Waters for domestic uses may be divided into two general classes; hard waters and soft waters. Hard waters can be either permanently hard, temporarily hard, or both permanently and temporarily hard. By hardness of water is meant its soap destroying or neutralizing power, which is due to the presence of carbonates or sulphates of lime or magnesia. A large degree of permanent hardness indicates a bad water. Permanently hard waters contain sulphates of lime or magnesia in solution; temporarily hard waters contain carbonates of lime or magnesia in solution, and both permanently and temporarily hard waters contain sulphates and carbonates of lime or magnesia in solution.

Hardness of water is measured in degrees (Clark-Wanklyn), and each degree of hardness corresponds to one grain of carbonate of lime or magnesia to one English gallon of water. Hardness expressed in parts per 100,000 can be converted to Clark's scale by multiplying the hardness by .7. The reason for this is Clark's scale gives the results in grains per English gallon, and there are 70,000 grains in an English or imperial gallon.

Example

How many degrees hardness (Clark) in water that is 20 parts hard per 100,000? .

Solution-.7 X 20 = 14 degrees Clark. - Ans.

Conversely, hardness expressed in degrees (Clark) can be changed to parts per 100,000 by dividing the degrees of hardness by .7.

Example

How many parts of hardness per 100,000 in water that contains 14 degrees of hardness?

Solution

14/.7 = 20 parts per 100,000.-Ans.

Hardness expressed in parts per 100,000 can be changed to grains per United States gallon by multiplying the hardness by .584. The reason for this is that a United States gallon contains approximately 58,400 grains. Conversely, hardness expressed in grains per United States gallon can be changed to parts per 100,000 by dividing the grains of hardness by . 584, or, where great refinement of calculation is not required, by the constant .6.

Example

How many grains of hardness per United States gallon in water that contains 20 parts per 100,000?

Solution

20 X .584 = 11.68 grains per gallon. - Ans.

Example

How many parts of hardness per 100,000 in water that contains 11.68 grains per United States gallon?

Solution

11.68/ .584 = 20 parts per 100,000. = Ans.

The manner of determining the degree of hardness in water is as follows: Seventy cubic centimeters* of water are placed in a clean glass bottle large enough to hold two or three times that quantity. A clear solution of soap of standard strength is then added, a little at a time, from a graduated tube, and the mixture briskly shaken. On some waters a slight lather will form at first, which will quickly disappear, or if the water is very hard a curd will form. More soap should then be added, shaking the bottle after each addition until the lather formed is sufficiently permanent to stand for five minutes. The number of cubic centimeters of soap solution added, less one, indicates the hardness of the water in degrees. The one cubic centimeter is deducted because even distilled water requires a small quantity of soap to make it lather.

♦Table for converting American and metric measures in appendix.

Table XIV - Hardness Of Waters

Standard Soap Solution is of such strength that one cubic centimeter contains sufficient soap to exactly neutralize one millogram of dissolved carbonate of lime. It is made by mixing half an ounce of finely shredded castile, or mottled soap, with two pints of methylated spirits and one pint of distilled water. The mixture should be kept at ordinary temperature, and allowed to stand for a few hours, occasionally shaking, then passed through a filter of blotting paper. Before using the solution it should be tested by means of water of known hardness. In case the solution is too strong it should be diluted with spirits and water until the strength is just right.

Soft water contains no mineral impurities. Rain water is the purest kind of natural soft water.

The character of water, its corresponding degree of hardness and chemical substance causing the hardness, rated as equivalent to grains carbonate of lime, may be found in Table XIV.