E. J. H. BIRCH Bishop Wordsworth's School, Salisbury, England
T h e main things to teach beginners about the hardness of water are the causes, disadvantages, and remedies for the usual types of temporary and permanent hardness. Whereas calcium and magnesium sulfates and chlorides are soluble to a greater or less degree in water, the corresponding carbonates are practically insoluble, unless the water is first saturated with carbon dioxide. Temporary hardness caused by carbonates can be removed by boiling. For the removal of permanent hardness, due to sulfates and chlorides, an exchange reaction must be used with a soap, a carbonate, or a zeolitic ion exchange medium. The effect of the hard water on soap must be shown and also the domestic and commercial effects of hard water, such as boiler scale and impurities in laundering. To complete the picture in the student's mind it must be pointed out that complete purification of the water can be obtained by distillation and, in some cases, by anion, followed by cation exchange media. he great disedvantage from the teacher's point of view is that the results can seldom be demonstrated without titration of the hard water with soap or evaporation to dryness, complications which are time consuming and apt to be distracting. The use of cobalt instead of calcium as the hardening ion in demonstrations has considerable advantages since its presence or absence can be seen a t once by its pink color. The use of cobalt also reminds the class that any ion which has an insoluble stearate hardens water, and that iron, chemically very similar to cobalt, is often found in natural hard waters. Cobalt is especially suitable for demonstration, since it not only has a distinct color, even when fairly dilute, but its carbonate is ~articularlrlv soluble in carbon dioxide s~lution.'.~ The disadvantage in the use of cobalt lies in the fact that the "cobalt chalk" used and the scale precipitated are senerallv basic: although thev effervesce with acids they do not correspond formally with calcium carbonate. In elementary classes it is best to deal with the cbemistry of the calcium compound only, pointing out that the- demonstration with cobalt merelv makes similar reactions visible. With more advanced classes the formation of insoluble and often basic carbonates can be discussed. 1M E L ~ R J., W., "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Longman's, Green & Co., 1947, p. 810. *Sm~aov,A. A,, Mem. soc. wsse mindral, 67, 465 (1938);
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Chemical Abstracts, 35,3568 (9).
The Preparation of "Cobalt Chalk." A suitable precipitate was obtained by the action of a solution of sodium bicarbonate on a solution of cobaltons chloride, filtering, and washing with water. The residue was dried in a desiccator for several days, since it begins to decompose a t the temperature of the steam oven. A rough analysis showed the precipitate to be basic and probably a mixture. (The ratio of Co: COz found was 1:0.35, 1:0.37, whereas CoCOs requires 1:0.74, COCO~CO(OH)~ and the usual basic carbonate C O C O ~ , ~ C O ( O1H )~ :0.19). After drying, the precipitate had a suitable "chalkyJ' appearance and a purplish pink color and gave a satisfactory effervescencewhen treated with acid. The Hardening of Water. In nature, wateris hardened by the passage of rain water containing dissolved carbon dioxide through layers of chalk or of slightly soluble gypsum. This may be simulated with cobalt by allowing water to percolate through a filter bed containing sand, the precipitated carbonate, and, for permanent hardness, a little cobaltous chloride. A glass tube, 9 inches long and about to a/4 inch O.D. is closed a t its lower end with a bored cork, having a circle of filter paper placed over it. The tube is filled to half its height with clean sand in which a little of the "cobalt chalk" is mixed (about 1 to 2 g. per tube). The tube is then conveniently fitted into a filter flask, though an ordinary conical flask will suffice if the outer cork is slotted. Four of these tubes and flasks are prepared and in two of them a few crystals of cobaltous chloride are
APRIL, 1949
sprinkled on the surface of the saud. Through two of the tubes-one with and one without the chloricle-cold boiled distilled water is allowed to percolate, and through the other two a saturated solution of carbon dioxide percolates. The solutionis kept saturated by passing in acid-free carbon dioxide, conveniently made by heating sodium bicarbonate, or from a Kipp generator after careful washing of the gas. The arraugement is shown in Figure 1. With a fresh sample of precipitated carbonate it is as well to make sure that there is no soluble cobalt in the filter bed by passing cold, boiled distilled water through it until it is colorless, before the class meets. The class notices that only in one case is the percolate colorless, showing the insolubility of a carbonate in pure water. Samples of the percolates from all the tubes can then be issued to individuals or small groups for investigation of the effects of softening processes. A very much stronger solution of the carbonate in carbon dioxide solution can be made by covering the precipitate with water in a flask, saturating with carbon dioxide, corking tightly, and allowing to stand with occasional shaking. This is useful as a stock solution, in case the demonstrated percolate is insufficient for class experiments. Softening by Boiling. The class should individually boil all the filtrates and notice: (1) the loss of carbon dioxide by the solutions saturated with it; (2) the pink precipitate (basic carbonate) from these same solutions; and (3) the fact that two per~olat~es, onecolorlessandone pink, remain unchanged. Subsequent filtration shows that two of the original percolates are now colorless and two pink, indicating the presence of a hardening ion. Softening by Adding Soda. The complete precipitation of the cobaltous chloride solution by a solution of sodium carbonate can also be carried out as a class experiment, and the softening of the water is shown by filtering the solution and noticing that all four original percolates have become colorless. This stage-by-stage removal of the unwanted ion, and the process of hardening, may be summarized for the students in a diagram such as that given in Figure 2. If an excess of soda has been added and the filtrate is really colorless, the class should taste a drop of it or evaporate a little on a metal spatula so that they can realize the presence of sodium salts in the softened water. Action of Soap on the Hard Water. The use of water hardened with the "cohalt chalk" has no advantages over the use of ordinary hard water when demonstrating the formation of an insoluble scum on the addition of a solution of soap. The experiment should nevertheless be carried out with some of the water so hardened, so that the students may see that the primary definition of hardness in water (the property which causes precipitation of stearates, etc.) is not confined to the more usual salts of calcium and magnesium but belongs to. almost any metal except the alkali metals. If there is plenty df time available it should also be useful for the class to titrate their samples of cobalt hard water
WATER
SOLUTION
FILTER AND ADD SODIUM CARBONATE
J
J.
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L
PERMANENT HARDNESS
am
COLORED SOLUTION
with a standard soap solution. Carrying out this test on the demonstration material and later upon naturally hard water will convince them that what they have seen in the demonstration material is also taking place, though invisibly, in the natural water. Zeolilic Softening. It is in the teaching of the "ionexchange" softening of water that the use of cobalt hard water has the most advantages, since in using colorless cations it is almost impossible to demonstrate what is really taking place, and any demonstration that is attempted must depend upon some form of "secondhand" analysis, whether with soap solution or by a precipitation reaction. A suitable apparatus for the demonstration of ionexchange softening of water is illustrated in Figure 3. Such an apparatus is simple to use, and in the successive opera%ons of softening, regeneration, and flushing, the bed is lifted from time to time. The operations
JOURNAL OF CHEMICAL EDUCATION
may be carried on by a member of the class, continuously during the lesson, or it can be used as a conversazione experiment. By the choice of a suitable size of apparatus the whole may be supported with one retort stand and a number of clamps. The central tube is closed a t each end by two-hole mbber stoppers, and the bed of exchange material such as Permntit is supported by a layer of glass beads between pieces of filter paper. In normal operations, stopcocks 1 and 2 are open and the pink-colored hard solution from the funnel runs out colorless from the bottom, while the pink color is transferred to the previously colorless zeolite. This effect is apparent extremely rapidly, and while a longer time is desirable for a complete lesson on the water softener a satisfactory demonstration and lesson can be given in a forty-minute period. When the zeolite is all pink, or when it is desired to
demonstrate the regeneration of the material, stopcocks 1 and 2 are closed and 3 and 4 are opened. If the tubes are always kept full of liquid this will be a comparatively rapid process, but if not there will be a slow start until the tube to stopcock 4 can as a siphon. When the regenerating liquid is a fairly strong brine, the fading of the pink color in the zeolite and its appearance in the liquid from 4 is very noticeable to a class, and fixes the regeneration process in their minds. The flushing is carried out by percolating soft water through 1 and 2 until the liquid gives no reaction with silver nitrate solution. 'I Two-Color" Softeninr. An amusing experiment that can be carried out with the zeolite demonstration apparatus is to percolate a purple-colored solution of cobalt chloride and copper sulfate. The effect is essentially chromatographic and a blue layer in the zeolite is found to be followed by a pink layer, as the copper ion is adsorbed more strongly than the cobalt. This may be pointed out as an exception to the old maxim that one cannot separate a solution of a mixture mechanically, and can form an introduction, a t a later stage, to the subject of chromatography. A one-stage percolation of a mixture of cobalt and nickel salts through Permutit does not give an easily recognizable separation, and although separation can be effected by use of other media,3 e. g., active aluminum oxide, it is not wise to use nickel and cobalt as the two ions in this experiment. The "Follow-up." It is obviously necessary not to leave the subject of water hardness to be demonstrated merely by "cobalt-chalk." The properties of naturally hard water must be examined by the class and they must make the usual experiments of soap titration and softening. They will, however, bring a more intelligent understanding to bear if they have actually seen another unwanted ion going into and coming out of solution in an analogous manner. a
FILLINGER, H. H., J. CHEM.EDUC., 24,444 (1947).
