372

COLOURED TESTS-VARIETIES OF SALTS.

the action of an acid, whilst the colour of litmus reddened by an acid becomes blue if it be mixed with an alkali. The yellow colour of turmeric is changed to brown when mixed with an alkali, but the yellow is restored if the alkali be caused to combine with an acid. A salt which affects neither the blue of litmus nor the yellow of turmeric is said to have a neutral reaction. But chemists are in the habit of regarding many salts as neutral in composition which are not neutral in their action upon coloured tests. The basic properties of different metallic oxides vary considerably in intensity. Equal quantities of the same acid, according as it is neutralized by equivalent quantities of a weak base or of a strong one, will differ considerably in their action upon coloured tests: for example, 62 parts of nitric acid radicle in combination with 39 parts of potassium furnish nitrate of potassium (KNO3), which, when dissolved in water, does not affect the colour either of blue or of reddened litmus-paper. is therefore neutral in its reactions upon coloured tests.

It

Salts of nitric acid in which I atom of a monad or uniequivalent metal like potassium, sodium, or silver displaces the hydrogen in 1 atom of the acid HNO, or in which I atom of a dyad or biequivalent metal such as lead or copper displaces the hydrogen of 2 atoms of nitric acid (2 HN→), are regarded as neutral in composition whatever may be their action upon vegetable colours. If, for instance, 223 parts of oxide of lead be made to act upon 126 parts of nitric acid, nitrate of lead and water will be formed; Pb">+2 HNO3=H2O+Pb′′2 NO,, and the salt is neutral in composition, though if dissolved in water it reddens litmus, and has an acid reaction. The same thing is true, also, in the case of nitrate of copper.

The change in the tint of the coloured test is therefore not to be regarded as an absolute proof of neutrality or acidity in a salt. The change of colour which the litmus experiences, even from a salt of neutral composition, is readily explained. Blue litmus is itself a species of salt, formed by the combination of the metals of one of the alkalies or earths with the radicle of a feeble vegetable acid which is naturally of a red colour, but which becomes blue when it is neutralized by an alkali. When a powerful acid, such as the nitric or the sulphuric, is mixed with this blue colouring matter, the radicle of the strong acid seizes upon the basyl which the litmus contains, and sets free the litmus acid which appears of its natural red hue; but on the addition of an alkali the blue is restored by the reaction of the newly added base upon the litmus acid and the formation of the litmus

NEUTRAL, OR NORMAL SALTS.

373 salt of the alkaline metal. Again, if a salt with a strong acid radicle and a comparatively feeble basyl be mixed with the blue litmus, the strong radicle of the salt seizes upon the part of the basyl which is in combination with the litmus, and liberates the litmus acid, which appears of a more or less intense red, according as the basyl of the neutral salt has given up more or less of its acid radicle.

For analogous reasons it sometimes happens that a salt which is neutral in composition may exhibit characters in which the basyl preponderates to a greater or less extent. Carbonate of potassium (K,Є3) is neutral in composition, but it appears to be basic in its action upon the yellow colour of turmeric-paper, which it renders powerfully brown, and it immediately restores the blue tinge to reddened litmus. This ambiguity in the use of the word neutral, may, however, be entirely obviated by describing as normal salts the salts above-mentioned as neutral in composition; employing the term neutral solely with reference to the action of the body upon coloured tests; and in this sense we shall hereafter use these terms.

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(554) Polybasic Acids-Acid Salts.—If a quantity of oxalic acid be divided into two equal portions, one of which is dissolved in water, and mixed with a solution of hydrate of potash until the liquid becomes neutral in its reaction upon litmus, a salt is formed which, on evaporation, may be obtained crystallized in six-sided prisms, which consist of the normal oxalate of potassium, (K€ Ꮎ H Ꮎ) . If this salt be redissolved in water, and the second portion of oxalic acid be added to it, chemical union of the two bodies will occur; the liquid so obtained will be found to have a sour taste, to redden litmus powerfully, and on evaporation to yield a new salt, which crystallizes in rhomboidal prisms, containing exactly half as much potassium in proportion to the acid as the first salt; this is the binoxalate, or acid oxalate, of potassium ( KH€ Ꮎ Ꮋ Ꮎ) ; for K,€ Ꮎ, H, Ꮎ + H,€ Ꮎ, H Ꮎ = 2 (KH€,O,H,O).

2 49

2 2

Again, if the normal sulphate of potassium (K,SO) be dissolved in hot sulphuric acid, tabular plates of a new, fusible, and strongly acid salt, will crystallize out as the liquid cools, and the bisulphate or acid sulphate of potassium will be formed (KHSO); for, K2SO+H2SO1=2 KHSO4 This salt contains only half the amount of potassium which is present in the normal sulphate. an attempt be made to form a similar salt by dissolving nitrate of potassium in nitric acid, the experiment will fail, for the nitre will be found to crystallize out unchanged.

If

It is thus apparent that there are certain acids which furnish salts containing only one proportion of the metallic basyl united with the radicle of the acid, whilst there are other acids in which the radicle has the power of combining with the basyl in two proportions, forming two classes of salts, one of which is neutral. in its reaction, the other is acid.

It usually happens that such acid salts contain, in addition to the salt radicle and basyl, a certain quantity of hydrogen, as occurs, for instance, in the acid sulphate and the acid oxalate of potassium. This hydrogen is not to be regarded as present in the form of water of crystallization: it discharges a more important function, for it takes the place of one of the atoms of the metal on these occasions and it is because the basic properties of hydrogen are so feeble, that the acid character predominates to so great a degree in such salts.

:

Acids in the molecule of which a single atom of hydrogen admits of displacement by a single atom of a metallic monad such as potassium, are said to be monobasic. Of this description the hydrochloric acid HCl, the nitric, HNO,, and the acetic, H¤ ̧н ̧Ð1⁄2 acids are examples.

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If, however, the molecule of the acid contains 2 atoms of hydrogen, susceptible of displacement by two atoms of a monad like potassium, or by one atom of a dyad like zinc, the acid is said to be dibasic, like the sulphuric, H,SO, the oxalic, H2,Ðμ and the tartaric, H,CHO acids. Many of the vegetable acids belong to this class.

Again, if the molecule of the acid contains 3 atoms of hydrogen susceptible of displacement by 3 atoms of a monad like potassium or silver, or by one atom of a triad like bismuth, such an acid is said to be tribasic. Common phosphoric acid, H.PO,, is a good instance of this kind; and among the organic acids the citric, HEH., may be mentioned.

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Acids which allow the substitution of more than one atom of hydrogen by a corresponding number of atoms of a metallic monad (whether 2, 3, or 4 atoms) are said to be polybasic.

Nitrate of potassium (KNO3) affords an instance of the formation of a normal salt by the action of the oxide of a monad or uniequivalent metal upon a monobasic acid; whilst nitrate of lead (Pb" 2 NO3) illustrates the case of a normal salt formed by the reaction of the protoxide of a biequivalent or dyad metal upon a monobasic acid; and carbonate of potassium (K,CO) affords an example of the formation of a normal salt by the reaction of the oxide of a monad metal with a dibasic acid. These three varieties

SALTS FORMED FROM SESQUIOXIDES.

include the most common forms of normal salt.

375

When a dibasic

acid acts upon the protoxide of a metallic dyad, a salt is formed similar to that obtained by the reaction of sulphuric acid upon hydrate of lime, where two atoms of hydrogen in the acid are replaced by 1 atom of the dyad or biequivalent metal calcium €a H,Ꮎ, + H, ᎦᎾ =2 H, Ꮎ + €a ᎦᎾ

2

2

;

which are formed by the In certain cases the sesqui

But numerous salts are known action of sesquioxides upon the acids. oxides act upon the acids just as the protoxides do. Thus the sesquioxide of antimony and uranium, as well as those of bismuth, iron, and aluminum, in certain circumstances yield a sulphate by the action of the two bodies in the proportions indicated in the following equations :—

¤‚Ð ̧ +H¿§0 ̧=(U‚Ð1⁄2)” §0+H2O

2 3

4

2

The

These sulphates, however, with the exception of the first, are all insoluble; the oxide of uranium (Н1⁄2Ð1⁄2)”, (uranyl, as Peligot has termed it) has been isolated; indeed it was for some years mistaken for metallic uranium, till Peligot showed it to be a compound, and obtained the metal itself. The other oxides of the formulæ corresponding to uranyl have not been isolated. foregoing salts are generally regarded as basic salts, derived from the sesquioxides. Many of them retain water (not shown in the formula), and may be conveniently represented either by the general formula M,O,,SO,, or they may, by trebling the formulæ just given above, be represented in the following manner :

In the majority of instances, when a dibasic acid, such as the sulphuric, acts upon a sesquioxide, such as alumina or ferric oxide, three atoms of the dibasic acid are required to form a normal soluble salt, in which case the metal becomes terequivalent, and two atoms of the metal are therefore equivalent to 6 atoms of hydrogen. For example, (Al””)1⁄2Ð ̧+3 H2SO yield 3 H2O+ (Al), 3 SO, and under these particular circumstances iron itself, instead of being biequivalent, as in the ferrous salts, becomes terequivalent in these ferric salts. (Fe")¿Ð ̧+3 H2SO yield 3 H2O+

2

376

2

METALS WITH VARIABLE EQUIVALENTS.

(Fe"), 3 SO; thus affording an example of the remarkable fact of a metal possessing two different equivalents.

Other metals besides iron exhibit this power of assuming two different equivalents: chromium and cerium, for example, in particular cases may be biequivalent, whilst in other cases they are undoubtedly terequivalent metals; for instance, they are biequivalent in

2 3 ᎦᎾ .

In the protoxides and salts corresponding to them, these metals are biequivalent; whilst in the sesquioxides and the salts corresponding to them the metals are terequivalent. Tin, platinum, and palladium, again, in certain cases are biequivalent, in others quadrequivalent.

The normal salts derived from these different sesquioxides by the action of acids upon them, have usually an acid taste, and redden litmus powerfully. This is well seen in the ferric and aluminic sulphates.

It is not necessary that the two or three atoms of basyl which the salts of the dibasic or tribasic acids contain should consist of the same metal. Indeed, it has already been shown, in the case of the various phosphates, that several basyls may coexist in the same salt, in definite proportions. There is, for example, a pyrophosphate of sodium and hydrogen, composed of Na2H,P2, in which two atoms of hydrogen supply the place of their equivalent of sodium; and in the microcosmic salt (Na,H,N,H,Pе1 · 4 H2O), we have a tribasic phosphate of sodium, ammonium, and hydrogen, where each of the three atoms of basyl differs from the others.

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22

Now it frequently happens that hydrogen is one of the basyls present in the salt, and when such is the case, the salt, when dissolved in water, often has a sour taste, and reddens litmuspaper strongly. It is in this way that the most common variety of acid salts is formed. Cream of tartar, or, as it is often called, bitartrate of potash, offers a good illustration of this kind of salt. Cream of tartar is a sparingly soluble crystallizable compound, of an agreeable acidulous taste; it consists of KHЄH and is, in fact, a dibasic tartrate of potassium and hydrogen: if now it be dissolved in hot water, and another equivalent proportion of caustic potash be added, the hydrogen is displaced by the second atom of potassium, all the acid taste disappears, and

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