To Determine the Permanent Hardness boil 70 c.c. of the water in a flask for half-an-hour; allow the precipitated carbonates of calcium and magnesium to settle. Some of the latter will be re-dissolved. Carefully decant, and make up the liquid to the original 70 c.c. with distilled water. Filter through fine filter paper and estimate hardness as above.
The amount of soap wasted in consequence of the hardness of water is very great. Thus, in the case of water of one degree of hardness, as every gallon contains one grain of chalk, 7,000 gallons would contain 7,000 grains—that is, a pound. But every grain of chalk wastes 8 or 9 grains of soap; therefore, a pound of chalk, contained in 7,000 gallons, would waste about 8½ pounds of soap. But nearly all waters are harder than this, and they not uncommonly possess a hardness of 20° or more. If the hardness be 20°, the waste would be 170 pounds of soap. This quantity would be easily used annually in a family of seven or eight persons, if we include the washing of clothes. The amount of money thus wasted can be easily estimated.
Not only does soft water require less soap, but it is much more suitable for making tea and soup, and for boiling meat and vegetables—both time and fuel being saved. The reason why better tea is made when a little carbonate of soda is added to the water is that the chalk is by this means precipitated.
Carbonate of calcium is precipitated from water by boiling it; carbonic acid being driven off, the neutral salt falls to the bottom of the vessel. This is the origin of the “fur” inside kettles, which lessens their conductivity to heat, and renders necessary a greater consumption of fuel.
The chalk may also be removed by adding to the water, while still in the reservoir, some milk of lime—that is, quicklime made into a milky solution with water. This is done on a large scale at various waterworks. The reaction may be expressed thus:—
Calcium bicarbonate + calcium oxide = calcium carbonate + calcium carbonate.
The calcium carbonate, as it is precipitated, carries down with it organic and other matters, thus clearing and purifying the water.
The hardness due to calcium sulphate is not removable by boiling. It is, therefore, called permanent hardness, to distinguish it from the temporary hardness of chalk waters, which is removable by boiling. It may, however, be partially removed by the addition of washing soda to the water, as well as the nitrate and chloride of calcium which are also present. The magnesium salts are not removable by boiling or soda. This is shown by the fact that the “fur” inside kettles does not usually contain magnesium salts.
The amount of hardness varies greatly in different waters. In the deep wells in magnesium limestone, it varies from 14°-57°; in the deep wells from chalk beds, it varies from 13° to 27° and may be higher. In the water from Bala Lake, Wales, the temporary hardness is 0°·1, the permanent hardness 0°·3; in the Loch Katrine water there is no temporary hardness, 0°·9 permanent hardness; in the water from the new red sandstone (Nottingham), the temporary hardness is 9°·6, permanent 10°·2; in a chalk spring at Ryde, temporary hardness 16°·7, permanent 3°·9 (Wanklyn). The total hardness in the metropolitan water supplies from the rivers Thames and Lea, varies from 13°·2 (Southwark Company) to 14°·6 (New River Company); in the Kent Deep Wells 20°·1; in deep wells from the chalk at Brighton it varies from 12° to 13°. In all these, the hardness is chiefly temporary.
The amount of permanent hardness is always great in water from clays, as the London, Oxford, Kimmeridge, and Lower Lias clays; or in places where there are large deposits of calcium sulphate, as at Montmartre, near Paris (hence the name Plaster of Paris, given to desiccated calcium sulphate). Water from fissures in the clay often contains, also, a large amount of organic matter.