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Special Report on Mineral Waters (1902)

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Part I--General Discussion of Mineral Waters, continued

Chapter V--The Theory of Solutions

by Prof. H. P. Cady

The modern theory of solutions is of especial interest in connection with a discussion of mineral waters, because of the light which it throws upon the condition and properties of the dissolved mineral matter.

Solutions Defined

A solution may be defined as a homogenous mixture of two or more substances which resembles a chemical compound in that the constituents cannot be separated by ordinary mechanical means, but which differs from a chemical compound in that the relative proportions of the constituents may vary between wide limits. Solutions may be divided into two classes: First, solutions which will conduct electricity; second, solutions which will not conduct electricity. Solutions of the first class are of the most interest here because in this class are included solutions of acids, bases and salts in water, and hence it embraces all mineral waters which are simply aqueous solutions of various mixtures of the above-mentioned substances.

Theory of Ionization

There are a good many reasons for believing that in solutions which will conduct electricity the molecules of the dissolved substances are broken up by the action of the solvent into smaller parts, called ions. This belief rests upon a great body of experimental facts which are too numerous and complex to be discussed here. According to this theory, when common salt, sodium chlorid, is dissolved in water, the molecules of sodium chlorid are broken up by the action of the solvent into sodium ions and chlorin ions, each of which is entirely free, uncombined, and independent of the others. These ions are not to be confused with the atoms of the elements, for they differ from them markedly in properties; for instance, metallic sodium attacks water vigorously, the sodium ion does not; chlorin is a gas having a greenish-yellow color, disagreeable odor, etc., chlorin ion has none of these properties. This great difference is due to the fact that the ions are very heavily charged with electricity.

Action of Solvents

Comparatively few solvents have this power of breaking up or dissociating dissolved substances, and of these water is the most powerful. But by no means all substances can be broken up into ions, even when dissolved in water; sugar, glycerol, alcohol, etc., are not dissociated to a perceptible degree, and their solutions do not conduct electricity.

Degree of Dissociation

The degree of dissociation of an ionizable suhstance depends upon the concentration of the solution; the more dilute the latter is, the greater the degree of dissociation. In solutions as dilute as most mineral waters. the substances may be regarded as being completely dissociated, and therefore the properties of such mineral waters are simply the sum of the properties of the ions present.

Combination of Elements in Water Analysis

The question of how to combine the results of an analysis of a mineral water has always been a vexed one. But with the modern view of solutions all these difficulties disappear, for most waters are so dilute that the substances are not combined but are present as ions; or if the solution is more concentrated, the same ions will be present, together with all the possible salts. For instance, if a dilute solution of sodium chlorid which will contain simply sodium ions and chlorin ions but not an appreciable quantity of the compound sodium chlorid, be mixed with a dilute solution of magnesium sulfate, which will contain simply magnesium ions and sulfate ions, there will be no reaction, and the solution will contain only these four ions and none of their possible compounds. Such a solution is absolutely identical in physical, chemical and therapeutic properties with one which is made by dissolving in the same quantity of water an equivalent quantity of sodium sulfate and magnesium chlorid, and it would be impossible for anyone to detect the least difference between the two solutions. If the solution is more concentrated so that the dissociation is not complete, there will be present the four ions--sodium, magnesium, chlorin, and sulfate, together with the four possible salts--sodium chlorid, sodium sulfate, magnesium chlorid, magnesium sulfate. Further, two of such solutions will be absolutely identical, no matter which pair of salts be dissolved. Under these circumstances, it would be obviously incorrect to report the substances as present either as sodium chlorid and magnesium sulfate, or as sodium sulfate and magnesium chlorid. The only rational way is to report them as ions, aud this is the method that has been given prominence in this work, although the other methods have been followed also for comparison.

Therapeutic Properties Rest Largely in the Ions

The therapeutic propeties of a mineral water are the sum of the properties of the ions, together with those of the undissociated portions of the salts present. Most mineral waters are so dilute that the undissociated portion of the salt may be neglected and the attention directed merely to the ions. The therapeutic properties of the various ions will be discussed in another portion of this work.

Occurrence of Ions in Mineral Waters

It is of interest to notice that the following ions which are required by animals for their growth and nutrition are all present in mineral waters, namely: Na, Cl, CO3, Ca, K, Mg, I, Fe, PO4, and SO4. Mineral salts are also carried by water to the rootlets of plants, and the following ions which are necessary to their growth are present in waters: K, Mg, PO4, CO3, with Ca, in most cases, and Mn often; NO3 and SO4 are useful as nutrients.

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Kansas Geological Survey, Geology
Placed on web April 7, 2017; originally published 1902.
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