EUTECH INSTRUMENTS PTE LTD.
TABLE OF CONTENTS TABLE OF CONTENTS .................................................................................................................................1 GENERAL INSTRUCTIONS .........................................................................................................................1 Introduction.................................................................................................................................................1 Required Equipment .......................
SPECIFICATIONS ........................................................................................................................................19 ORDERING INFORMATION......................................................................................................................
GENERAL INSTRUCTIONS Introduction Eutech Instruments Ammonia Gas-Sensing Electrode is used to quickly, simply, accurately, and economically measure dissolved ammonia in aqueous solutions. It can also be used to measure the ammonium ion after conversion to ammonia or organic nitrogen from Kjeldahl digestion of the sample. The measurement is not affected by sample color or turbidity and samples do not need to be distilled.
. Eutech Ionic Strength Adjuster (ISA) Solution, 5M NaOH/.05M Disodium EDTA/10% Methanol with Color Indicator, Code No. EC-ISA-AA2-BT. To prepare this solution from your own laboratory stock, half fill a 1,000 ml beaker with distilled water and add 200 grams of reagent-grade NaOH. Stir the solution to dissolve the pellets, add 18.61 grams of disodium EDTA, and stir the solution again until all solids have dissolved. Allow solution to cool.
4. Place the bottom cap gently over the membrane onto the threads and screw the bottom cap on until finger-tight. Check that the membrane is free of wrinkles and holes or else repeat the above steps. 5. Using the syringe provided, fill the outer body with approximately 2 ml of inner filling solution. Place glass inner body into epoxy outer body containing the internal filling solution and screw on the upper cap until finger-tight.
5. To another 150 ml beaker, add 100 ml of the higher value standard and 2 ml of ISA. Place the beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode tips into the solution. 6. Adjust the meter to the concentration of the standard and fix the value in the memory. 7. Read the electrode slope according to the meter manufacturer's instructions. Correct electrode operation is indicated by a slope of 90-100%.
dissolved species below 1M. If the total level is greater than 1M, the sample should be diluted before measurement. For further explanation, see the section entitled Effect of Dissolved Species. Units of Measurement Measurement of ammonia can be expressed in units of moles/liter, ppm as nitrogen, ppm as ammonia, or other convenient concentration unit. Table 1 lists conversion units. TABLE 1: Concentration Unit Conversion Factors moles/liter 10-2 10-3 10-4 ppm N ppm NH3 140.0 14.0 1.4 170.0 17.0 1.
5. Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against the concentration (log axis). Extrapolate the calibration curve down to about 1.0 x 10-5M. A typical calibration curve can be found in Figure 1. A calibration curve is constructed on semi-logarithmic paper when using the pH/mV meter in the millivolt mode. The measured electrode potential in mV (linear axis) is plotted against the standard concentration (log axis).
2. 3. Place the more dilute solution on the magnetic stirrer and begin stirring at a constant rate. Assure that the meter is in the concentration mode. Lower the electrode tip into the solution. 4. Adjust the meter to the concentration of the ammonia standard and fix the value in the memory according to the meter manufacturer's instructions after stabilization of the reading. 5. Rinse the electrode with distilled water and blot dry. 6.
1. Measure out 100 ml of a pH 4 buffer solution, add it to a 150 ml beaker, place the beaker on the magnetic stirrer, and begin stirring. Place the electrode tip in the solution for about 3 minutes. 2. By serial dilution, prepare a 10-2M or 100 ppm ammonia standard by diluting the 0.1M or 1,000 ppm standard solution. 3. To a 2500 ml erlenmeyer flask, add 1,000 ml of distilled water and 10 ml of 10M NaOH. Place on magnetic stirrer and begin stirring.
Since water vapor reacts with ammonia in the gas phase, placing the ammonia electrode in a closed system containing water vapor will allow measurements of solutions above 10-3M (14 ppm) to be taken. Ammonia in samples containing non-aqueous solutions or surfactants can be measured by adjusting the sample pH to 11 to 13 with 10M NaOH. After measuring the sample, transfer it to an erlenmeyer flask large enough to contain approximately 2/3 volume of air after sample addition.
Complexation Metal complexes are formed with ammonia and a number of metal ions. Complexes of mercury, copper, gold, silver, nickel, zinc, cobalt, and cadmium are removed in the form of hydroxide complexes or precipitates in basic solution. When ammonia concentration is below 10-3M and hydroxide is present at the 10-1M level, the only appreciable complex formed will be between mercury and ammonia.
Limits of Detection The upper limit of detection in pure ammonia solutions is 1M. Ammonia is rapidly lost to the air above a concentration of 1M. Dilution may be used if ammonia concentrations are above 1M. Also dilute samples between 1M and 10-1M or calibrate the electrode at 4 or 5 intermediate points. The lower limit of detection is around 1X10-6 M. Refer to Figure 1 for a comparison of the theoretical response to the actual response at low levels of ammonia.
The ammonia that diffuses through the membrane dissolves in the internal filling solution, reacting reversibly with water in the filling solution, to a small extent: NH3 + H2O ⇒ NH4+ + OHThe equilibrium equation gives rise to the equilibrium constant in the following equation: [NH4+] [OH-] constant = ⎯⎯⎯⎯⎯⎯⎯ [NH3] The ammonium ion concentration can be considered fixed, since the internal filling solution contains ammonium chloride at a sufficiently high level.
It is possible to calculate the ratio of ammonia to ammonium ion, theoretically, if the pH is known. A. Martell and R. Smith in Critical Stability Constants, Plenum Press, New York, 1974, state: [NH4+] [NH4+] ⎯⎯⎯⎯ = ⎯⎯⎯⎯ K ~ 10-9.3 [NH3] 10-pH [NH3] [H3O+] at 25oC, X = 0.1 where pH ~ 9.3 The ratio of ammonium to ammonia is given by: [NH4+] ⎯⎯⎯⎯ = K-pH = 10 9.3-pH [NH3] Both temperature and ionic strength will cause the exact value of K to vary. If, for example, while the pH at 25oC and X = 0.1 is 9.
Glassware Clean glassware is essential for good measurement. Be sure to wash the glassware well with a mild detergent and rinse very well with distilled or deionized water. Clean glassware will drain without leaving water droplets behind. Electrodes The electrodes may be checked by using the procedure found in the section entitled Electrode Slope Check. 1. Be sure to use either distilled or deionized water when following the procedures given in Electrode Slope Check. 2.
Technique Be sure that the electrode's limit of detection has not been exceeded. Be sure that the analysis method is clearly understood and is compatible with the sample. Refer to the instruction manual again. Reread the sections GENERAL PREPARATION and ELECTRODE CHARACTERISTICS. If trouble still persists, call Eutech Instruments Pte Ltd. at (65) 6778-6876 and ask for the Customer Services Department.
TROUBLESHOOTING HINTS Symptom Out of Range Reading Possible Causes defective meter Next Step perform meter checkout procedure (see meter instruction manual) defective inner body refer to Checking the Electrode Inner Body electrode not plugged in properly unplug electrode and reset internal filling solution not added fill outer body of electrode with proper amount of internal filling solution air bubble on membrane remove bubble by re-dipping electrode electrode not in solution Noisy or Unstable i
high temperatures membrane failure (wet, perforation, discoloration) replace membrane samples and standards standards not at constant temperature allow samples and standards to come to room temperature before use heat generated by magnetic stirrer place insulating material between stirrer and beaker defective inner body refer to Checking the Electrode Inner Body Low Slope or No Slope electrode exposed to air for extended period hold electrode by outer body and pull on electrode cable.
value correctly incorrect standards prepare fresh standards wrong units used apply correct conversion factor: 10-3M = 17 ppm as NH3 = 14 ppm as N buffer added to standards and not samples add same proportions of buffer to standards and samples sample carryover rinse electrodes thoroughly between samples Checking the Electrode Inner Body If the electrode slope is found to be low during operation, the following solutions will be necessary to check the inner body: - pH 4 Buffer (0.
SPECIFICATIONS Concentration Range: 5 x 10-7M to 1 M NH3 (0.01 to 17,000 ppm NH3) (0.01 to 14,000 ppm as N) pH Range: above 11 Temperature Range: 0o to 50oC Inner Body Resistance: ~1,000 Mohm Reproducibility: + 2% Size: 110 mm length; 12 mm diameter; 1 m cable length Storage: Store electrodes in 0.1M NH4Cl ORDERING INFORMATION CODE NO.
