145
Ionic Interactions
5. The electrical conductance of the solution is increased by 10,000 (10 –6 ohm –1 cm –1 to
4.68 × 10 –2 Ω –1 cm –1 ). This indicates that the ions are able to carry a charge through
solution.
6. The temperature of the maximum density is lowered by 8°C to –4°C. This (like
seawater) is below the freezing point. These results indicate that the hydration
process breaks down the water structure.
7. The osmotic pressure (~26 atm) is created. This pressure is related to the vapor
pressure, freezing point, and boiling point effects (it is a colligative property).
It can be determined from
π = −(
)
RT V
a
/
ln
H O
H O
2
2
(4.15)
where a H 2 O is the activity of water in the solution (P/P H 2 O ), and V H 2 O is the molar
volume of water in the solution (V H 2 O = MW/ρ). This osmotic pressure can act
as a driving force for the diffusion of water through membranes. The H 2 O molecules interact very strongly with ions. Many electrolytes hold onto their water
molecules with such tenacity that the solids crystallize with a definite number of
water molecules.
An understanding of ion–water interactions and ion–ion interactions is thus a prerequisite for making any progress in understanding the chemistry of seawater. One might
ask the question, how many H 2 O molecules are hydrated to the Na + or Cl – ions? Estimates
range from 2 to 70 depending on how the measurements were made. This large variety
of numbers is due to the hydration volume around an ion not having a definite boundary. Thus, some methods count the H 2 O molecules that are not firmly bound by the ion.
Another question one might ask is, how many of the hydrated water molecules move when
the ion moves? To answer this question, we must know the exact time that an H 2 O molecule remains on an ion. This is normally not long, and it is probably not realistic to try to
distinguish between the stationary and the kinetic hydration atmosphere of an ion.
For most of the ions of importance in seawater chemistry, the major factor determining
the extent or strength of hydration is the charge density (the Z/ r ratio). A higher charge
density of an ion will lead to greater hydration.
4.4.1 electrostriction
Another unexpected phenomenon on the formation of an NaCl solution is electrostriction.
For example, the density of solid NaCl is 2.165 g cm –1 . The volume of 35 g of NaCl would
thus occupy 16.2 cm 3 mol –1 of space. The density of water at 25°C is 0.997 g cm –1 ; thus, the
volume of 965 g of water is 967.9 cm 3 . If the volumes are conservative, on mixing the solution would have a volume of 16.2 + 967.9 cm 3 = 984.1 cm 3 . Since the density of the solution is
1.0232 g cm –3 , the actual volume is 977.3 cm 3 . Thus, the volume of the solution is decreased
by 984.1 – 977.3 = 6.8 cm 3 . This decrease in volume is called electrostriction and is caused by
ion–water interactions. The ion draws the water molecules inward, compressing the solvent.
The water molecules near an ion have a higher density than the bulk water. This effect is
important for two reasons: (a) This hydration changes the mobility of ions, and (b) the effect
of pressure on ionic equilibria forces reactions to the smallest volume. Since ions in solution
have a smaller effective volume, pressure will force solids to have a higher solubility.
Ionic Interactions
5. The electrical conductance of the solution is increased by 10,000 (10 –6 ohm –1 cm –1 to
4.68 × 10 –2 Ω –1 cm –1 ). This indicates that the ions are able to carry a charge through
solution.
6. The temperature of the maximum density is lowered by 8°C to –4°C. This (like
seawater) is below the freezing point. These results indicate that the hydration
process breaks down the water structure.
7. The osmotic pressure (~26 atm) is created. This pressure is related to the vapor
pressure, freezing point, and boiling point effects (it is a colligative property).
It can be determined from
π = −(
)
RT V
a
/
ln
H O
H O
2
2
(4.15)
where a H 2 O is the activity of water in the solution (P/P H 2 O ), and V H 2 O is the molar
volume of water in the solution (V H 2 O = MW/ρ). This osmotic pressure can act
as a driving force for the diffusion of water through membranes. The H 2 O molecules interact very strongly with ions. Many electrolytes hold onto their water
molecules with such tenacity that the solids crystallize with a definite number of
water molecules.
An understanding of ion–water interactions and ion–ion interactions is thus a prerequisite for making any progress in understanding the chemistry of seawater. One might
ask the question, how many H 2 O molecules are hydrated to the Na + or Cl – ions? Estimates
range from 2 to 70 depending on how the measurements were made. This large variety
of numbers is due to the hydration volume around an ion not having a definite boundary. Thus, some methods count the H 2 O molecules that are not firmly bound by the ion.
Another question one might ask is, how many of the hydrated water molecules move when
the ion moves? To answer this question, we must know the exact time that an H 2 O molecule remains on an ion. This is normally not long, and it is probably not realistic to try to
distinguish between the stationary and the kinetic hydration atmosphere of an ion.
For most of the ions of importance in seawater chemistry, the major factor determining
the extent or strength of hydration is the charge density (the Z/ r ratio). A higher charge
density of an ion will lead to greater hydration.
4.4.1 electrostriction
Another unexpected phenomenon on the formation of an NaCl solution is electrostriction.
For example, the density of solid NaCl is 2.165 g cm –1 . The volume of 35 g of NaCl would
thus occupy 16.2 cm 3 mol –1 of space. The density of water at 25°C is 0.997 g cm –1 ; thus, the
volume of 965 g of water is 967.9 cm 3 . If the volumes are conservative, on mixing the solution would have a volume of 16.2 + 967.9 cm 3 = 984.1 cm 3 . Since the density of the solution is
1.0232 g cm –3 , the actual volume is 977.3 cm 3 . Thus, the volume of the solution is decreased
by 984.1 – 977.3 = 6.8 cm 3 . This decrease in volume is called electrostriction and is caused by
ion–water interactions. The ion draws the water molecules inward, compressing the solvent.
The water molecules near an ion have a higher density than the bulk water. This effect is
important for two reasons: (a) This hydration changes the mobility of ions, and (b) the effect
of pressure on ionic equilibria forces reactions to the smallest volume. Since ions in solution
have a smaller effective volume, pressure will force solids to have a higher solubility.