Conductivity 电导率测试原理

Medium and high conductivity in aqueous solutions (mS/cm) 1 10 100 sewage food acid, base 1000
Probes
•2-electrode cell: Designed for use in low conducting liquids, different shapes of electrodes (plates, concentric pipes), range between 0.0001 µS/cm and 200 µS/cm. •4-electrode cell: Middle-range and high-range conductivity up to 2 S/cm.
USP 29
Regulation to test pure water for pharmaceutical purposes by conductivity measurement: •Hardware requirements: Uncertainties of conductivity cell max. 2 %, meter max. 0,1 %, minimum resolution 0,1 µS/cm. Three-stage procedure: 1. Online-measurement, according to a chart with corresponding conductivity/temperature values, which must not be exceeded. 2. Intense stirring of the sample on air, T = 25 ° , if conductivity < 2,1 µS/cm C results comply with USP 29 . 3. If not, add KCl and take pH at 25 ° Chart supp lies corresponding conductivity. C. If pH < 5 or pH > 7, water is not suitable.
And now two terms often used in context with conductivity: TDS (Total Dissolved Solids): Gives the amount of dissolved ions in mg/l if content is known (ion charge!) Procedure: Works with nlF, residue has to be determined and the correction factor has to be adjusted between 0,4 and 1,0. This factor regards the charge of ions.
A K=
l A
Hale Waihona Puke Baidu
Electrolytic conductance:
κ = GK
κ [Ω-1 m-1]=[S/m] Ω
Examples of low conductivity (µS/cm) 0,055 Pure water Water for pharmaceutical purposes Ion exchanger desalination Drinking water 1 10 100 1000
KCl → K + + Cl−
Electrolytic conductivity κ depends on concentration c and the molar conductivity λ: κ=λ.C λ is the sum of ionic movability I+/- of anions and cations (valid only for strongly diluted solutions). Else: Square root rule of Kohlrausch λ = λ0 + a c or Debye-Hückel-Onsager-theory of strong electrolytes. Ionic movability depends on: charge, ion radius, number of hydratation, viscosity, temperature.
Salinity: Number without dimension, was created for examination of sea water and other salty waters, standard is a solution of 32.4356 g/kg KCl which leads to the defined value S = 35.000 at 15 ° S depends also o n temperature and pressure, C. normally corrected in the lab only for temperature.
U
E1
E2
E3 d
E4
Influences on conductivity: •Charge per ion •Degree of dissociation: available amount of charges •Concentration: absolute number of charges, interaction of ions •Temperature: see below •Polarization: Depends on concentration, results in wrong measurements because of ionic interaction on the surface of the electrode (Helmholtz´ double layer): The measured value is less than the real one. Measure: High-frequency voltage, non-polarizing materials (stainless steel, graphite etc.) •Viscosity: influences movability of ions
Influences on conductivity: •Chemical reactions: unpredictable behavior •Impedance: Effect at very low conductivity by cables and meter electronics
Temperature coefficient: Temperature dependence of conductivity: •Linear compensation: solutions of salts, acids and bases •Non-linear compensation for natural waters (EN 27888) •Non-linear compensation for pure water •Non-linear compensation for special liquids (user-defined) •Without compensation: USP 29 Refer to a fixed temperature (20 ° or 25 ° C C)
2-cell electrode
Amount of the voltage between the two plates depends on the conductivity of the solution
E1
E2
4-electrode cell
Current supply via E1 and E2. Electrodes E3 and E4 measure the loss of voltage along d. A measure of the resistance or conductivity is the amount of the supplied current.
Conductivity
Introduction •More than 100 years old measuring technique •Sum parameter •Substances with strong polar or ionic bonding dissolve in water to ions („electrolytic dissociation“)
Principles of Measuring
Liquids with dissolved ions are conductors
e-
+
-
eanode
cathode
Ohm´s Law
Resistance: Conductance: R=U I
1 G=R
[Ohm; Ω] [Siemens; S]
Volume resistivity: Cell factor/cell constant K l
Calibration
Uncertainties of probes are 1,5 to 2 % of the cell constant. •Calibration normal seldom necessary because of resistant electrode material •For high accuracy measurements probes can be tested with standard solutions and -if necessary- cell constants be electronically adjusted. •EN 27888 lays down a testing every six months •Use commercial standards (avoid danger of pollution). •Avoid damage of the electrode surfaces •Meter can be tested with precision resistors.
Example for calculation of (linear) temperature compensation:
κ κM ∆κ κR ∆T
a=
∆κ × 1 × 100 % ∆T κR κ κR = (1+ aM T ) ∆
TR
TM
T
Idea: Conductivity strongly depends on temperature, and is predictable in a certain range. Reference temperature: comparability of different samples at one temperature.