CHEMICAL HYDRATION NUMBERS JACOB KIELLAND* Porsgrunn, Norway
T
HE effect commonly termed chemical hydration is caused by specific force interactions between the solute and the dipolar solvent, with the result that a number of water molecules orient themselves around each solute molecule, forming a complex aquo-ion.? Ulich (1)has calculated the chemical hydration numbers of a series of ions by an entropy deficiency method. The results are shown in Table 1, which has been partly revised and enlarged by the author, using the recent entropy values of aqueous ions given by Latimer (Z), together with those of gaseous ions recommended by Kelley (3). It is worth noting that the corrected hydration numbers for Mg++and Ca++ are in still better accordance with the values calculated from conductance data than are those reported by Ulich. Hydration numbers of mercuric, hydroxyl (4), and ferric ions have also been calculated and included in Table 1. TABLE 1
HYDRATION NVHBBIS OP I O N S
IN WATBB AT
25'
Ion H i Li+ Nai K i Rht Agi TIt P- CI- Brc J - OH- S-n 4 5 31/2 2 2 3% 2 5 2 l l / l l / 2 7 4 lonMg+'Ca++Ba++Zn++CdtiHg**CuiiFctiSn*+Pb**Hg~++ 2 8ya 7 11 9 83/2 lo'/. 11 8 7 7
Fe**+Al*++
I* 1. 4 .. * Research chemist. t Compare for instance N.F.HALL,Chcm. Reu., 19,89 (1936),
and several papers of H. BRINTZINGER, Z. anorg. allgem. Chem. (1934-36).
It must he pointed out, however, that the total entropy of hydration is an integrated effect upon all of the surrounding water molecules, and the assumption of a constant entropy deficiency per water molecule cannot be strictly justified. Accordingly the hydration numhers obtained by the method of Ulich must be regarded as mean stoichiometric ones only; but in many cases they may not the less be of some value, for example in connection with the problem of aquo-ions and other complex ions. The distinction between the chemical and the total hydration is emphasized by the following comparison : Sourre Total hydration numbers
H . Brintdnger
Li*
22
No+
16
F - B++ C o ~ * A l s ' F c * + 83/1 19 18 18 6 18 18 1Z1/2 87
33
86
90
Further, it would be of interest to obtain values of the hydration numhers also for non-electrolytes, polar ones as well as non-polar. The author has carried out such a calculation, based upon the entropies of hydration given by Butler and Reid (5). The decrease in entropy per gram-mole of water forming the hydrate was tentatively taken equal to 6 E.W. as suggested by Ulich (I). Accordingly, one obtains the hydration number by dividing the difference between the entropy of aqueous
ions and the corresponding gaseous ones by six. The results are given in Table 2. TABLE 2 ENTROPIBS OF HYDRATION AND HYD%ATIOA NVMBBLS OP SOMEG&SBS AND O a ~ a h - r COMPOU~TIS c IN W A T AT ~ 25'
Su6stancc Inert gascs Helium Neon kg00
Krypton
xenon Radon common gsseo Hydrogen Nitrogen
oxygen
Carbon moooxide Carbon dioxide Carbonyl ~ulfide Methane Ethane Ethylene Acetylene Methyl chloride Aliphatic alcohols Methyl Ethyl n-Propyl i-PropyL n-Bufyl i-Eutyl Sec.-B"tyl 1rrl.-Bufvl
evident that the hydration effects must have their origin both in van der Waals forces as well as in dipole or multipole interactions.* Such forces one must also take into account in the case of electrolytes. Hence it is natural that many objections (6) have already been raised against the validity of the Born-Bjerrum equation for the beat of hydration of electrolytes, which takes into consideration electrostatic effects only. In their investigations concerning univalent organic electrolytes, McBain and Betz (7) have found that association, due to van der Waals cohesive forces, is so dominant that it completely submerges the ordinary features of interionic attraction. It is also interesting to note that Kortiim (a), investigating the light absorption in aqueous solutions, has found salt effects which for non-electrolytes are of the same order of magnitude as those for electro1ytes.t Kortiim (8) concluded that the van der Waals forces are most probably of greatest importance for solving the remaining problems concerning solutions of electrolytes. ADDITIVITY OF HYDRATION NUMBERS
It may be of interest that the hydration numbers for aliphatic compounds given in Table 2 have proved to be approximately additive.: Using the following figures, which necessarily must he regarded as only preliminary ones,
Ethyl acetate Acetone Glycerol Chloroform
Atomic groufi -OH Hydration number 2 DISCUSSION
Perhaps the most striking feature, when examining the hydration numbers, is the fact that the degrees of hydration of these non-electrolytes seem to be quite as great as those of the common low-valent ionic species given in Table 1. Even the inert gases are hydrated, and to an extent corresponding to the least hydrated ions. The higher aliphatic alcohols have hydration numbers corresponding to the most hydrated monovalent ions, while glycerol must he compared with the divalent ones. Since non-electrolytes, and even the inert gases, have been shown to be hydrated to a considerable extent, it is
-NH. 2
=CO -C1 1 1
-CH3 1
=CHn =CH 1 '!P
one obtains the values recorded in Table 2 (last column) in good accordance with the hydration numbers actually found by the entropy deficiency method (first column). SUMMARY
Hydration numbers of thirty-two gases and organic compounds in water have been calculated by the entropy deficiency method, and compared with those for ionic species. In the case of the organic compounds examined, approximate additivity has been shown, and some preliminary values for different atomic groups are suggested.
LITERATURE CITED
(1) ULICR,Z. Elektrockem., 36, 497 (1930). (2) LATIWR, Chem. Rew'ews, 18, 349 (1936). (3) KELLEY, Bureau of Mines Bulletin No. 350. (4) Entropy of hydroxyl taken from the paper of JOEINSTONE AND DAWSON, I . A m . Chem. Soc., 55,2744 (1933). (51 BUTLEICANE REID.I. Chem. Soc.. 1936. 1171. (6j Vom, Trans. ~ a r a d a ySoc., 32 ( ~ e p t . ;1936). See also p. 668 in LANGE AND MARTIN, Z. Elektrochem., 42,662 (1936). 17) M ~ R A AN" ~ NR T . ~Am Chrm~. ~ .Tor. 119%). \., -..a , rz . ~~. . ~ 57. -. . , 1905 -.-. ~ -.-., ( 8 ) KonTiiM. Z. Elekfrochem.. 42. 287 (19361.
of the dipole of the water molecule and the dipole which it induces in the normally nan-polar atom. The non-existence of crystalline hydrates of helium and neon were attributed to the low polarizahilities of these atoms. With the entropy deficiency method, however, we are able to detect hydrates also for these gases. t That constitutive influences in several cases seem to overc o k e the effect of the charge, have also been emphasized in other connectionsfor instance, by N . F. HALL,Chem. Rev., 19, 89 119.161. Werner's series of basic cations. \ - ~ ~when ~ , discussin* . .t Comoare additivitj of other thermodynamic ropert ties of organic cbmpaunds, f&exarnple, by PAKKS AND H ~ F M ~ N"The ,
