OPERATING INSTRUCTIONS MI-730, 8-730, 16-730

MI-730, 8-730, 16-730  
Oxygen Microelectrodes
Operating Instructions

Included in the  Electrode Kit

1   Oxygen Electrode with membrane housing
6   Replaceable membrane housings
1   Bottle of electrolyte
1   Filling tip
1   Set of instructions
5   Polishing pads

Electrode Assembly and Preparation

  1.  Remove the acrylic housing from the Oxygen electrode by unscrewing it from the electrode body.  This housing, with affixed Teflon membrane, does not contain the necessary oxygen electrolyte solution.

  2. Add oxygen electrolyte solution to the acrylic housing.  This is accomplished by     attaching a filling tip to the bottle of electrolyte and adding the electrolyte to the housing to a minimum height of 6 mm.  Gently place the bubble-free end of the filling tip against the Teflon membrane and release the electrolyte to the minimum height.

  3. Insert the housing on to the Oxygen electrode, being careful not to trap any air bubbles near the electrode tip.  The housing is screwed clockwise into the body of the electrode until it stops.  Check the tip of the electrode for proper seating of the housing against the electrode.  The electrode is properly seated if the electrode protrudes slightly beyond the end of the housing.

Calibration

Calibration of the electrode requires the use of two standard gases with percent values that are close to the percent values of Oxygen to be measured.  Common values used are 0% Oxygen for zeroing and 21% Oxygen (Ambient Air) for sloping or gain.

 A.  Calibration for gas samples: When samples to be measured are gaseous, then calibration should be performed with humidified gases.  Two possible setups for calibrating the electrode are shown below.  Keep the tip of the electrode as far as possible from the surface of the water.  The bubbling rate of the gas should be slow (3 - 6 bubbles per second).  Although  bubbling at a faster rate will flush the chamber more quickly, it will also cause a cooling effect on the electrode. (Insert Picture)

  1. Bubble the 0% gas through the chamber and adjust the zero of the meter after a stable reading is obtained.  It may take up to 15 minutes to de-gas the chamber of contaminants, however bubbling the gas vigorously will cause droplets to collect on the tip of the electrode.  This will make the electrode response time appear to be slow.

  2. Bubble the 21% gas (or any other percent value you decided to use for your application) through the chamber until a stable reading is obtained.  Adjust the calibration control to 21%.  This procedure of alternating between the two gases should be continued until you become confident of stability and reproducibility.  The electrode is now ready to use.

B.  Calibration for liquid samples: To decrease calibration time, two separate calibration chambers should be used.  One for the 0% gas and another for the sloping gas such as 21%.  When setting up the calibration chambers initially, it will take up to 30 minutes to flush each chamber to obtain a steady state oxygen level and a constant temperature.  Again the bubbling rate should be carefully regulated (3 - 6 bubbles per second) so that both calibrating liquids are at the same temperature.

Calibrating standards and samples must be at the same temperature for accurate Oxygen measurements.

  1. Immerse the tip of the electrode into the 0% standard and adjust the zero of the meter after a stable reading is obtained.

  2. Remove the electrode from the first standard and place it into the second standard.  Adjust the calibration control to the value of the second standard (ex: 21%) Alternate between the two standards until you become confident of stability and reproducibility.  The electrode is now ready to use.

Handling

When necessary, the membrane of the electrode can be replaced by following the assembly procedure above.  When removing and replacing a membrane as well as when calibrating or making measurements, be careful not to apply pressure against the internal electrode.  Any excessive pressure against the internal electrode can cause the electrode to crack rendering it useless and unrepairable.

Cleaning

When using the electrode in solutions containing protein, the electrode should be soaked in an enzyme cleaning solution such as Terg-a-zyme (Alconox, Inc.) after each use for a couple of minutes to remove the protein from the membrane surface.  This will prolong the useful life of the membrane.

Storage

Always clean and rinse the electrode before storing:

Long-term (over 2 month): Remove the membrane housing from the electrode.  Rinse the internal electrode with distilled water and pat dry.  Place a new, unfilled membrane housing over the internal electrode and attach loosely (Do not seat completely).  This membrane will serve to keep the dust off of the electrode tip. 

 

Short-term: The electrode can be left in room air with membrane housing still attached.

Output Conversion

Formula for conversion of % oxygen to solubility in moles/liter:

S = (a/22.414) x (760-p)/760) x (r%/100)

S = solubility of gas in moles per liter                  

a = absorption coefficient of gas at temperature

P = vapor pressure of water at temperature      

r% = actual reading in percent Oxygen                  

Temperature (T) vs. absorption coefficient (a) for oxygen in water:

 
Degrees
 Celsius
Absorption
 Coefficient
Degrees
Celsius
Absorption
 Coefficient

5

0.04287

19

0.03161

6

0.04180

20

0.03102

7

0.04080

21

0.03044

8

0.03983

22

0.02988

9

0.03891

23

0.02934

10

0.03802

24

0.02881

11

0.03718

25

0.02831

12

0.03637

26

0.02783

13

0.03559

27

0.02736

14

0.03486

28

0.02691

15

0.03415

29

0.02649

16

0.03348

30

0.02608

17

0.03283

35

0.02440

18

0.03220

40

0.02306