Chapter X: Sodium Chloride--Berthollet's Laws--Hydrochloric Acid (2)

[24] If MX and NY represent the molecules of two salts, and if there be
_no third substance_ present (such as water in a solution), the
formation of XY would also be possible; for instance, cyanogen,
iodine, &c. are capable of combining with simple haloids, as well
as with the complex groups which in certain salts play the part of
haloids. Besides which the salts MX and NY or MY with NX may form
double salts. If the number of molecules be unequal, or if the
valency of the elements or groups contained in them be different,
as in NaCl + H_{2}SO_{4}, where Cl is a univalent haloid and
SO_{4} is bivalent, then the matter may be complicated by the
formation of other compounds besides MY and NX, and when a solvent
participates in the action, and especially if present in large
proportion, the phenomena must evidently become still more
complex; and this is actually the case in nature. Hence while
placing before the reader a certain portion of the existing store
of knowledge concerning the phenomena of double saline
decompositions, I cannot consider the theory of the subject as
complete, and have therefore limited myself to a few data, the
completion of which must be sought in more detailed works on the
subject of theoretical chemistry, without losing sight of what has
been said above.

[24 bis] When the mixture of potassium nitrate and sodium acetate was
heated by Spring to 100°, it was completely fused into one mass,
although potassium nitrate fuses at about 340° and sodium nitrate
at about 320°.

When Berthollet enunciated his doctrine the present views of atoms and molecules had yet to be developed, and it is now necessary to submit the matter to examination in the light of these conceptions; we will therefore consider the reaction of salts, taking M and N, X and Y as equivalent to each other--that is, as capable of replacing each other 'in toto,' as Na or K,, 1/2Ca or 1/2Mg (bivalent elements) replace hydrogen.

And since, according to Berthollet's doctrine, when _m_MX of one salt comes into contact with _n_NY of another salt, a certain quantity _x_MY and _x_NX is formed, there remains _m_-_x_ of the salt MX, and _n_-_x_ of the salt NY. If _m_ be greater than _n_, then the maximum interchange could lead to _x_ = _n_, whilst from the salts taken there would be formed _n_MY + _n_NX + (_m_-_n_)MX--that is, a portion of one only of the salts taken would remain unchanged because the reaction could only proceed between _n_MX and _n_NY. If _x_ were actually equal to _n_, the mass of the salt MX would not have any influence on the _modus operandi_ of the reaction, which is equally in accordance with the teaching of Bergmann, who supposed double reactions to be independent of the mass and determined by affinity only. If M had more affinity for X than for Y, and N more affinity for Y than for X, then according to Bergmann there would be no decomposition whatever, and _x_ would equal 0. If the affinity of M for Y and of N for X were greater than those in the original grouping, then the affinity of M for X and of N for Y would be overcome, and, according to Bergmann's doctrine, complete interchange would take place--_i.e._ _x_ would equal _n_. According to Berthollet's teaching, a distribution of M and N between X and Y will take place in every case, not only in proportion to the degrees of affinity, but also in proportion to the masses, so that with a small affinity and a large mass the same action can be produced as with a large affinity and a small mass. Therefore, (1) _x_ will always be less than _n_ and their ratio _x_/_n_ less than unity--that is, the decomposition will be expressed by the equation, _m_MX + _n_NY = (_m_-_x_)MX + (_n_-_x_)NY + _x_MY + _x_NX; (2) by increasing the mass _m_ we increase the decomposition--that is, we increase _x_ and the ratio _x_/(_n_-_x_), until with an infinitely large quantity m the fraction _x_/_n_ will equal 1, and the decomposition will be complete, however small the affinities uniting MY and NX may be; and (3) if _m_ = _n_, by taking MX + NY or MY + NX we arrive at one and the same system _in either case_: (_n_-_x_)MX + (_n_-_x_)NY + _x_MY + _x_NX. These direct consequences of Berthollet's teaching are verified by experience. Thus, for example, a mixture of solutions of sodium nitrate and potassium chloride in all cases has entirely the same properties as a mixture of solutions of potassium nitrate and sodium chloride, of course on condition that the mixed solutions are of identical elementary composition. But this identity of properties might either proceed from one system of salts passing entirely into the other (Bergmann's hypothesis) in conformity with the predominating affinities (for instance, from KCl + NaNO_{3} there might arise KNO_{3} + NaCl, if it be admitted that the affinities of the elements as combined in the latter system are greater than in the former); or, on the other hand, it might be because both systems by the interchange of a portion of their elements give one and the same state of equilibrium, as according to Berthollet's teaching. Experiment proves the latter hypothesis to be the true one. But before citing the most historically important experiments verifying Berthollet's doctrine, we must stop to consider the conception _of the mass_ of the reacting substances. Berthollet understood by mass the actual relative quantity of a substance; but now it is impossible to understand this term otherwise than as the number of molecules, for they act as chemical units, and in the special case of double saline decompositions it is better to take it as the number of equivalents. Thus in the reaction NaCl + H_{2}SO_{4} the salt is taken in one equivalent and the acid in two. If 2NaCl + H_{2}SO_{4} act, then the number of equivalents are equal, and so on. The _influence of mass_ on the amount of decomposition _x_/_n_ forms the root of Berthollet's doctrine, and therefore we will first of all turn our attention to the establishment of this principle in relation to the double decomposition of salts.

About 1840 H. Rose[25] showed that water decomposes metallic sulphides like calcium sulphide, CaS, forming hydrogen sulphide, H_{2}S, notwithstanding the fact that the affinity of hydrogen sulphide, as an acid, for lime, CaH_{2}O_{2}, as a base, causes them to react on each other, forming calcium sulphide and water, CaS + 2H_{2}O. Furthermore, Rose showed that the greater the amount of water acting on the calcium sulphide, the more complete is the decomposition. The results of this reaction are evident from the fact that the hydrogen sulphide formed may be expelled from the solution by heating, and that the resulting lime is sparingly soluble in water. Rose clearly saw from this that such feeble agents, in a chemical sense, as carbonic anhydride and water, by acting in a mass and for long periods of time in nature on the durable rocks, which resist the action of the most powerful acids, are able to bring about chemical change--to extract, for example, from rocks the bases, lime, soda, potash. The influence of the mass of water on antimonious chloride, bismuth nitrate, &c., is essentially of the same character. These substances give up to the water a quantity of acid which is greater in proportion as the mass of the water acting on them is greater.[25 bis]

[25] H. Rose is more especially known for his having carefully studied
and perfected several methods for the exact chemical analysis of
many mineral substances. His predecessor in this branch of
research was Berzelius, and his successor Fresenius.

[25 bis] Historically the influence of the mass of water was the first
well-observed phenomenon in support of Berthollet's teaching, and
it should not now be forgotten. In double decompositions taking
place in dilute solutions where the mass of water is large, its
influence, notwithstanding the weakness of affinities, must he
great, according to the very essence of Berthollet's doctrine.

As explaining the action of the mass of water, the experiments of
Pattison Muir (1879) are very instructive. These experiments
demonstrate that the decomposition of bismuth chloride is the more
complete the greater the relative quantity of water, and the less
the mass of hydrochloric acid forming one of the products of the
reaction.

Barium sulphate, BaSO_{4}, which is insoluble in water, when fused with sodium carbonate, Na_{2}CO_{3}, gives, but not completely, barium carbonate, BaCO_{3}, (also insoluble), and sodium sulphate, Na_{2}SO_{4}. If a solution of sodium carbonate acts on precipitated barium sulphate, the same decomposition is also effected (Dulong, Rose), but it is restricted by a limit and requires time. A mixture of sodium carbonate and sulphate is obtained in the solution and a mixture of barium carbonate and sulphate in the precipitate. If the solution be decanted off and a fresh solution of sodium carbonate be poured over the precipitate, then a fresh portion of the barium sulphate passes into barium carbonate, and so by increasing the mass of sodium carbonate it is possible to entirely convert the barium sulphate into barium carbonate. If a definite quantity of sodium sulphate be added to the solution of sodium carbonate, then the latter will have no action whatever on the barium sulphate, because then a system in equilibrium determined by the reverse action of the sodium sulphate on the barium carbonate and by the presence of both sodium carbonate and sulphate in the solution, is at once arrived at. On the other hand, if the mass of the sodium sulphate in the solution be great, then the barium carbonate is reconverted into sulphate until a definite state of equilibrium is attained between the two opposite reactions, producing barium carbonate by the action of the sodium carbonate and barium sulphate by the action of the sodium sulphate.

Another most important principle of Berthollet's teaching is the existence of _a limit of exchange decomposition_, or _the attainment of a state of equilibrium_. In this respect the determinations of Malaguti (1857) are historically the most important. He took a mixture of solutions of equivalent quantities of two salts, MX and NY, and judged the amount of the resulting exchange from the composition of the precipitate produced by the addition of alcohol. When, for example, zinc sulphate and sodium chloride (ZnSO_{4} and 2NaCl) were taken, there were produced by exchange sodium sulphate and zinc chloride. A mixture of zinc sulphate and sodium sulphate was precipitated by an excess of alcohol, and it appeared from the composition of the precipitate that 72 per cent. of the salts taken had been decomposed. When, however, a mixture of solutions of sodium sulphate and zinc chloride was taken, the precipitate presented the same composition as before--that is, about 28 per cent. of the salts taken had been subjected to decomposition. In a similar experiment with a mixture of sodium chloride and magnesium sulphate, 2NaCl + MgSO_{4} or MgCl_{2} + Na_{2}SO_{4}, about half of the metals underwent the decomposition, which may be expressed by the equation 4NaCl + 2MgSO_{4} = 2NaCl + MgSO_{4} + Na_{2}SO_{4} + MgCl_{2} = 2Na_{2}SO_{4} + 2MgCl_{2}. A no less clear limit expressed itself in another of Malaguti's researches when he investigated the above-mentioned reversible reactions of the insoluble salts of barium. When, for example, barium carbonate and sodium sulphate (BaCO_{3} + Na_{2}SO_{4}) were taken, then about 72 per cent. of the salts were decomposed, that is, were converted into barium sulphate and sodium carbonate. But when the two latter salts were taken, then about 19 per cent. of them passed into barium carbonate and sodium sulphate. Probably the end of the reaction was not reached in either case, because this would require a considerable time and a uniformity of conditions attainable with difficulty.

Gladstone (1855) took advantage of the colour of solutions of different ferric salts for determining the measure of exchange between metals. Thus a solution of ferric thiocyanate has a most intense red colour, and by making a comparison between the colour of the resulting solutions and the colour of solutions of known strength it was possible to judge to a certain degree the quantity of the thiocyanate formed. This colorimetric method of determination has an important significance as being the first in which a method was applied for determining the composition of a solution without the removal of any of its component parts. When Gladstone took equivalent quantities of ferric nitrate and potassium thiocyanate--Fe(NO_{3})_{3} + 3KCNS--only 13 per cent. of the salts underwent decomposition. On increasing the mass of the latter salt the quantity of ferric thiocyanate formed increased, but even when more than 300 equivalents of potassium thiocyanate were taken a portion of the iron still remained as nitrate. It is evident that the affinity acting between Fe and NO_{3} and between K and CNS on the one hand, is greater than the affinity acting between Fe and CNS, together with the affinity of K for NO_{3}, on the other hand. The investigation of the variation of the fluorescence of quinine sulphate, as well as the variation of the rotation of the plane of polarisation of nicotine, gave in the hands of Gladstone many proofs of the entire applicability of Berthollet's doctrine, and in particular demonstrated the influence of mass which forms the chief distinctive feature of the teaching of Berthollet, teaching little appreciated in his own time.

At the beginning of the year 1860, the doctrine of the limit of reaction and of the influence of mass on the process of chemical transformations received a very important support in the researches of Berthelot and P. de Saint-Gilles on the formation of the ethereal salts RX from the alcohols ROH and acids HX, when water is also formed. This conversion is essentially very similar to the formation of salts, but differs in that it proceeds slowly at the ordinary temperature, extending over whole years, and is not complete--that is, it has a distinct limit determined by a reverse reaction; thus an ethereal salt RX with water gives an alcohol ROH and an acid HX--up to that limit generally corresponding with two-thirds of the alcohol taken, if the action proceed between molecular quantities of alcohol and acid. Thus common alcohol, C_{2}H_{5}OH, with acetic acid, HC_{2}H_{3}O_{2}, gives the following system rapidly when heated, or slowly at the ordinary temperature, ROH + HX + 2RX + 2H_{2}O, whether we start from 3RHO + 3HX or from 3RX + 3H_{2}O. The process and completion of the reaction in this instance are very easily observed, because the quantity of free acid is easily determined from the amount of alkali requisite for its saturation, as neither alcohol nor ethereal salt acts on litmus or other reagent for acids. Under the influence of an increased mass of alcohol the reaction proceeds further. If two molecules of alcohol, RHO, be taken for every one molecule of acetic acid, HX, then instead of 66 p.c., 83 p.c. of the acid passes into ethereal salt, and with fifty molecules of RHO nearly all the acid is etherised. The researches of Menschutkin in their details touched on many important aspects of the same subject, such as the influence of the composition of the alcohol and acid on the limit and rate of exchange--but these, as well as other details, must be looked for in special treatises on organic and theoretical chemistry. In any case the study of etherification has supplied chemical mechanics with clear and valuable data, which directly confirm the two fundamental propositions of Berthollet; the influence of mass, and the limit of reaction--that is, the equilibrium between opposite reactions. The study of numerous instances of dissociation which we have already touched on, and shall again meet with on several occasions, gave the same results. With respect to double saline decompositions, it is also necessary to mention the researches of Wiedemann on the decomposing action of a mass of water on the ferric salts, which could be determined by measuring the magnetism of the solutions, because the ferric oxide (soluble colloid) set free by the water is less magnetic than the ferric salts.

A very important epoch in the history of Berthollet's doctrine was attained when, in 1867, the Norwegian chemists, Guldberg and Waage, expressed it as an algebraical formula. They defined the active mass as the number of molecules contained in a given volume, and assumed, as follows from the spirit of Berthollet's teaching, that the action between the substances was equal to the product of the masses of the reacting substances. Hence if the salts MX and NY be taken in equivalent quantities (_m_ = 1 and _n_ = 1) and the salts MY and NX are not added to the mixture but proceed from it, then if _k_ represent the coefficient of the rate of the action of MX on NY and if _k_´ represent the same coefficient for the pair MY and NX, then we shall have at the moment when the decomposition equals x a measure of action for the first pair: _k_(1-_x_)(1-_x_) and for the second pair _k´xx_, and a state of equilibrium or limit will be reached when _k_(1-_x_)^2 = _k_´_x_^2, whence the ratio _k_/_k_´ = [_x_/(1-_x_)]^2. Therefore in the case of the action of alcohol on an acid, when _x_ = 2/3, the magnitude _k_/_k_´ = 4, that is, the reaction of the alcohol on the acid is four times as fast as that of the ethereal salt on water. If the ratio _k_/_k_´ be known, then the influence of mass may be easily determined from it. Thus if instead of one molecule of alcohol two be taken, then the equation will be _k_(2-_x_)(1-_x_) = _k´xx_, whence _x_ = 0·85 or 85 percent., which is close to the result of experiment. If 300 molecules of alcohol be taken, then x proves to be approximately 100 per cent., which is also found to be the case by experiment.[26]

[26] From the above it follows that an excess of acid should influence
the reaction like an excess of alcohol. It is in fact shown by
experiment that if two molecules of acetic acid be taken to one
molecule of alcohol, 84 p.c. of alcohol is etherified. If with a
large preponderance of acid or of alcohol certain discrepancies
are observed, their cause must be looked for in the incomplete
correspondence of the conditions and external influences.

But it is impossible to subject the formation of salts to any process directly analogous to that which is so conveniently effected in etherification. Many efforts have, however, been made to solve the problem of the measure of reaction in this case also. Thus, for example, Khichinsky (1866), Petrieff (1885), and many others investigated the distribution of metals and haloid groups in the case of one metal and several haloids taken in excess, as acids; or conversely with an excess of bases, the distribution of these bases with relation to an acid; in cases where a portion of the substances forms a precipitate and a portion remains in solution. But such complex cases, although they in general confirm Berthollet's teaching (for instance, a solution of silver nitrate gives some silver oxide with lead oxide, and a solution of nitrate of lead precipitates some lead oxide under the action of silver oxide, as Petrieff demonstrated), still, owing to the complexity of the phenomena (for instance, the formation of basic and double salts), they cannot give simple results. But much more instructive and complete are researches like those made by Pattison Muir (1876), who took the simple case of the precipitation of calcium carbonate, CaCO_{3}, from the mixture of solutions of calcium chloride and sodium or potassium carbonate, and found in this case that not only was the rate of action (for example, in the case of CaCl_{2} + Na_{2}CO_{3}, 75 per cent. of CaCO_{3} was precipitated in five minutes, 85 per cent. in thirty minutes, and 94 per cent. in two days) determined by the temperature, relative mass, and amount of water (a large mass of water decreases the rate), but that the limit of decomposition was also dependent on these influences. However, even in researches of this kind the conditions of reaction are complicated by the non-uniformity of the media, inasmuch as a portion of the substance is obtained or remains in the form of a precipitate, so that the system is heterogeneous. The investigation of double saline decompositions offers many difficulties which cannot be considered as yet entirely overcome. Although many efforts have long since been made, the majority of the researches were carried on in aqueous solutions, and as water is itself a saline compound and able to combine with salts and enter into double decomposition with them, such reactions taking place in solutions in reality present very complex cases.[27] In this sense the reaction between alcohols and acids is much more simple, and therefore its significance in confirmation of Berthollet's doctrine is of particular importance. The only cases which can be compared with these reactions for simplicity are those exchange decompositions investigated by G. G. Gustavson, which take place between CCl_{4} and RBr_{n} on the one hand, and CBr_{4} and RCl_{n} on the other. This case is convenient for investigation inasmuch as the RCl_{n} and RBr_{n} taken (such as BCl_{3}, SiCl_{4}, TiCl_{4}, POCl_{3}, and SnCl_{4}) belong to those substances which are decomposed by water, whilst CCl_{4} and CBr_{4} are not decomposed by water; and therefore, by heating, for instance, a mixture of CCl_{4} + SiBr_{4} it is possible to arrive at a conclusion as to the amount of interchange by treating the product with water, which decomposes the SiBr_{4} left unchanged and the SiCl_{4} formed by the exchange, and therefore by determining the composition of the product acted on by the water it is possible to form a conclusion as to the amount of decomposition. The mixture was always formed with equivalent quantities--for instance, 4BCl_{3} + 3CBr_{4}. It appeared that there was no exchange whatever on simple intermixture, but that it proceeded slowly, when the mixture was heated (for example, with the mixture above mentioned at 123° 4·86 per cent. of Cl was replaced by Br after 14 days' heating, and 6·83 per cent. after 28 days, and 10·12 per cent. when heated at 150° for 60 days). A limit was always reached which corresponded with that of the complemental system; in the given instance the system 4BBr_{3} + 3CCl_{4}. In this last 89·97 per cent. of bromine in the BBr_{3} was replaced by chlorine; that is, there were obtained 89·97 molecules of BCl_{3} and there remained 10·02 molecules of BBr_{3}, and therefore the same state of equilibrium was reached as that given by the system 4BCl_{3} + 3CBr_{4}. Both systems gave one and the same state of equilibrium at the limit, which is in agreement with Berthollet's doctrine.[28]

[27] As an example two methods may be mentioned, Thomsen's and
Ostwald's. Thomsen (1869) applied a thermochemical method to
exceedingly dilute solutions without taking the water into further
consideration. He took solutions of caustic soda containing
100H_{2}O per NaHO, and sulphuric acid containing 1/2H_{2}SO_{4} +
100H_{2}O. In order that these solutions may be mixed in such
quantities that atomic proportions of acid and alkali would act,
for forty grams of caustic soda (which answers to its equivalent)
there should be employed 49 grams of sulphuric acid, and then
+15,689 heat units would be evolved. If the normal sodium sulphate
so formed be mixed with _n_ equivalents of sulphuric acid, a
certain amount of heat is absorbed, namely a quantity equal to
(_n_.1650)/(_n_ + 0·8) heat units. An equivalent of caustic soda,
in combining with an equivalent of nitric acid, evolves +13,617
units of heat, and the augmentation of the amount of nitric acid
entails an absorption of heat for each equivalent equal to -27
units; so also in combining with hydrochloric acids +13,740 heat
units are absorbed, and for each equivalent of hydrochloric acid
beyond this amount there are absorbed -32 heat units. Thomsen mixed
each one of three neutral salts, sodium sulphate, sodium chloride
and sodium nitrate, with an acid which is not contained in it; for
instance, he mixed a solution of sodium sulphate with a solution
of nitric acid and determined the number of heat units then
absorbed. An absorption of heat ensued because a normal salt was
taken in the first instance, and the mixture of all the above
normal salts with acid produces an absorption of heat. The amount
of heat absorbed enabled him to obtain an insight into the process
taking place in this mixture, for sulphuric acid added to sodium
sulphate absorbs a considerable quantity of heat, whilst
hydrochloric and nitric acids absorb a very small amount of heat
in this case. By mixing an equivalent of sodium sulphate with
various numbers of equivalents of nitric acid, Thomsen observed
that the amount of heat absorbed increased more and more as the
amount of nitric acid was increased; thus when HNO_{3} was taken
per 1/2Na_{2}SO_{4}, 1,752 heat units were absorbed per equivalent
of soda contained in the sodium sulphate. When twice as much
nitric acid was taken, 2,026 heat units, and when three times as
much, 2,050 heat units were absorbed. Had the double decomposition
been complete in the case where one equivalent of nitric acid was
taken per equivalent of Na_{2}SO_{4} then according to calculation
from similar data there should have been absorbed -2,989 units of
heat, while in reality only -1,752 units were absorbed. Hence
Thomsen concluded that a displacement of only about two-thirds of
the sulphuric acid had taken place--that is, the ratio _k_ : _k_´
for the reaction 1/2Na_{2}SO_{4} + HNO_{3} and NaNO_{3} +
1/2H_{2}SO_{4} is equal, as for ethereal salts, to 4. By taking
this figure and admitting the above supposition, Thomsen found
that for all mixtures of soda with nitric acid, and of sodium
nitrate with sulphuric acid, the amounts of heat followed Guldberg
and Waage's law; that is, the limit of decomposition reached was
greater the greater the mass of acid added. The relation of
hydrochloric to sulphuric acid gave the same results. Therefore
the researches of Thomsen fully confirm the hypotheses of Guldberg
and Waage and the doctrine of Berthollet.

Thomson concludes his investigation with the words: (_a_) 'When
equivalent quantities of NaHO, HNO_{3} (or HCl) and 1/2H_{2}SO_{4}
react on one another in an aqueous solution, then two-thirds of
the soda combines with the nitric and one-third with the sulphuric
acid; (_b_) this subdivision repeats itself, whether the soda be
taken combined with nitric or with sulphuric acid; (_c_) and
therefore nitric acid has double the tendency to combine with the
base that sulphuric acid has, and hence in an aqueous solution it
is a stronger acid than the latter.'

'It is therefore necessary,' Thomsen afterwards remarks, 'to have
an expression indicating the tendency of an acid for the
saturation of bases. This idea cannot be expressed by the word
_affinity_, because by this term is most often understood that
force which it is necessary to overcome in order to decompose a
substance into its component parts. This force should therefore be
measured by the amount of work or heat employed for the
decomposition of the substance. The above-mentioned phenomenon is
of an entirely different nature,' and Thomsen introduces the term
_avidity_, by which he designates the tendency of acids for
neutralisation. 'Therefore the avidity of nitric acid with respect
to soda is twice as great as the avidity of sulphuric acid. An
exactly similar result is obtained with hydrochloric acid, so that
its avidity with respect to soda is also double the avidity of
sulphuric acid. Experiments conducted with other acids showed that
not one of the acids investigated had so great an avidity as
nitric acid; some had a greater avidity than sulphuric acid,
others less, and in some instances the avidity = 0.' The reader
will naturally see clearly that the path chosen by Thomsen
deserves to be worked out, for his results concern important
questions of chemistry, but great faith cannot be placed in the
deductions he has already arrived at, because great complexity of
relations is to be seen in the very method of his investigation.
It is especially important to turn attention to the fact that all
the reactions investigated are reactions of double decomposition.
In them A and B do not combine with C and distribute themselves
according to their affinity or avidity for combination, but
reversible reactions are induced. MX and NY give MY and NX, and
conversely; therefore the affinity or avidity for combination is
not here directly determined, but only the difference or relation
of the affinities or avidities. The affinity of nitric acid not
only for the water of constitution, but also for that serving for
solution, is much less than that of sulphuric acid. This is seen
from thermal data. The reaction N_{2}O_{5} + H_{2}O gives +3,600
heat units, and the solution of the resultant hydrate, 2NHO_{3},
in a large excess of water evolves +14,986 heat units. The
formation of SO_{3} + H_{2}O evolves +21,308 heat units, and the
solution of H_{2}SO_{4} in an excess of water 17,860--that is,
sulphuric acid gives more heat in both cases. The interchange
between Na_{2}SO_{4} and 2HNO_{3} is not only accomplished at the
expense of the production of NaNO_{3}, but also at the expense of
the formation of H_{2}SO_{4}, hence the affinity of sulphuric acid
for water plays its part in the phenomena of displacement.
Therefore in determinations like those made by Thomsen the water
does not form a medium which is present without participating in
the process; it also takes part in the reaction. (Compare Chapter
IX., Note 14.)

Whilst retaining essentially the methods of Thomsen, Ostwald
(1876) determined the variation of the sp. gr. (and afterwards of
volume), proceeding in the same dilute solutions, on the
saturation of acids by bases, and in the decomposition of the
salts of one acid by the other, and arrived at conclusions of just
the same nature as Thomsen's. Ostwald's method will be clearly
understood from an example. A solution of caustic soda containing
an almost molecular (40 grams) weight per litre had a specific
gravity of 1·04051. The specific gravities of solutions of equal
volume and equivalent composition of sulphuric and nitric acids
were 1·02970 and 1·03084 respectively. On mixing the solutions of
NaHO and H_{2}SO_{4} there was formed a solution of Na_{2}SO_{4}
of sp. gr. 1·02959; hence there ensued a decrease of specific
gravity which we will term Q, equal to 1·04051 +
1·02970-2(1·02959) = 0·01103. So also the specific gravity after
mixture of the solutions of NaHO and HNO_{3} was 1·02633, and
therefore Q = 0·01869. When one volume of the solution of nitric
acid was added to two volumes of the solution of sodium sulphate,
a solution of sp. gr. 1·02781 was obtained, and therefore the
resultant decrease of sp. gr.

Q_{1} = 2(1·02959) + 1·03084-3(1·02781) = 0·00659.

Had there been no chemical reaction between the salts, then
according to Ostwald's reasoning the specific gravity of the
solutions would not have changed, and if the nitric acid had
entirely displaced the sulphuric acid Q_{2} would be =
0·01869-0·01103 = 0·00766. It is evident that a portion of the
sulphuric acid was displaced by the nitric acid. But the measure
of displacement is not equal to the ratio between Q_{1} and Q_{2},
because a decrease of sp. gr. also occurs on mixing the solution
of sodium sulphate with sulphuric acid, whilst the mixing of the
solutions of sodium nitrate and nitric acid only produces a slight
variation of sp. gr. which falls within the limits of experimental
error. Ostwald deduces from similar data the same conclusions as
Thomsen, and thus reconfirms the formula deduced by Guldberg and
Waage, and the teaching of Berthollet.

The participation of water is seen still more clearly in the
methods adopted by Ostwald than in those of Thomsen, because in
the saturation of solutions of acids by alkalis (which Kremers,
Reinhold, and others had previously studied) there is observed,
not a contraction, as might have been expected from the quantity
of heat which is then evolved, but an expansion, of volume (a
decrease of specific gravity, if we calculate as Ostwald did in
his first investigations). Thus by mixing 1,880 grams of a
solution of sulphuric acid of the composition SO_{3} + 100H_{2}O,
occupying a volume of 1,815 c.c., with a corresponding quantity of
a solution 2(NaHO + 5H_{2}O), whose volume = 1,793 c.c., we obtain
not 3,608 but 3,633 c.c., an expansion of 25 c.c. per gram
molecule of the resulting salt, Na_{2}SO_{4}. It is the same in
other cases. Nitric and hydrochloric acids give a still greater
expansion than sulphuric acid, and potassium hydroxide than sodium
hydroxide, whilst a solution of ammonia gives a contraction. The
relation to water must be considered as the cause of these
phenomena. When sodium hydroxide and sulphuric acid dissolve in
water they develop heat and give a vigorous contraction; the water
is separated from such solutions with great difficulty. After
mutual saturation they form the salt Na_{2}SO_{4}, which retains
the water but feebly and evolves but little heat with it, i.e., in
other words, has little affinity for water. In the saturation of
sulphuric acid by soda the water is, so to say, displaced from a
stable combination and passes into an unstable combination; hence
an expansion (decrease of sp. gr.) takes place. It is not the
reaction of the acid on the alkali, but the reaction of water,
that produces the phenomenon by which Ostwald desires to measure
the degree of salt formation. The water, which escaped attention,
itself has affinity, and influences those phenomena which are
being investigated. Furthermore, in the given instance its
influence is very great because its mass is large. When it is not
present, or only present in small quantities, the attraction of
the base to the acid leads to contraction, and not expansion.
Na_{2}O has a sp. gr. 2·8, hence its molecular volume = 22; the
sp. gr. of SO_{3} is 1·9 and volume 41, hence the sum of their
volumes is 63; for Na_{2}SO_{4} the sp. gr. is 2·65 and volume
53·6, consequently there is a contraction of 10 c.c. per
gram-molecule of salt. The volume of H_{2}SO_{4} = 53·3, that of
2NaHO = 37·4; there is produced 2H_{2}O, volume = 36, +
Na_{2}SO_{4}, volume = 53·6. There react 90·7 c.c., and on
saturation there result 89·6 c.c.; consequently contraction again
ensues, although less, and although this reaction is one of
substitution and not of combination. Consequently the phenomena
studied by Ostwald depend but little on the measure of the
reaction of the salts, and more on the relations of the dissolved
substances to water. In substitutions, for instance 2NaNO_{3} +
H_{2}SO_{4} = 2HNO_{3} + Na_{2}SO_{4}, the volumes vary but
slightly: in the above example they are 2(38·8) + 53·3 and 2(41·2)
+ 53·6; hence 131 volumes act, and 136 volumes are produced. It
may be concluded, therefore, on the basis of what has been said,
that on taking water into consideration the phenomena studied by
Thomsen and Ostwald are much more complex than they at first
appear, and that this method can scarcely lead to a correct
interpretation as to the distribution of acids between bases. We
may add that P. D. Chroustcheff (1890) introduced a new method for
this class of research, by investigating the electro-conductivity
of solutions and their mixtures, and obtained remarkable results
(for example, that hydrochloric acid almost entirely displaces
formic acid and only 2/3 of sulphuric acid), but details of these
methods must be looked for in text-books of theoretical
chemistry.

[28] G. G. Gustavson's researches, which were conducted in the
laboratory of the St. Petersburg University in 1871-72, are among
the first in which the measure of the affinity of the elements for
the halogens is recognised with perfect clearness in the limit of
substitution and in the rate of reaction. The researches conducted
by A. L. Potilitzin (of which mention will be made in Chapter XI.,
Note 66) in the same laboratory touch on another aspect of the
same problem which has not yet made much progress, notwithstanding
its importance and the fact that the theoretical side of the
subject (thanks especially to Guldberg and Van't Hoff) has since
been rapidly pushed forward. If the researches of Gustavson took
account of the influence of mass, and were more fully supplied
with data concerning velocities and temperatures, they would be
very important, because of the great significance which the case
considered has for the understanding of double saline
decompositions in the absence of water.

Furthermore Gustavson showed that the greater the atomic weight of
the element (B, Si, Ti, As, Sn) combined _with chlorine_ the
greater the amount of chlorine replaced by bromine by the action
of CBr_{4}, and consequently the less the amount of bromine
replaced by chlorine by the action of CCl_{4} on bromine
compounds. For instance, for chlorine compounds the percentage of
substitution (at the limit) is--

BCl_{3} SiCl_{4} TiCl_{4} AsCl_{3} SnCl_{4}
10·1 12·5 43·6 71·8 77·5

It should he observed, however, that Thorpe, on the basis of his
experiments, denies the universality of this conclusion. I may
mention one conclusion which it appears to me may be drawn from
the above-cited figures of Gustavson, if they are subsequently
verified even within narrow limits. If CBr_{4} be heated with
RCl_{4}, then an exchange of the bromine for chlorine takes place.
But what would be the result if it were mixed with CCl_{4}?
Judging by the magnitude of the atomic weights, B = 11, C = 12, Si
= 28, about 11 p.c. of the chlorine would be replaced by bromine.
But to what does this point? I think that this shows the existence
of a motion of the atoms in the molecule. The mixture of CCl_{4}
and CBr_{4} does not remain in a condition of static equilibrium;
not only are the molecules contained in it in a state of motion,
but also the atoms in the molecules, and the above figures show
the measure of their translation under these conditions. The
bromine in the CBr_{4} is, _within the limit_, substituted by the
chlorine of the CCl_{4} in a quantity of about 11 out of 100: that
is, a portion of the atoms of bromine previously to this moment in
combination with one atom of carbon pass over to the other atom of
carbon, and the chlorine passes over from this second atom of
carbon to replace it. Therefore, also, in the homogeneous mass
CCl_{4} all the atoms of Cl do not remain constantly combined with
the same atoms of carbon, and _there is on exchange of atoms
between different molecules in a homogeneous medium also_. This
hypothesis may in my opinion explain certain phenomena of
dissociation, but though mentioning it I do not consider it worth
while to dwell upon it. I will only observe that a similar
hypothesis suggested itself to me in my researches on solutions,
and that Pfaundler enunciated an essentially similar hypothesis,
and in recent times a like view is beginning to find favour with
respect to the electrolysis of saline solutions.

Thus we now find ample confirmation from various quarters for the following rules of Berthollet, applying them to double saline decompositions: 1. From two salts MX and NY containing different haloids and metals there result from their reaction two others, MY and NX, but such a substitution will not proceed to the end unless one product passes from the sphere of action. 2. This reaction is limited by the existence of an equilibrium between MX, NY, MY, and NX, because a reverse reaction is quite as possible as the direct reaction. 3. This limit is determined both by the measure of the active affinities and by the relative masses of the substances as measured by the number of the reacting molecules. 4. Other conditions being constant, the chemical action is proportional to the product of the chemical masses in action.[29]

[29] Berthollet's doctrine is hardly at all affected in principle by
showing that there are cases in which there is no decomposition
between salts, because the affinity may be so small that even a
large mass would still give no observable displacements. The
fundamental condition for the application of Berthollet's
doctrine, as well as Deville's doctrine of dissociation, lies in
the reversibility of reactions. There are practically irreversible
reactions (for instance, CCl_{4} + 2H_{2}O = CO_{2} + 4HCl), just
as there are non-volatile substances. But while accepting the
doctrine of reversible reactions and retaining the theory of the
evaporation of liquids, it is possible to admit the existence of
non-volatile substances, and in just the same way of reactions,
without any visible conformity to Berthollet's doctrine. This
doctrine evidently comes nearer than the opposite doctrine of
Bergmann to solving the complex problems of chemical mechanics for
the successful solution of which at the present time the most
valuable help is to be expected from the working out of data
concerning dissociation, the influence of mass, and the
equilibrium and velocity of reactions. But it is evident that from
this point of view we must not regard a solvent as a
non-participant space, but must take into consideration the
chemical reactions accompanying solution, or else bring about
reactions without solution.

Thus if the salts MX and NY after reaction partly formed salts MY and NX, then a state of equilibrium is reached and the reaction ceases; but if one of the resultant compounds, in virtue of its physical properties, passes from the sphere of action of the remaining substances, then the reaction will continue. This exit from the sphere of action depends on the physical properties of the substance and on the conditions under which the reaction takes place. Thus, for instance, the salt NX may, in the case of reaction between solutions, separate as a precipitate, an insoluble substance, while the other three substances remain in solution, or it may pass into vapour, and in this manner also pass away from the sphere of action of the remaining substances. Let us now suppose that it passes away in some form or other from the sphere of action of the remaining substances--for instance, that it is transformed into a precipitate or vapour--then a fresh reaction will set in and a re-formation of the salt NX. If this be removed, then, although the quantity of the elements N and X in the mass will be diminished, still, according to Berthollet's law, a certain amount of NX should be again formed. When this substance is again formed, then, owing to its physical properties, it will again pass away; hence the reaction, in consequence of the physical properties of the resultant substances, is able to proceed to completion notwithstanding the possible weakness of the attraction existing between the elements entering into the composition of the resultant substance NX. Naturally, if the resultant substance is formed of elements having a considerable degree of affinity, then the complete decomposition is considerably facilitated.

Such a representation of the _modus operandi_ of chemical transformations is applicable with great clearness to a number of reactions studied in chemistry, and, what is especially important, the application of this aspect of Berthollet's teaching does not in any way require the determination of the measure of affinity acting between the substances present. For instance, the action of ammonia on solutions of salts; the displacement, by its means, of basic hydrates insoluble in water; the separation of volatile nitric acid by the aid of non-volatile sulphuric acid, as well as the decomposition of common salt by means of sulphuric acid, when gaseous hydrochloric acid is formed--may be taken as examples of reactions which proceed to the end, inasmuch as one of the resultant substances is entirely removed from the sphere of action, but they in no way indicate the measure of affinity.[30]

[30] Common salt not only enters into double decomposition with acids
but also _with every salt_. However, as clearly follows from
Berthollet's doctrine, this form of decomposition will only in a
few cases render it possible for new metallic chlorides to be
obtained, because the decomposition will not be carried on to the
end unless the metallic chloride formed separates from the mass of
the active substances. Thus, for example, if a solution of common
salt be mixed with a solution of magnesium sulphate, double
decomposition ensues, but not completely, because all the
substances remain in the solution. In this case the decomposition
must result in the formation of sodium sulphate and magnesium
chloride, substances which are soluble in water; nothing is
disengaged, and therefore the decomposition 2NaCl + MgSO_{4} =
MgCl_{2} + Na_{2}SO_{4} cannot proceed to the end. However, the
sodium sulphate formed in this manner may be separated by freezing
the mixture. The complete separation of the sodium sulphate will
naturally not take place, owing to a portion of the salt remaining
in the solution. Nevertheless, this kind of decomposition is made
use of for the preparation of sodium sulphate from the residues
left after the evaporation of sea-water, which contain a mixture
of magnesium sulphate and common salt. Such a mixture is found at
Stassfurt in a natural form. It might be said that this form of
double decomposition is only accomplished with a change of
temperature; but this would not be true, as may be concluded from
other analogous cases. Thus, for instance, a solution of copper
sulphate is of a blue colour, while a solution of copper chloride
is green. If we mix the two salts together the green tint is
distinctly visible, so that by this means the presence of the
copper chloride in the solution of copper sulphate is clearly
seen. If now we add a solution of common salt to a solution of
copper sulphate, a green coloration is obtained, which indicates
the formation of copper chloride. In this instance it is not
separated, but it is immediately formed on the addition of common
salt, as it should be according to Berthollet's doctrine.

The complete formation of a metallic chloride from common salt can
only occur, judging from the above, when it separates from the
sphere of action. The salts of silver are instances in point,
because the silver chloride is insoluble in water; and therefore
if we add a solution of sodium chloride to a solution of a silver
salt, silver chloride and the sodium salt of that acid which was
in the silver salt are formed.

As a proof that double decompositions like the above are actually accomplished in the sense of Berthollet's doctrine, the fact may be cited that common salt may be entirely decomposed by nitric acid, and nitre may be completely decomposed by hydrochloric acid, just as they are decomposed by sulphuric acid; but this only takes place when, in the first instance, an excess of nitric acid is taken, and in the second instance, an excess of hydrochloric acid, for a given quantity of the sodium salt, and when the resultant acid passes off. If sodium chloride be put into a porcelain evaporating basin, nitric acid added to it, and the mixture heated, then both hydrochloric and nitric acids are expelled by the heat. Thus the nitric acid partially acts on the sodium chloride, but on heating, as both acids are volatile, they are both converted into vapour; and therefore the residue will contain a mixture of a certain quantity of the sodium chloride taken and of the sodium nitrate formed. If a fresh quantity of nitric acid be then added, reaction will again set in, a certain portion of hydrochloric acid is again evolved, and on heating is expelled together with nitric acid. If this be repeated several times, it is possible to expel all the hydrochloric acid, and to obtain sodium nitrate only in the residue. If, on the contrary, we take sodium nitrate and add hydrochloric acid to it in an aqueous solution, a certain quantity of the hydrochloric acid displaces a portion of the nitric acid, and on heating the excess of hydrochloric acid passes away with the nitric acid formed. On repeating this process, it is possible to displace the nitric acid with an excess of hydrochloric acid, just as it was possible to displace the hydrochloric acid by an excess of nitric acid. The influence of the mass of the substance in action and the influence of volatility are here very distinctly seen. Hence it may be affirmed that sulphuric acid does not displace hydrochloric acid because of an especially high degree of affinity, but that this reaction is only carried on to the end because the sulphuric acid is not volatile, whilst the hydrochloric acid which is formed is volatile.

The preparation of hydrochloric acid in the laboratory and on a large scale is based upon these data. In the first instance, an excess of sulphuric acid is employed in order that the reaction may proceed easily at a low temperature, whilst on a large scale, when it is necessary to economise every material, equivalent quantities are taken in order to obtain the normal salt Na_{2}SO_{4} and not the acid salt, which would require twice as much acid. The hydrochloric acid evolved is a gas which is very soluble in water. It is most frequently used in practice in this state of solution under the name of _muriatic acid_.[31]

[31] The apparatus shown in fig. 46 (Chapter VI., Note 12) is generally
employed for the preparation of small quantities of hydrochloric
acid. Common salt is placed in the retort; the salt is generally
previously fused, as it otherwise froths and boils over in the
apparatus. When the apparatus is placed in order sulphuric acid
mixed with water is poured down the thistle funnel into the
retort. Strong sulphuric acid (about half as much again as the
weight of the salt) is usually taken, and it is diluted with a
small quantity of water (half) if it be desired to retard the
action, as in using strong sulphuric acid the action immediately
begins with great vigour. The mixture, at first without the aid of
heat and then at a moderate temperature (in a water-bath), evolves
hydrochloric acid. Commercial hydrochloric acid contains many
impurities; it is usually purified by distillation, the middle
portions being collected. It is purified from arsenic by adding
FeCl_{2}, distilling, and rejecting the first third of the
distillate. If free hydrochloric acid gas be required, it is
passed through a vessel containing strong sulphuric acid to dry
it, and is collected over a mercury hath.

Phosphoric anhydride absorbs hydrogen chloride (Bailey and Fowler,
1888; 2P_{2}O_{3} + 3HCl = POCl_{3} + 3HPO_{3}) at the ordinary
temperature, and therefore the gas cannot he dried by this
substance.

In chemical works the decomposition of sodium chloride by means of sulphuric acid is carried on on a very large scale, chiefly with a view to the preparation of normal sodium sulphate, the hydrochloric acid being a bye-product.[31 bis] The furnace employed is termed a _salt cake furnace_. It is represented in fig. 65, and consists of the following two parts: the pan B and the roaster C, or enclosed space built up of large bricks _a_ and enveloped on all sides by the smoke and flames from the fire grate, F. The ultimate decomposition of the salt by the sulphuric acid is accomplished in the roaster. But the first decomposition of sodium chloride by sulphuric acid does not require so high a temperature as the ultimate decomposition, and is therefore carried on in the front and cooler portion, B, whose bottom is heated by gas flues. When the reaction in this portion ceases and the evolution of hydrochloric acid stops, then the mass, which contains about half of the sodium chloride still undecomposed, and the sulphuric acid in the form of acid sodium sulphate, is removed from B and thrown into the roaster C, where the action is completed. Normal sodium sulphate, which we shall afterwards describe, remains in the roaster. It is employed both directly in the manufacture of glass, and in the preparation of other sodium compounds--for instance, in the preparation of soda ash, as will afterwards be described. For the present we will only turn our attention to the hydrochloric acid evolved in B and C.

[31 bis] In chemical works where sulphuric acid of 60° Baumé (22 p.c.
of water) is employed, 117 parts of sodium chloride are taken to
about 125 parts of sulphuric acid.