The MI-740, 8-740 or the16-740 Electrode Kit contains the following:

1   Electrode with membrane housing
6   Replaceable membrane housings
1   Bottle of electrolyte
1   Bottle of NH₃ -free NaOH solution
1   Filling Tip
1   Set of Instructions

Electrode Assembly and Preparation

1. Remove the acrylic housing from the ammonia electrode by unscrewing the housing from the electrode body.  This housing, with affixed Teflon membrane, does not contain the necessary 740 electrolyte solution.

2.  Add 740 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 ammonia 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 ammonium chloride standards that bracket the concentration range expected in your samples.  A working calibration curve must be constructed using semilog graph paper.  Following the calibration procedure is an example of a working calibration curve.

Meter requirement:  Any research grade pH meter with millivolt mode capability.  Set meter to millivolt mode.

The MI-740 is standardized using ammonium chloride (NH₄Cl) solutions.  A 55 mv difference (approximate) will occur between each tenfold change in concentration.  For example, test in  0.0001 N NH₄Cl,  0.01 N NH₄Cl  and  0.1 N NH₄Cl.

A.  Calibration procedure for liquid samples:

The use of dram vials is probably the easiest way to calibrate the MI-740 Micro-ammonia electrode.

1.  Fill each of four vials to the neck of the bottle with ammonium chloride solutions listed above and label the vials  (Ex: 0.0001, 0.001. 0.01 etc.)

2.  Place 2-3 drops of the NaOH solution in the first vial.  Cap the vial immediately and shake vigorously for a few seconds.  Do the same for each remaining vial.

3.  Take the vial labeled 0.1 and remove the cap.  Place the electrode into the solution immediately and record the millivolt reading once it stabilizes (approximately 2 - 3 minutes).

4.  Remove the electrode from the 0.1 vial, dab the excess solution off of the electrode using a paper towel, and place it into the 0.01 vial.  Re-cap the 0.1 vial.  Record the millivolt reading after stabilization.  Repeat this step for the 0.001 and the 0.0001 standards.

5.  The 0.001 and the 0.0001 vials tend to lose ammonia easily once opened, therefore the standards in these vials may need to be replaced after each 2 - 3 minute calibration check.

6.  Repeat this procedure until the millivolt readings are reproducible (usually about 3 repetitions).

7.  Soak the electrode in the treated standard that is closest to the ammonia level expected in your samples between calibration and samples.

8.  Because the membrane is permeable to water vapor, the electrode voltage may drift slightly (1 -2 mv) over time.  We recommend a 1 point calibration check between samples in the standard that is closest to your expected concentration levels in your samples.  This will allow you to compensate for any voltage shifts in your calibration curve.

B.  Calibration for gas samples:

When calibrating for gas samples, use the above procedure with one exception.  Place the tip of the electrode in the headspace directly above the liquid in the vials.

Remember:  For best results, do the following:

1.  Always use standards that bracket the concentration range that you expect to have in your samples.

2.  Always mix your standards to closely match the composition of your samples.

3.  After calibrating, always keep the tip of the electrode in a treated standard between samples.

(Insert Calibration Curve)

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.

Storing

Always clean and rinse the electrode before storing:

Long-term (over 1 week):  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 sensing tip of the electrode with membrane housing still attached can be left soaking in a small amount of the 740 electrolyte.  Because of the high water vapor transport rate of the membrane, storing of the assembled electrode in air overnight will cause the membrane housing to dry out completely.

Sample Handling

The electrode measures dissolved ammonia gas, therefore handling of your sample is critical for accurate measurement.  Samples should be measured immediately after collection and treatment with the  NaOH solution to minimize exposure of the sample to room air.  When samples need to be stored for a time before measurement, acidify the sample to a pH of 5 or 6.  When ready for measuring,  add several drops of the  NaOH solution supplied to adjust the pH to 11 or 12 to convert all of the ammonium ion to dissolved ammonia gas.