redox potential electrodes

7/22/2019 Redox Potential Electrodes

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Redox Potential Electrodes

Platinum, Gold and Silver Electrodes

Request InfoGet a Quote

Redox is an ideal solution for testing the oxidation/reduction

in factory and industrial waste (chromium and cyanide

concentration), fish farming and aquariums (water quality),

and swimming pools (sanitation levels).

InLab ORP/Redox

Electrodes

Precise and reliable - whatever

the application, the InLab range offers the

right ORP electrode.

LE ORP/Redox

Electrodes
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The BASi Oxygen Electrode.

Welcome to the dissolved oxygen (D.O.) theory Page.

Basic principles of Polagrography cell (electrode) Polar gram EIDs dissolved oxygen electrode picture Electrode reactions Number of electrons involved Calibration Calibration in air saturated with water vapor Calibration in air saturated water Checking the sensor function Measurement and analytical quality assurance Cleaning of sensors Regeneration of sensors Polarization periods (startup periods) prior to measurement Approach flow Correction for salt content Influence of interfering gases Solubility functions Checking the oxygen meter and or logger Dissolved oxygen Applications Practical experiments
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Dissolved Oxygen:

The air we breathe contains about 20% oxygen. Fish and otheraquatic organisms require oxygen as well. The term Dissolved

Oxygen (DO or D.O.) refers to the amount of free oxygendissolved in water which is readily available to respiring aquaticorganisms.

Water quality standards often express minimum concentrations ofdissolved oxygen which must be maintained in order to supportlife as well as be of beneficial use. Levels of dissolved oxygenbelow 4-5 milligrams per liter affect fish health and levels below 2milligrams per liter can be lethal to fish.

dditionally, biochemical oxygen demand (BOD) is commonlyused with reference to effluent discharges and is a common,environmental procedure for determining the extent to whichoxygen within a sample can support microbial life.

The test for BOD is especially important in waste watertreatment, food manufacturing, and filtration facilities where theconcentration is crucial to the overall process and end products.High concentrations of DO predict that oxygen uptake bymicroorganisms is low along with the required break down ofnutrient sources in the medium.

Basic principles of Polagrography cell:

Liquid and Air state of equilibrium is reached when the partialpressure of oxygen, i.e. the part of the total pressure that is dueto oxygen, is equal in air and in liquid. The liquid is then

saturated with oxygen.

Figure 1.1 Air and liquid oxygen equilibrium
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Polargram:

When an electrode of noble metal such as platinum or gold is made 0.6 to 0.8 V negative

with respect to a suitable reference electrode such as AgAgCl or ancalomel electrode in a neutral KCI solution (see Figure 1.2), theoxygen dissolved in the liquid is reduce at the surface of thenoble metal.

Figure 1.2 Polarographhy diagram

This above phenomenon can be observed from a current to

voltage diagram called a polarogram of the electrode. As shownin Figure 1.3a, the negative voltage applied to the noble metalelectrode (called the cathode) is increased, the current increasesinitially but soon it becomes saturated. In this plateau region ofthe polarogram, the reaction of oxygen at the cathode is so fastthat the rate of reaction is limited by the diffusion of oxygen tothe cathode surface. When the negative bias voltage is furtherincreased, the current output of the electrode increases rapidlydue to other reactions, mainly, the reduction of water to

hydrogen. If a fixed voltage in the plateau region (for example, -0.6V) is applied to the cathode, the current output of the


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electrode can be linearly calibrated to the dissolved oxygen(Figure 1.3b). It has to be noted that the current is proportionalnot to the actual concentration but to the activity or equivalentpartial pressure of dissolved oxygen, which is often referred to as

oxygen tension. A fixed voltage between -0.6 and -0.8 V isusually selected as the polarization voltage when using Ag/AgClas the reference electrode or any other EID's dissolved oxygenelectrodes.

Additionally for physical and chemical correctness, partialpressure in a liquid actually refers to the fugacity. In the pressurerange relevant to the measurements at hand, it is acceptable to

equate the two values and this allows us to restrict the followingconsiderations to the partial pressure. In dry, atmospheric air,

the partial pressure of oxygen is 20.95% of the air pressure. Thisvalue is reduced over a water surface because water vapor has itsown vapor pressure and a corresponding partial pressure.

Figure 1.3 (a) Current to voltage diagram at differentoxygen tension; (b) Calibration obtained at a fixedpolarization voltage of 600 mV.

When the cathode, the reference electrode, and the electrolyte

are separated from the measurement medium by a polymermembrane, which is permeable to the dissolved gas but not tomost of the ions and other species, and when most of the masstransfer resistance is confined in the membrane, EIDs electrode


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EIDs polaragraphoc dissolved oxygen electrode picture:

EIDs ELECTRODER - ABS body Dissolved Oxygen Sensor (ADO)

EIDs dissolved oxygen, Probe, polaragraphic, ABS body, 12mm *120mm, with 10K Negative Temperature Compensation

Figure 1.5 Basic Polarographhy-electrode

Electrode reactions:

For our polarographic electrodes, the reaction proceeds asfollows:

Cathodic reaction: 02+ 2H20 + 2e

-

H2O

2+ 2OH

-

H202+ 2e

--> 20H-

Anodic reaction: Ag + Cl-AgCl + e- Overall reaction: 4Ag + 02+ 2H2O+ 4 Cl-4 AgCl+ 4 OH-

The reaction tends to produce alkalinity in the medium togetherwith a small amount of hydrogen peroxide.


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Figure 1.4b Two-point calibration

The rate at which oxygen enters a dissolved oxygen probe is afunction of:

the concentration of oxygen in the sample the diffusion coefficient/permeability of the

membrane (function of temperature

Altitude(ft)

Pressure(mm Hg)

CorrectionCalibrationCorrection

Factor

-540 775 1.02

SeaLevel 760 1

500 746 0.98

1000 732 0.96

1500 720 0.95

2000 707 0.93

2500 694 0.91

3000 681 0.9

3500 668 0.88

4000 656 0.86


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4500 644 0.85

5000 632 0.83

5500 621 0.82

6000 609 0.8

Table 1: Oxygen Value Corrected for Pressure (25 C)

Relative Humidity and temperature effect andTemperature compensation

%.

Temperature(Celsius)

DO (100%R.H.)(ppm,

mg/L)

DO (0%R.H.)(ppm,

mg/L)

0 14.6 14.66

1 14.19 14.26

2 13.81 13.89

3 13.44 13.53

4 13.09 13.185 12.75 12.85

6 12.43 12.54

7 12.12 12.23

8 11.83 11.94

9 11.55 11.66

10 11.27 11.4

11 11.01 11.14

12 10.76 10.913 10.52 10.66

14 10.29 10.44

Table 2 above: Dissolved Oxygen Solubility vs.Temperature


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Temperature (Celsius)

790775760745730715700685670665

015.2

14.9

14.6

14.3

1413.7

13.4

13.2

12.9

12.6

1 14.8

14.5

14.2

13.9

13.6

13.3

13.1

12.8

12.5

12.2

214.4

14.1

13.8

13.5

13.3

1312.7

12.4

12.2

11.9

3 1413.7

13.4

13.2

12.9

12.6

12.4

12.1

11.8

11.6

413.6

13.4

13.1

12.8

12.6

12.3

12.1

11.8

11.5

11.3

5 13.3 13 12.8 12.5 12.2 12 11.7 11.5 11.2 11

612.9

12.7

12.4

12.2

11.9

11.7

11.4

11.2

10.9

10.7

712.6

12.4

12.1

11.9

11.6

11.4

11.2

10.9

10.7

10.4

812.3

12.1

11.8

11.6

11.4

11.1

10.9

10.7

10.4

10.2

9 1211.

8

11.

6

11.

3

11.

1

10.

9

10.

6

10.

4

10.

2

9.9

4

1011.7

11.5

11.3

1110.8

10.6

10.4

10.1

9.92

9.69

1111.5

11.2

1110.8

10.6

10.4

10.1

9.91

9.69

9.47

1211.2

1110.8

10.5

10.3

10.1

9.99.68

9.47

9.25

1310.9

10.7

10.5

10.3

10.1

9.89

9.68

9.47

9.26

9.04

1410.7

10.5

10.3

10.1

9.88

9.67

9.46

9.26

9.05

8.85

1510.5

10.3

10.1

9.87

9.67

9.46

9.26

9.06

8.86

8.65

1610.3

10.1

9.85

9.65

9.45

9.26

9.06

8.86

8.66

8.46

17 109.84

9.65

9.46

9.26

9.07

8.87

8.68

8.48

8.29

18 9.83

9.64

9.45

9.26

9.07

8.88

8.69

8.5 8.31

8.12


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199.63

9.45

9.26

9.07

8.89

8.78.51

8.33

8.14

7.95

209.44

9.25

9.07

8.89

8.78.52

8.34

8.15

7.97

7.79

21 9.26

9.08

8.9 8.72

8.54

8.36

8.18

8 7.82

7.64

229.07

8.98.72

8.54

8.37

8.19

8.01

7.84

7.66

7.48

238.91

8.73

8.56

8.39

8.21

8.04

7.86

7.69

7.52

7.34

Temperature(Celsius)

0 15.2 14.9 14.6 14.3 14 13.7 13.4 13.2 12.9 12.61 14.8 14.5 14.2 13.9 13.6 13.3 13.1 12.8 12.5 12.2

2 14.4 14.1 13.8 13.5 13.3 13 12.7 12.4 12.2 11.9

3 14 13.7 13.4 13.2 12.9 12.6 12.4 12.1 11.8 11.6

4 13.6 13.4 13.1 12.8 12.6 12.3 12.1 11.8 11.5 11.3

5 13.3 13 12.8 12.5 12.2 12 11.7 11.5 11.2 11

6 12.9 12.7 12.4 12.2 11.9 11.7 11.4 11.2 10.9 10.7

7 12.6 12.4 12.1 11.9 11.6 11.4 11.2 10.9 10.7 10.4

8 12.3 12.1 11.8 11.6 11.4 11.1 10.9 10.7 10.4 10.2

9 12 11.8 11.6 11.3 11.1 10.9 10.6 10.4 10.2 9.94

10 11.7 11.5 11.3 11 10.8 10.6 10.4 10.1 9.92 9.69

11 11.5 11.2 11 10.8 10.6 10.4 10.1 9.91 9.69 9.47

12 11.2 11 10.8 10.5 10.3 10.1 9.9 9.68 9.47 9.25

13 10.9 10.7 10.5 10.3 10.1 9.89 9.68 9.47 9.26 9.04

14 10.7 10.5 10.3 10.1 9.88 9.67 9.46 9.26 9.05 8.85

15 10.5 10.3 10.1 9.87 9.67 9.46 9.26 9.06 8.86 8.65

16 10.3 10.1 9.85 9.65 9.45 9.26 9.06 8.86 8.66 8.46

17 10 9.84 9.65 9.46 9.26 9.07 8.87 8.68 8.48 8.29

18 9.83 9.64 9.45 9.26 9.07 8.88 8.69 8.5 8.31 8.12

19 9.63 9.45 9.26 9.07 8.89 8.7 8.51 8.33 8.14 7.9520 9.44 9.25 9.07 8.89 8.7 8.52 8.34 8.15 7.97 7.79

21 9.26 9.08 8.9 8.72 8.54 8.36 8.18 8 7.82 7.64

22 9.07 8.9 8.72 8.54 8.37 8.19 8.01 7.84 7.66 7.48

23 8.91 8.73 8.56 8.39 8.21 8.04 7.86 7.69 7.52 7.34

Table 3 above: Oxygen concentration (ppm) for varying

pressures (mmHg) and temperatures (degrees Celsius) at

100% relative humidity

redox potential electrodes

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