PART I.

INTRODUCTORY.

PRELIMINARY REMARKS.

DEFINITION, GENERAL PRINCIPLES, OBJECTS, UTILITY, AND IMPORTANCE OF QUALITATIVE CHEMICAL ANALYSIS, THE CONDITIONS AND REQUIREMENTS FOR A SUCCESSFUL STUDY OF THAT SCIENCE.

CHEMISTRY is the science which treats of the various materials entering into the structure of the earth, their composition and decomposition, their mutual relations and deportment in general. A special branch of this science is designated Analytical Chemistry, inasmuch as it pursues a distinct and definite object-viz., the analysis of compound bodies, and the examination of their component elements. Analytical chemistry, again, is subdivided into two branches-viz, qualitative analysis, which simply studies the nature and properties of the component parts of bodies; and quantitative analysis, which ascertains the quantity of every individual element present. The office of qualitative analysis, therefore, is to exhibit the constituent parts of a substance of unknown composition in forms of known composition, from which the constitution of the body examined, and the presence of its several component elements may be positively inferred. The efficiency of its method depends upon two conditions-viz., it must attain the object in view with unerring certainty, and in the most expeditious manner. The object of quantitative analysis, on the other hand, is to exhibit the elements revealed by the qualitative investigation in forms which will permit the most accurate estimate of their weight, or to effect by other means the determination of their quantity.

These different ends are, of course, attained respectively by very different ways and means. The study of qualitative analysis must, therefore, be pursued separately from that of quantitative analysis, and must naturally precede it.

Having thus generally defined the meaning and scope of qualitative analysis, we have now still to consider, in the first place, the preliminary information required to qualify students for a successful cultivation of this branch of science, the rank which it holds in the domain of chemistry, the bodies that fall within the sphere of its operations, and its utility and importance; and, in the second place, the principal parts into which its study is divided.

It is, above all, absolutely indispensable for a successful pursuit of qualitative investigations, that the student should possess some knowledge of the chemical elements, and of their most important combinations, as well as of the principles of chemistry in general; and that he should combine with this knowledge some readiness in the apprehension of chemical processes. The practical part of this science demands, moreover, strict order, great neatness, and a certain skill in manipulation. If the student joins to these qualifications the habit of invariably ascribing the failures with which he may happen to meet, to some error or defect in his operations, or, in other words, to the absence of some condition or other indispensable to the success of the experiment-and a firm reliance on the immutability of the laws of nature cannot fail to create this habit -he possesses every requisite to render his study of analytical chemistry successful.

Now, although chemical analysis is based on general chemistry, and cannot be cultivated without some previous knowledge of the latter, yet, on the other hand, we have to look upon it as one of the main pillars upon which the entire structure of the science rests; since it is of almost equal importance for all branches of theoretical as well as of practical chemistry; and I need not expatiate here on the advantages which the physician, the pharmaceutist, the mineralogist, the rational farmer, the manufacturer, the artisan, and many others derive from it.

This consideration would surely in itself be sufficient reason to recommend a thorough and diligent study of this branch of science, even if its cultivation lacked those attractions which yet it unquestionably possesses for every one who devotes himself zealously and ardently to it. The human mind is constantly striving for the attainment of truth; it delights in the solution of problems; and where do we meet with a greater variety of them, more or less difficult of solution, than in the province of chemistry? but as a problem to which, after long pondering, we fail to discover the key, wearies and discourages the mind: so, in like manner, do chemical investigations, if the object in view is not attained-if the results do not bear the stamp of truth, of unerring certainty. A half-knowledge is therefore, as indeed in every department of science, but more especially here, to be considered worse than no knowledge at all; and a mere superficial cultivation of chemical analysis is consequently to be particularly guarded against.

A qualitative investigation may be made with a twofold view-viz., either, 1st, to prove that a certain body is or is not contained in a substance, e.g. lead in wine; or, 2nd, to ascertain all the constituents of a chemical compound or mixture. Any substance whatever may of course become the object of a chemical analysis.

In the present work, however, we purpose to confine ourselves to those elements and compounds which are more generally employed in pharmacy, in the arts and manufactures, and in agriculture.

The study of qualitative analysis is most properly divided into four principal parts-viz.,

1. CHEMICAL OPERATIONS.

2. REAGENTS AND THEIR USES.

3. DEPORTMENT OF THE VARIOUS BODIES WITH REAGENTS.

4. SYSTEMATIC COURSE OF QUALITATIVE ANALYSIS.

It will now be readily understood that the pursuit of chemical

analysis requires practical skill and ability, as well as theoretical knowledge; and that, consequently, a mere speculative study of that science can be as little expected to lead to success as purely empirical experiments. To attain the desired end, theory and practice must be combined.

SECTION I.

OPERATIONS.

§ 1.

THE operations of analytical chemistry are essentially the same as those of synthetical chemistry, though modified to a certain extent to adapt them to the different object in view, and to the small quantities operated upon in analytical investigations.

The following are the principal operations in qualitative analysis.

§ 2.

1. SOLUTION.

The term "solution," in its widest sense, denotes the perfect union of a body, no matter whether gaseous, liquid, or solid, with a fluid, resulting in a homogeneous liquid. However, when the substance dissolved is gaseous, the term "absorption" is more properly made use of; and the solution of one fluid in another is more generally called a mixture. The application of the term solution, in its usual and more restricted sense, is confined to the perfect union of a solid body with a fluid.

A solution is the more readily effected the more minutely the body to be dissolved is divided. The fluid by means of which the solution is effected, is called the solvent. We call the solution chemical, where the solvent enters into chemical combination with the substance dissolved; simple, where no definite combination takes place.

In a simple solution the dissolved body exists in the free state, and retains all its original properties, except those dependent on its form and cohesion; it separates unaltered when the solvent is withdrawn. Common salt dissolved in water is a familiar instance of a simple solution. The salt in this case imparts its peculiar taste to the fluid. On evaporating the water, the salt is left behind in its original form. A simple solution is called saturated when the solvent has received as much as it can hold of the dissolved substance. But as fluids dissolve generally larger quantities of a substance, the higher their temperature, the term saturated, as applied to simple solutions, is only relative, and refers invariably to a certain temperature. It may be laid down as a general rule, that elevation of temperature facilitates and accelerates simple solution.

A chemical solution contains the dissolved substance not in the same state nor possessed of the same properties as before; the dissolved body is no longer free, but intimately combined with the solvent, which latter also has lost its original properties; a new substance has thus been produced, and the solution manifests therefore now the properties of this new substance. A chemical solution also may be accelerated by elevation of temperature; and this is indeed usually the case, since heat generally

promotes the action of bodies upon each other. But the quantity of the dissolved body remains always the same in proportion to a given quantity of the solvent, whatever may be the difference of temperaturethe combining proportions of substances being invariable, and altogether independent of the gradations of temperature.

The reason of this is, that in a chemical solution the solvent and the body upon which it acts have invariably opposite properties, which they strive mutually to neutralize. Further solution ceases as soon as this tendency of mutual neutralization is satisfied. The solution is in this case also said to be saturated or, more properly, neutralized, and the point which denotes it to be so is termed the point of saturation or neutralization. The substances which produce chemical solutions are, in most cases, either acids or alkalies. With few exceptions, they have first to be converted to the fluid state by means of a simple solvent. When the opposite properties of acid and base are mutually neutralized, and the new compound is formed, the actual transition to the fluid state will ensue only if the new compound possesses the property of forming a simple solution with the liquid present; e. g. when solution of acetic acid in water is brought into contact with oxide of lead, there ensues, first, a chemical combination of the acid with the oxide, and then a simple solution of the new-formed acetate of lead in the water of the

menstruum.

In pharmacy, solutions are often made in a mortar having a lip, by triturating the body to be dissolved with the solvent added gradually in small quantities at a time; in chemical laboratories solutions are rarely made in this manner, but generally by digesting or heating the substance to be dissolved with the fluid in beaker-glasses, flasks, test-tubes, or dishes. In the preparation of chemical solutions, the best way generally is to mix the body to be dissolved in the first place with water (or with whatever other indifferent fluid may happen to be used), and then gradually add the chemical agent. By this course of proceeding a large excess of the latter is avoided, an over-energetic action guarded against, the process greatly facilitated, and complete solution ensured, which is a matter of some importance, as it will not seldom happen in chemical combinations that the product formed refuses to dissolve if an excess of the chemical solvent is present; in which case the molecules first formed of the new salt, being insoluble in the menstruum present, gather round and enclose the portion still unacted on, weakening thereby or preventing altogether further chemical action upon them. Thus, for instance, Witherite (carbonate of baryta) dissolves readily when, after being reduced to powder, water is poured upon it, and hydrochloric acid gradually added; but it dissolves with difficulty and imperfectly when projected into a concentrated solution of hydrochloric acid in water, for chloride of barium will indeed dissolve in water, but not in hydrochloric acid.

CRYSTALLIZATION and PRECIPITATION are the reverse of solution, since they have for their object the conversion of a fluid or dissolved substance to the solid state. As both generally depend on the same cause, viz., on the absence of a solvent, it is impossible to assign exact limits to either; in many cases they merge into one another. We must, however, consider them separately here, as they differ essentially in their extreme forms, and as the special objects which we purpose to attain by their application are generally very different.