41603

Hardness of Drinking Water

Лабораторная работа

Химия и фармакология

Shchetynsk ICS 405 Lbortory work 3 Hrdness of Drinking Wter im: to reserch the types of the hrdness of drinking wter. Theoreticl informtion Sources of Hrdness Minerls in Drinking Wter Wter is good solvent nd picks up impurities esily. Pure wter tsteless colorless nd odorless is often clled the universl solvent. When wter is combined with crbon dioxide to form very wek crbonic cid n even better solvent results.

Английский

2013-10-24

53.38 KB

0 чел.

Done by T. Shchetynska, IACS 405

Laboratory work #3

Hardness of Drinking Water

Aim: to research the types of the hardness of drinking water.

Theoretical information

Sources of Hardness Minerals in Drinking Water

Water is a good solvent and picks up impurities easily.  Pure water - tasteless, colorless, and odorless - is often called the universal solvent. When water is combined with carbon dioxide to form very weak carbonic acid, an even better solvent results. As water moves through soil and rock, it dissolves very small amounts of minerals and holds them in solution. Calcium and magnesium dissolved in water are the two most common minerals that make water "hard."  The degree of hardness becomes greater as the calcium and magnesium content increases and is related to the concentration of multivalent cations dissolved in the water.

Origin of water "hardness"

Carbon dioxide reacts with water to form carbonic acid (1) which at ordinary environmental pH exists mostly as bicarbonate ion (2). Microscopic marine organisms take this up as carbonate (4) to form calcite skeletons which, over millions of years, have built up extensive limestone deposits. Groundwaters, made slightly acidic by CO2 (both that absorbed from the air and from the respiration of soil bacteria) dissolve the limestone (3), thereby acquiring calcium and bicarbonate ions and becoming "hard". If the HCO3– concentration is sufficiently great, the combination of processes (2) and (4) causes calcium carbonate ("lime scale") to precipitate out on surfaces such as the insides of pipes. (Calcium bicarbonate itself does not form a solid, but always precipitates as CaCO3.)

Types of hard water

Temporary (carbonate) hardness is caused by a combination of calcium ions and bicarbonate ions in the water. It can be removed by boiling the water or by the addition of limewater (calcium hydroxide). Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling.

The following is the equilibrium reaction when calcium carbonate (CaCO3) is dissolved in water:

CaCO3(s) + CO2(aq) + H2O ⇋ Ca2+(aq) + 2HCO3-(aq)

Upon heating, less CO2 is able to dissolve into the water (see Solubility). Since there is not enough CO2 around, the reaction cannot proceed from left to right, and therefore the CaCO3 will not dissolve as rapidly. Instead, the reaction is forced to the left (i.e., products to reactants) to re-establish equilibrium, and solid CaCO3 is formed. Boiling the water will remove hardness as long as the solid CaCO3 that precipitates out is removed. After cooling, if enough time passes, the water will pick up CO2 from the air and the reaction will again proceed from left to right, allowing the CaCO3 to "re-dissolve" into the water.

Permanent (common) hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence in the water of calcium and magnesium sulfates and/or chlorides which become more soluble as the temperature rises. Despite the name, permanent hardness can be removed using a water softener or ion exchange column, where the calcium and magnesium ions are exchanged with the sodium ions in the column.

Hard water causes scaling, which is the left-over mineral deposits that are formed after the hard water had evaporated. This is also known as limescale. The scale can clog pipes, ruin water heaters, coat the insides of tea and coffee pots, and decrease the life of toilet flushing units.

In industrial settings, water hardness must be constantly monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that comes in contact with water. Hardness is controlled by the addition of chemicals and by large-scale softening with zeolite (Na2Al2Si2O8.xH2O) and ion exchange resins.

Control of water hardness

There are two ways to help control water hardness: use a packaged water softener or use a mechanical water softening unit.

Packaged water softeners are chemicals that help control water hardness. They fall into two categories: precipitating and non-precipitating.

Precipitating water softeners include washing soda and borax. These products form an insoluble precipitate with calcium and magnesium ions. The mineral ions then cannot interfere with cleaning efficiency, but the precipitate makes water cloudy and can build up on surfaces. Precipitating water softeners increase alkalinity of the cleaning solution and this may damage skin and other materials being cleaned.

Non-precipitating water softeners use complex phosphates to sequester calcium and magnesium ions.  There is no precipitate to form deposits and alkalinity is not increased. If used in enough quantity, non-precipitating water softeners will help dissolve soap curd for a period of time.

Mechanical water softening units can be permanently installed into the plumbing system to continuously remove calcium and magnesium.  Water softeners operate on the ion exchange process. In this process, water passes through a media bed, usually sulfonated polystyrene beads. The beads are supersaturated with sodium. The ion exchange process takes place as hard water passes through the softening material. The hardness minerals attach themselves to the resin beads while sodium on the resin beads is released simultaneously into the water. When the resin becomes saturated with calcium and magnesium, it must be recharged. The recharging is done by passing a salt (brine) solution through the resin. The sodium replaces the calcium and magnesium which are discharged in the waste water. Hard water treated with an ion exchange water softener has sodium added. According to the Water Quality Association (WQA), the ion exchange softening process adds sodium at the rate of about 8 mg/liter for each grain of hardness removed per gallon of water.

For example, if the water has a hardness of 10 grains per gallon, it will contain about 80 mg/liter of sodium after being softened in an ion exchange water softener if all hardness minerals are removed.

Because of the sodium content of softened water, some individuals may be advised by their physician, not to install water softeners, to soften only hot water or to bypass the water softener with a cold water line to provide unsoftened water for drinking and cooking; usually to a separate faucet at the kitchen sink.

Softened water is not recommended for watering plants, lawns, and gardens due to its sodium content.

Although not commonly used, potassium chloride can be used to create the salt brine. In that case potassium rather than sodium is exchanged with calcium and magnesium.

Before selecting a mechanical water softener, test water for hardness and iron content. When selecting a water softener, the regeneration control system, the hardness removal capacity and the iron limitations are three important elements to consider.

There are three common regeneration control systems. These include a time-clock control (you program the clock to regenerate on a fixed schedule); water meter control (regenerates after a fixed amount of water has passed through the softener); and hardness sensor control (sensor detects hardness of the water leaving the unit, and signals softener when regeneration is needed).

Hardness removal capacity, between regenerations, will vary with units. Softeners with small capacities must regenerate more often. Your daily softening need depends on the amount of water used daily in your household and the hardness of your water. To determine your daily hardness removal need, multiply daily household water use (measured in gallons) by the hardness of the water (measured in grains per gallon).

Iron removal limitations will vary with water softener units. If the iron level in your water exceeds the maximum iron removal capacity recommended by the manufacturer of the unit you are considering, iron may foul the softener, eventually causing it to become plugged.

Laboratory procedure

1. To determine the common hardness of water we take:

25 ml water

75 ml distilled water

several crystals of indicators

5 ml ammonia buffer

We titrate this substance with the help of versene till the substance changes its color. In our case the color of magenta changes dark blue one.

2. To determine the carbonate hardness we take:

25 ml water

75 ml distilled water

2 drops of methyl orange

Titrate (adding HCl) till the substance of orange color changes the color into rose.

3. Calculate the coefficient of hardness:

- for common hardness

- for carbonate hardness

Conclusion:

Hard water is water that has high mineral content (in contrast with soft water). Hard water minerals primarily consist of calcium (Ca2+), and magnesium (Mg2+) metal cations, and sometimes other dissolved compounds such as bicarbonates and sulphates. Calcium usually enters the water as either calcium carbonate (CaCO3), in the form of limestone and chalk, or calcium sulphate (CaSO4), in the form of other mineral deposits. The predominant source of magnesium is dolomite (CaMg(CO3)2). Hard water is generally not harmful to one's health.


 

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