Temporary hardness of water

It is the cause of temporary hardness of water, because the calcium ions react with soap to give scum. Calcium hydrogencarbonate is unstable when heated and decomposes to give solid calcium carbonate. This explains why temporary hardness is removed by boiling and the formation of scale in kettles and boilers. 

On heating, this process is reversed. M ne-sium hydrogencarbonate is one of the compounds responsible for temporary hardness of water. 

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... 

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

Temporary hardness. Probably because of the ease with which, these impurities may be removed, the acid carbonates of calcium and magnesium (and in some cases ferrous iron) are said to constitute temporary hardness in water. This type of hardness may be removed by boiling the water, whereupon the soluble acid carbonates are converted to the insoluble normal carbonates, which may be removed by filtration ... 

Temporary and permanent hardness of water signifies whether the water is in the form of snow or ice. 

They also deposit insoluble calcium or magnesium carbonate onto heated surfaces to cause scale buildup, so that heating for removal is not of practical use. Interestingly, addition of calcium in the form of slaked lime (Ca(OH)2) can efficiently remove much of the temporary hardness of natural water supplies (Eqs. 5.12-5.14). 

The presence of salts of calcium or magnesium in solution can be most undesirable in many finishing processes. These salts are responsible for hardness of water and they lead to the formation of insoluble precipitates with soaps which may be deposited on the goods, causing discoloration. Calcium and magnesium compounds are commonly present as sulphates and chlorides. Bicarbonates produce what is known as temporary hardness sulphates and chlorides are the cause of permanent hardness. 

In stating the hardness of water it is usual to express it in terms of calcium carbonate, and this applies to both temporary and permanent hard-... 

USE As acid-base indicator in 0.2% soln in alcohol. pH 4.4 red 6.4 blue. Satisfactory for titrating mineral acids, strong bases, many alkaloids determining alkalinity and temporary hardness in water analysis. Biological stain dye for wool, silk- Not adapted for carbonates, weak inorganic and organic acids, weak bases. Lacmoid is more sensitive than litmus, particularly in form of test paper. 

MgCOj + H2O + CO2 Mg(HC03)2 It is a cause of temporary hardness in water. 

Hardness of water is usually expressed by assuming that all the hardness is due to dissolved calcium carbonate, which is present as ions. It can be estimated by titration with a standard soap solution or with edta. See also permanent hardness detergents temporary hardness water softening. 

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. 

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... 

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