Temporary water hardness

Lime is the largest toimage chemical used in the treatment of potable and industrial water supplies. In conjunction with alum or iron salts it is used to congulate suspended solids and remove turbidity. It is also used in water softening to remove temporary (bicarbonate) hardness. Typical reactions are ... 

What is meant by water hardness Explain the difference between temporary and permanent hardness. 

The scale formed under moderate temperatures is usually due to temporary (bicarbonate) hardness being converted into calcium carbonate, which occurs on heating or increase in alkalinity sufficient to result in calcium carbonate saturation. The solubility of calcium carbonate also affects corrosion since the alkalinity of dissolved carbon dioxide in the water is greatly reduced as the saturation equilibrium is approached. Ideally, at equilibrium the various forms of carbon dioxide (free C02, bicarbonate and carbonate) are so balanced that they cause neither scale nor corrosion. 

Water hardness is the total calcium and magnesium ion concentration in a water sample and is expressed as the concentration of calcium carbonate. Temporary hardness is that part of the total hardness that disappears on boiling. Whilst not being accepted as a standard method, the use of ion-selective electrodes allows a rapid measurement of water hardness and can be used to determine changes in hardness. The direct potentiometric method is not recommended for the ion-selective electrode but an indirect potentiometric method involving ethylenediaminetetraacetic acid titration is recommended. The ion-selective electrode that is used is a liquid ion-exchange electrode that responds to the divalent ions magnesium and calcium. 

The hydrogencarbonate ion, produced in nature by this reaction, is one of the main causes of temporary hardness in water. Carbon dioxide is fairly soluble in water, 1 cm dissolving 1.7 cm of the gas at stp. The variation of solubility with pressure does not obey Henry s law, since the reaction... 

Magnesium and calcium hydrogencarbonates are known in solution and are responsible for temporary hardness in water. 

Such water, and also that containing salts of multipositive metals, (usually sulphates), is said to be hard since it does not readily produce a lather with soap. Experiments with alkali metal salts can be performed to verify that the hardness is due to the presence of the multipositive metal ions and not to any of the anions present. The hardness due to calcium and magnesium hydrogencarbonates is said to be temporary since it can be removed by boiling ... 

The base exchange process removes both the temporary and permanent hardness salts from the water by allowing the water to flow through resin beads containing sodium zeolite, Na2Z. 

The dealkalization process removes the temporary hardness in water. This uses an acid resin bed for regeneration—in this case sulfuric acid (H2SO4). 

Many analyses quote total hardness. Some give temporary hardness (or carbonate hardness) and permanent hardness (or non-carbonate hardness), usually in consistent units so that the values can be added together to give the total hardness. The total hardness is actually the quantity of calcium (Ca) - - magnesium (Mg) in the water. If the total is not given directly, the values given for these two constituents must be added, after conversion to mg/1 as CaC03 if necessary. 

Temporary or carbonate hardness and alkalinity frequently, but not always, mean the same thing (see below). Bicarbonate or hydrogen carbonate is a more scientific term since alkalinity is actually the concentration of the bicarbonate (HCO3") ion in the water. 

Another reaction of bicarbonate is that in boiling water it combines with any hardness present to produce scale, while releasing CO2 into the steam. This hardness is called the temporary hardness. Its concentration therefore depends on the lessor amounts of either hardness or bicarbonate. Most waters contain more hardness than bicarbonate, so that the temporary hardness is usually equal to the bicarbonate content. The temporary hardness quoted in analyses is often the only information available on the bicarbonate content of the water. 

Most natural waters contain more hardness than bicarbonate. Only a few sources in the UK, usually from wells in sandstone strata, contain more alkalinity than hardness. In most cases the temporary hardness greatly exceeds the permanent hardness. This is especially tme of the hard alkaline waters, which come from chalk and limestone measures. 

This three-stage process is used for waters of high alkalinity and hardness. It actually removes most of the temporary hardness and so reduces the TDS of the water. However, in the process it increases the proportion of silica in the remainder. Any residual temporary hardness and the permanent hardness are softened in a conventional softener. 

The effect is best illustrated by a numerical example (Table 31.4). Let us take the case of hard and alkaline deep well water such as that found to the north of London, whose main characteristics are shown in the first column of Table 31.4. The second column shows its quality after de-alkalization has removed nine-tenths of the temporary hardness and converted it into CO2 gas. This is removed from the water by stripping it with air in a packed degassing column, and the product then softened in the third stage to yield the product shown in the third column. 

Carbon dioxide, from the decomposition in the boiler of temporary hardness salts present in some waters, causes corrosion of steel steam pipework and cast iron valves and traps. Corrosion inhibitors may be used, but the choice of inhibitor must take into account the other materials in the system. Neutralizing amines such as morpholine or cyclohexylamine are commonly used. 

DETERMINATION OF THE TOTAL HARDNESS (PERMANENT AND TEMPORARY) OF WATER... 

Calculate the temporary hardness of the water by subtracting the permanent hardness from the total hardness. 

Temporary hardness or carbonate hardness is removed by heating water to precipitate the carbonate salt (e.g., bicarbonate hardness salts) and is equal to or less than the total alkalinity. 

The principal temporary hardness salt in raw water is calcium bicarbonate, formed by dissolution of limestone (calcium carbonate) by... 

Primarily the sum of Ca and Mg salts in water, although it may include other metal salts such as Al, Mn, Sr, and Zn. Temporary hardness (carbonate hardness) is that portion of the total hardness that can combine with C03 or HC03. The balance is non-carbonate or permanent hardness and is caused by Ca or Mg nitrates/sul-fates/chlorides, etc. Permanent hardness is equivalent to the excess of hardness over alkalinity. 

Temporary hard water contains divalent cations, such as Ca2+, Mg2+, and Fe2+ and the bicarbonate anion, HC03 . Heating produces, H20(1), C02 (g), and a carbonate precipitate. Permanent hard water does not form a precipitate upon heating the ions it contains, such as S042 are thermally stable. 

Temporary hard water is softened by the addition of an alkaline (basic) material. All the substances listed form alkaline solutions except NH4C1. NH4+ hydrolyzes to form an acidic solution, and thus, cannot be used to soften temporary hard water. 

---