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thetical example, where M stands for a basic radical, such as one of the metals20 MC1+19 KHO=19(MHO), MC1+19KCI.
Basic salt. With regard to solvents ;-the action of these agents is exceedingly obscure; they appear to fulfil an office intermediate between chemical union and mechanical mixture. The student had better regard them simply as agents for presenting bodies in a liquid form, and as rarely participating in the chemical actions which take place in their very midst: of course they do occasionally exert very powerful chemical actions; but these instances can be readily discriminated. The more stable the equilibrium of a body, the less chemical influence is it likely to exert; and thus water is far less likely to interfere than the acids, and is for this reason always employed wherever it is practicable, as the solvent to which the least objection can be raised.
REAGENTS are those substances which by admixture we bring to act upon the bodies we desire to analyse, in such a manner as to produce certain phenomena which shall prove indubitably the presence of the substance sought for. If in such circumstances the expected effect is not produced, we must infer the absence of the object of our search.
It is obviously, therefore, of the greatest importance that the reagents which are employed in chemical analysis should, if not absolutely pure, at least be free from substances which would interfere with the indications which they are employed to give. The amount and the nature of the impurities which they contain should be accurately known.
Most of the reagents in common use are met with in commerce of sufficient purity to allow of their application in all but the more delicate operations of analysis. For certain special cases, however, as in legal investigations for the purpose of ascertaining the presence or absence of poison, too much care cannot be bestowed upon the preparation of absolutely pure reagents; and in such instances it is generally necessary for the chemist to ensure the purity of
the materials he employs by preparing, or at least scrupulously purifying them himself.
The ordinary methods of purification are briefly these: Sublimation, Crystallization, Precipitation, and Distillation.
SUBLIMATION can only be resorted to when it is desired to free a body solid at ordinary temperatures, but volatile at a higher degree of heat, from nonvolatile impurities; iodine is usually purified in this way: many mercury and ammonium salts might also be thus freed from accompanying foreign matters which were not volatile. In the sublimation of many substances it is simply necessary to place the substance in a dish over which is fixed a tall beaker or bell-jar, which may be luted or otherwise attached to the dish below; if the dish containing the substance is now heated, the volatile substance (iodine, oxalic acid, &c., as the case may be) rises in vapour,--speedily however condensing on the cold surface of the receiver above.
CRYSTALLIZATION is perhaps the most commonly adopted of all methods of purification. In general, substances dissolve more abundantly in hot than in cold solvents. If then at some given temperature a menstruum, as water, alcohol or æther, be saturated with any solid body (i.e. supplied with it until no more is dissolved), the solution perfectly transparent at that degree of heat, will, as soon as the temperature diminishes, deposit some of the dissolved matter in the solid form. In the same manner let a given bulk of a saturated solution of almost any substance be taken, and without increasing its temperature, let its volume be diminished to one half (as by slow spontaneous evaporation), then a considerable portion of the solid matter before held in solution will be found to have separated in the form of crystals. Upon these two facts is based the method of purification by crystallization.
Salts differ much as to their solubility in the same menstruum; and by taking advantage of the facts which experimenters have accumulated upon this point, we can separate different compounds with considerable accuracy by evaporating the solution containing the mixture, we will suppose of two salts, to that degree of concentration at which one of them will almost entirely separate, while the other will remain almost as wholly in solution. By the repetition of this process upon the already partially purified product of the first crystallization, greater purity is attained. But with many articles of commerce one carefully conducted crystallization is often found sufficient. The technical name for the liquid remaining after the separation of the crystals is "mother-liquor," while the successive products of the crystallizing process are spoken of as “crops” of crystals.
PRECIPITATION is frequently employed when the substance constituting the impurity is known to form an insoluble compound on the addition of some reagent the introduction of which does not interfere with the efficiency of the reagent we seek to purify. Let it be supposed that chloride of ammonium (NH, Cl) is the substance with which we wish to deal. Now the impurity which this salt commonly contains is chloride of iron (FeCl); to separate this the substance is dissolved in water, and a few drops of sulphide of ammonium, (NH), S, are added to the solution ; the black sulphide of iron (Fe, S) is formed and precipitated, its removal being effected by passing the liquid through a filter, while the reagent added, so far from contaminating the chloride of ammonium by the decomposition, itself helps to form an additional quantity of that salt, the ammonium originally combined with the sulphur uniting with the chlorine of the chloride of iron (NH), 8+2FeCl
=Fe, S+2NH, Cl: if any excess of the alkaline sulphide has been added, by boiling the solution it may be volatilized and thus removed. However, it is not often possible thus to remove the excess of the substances introduced in order to precipitate impurities; but care can then be taken that the foreign substances unavoidably brought in shall be such as do not interfere with the reactions which the purified substance is intended to exhibit. Sometimes, on the other hand, the pure substance is precipitated by an appropriate reagent, leaving the impurities in solution, being therefore just the reverse of the preceding process.
DISTILLATION is always used to separate a liquid which vaporizes at a certain temperature from other liquid or solid matters which are not volatile at that temperature. Most liquids when heated to a certain point which is peculiar to each substance, boil and are converted into vapour, and in this condition of vapour they remain as long as that degree of heat is maintained, but no sooner does it diminish, than they again return to the liquid form of matter. The apparatus employed is generally the following:-A is the retort
in which the liquid is placed, the temperature being indicated by the thermometer B, and regulated accordingly; the only way of escape for the vapour which rises from the boiling liquid is through the neck C into the apparatus DD, which is a glass tube passed through an outer tube of zinc or copper, dd, the space between these two tubes being occupied by cold water; the vapour is thereby cooled completely, and consequently reduced to the liquid state, in which condition it flows through the adapter E, and issues from the
point F into the receiver G. By this process of distillation several liquids of different degrees of volatility may be separated, the “distillates” being preserved apart, while non-volatile substances, whether solids or liquids, in sus. pension or solution, may be removed, since they remain in the retort when the volatile matters have passed over; this separation is of course more easily effected than that of volatile liquids from liquids of different volatility.
We here give a list of the reagents employed in analysis, separating, however, those commonly used from those employed for special purposes.
LIST OF REAGENTS EMPLOYED IN CHEMICAL ANALYSIS. The reagents to which an asterisk is affixed are employed in the solid state :
the remainder dissolved in water.
BASIC ELEMENTS. *Zinc (Zn).
SALTS OF POTASSIUM. *Nitrate of potassium (KNO3).
Chromate of potassium (KCrO2). *Cyanide of potassium (KCN).
Acetate of potassium (KĀ). *Hydrate of potassium (KHO).
Ferrocyanide of potassium (K, Cfy). *Carbonates of potassium and sodium
(K, CO, +Na, CO3).
SALTS OF SODIUM. *Carbonate of sodium (Na, CO2). *Biborate of sodium or borax
(Na, Bo, 0,). Phosphate of sodium (Na, HPO.).
SALTS OF AMMONIUM. Chloride of ammonium (NH, CI). Hydrate of ammonium (NH, HO). Sulphydrate of ammonium
(NH, HS). Carbonate of ammonium
| (NHJCO). Oxalate of ammonium ([NH.], ).
SALTS OF BARIUM. Chloride of barium (BaCl). Hydrate of barium (BaHO).
BASIC ELEMENTS. *Iron (Fe). *Copper (Cu).
SALTS OF POTASSIUM. Iodide of potassium (KI). Sulphocyanide of potassium
(KCNS). Silicate of potassium (KSIO, ?). Sulphydrate of potassium (KHS). Sulphate of potassium (K, SO.). Acid metantimoniate of potassium
(K, H, Sb, 0-). Ferricyanide of potassium (K,Cfdy).
SALTS OF SODIUM. Acetate of sodium (NaĀ). Hydrate of sodium (NaHO). Sulphydrate of sodium (NaHS). Sulphite of sodium (Na, SO2). Phosphate of sodium and ammonium
SALTS OF AMMONIUM. Molybdate of ammonium
(NH, MoO,). Acetate of ammonium (NH, A). Phosphate of ammonium ([NH] HPO4)
SALTS OF BARIUM. *Fluoride of barium (BaF).
SALTS OF CALCIUM. Chloride of calcium (CaCl). *Oxide of calcium (Ca, O). Sulphate of calcium (Ca, SO.).
SALTS OF IRON. * Ferrous sulphate (Fe, SO2). Ferric chloride (Fe, C12).
SALT OF COBALT. Nitrate of cobalt (CONO3).
SALT OF SILVER. Nitrate of silver (AgNO3).
SALT OF LEAD. Acetate of lead (Pb Ā).
SALT OF PLATINUM.
SALT OF GOLD.
SALTS OF HYDROGEN.
ACID ELEMENT. *Carbon (C).
*Oxide of barium (Ba, O).
SALT OF MAGNESIUM. Sulphate of magnesium (Mg, SO.).
SALTS OF COPPER. Cuprous sulphate (Cu, SO2). Cupric sulphate (Cu, SO4).
SALTS OF MERCURY. Mercurous nitrate (Hg, NO3). Mercuric chloride (HgCl). Mercuric oxide (Hg, O).
SALT OF BISMUTH Oxyhydrate of bismuth (Bi H, 03, Bi, 03).
SALT OF PALLADIUM. Chloride of palladium (PdCl).
SALTS OF ETHYLE.
SALTS OF HYDROGEN.
ORGANIC BODIES. *Sugar (C12H,2011). *Gelatine ?
Blue litmus paper.
ORGANIC BODY. *Starch (C12H2,01).