg / l CaCO 3.

Feel like everything is not easy ?! Therefore, I will give a table that allows you to compare and translate some degrees of rigidity into others:

Table 1

Name of units	Mg-eq / L	Degree of rigidity
		Deutsch	French	American	English
1 mEq / L	1	2.804	5.005	50.045	3.511
1 German degree dH	0.3566	1	1.785	17.847	1.253
1 french degree	0.1998	0.560	1	10,000	0.702
1 american degree	0.0200	0.056	0.100	1	0.070
1 English degree	0.2848	0.799	1.426	14.253	1