Notes on analytical chemistry : for students in medicine / by Albert J. Bernays.
- Albert Bernays
- Date:
- 1886
Licence: Public Domain Mark
Credit: Notes on analytical chemistry : for students in medicine / by Albert J. Bernays. Source: Wellcome Collection.
99/160
![Tests for Hardness in Water. Permanent hardness is a consequence of the presence, in the water, of soluble salts of calcium and magnesium, \vhich are not decomposed, or preci])itated, on boiling, and which cannot therefore be i-emoved by this method. These salts are mainly the sulphates, chloriiles, and nitrates. Permanent hardness may be removed by precipitation with an alkaline carbonate, which throws down tlie insoluble carbonates, leaving, in solution, a salt of the alkaline metal, which does not render water hard. Thus:— CaSO. + Na^COs = Na^SO, + CaCOg. Hardnes.'? of both kinds is removed by distillation. Soap, which may be looked upon as consisting mainly of sodium stearate, has a softening action upon water similar to that of the alkaline carlxniates, save that, in this case insoluble calcium and magnesium stearates are precipitated, while the sodium goes into solution combined with the acid previously in combination with the calcium and magnesium. This reaction explains the phenomenon of the hardness of water, since the water will not lather with soap, until the whole of the calcium and magnesium are thrown out of solution as insoluble stearates. Upon this fact the method for the determination of the amount of hardness in a water is based. To determine the hardness of a water, a standard soap solution is required. For this dissolve a small quantity of a good curd soap in methylated spirit, and dilute somewhat, witli a mixture of two parts of methylated spirit, to one of water. Now make a solution of calcium chloride of known strength, so that the calcium, calctilated as CaCO,, shall be equal to (say) 7 grains per gallon. Pour the filtered st)lution of soap into a burette, and 100 c.c. of the standard calcium chloride solution in a bottle of about 250 c.c. capacity ; add the soap solution from the bm'ette in small quantities at a time, stopper and shake vigorously after each addition, until a permanent lather is formed. The amount of soap solution required to effect this is then read off from the burette, and the soap solution is diluted with the mixture of two parts methylated spirit, and one of water, until 100 c.c. of the standard calcium chloride solution require exactly 15 2 c.c. of the soap solution. Now, since the standard calcium chloride solution is equivalent to 7 grains per gallon of CaCO.j, and 100 c.c. of distilled water require 1 - 2 c.c. of soap solution to form a permanent lather, the remaining 14 c.c. of tho soap solution are equivalent to 7 grains per gallon of CaCOa; that is, 1 c.c. of the soap solution, in 100 c.c. of water, is equivalent to half a grain per gallon, of hardness calculated as CaC03, With this standard soap solution, the hardness of any water may be determined. Waters, which require more than 25 c.c. of the soap solution per 100 c.c. of the water, should be diluted, and a correction made accordingly.](https://iiif.wellcomecollection.org/image/b21499056_0099.jp2/full/800%2C/0/default.jpg)
