The Measurement of Electrolytic Conductivity

1

The Measurement ofElectrolytic

Conductivity

(Conductometry)

2

Outline

• Short introduction on conductometry: relevance, history, examples

• Theoretical background of electrolytic conductivity- what is influencing the conductivity?

• Measurement methods (contacting electrodes)- 2-pole and 4-pole sensors

• CONDUCELL• Conclusion

3

Conductivity

The conductometry is - beside of the measurement of pH -the most popular electrochemical method in process control. Measurements of electrolytic conductivity have been performed already 100 years ago. The technical background of this method has been developed end of the 19th century.

The conductometry is not a selective measurement, but measures the influence of all ions involved in a solution. This is why most of the conductivity values known for typical solutions are based on empirical measurements, and not theory.

4

0.1 1 10 100 1000

ultra pure water / boiler feed water

ion exchange waterSoftened water

Drinking water

Conductivity of aqueous solutions in the µS/cm range

Conductivity

5

Conductivity of aqueous solutions in the mS/cm range

1 10 100 1000

Waste water

Sea waterprocess water

conc. acids / bases

Conductivity

6

Conductivity – basic theory

A

R [Ω] = ρ [Ωm] l[m]A[m2]

The resistance R of an electrical wire is defined by its specific resistance ρ, the length l and the area A of the conductor.

l

7


Resistance R[Ω] = ρ [Ωm]

Conductance G [S] = 1 / R [Ω]

Specific conductance χ [Sm-1] = 1 / ρ

A[m2]l[m]

Zellenkonstante k[cm-1] = l[m]

8

Conductivity – basic theoryDue to their mobility, ions can transport electrical charge in a solution. This transport of electrical charge is an electrical current.

Na+Cl+

e- e-

cathode anode

9


When a DC Potential is applied between two electrodes, the cations (positively charged ions) will be attracted by the cathode (negatively polarized), and the anions will move to the anode.However, when applying a DC potential between anode and cathode, electrochemical reactions would occur at both electrodes (e.g. electrolysis of water, reduction of oxygen). These reactions are not desired, as they disturb the measurement of conductivity. Therefore, an AC potential is applied to the electrodes. The frequency is chosen such a way, that electrochemical reactions cannot follow. Typical frequency is 300 to 4‘000 Hz.

10


The electrolytic conductivity χ is a function of the concentration c and the mobility λ of all ions in solution.

χ = c • λχ = c • λ

11


It is a function of:• ionic charge• ion diameter• how many water is bound to the ion• temperature• viscosity of the solvent

The ionic mobility λ is a specific value for every ion that exists.

12

Example of a pure sodium chloride solution

λ+ (Na+) = 50 cm2 Ω-1 mol-1 at 25 °Cλ - (Cl-) = 76 cm2 Ω-1 mol-1 at 25 °Cc (NaCl) = 10-4 mol L-1 = 10-7 mol cm-3

Λ = λ+ + λ-

χ = c · Λ

χ = 126 cm2 Ω-1 mol-1 · 10-4 mol cm-3 = 12.6 μS cm-1


13

temperature [°C]

conductivity [mS/cm]



KCl 0,01 mol/l KCl 0,1 mol/l KCl 1 mol/l20 1.278 11.67 102.0721 1.305 11.91 104.0022 1.332 12.15 105.5423 1.359 12.39 107.8924 1.386 12.64 109.8425 1.413 12.88 111.8026 1.441 13.13 113.7727 1.468 13.37 115.7428 1.496 13.6229 1.524 13.8730 1.552 14.12

KCl solution with defined conductivity


14

• Be careful: the linear relationship between χ and c is valid only for diluted solutions

(concentration below 10-1 mol/L).

• At higher concentrations, there is interaction between the ions and also between ion and solvent. This results in a non-linear relationship between concentration and conductivity. These effects have been described and modelled by Kohlrausch and Debye-Hückel.


15

Conductivity – measurement

• Determination of the electrolytic conductivity by means of a measurement of the resistance between two platinum plates immersed in solution.

A

l

R [ m]

l[m]

A[m2] R [Ω]=l [m]

A [m2]χ [Sm-1]1

geometrical factorso-called cell constant k

16

Conductivity – measurement

Determination of the cell constant k

Due to effects at the edge of the platinum plates and due to a inhomogeneous electrical field, it is very difficult to determine the cell constant directly from the geometrical dimensions A and l. The cell constant must be defined by an experiment:

a) with calibration solutions of know conductivityb) with a comparison to a measurement with a second sensor sitting in the same solution. The second sensor has a known cellconstant.

17

Conductivity - sensors

Requirements for a conductivity sensor

• large measurement range with high linearity• large pressure and temperature range• integrated temperature sensor• resistant to corrosion• no maintenance• easy to clean

18


Two different conductivity sensors for contacting sensors:

• 2-pole sensors

• 4-pole sensors

19


2-pole sensors

advantage: low cell constant k can be realized. Accordingly, low conductivity values can be measured.

disadvantage: polarisation effects (capacitive) at the 2 electrodes lead to measurement errors; the measurement range is limited in the low conductivity range.

R [ m] A[m2] R [Ω] =

χ [Sm-1]k

20


2-pole sensors

Ι

Υ R [ m]

A[m2

]

Influence of parasitic capacitor

Influence of polarisationG

χ

21

CONDUCELL 2UP-PG 120

P/N 237600• 2-pole conductivity sensor• 1 … 200 μS/cm• 0 ... 130°C, p < 20 bar• cell constant ca. 0.4 cm-1

• open field• integrated Pt 1000• Glass, platinum, FDA-EPDM• VP-connector

22


4-pole sensors

advantage: large, linear measurement range

disadvantage: due to constructive reasons, low cell constants are difficult to realize.

4-pole principle

ΥΙ

23

CONDUCELL 4US-G125-62/25

P/N 237700• 4-pole conductivity sensor• 0.1 … 600’000 μS/cm• 0 ... 135°C, p < 6 bar• cell constant ca. 0.135 cm-1

• open field• integrated Pt 1000• surface quality: N6, electro-polished• DIN 1.4435, PEEK, FDA-EPDM• for 25 mm weld-in socket• 5 m fixed cable

24

CONDUCELL 4US-T150-100

P/N 237760• 4-pole conductivity-sensor• 0.1 … 600’000 μS/cm• 0 ... 135°C, p < 6 bar• cell constant ca. 0.135 cm-1

• open field• integrated Pt 1000• surface: N6, electro-polished• DIN 1.4435, PEEK, FDA-EPDM• Tri-Clamp 1.5”• 5 m fix cable

25

CONDUCELL 4US – linearity

CONDUCELL 4US / KNICK Stratos Cond 2401

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06Referenzleitfähigkeit [uS/cm]

gem

esse

ne L

eitfä

higk

eit [

uS/c

m]

Linearity over 6 decades !!

26

CONDUCELL 4US – compatibilityCONDUCELL 4US connected to 4 different instruments

deviation from reference value

-100

-80

-60

-40

-20

0

20

40

60

80

100

1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06

reference conductivity [uS/cm]

devi

atio

n fr

om re

fere

nce

valu

e [%

]

SIEMENS SIPAN 3WTW LF 539YOKOGAWA Model SC150KNICK Stratos Cond 2401KNICK Portamess 911 Cond

27

Conclusions

CONDUCELL is an excellent sensor with high linearity over 6 decades, dedicated for hygienic applications.

To take advantage of the high linearity, the appropriate transmitter has to be selected.

The flow-through cell must have a diameter > 50 mm, or must be constructed with a non-conductive material (e.g. PEEK).

28

Products coming up CONDUCELL 4USF-PG-120• 4-pole conductivity sensor• 12 mm diameter PEEK shaft• hygienic design• 1 μS/cm ... 500 mS/cm

CONDUCELL 4USF-PG-120 (WO 9690393) am Knick Stratos 2401 04.03.05

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06reference conductivity [uS/cm]

read

ing

on 2

401

CO

ND

[uS/

cm]

Pos. 1

29

Products coming up CONDUCELL flow-through• 4-pole conductivity sensor• flow-through (10 mm inner diameter)• Neumo BioConnect DN10• hygienic design

30

Products coming up CONDUCELL flow-through• excellent linearity 1 μS/cm ... 500 mS/cm

4-Pol LF Durchflusszelle, Knick Stratos 2401, 03.12.04

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06Referenz-Leitfähigkeit [uS/cm]

gem

esse

n 24

01 C

ON

D [u

S/cm

]

The Measurement of Electrolytic Conductivity

Documents

Transcript of The Measurement of Electrolytic Conductivity