Molar conductivity
The molar conductivity is characteristic for each type of ion in aqueous solutions, it is also directly proportional to the rates of migration of ions in electrolysis.
From the known molar boundary conductivities can be calculated in advance to which electrical conductivity should have certain salts in water. Or, if you only know the kind of salt, one can determine the concentration of a conductivity measurement.
Knowledge of the molar conductivity of the individual ions boundary is also important in the electrolysis. With knowledge of the molar conductivities boundary migration velocities of ions during electrolysis can be calculated. By conductivity measurements ( amperometry ) can also be of metabolism, the transport numbers or the nature of the emerging or unreacted ion track in electrolysis.
Molar conductivity
The more salt ions are in an aqueous solution, the better it conducts electricity. In other words, the electrical resistance of a solution decreases, the more salt ions are contained in it. Depending on the chemical nature of the salt ions, some ions conduct electricity very well, other ions worse. The specific resistance is dependent on the electrode size and distance of an electrolytic solution resistance value (resistivity ). The specific conductivity ( dimension -1cm -1 = S * cm -1) is the inverse of resistivity. In the not very concentrated solutions (up to about 1 mol / liter) is the specific conductivity of the salt concentration directly proportional. Contributes to the conductivity of a specific salt, depending on the concentration in a coordinate system, a straight line is obtained with a certain pitch.
Dividing the specific conductivity values at the respective concentrations by the respective concentration, one obtains the so-called molar conductivity (, dimension ( earlier) S · * cm2 * mol -1):
The quantity c is the concentration of the salt in distilled water ( dimension: mol / liter). If the new dimension of information are used, the values must be converted accordingly.
The molar conductivity has a characteristic value which is slightly dependent on the concentration depending on the chemical nature of the salt.
Molar conductivity boundary
Plotting the molar conductivity of various salts as a function of the square root of the corresponding concentration in a coordinate system, obtained straight line intersecting the axis of ordinate to a certain point. This point is the molar conductivity at infinite dilution limit. This point is a very characteristic constant depending on the nature of the ion species. The point on the ordinate ( the molar conductivity) is called the molar limiting conductivity. This relationship is only valid for strong ions ( Cl -, SO42 -, Na ).
The relationship for strong electrolytes was found by Friedrich Wilhelm Kohlrausch and is known as Kohlrausch cal square root law.
The law reads:
For Λ0 applies:
Here, ν and ν - stoichiometric ratios according to the empirical formulas and Λ0 Λ0 - and the boundary conductivities of the individual ions ( equivalent conductivities ).
To determine the molar limit conductivity of single-ion molar mass of a salt, an acid or base is determined by the number of charge carriers of the ion divided (formerly eq), so that salts with different stoichiometric factors - such as sodium sulfate and sodium chloride - can be compared.
Also previously unknown boundary conductivities of ions can thus be easily determined from the difference of known boundary conductivities.
For weak electrolytes, the Ostwald dilution law applies.
An improvement of the conductivity theory represents the Debye- Hückel law
An example for the determination of the molar conductivity of ion - boundary conductivities
For a NaCl solution is obtained using the table below a molar limiting conductivity of:
(NaCl ) = 1 * 50.1 S · cm2mol -1 1 * 76.8 S · cm2mol -1 = 126.9 S · cm2mol -1.
According to the example, a 0.01 molar sodium chloride solution also has a specific conductivity of about:
= 126.9 * 0.01 / 1000 S · cm -1 = 0.001269 S * cm -1.
For very accurate calculations one needs the Debye- Hückel - Onsager theory.
With inexpensive conductivity meters quickly and easily aqueous solutions can be investigated.
Ion boundary conductivities for the determination of transference numbers
From knowledge of the boundary conductivities, the migration rates and the so-called transference numbers of ions in electrolytic can be determined. At an electrolysis some ions migrate rapidly (eg, H , OH - ) other quite slow (Li , CH3COO - ).
For the transport numbers of cations applies:
.
For the transport numbers of anions applies:
.
The following applies:
.
A high transference number is thus synonymous with a high rate of migration of an ion. Certain ions can accumulate faster than in the other electrode chamber through different migration rates at an electrolysis in an electrode chamber. This can be determined by conductivity measurements.
Table border molar conductivities of ions
Molar boundary conductivities of ions at 298 K in dist. water
(? ) Own measurement. Oxalate shows unusual behavior in dilution with respect to the conductivity.
Applications
The Molar conductivity can be used to determine the salt or ion concentration in fabrics and in their aqueous solutions in the laboratory or as in the hobby. In agriculture and horticulture so that is estimated in the irrigation water and soil nutrient and fertilizer concentration.