Thermo Scientific 9512BNWP Instruction Manual

TABLE OF CONTENTS

GENERAL INFORMATION 1

Introduction 1
Required Equipment 1
Required Solutions 2

USING THE ELECTRODE 3

Set Up 3

Electrode Assembly 3
Electrode Preparation 4
Checking Electrode Operation (Slope) 7

Before Analysis 8

Units of Measurement 8
Sample Requirements 8
Measuring Hints 8
Sample Storage 9

Analytical Procedures 10

Analytical Techniques 10
Direct Calibration 11
Low Level Measurements By Direct Calibration 14
Known Addition 16

Electrode Storage 19

TROUBLESHOOTING 22

Troubleshooting Checklist 22
Troubleshooting Guide 26
Assistance 28

ELECTRODE CHARACTERISTICS 29

Electrode Response 29
Reproducibility 30
Temperature Effects 30
Interferences 30
pH Effects 31
Complexation 31
Effect of Dissolved Species 31
Electrode Life 31
Theory of Operation 32

ORDERING INFORMATION 37

SPECIFICATIONS 38

1

GENERAL INFORMATION

Introduction

The Orion 95-12 Ammonia Electrode allows fast, simple,
economical, and accurate measurements of dissolved ammonia in
aqueous solutions. This gas-sensing electrode can also be used to
measure the ammonium ion after conversion to ammonia, or
organic nitrogen after Kjeldahl digestion of the sample. Sample
color and turbidity do not affect the measurement, and samples
need not be distilled. Almost all anions, cations, and dissolved
species, other than volatile amines, do not interfere.

All apparatus and solutions required for measurement, electrode
characteristics and electrode theory are discussed in this manual.
General analytical procedures, low-level procedures and a method
for measuring ammonia in solutions that wet the membrane
are given.

The Orion 95-12 comes with the following:

• forty loose membranes

• reusable membrane cap

• tweezers for handling membranes

• dispensing cap

• electrode filling solution

• electrode instruction manual.

Operator instructions for Orion Meters are outlined in the individual
meter’s instruction manual. Our Technical Service Chemists can
be consulted for assistance and troubleshooting advice. Please
refer to Troubleshooting Section for information on contacting
Technical Services.

2

Required Equipment

Meter

The easiest type of meters to use is direct concentration readout
specific ion meter, such as Orion EA940, 920A, 920Aplus, 720A,
720Aplus, 710A, 710Aplus, 290A, 290Aplus or 370. If unavailable, a
pH/mV meter with readability to 0.1 mV could be used.

Magnetic Stirrer

Highly recommended for laboratory measurements.

Graph Paper

4-cycle semilogarithmic paper for preparing calibration curves
(for use with digital pH/mV laboratory meters)

Required Solutions

Distilled or Deionized Water

Water MUST be ammonia-free. Pass distilled or deionized water
through an ion-exchange column containing a strong acidic cation
exchange resin, such as Dowex 50W-X8.

Standard Solutions Orion

Ionic Strength Adjustor (ISA) 951211
Electrode Filling Solution 951202

0.1 M Ammonium Chloride Standard 951006
1000 ppm as Nitrogen Standard 951007

Solutions Prepared by Customer

For inner body check:

To check the operation of the electrode inner body.

Solution 1 — pH 4.01 Buffer with 0.1M NH

4

Cl (or 0.1 M NaCl)

Take 200 mL of the pH 4.01 buffer solution, Orion 910104, add 1.07 g
reagent-grade NH

4

Cl (or 1.16g reagent-grade NaCl), stir to mix and
label bottle as Solution 1. Store the buffer for repeated use.
Discard buffer if turbidity develops.

Solution 2 — pH 7.00 Buffer with 0.1M NH

4

Cl (0.1 M NaCl)

Take 200 mL of pH 7.00 buffer solution, Orion 910107, add 1.07 g
reagent-grade NH

4

Cl (or 1.16g reagent-grade NaCl), stir to mix and
label bottle as Solution 2. Store the buffer for repeated use.
Discard bottle if turbidity develops.

3

USING THE ELECTRODE

Set Up

Electrode Assembly

NOTE: Soak inner body in electrode filling solution for at
least two hours before assembling new electrode. For best
results, soak inner body overnight.

Assemble electrode according to instructions below. The
electrode is shipped dry with loose membranes and a loose
membrane cap. Avoid excessive handling of the membrane during
assembly; this may affect the membrane’s hydrophobic properties,
causing shortened membrane life. Use the tweezers provided.
A membrane will last from one week to several months depending
on usage.

NOTE: Membrane failure is characterized by a shift in
electrode potential, drift, and poor response. See
Troubleshooting section. Membrane failure may be apparent
on visual inspection as dark spots or discoloration of the
membrane.

4

Electrode Preparation

NOTE: Soak inner body in electrode filling solution for at
least two hours before assembling electrode. For best
results, soak inner body overnight.

1. While holding the probe vertically, unscrew the top cap (a)

and carefully remove the glass electrode inner body (b) from
the electrode outer body (c) (See Fig. 1). Set cap with inner
body aside carefully. Pour out remaining electrode filling
solution from outer body if applicable, new probe will not have
any filling solution in the body.

2. Unscrew membrane cap (d) from outer body (See Fig. 2).

Remove old membrane, if applicable.

3. Wearing gloves, use tweezers to carefully grasp corner of

white membrane from between paper separators (See Fig. 3).
Do not to touch center of membrane.

(a)

(b)

(c)

(d)

Fig. 1

(d)

Fig. 2

Fig. 3

4. With free hand, grasp electrode outer body with threads

oriented to the top. Align straight edge of membrane against
threaded shoulder and hold with thumb (See Fig. 4). With other
hand, gently stretch the membrane upwards (See Fig. 5), then
across the opening (See Fig. 6), then down to align other edge
with the opposite shoulder (See Fig. 7). While holding each
edge on both sides, gently stretch each serrated side of the
membrane out and down over threads making sure that
membrane surface is smooth and without wrinkles (See Fig. 8).
Smooth any loose material taking care not to touch center of
membrane (See Fig. 9).

5

Fig. 4

Fig. 7

Fig. 8

Fig. 5

Fig. 6

Fig. 9

6

NOTE: Do not overstretch the membrane. The membrane is
considered overstretched if the black body material can be
seen through the membrane. Discard membrane if
overstretched and replace with new membrane. Continue
from Step 3.

5. Replace membrane cap being careful not to touch membrane
center (See Fig. 10). Screw down half way and tuck any loose
membrane material under cap while twisting. Make sure cap
is fully screwed on.

6. Fill the outer body with 2.5 ml filling solution, or approximately
50 drops from the bottle (See Fig. 11).

7. Place inner body into outer body containing filling solution and
screw on upper cap (See Fig. 12).

8. Carefully shake fully assembled electrode as if it were a
clinical thermometer to remove bubbles. Gently pull spring
loaded cable back and slowly release to allow filling solution
to migrate between membrane and glass electrode inner body
(See Fig. 13).

9. Rinse the electrode well and wipe dry.

Fig. 10

Fig. 11

Fig. 12 Fig. 13

Checking Electrode Operation (Slope)

These are general instructions, which can be used with most
meters to check electrode operation. See individual meter
instruction manuals for more specific information.

This procedure measures electrode slope. Slope is defined as the
change in millivolts observed with every tenfold change in
concentration. Obtaining the slope value provides the best means
for checking electrode operation.

1. If electrodes have been stored dry, prepare the electrodes as
described under the section entitled Electrode Preparation.

2. Connect electrodes to the meter. For an electrode with a BNC
connector, turn the connector clockwise to attach to the
meter. For an electrode with a U.S. Standard connector, insert
reference pin-tip connector and the sensing electrode
connector into appropriate jacks on the meter. Non-Orion
meters may require special adaptors. Consult your meter
instruction manual.

3. Place 100 mL distilled water into a 150 mL beaker. Add 2 mL
pH-adjusting ISA, Orion 951211. Stir thoroughly. Set the meter
to the mV mode.

4. Rinse electrode with distilled water and place in the solution
prepared in step 3 above. To prevent air entrapment on the
membrane surface, be sure to use an electrode holder that
keeps the electrode at a 20˚ angle. If bubbles appear on the
sensing membrane, tap electrode gently to remove.

5. Select either 0.1 M or 1000 ppm ammonium chloride standard.
Pipet 1 mL of the standard into the beaker, stir thoroughly.
When a stable reading is displayed, record the electrode
potential in millivolts.

6. Pipet 10 mL of the same standard into the same beaker. Stir
thoroughly. When a stable reading is displayed, record the
electrode potential in millivolts.

7. Take the first potential reading and subtract it from the
second one. The difference should be in the range of

-54 to -60 mV/decade when the solution temperature is
between 20-25 ˚C. If the potential is not within this range,
refer to Troubleshooting.

7

Before Analysis

Units of Measurement

Ammonia can be measured in units of moles per liter, parts per
million as ammonia, parts per million as nitrogen, or any other
convenient unit (see Table 1).

Table 1
Concentration Unit Conversion Factors

Moles/Liter ppm as N ppm as NH

3

10

-4

1.40 1.70

10

-3

14.0 17.0

10

-2

140 170

10

-1

1400 1700

Sample Requirements

Samples must be aqueous; they must not contain organic solvents.
Consult Our Technical Service Chemists for use of the electrode in
unusual applications. Samples and standards should be at the
same temperature. A 1 ˚C difference in temperature will give rise
to about 2% measurement error. In all analytical procedures, pH­adjusting ISA must be added to all samples and standards
immediately before measurement. After addition of the ISA all
solutions should fall within a pH 11 to 14 range (solution should be
blue in this range) and have a total level of dissolved species
below 1 M. If the total level of dissolved species is above 1 M, see
section entitled Effect of Dissolved Species.

Measuring Hints

Minimize NH3loss from sample by following the recommendations
below:

• Store samples according to procedure in Sample Storage.

• Use beakers that minimize the ratio of surface area to volume.

• Keep beakers containing standards and samples covered
between measurements.

8

• Add 2 mL of pH-adjusting ISA to 100 mL of sample or standard
immediately before making measurements, and make sure the
blue color is observed.

• Between measurements, rinse the electrode with distilled
water.

• Be sure samples, standards, and electrodes are at the same
temperature.

• Samples and standards should be stirred using a magnetic
stirrer. Some magnetic stirrers generate enough heat to
change solution temperature. Placing a piece of insulating
material such as cork or styrofoam between the beaker and
the stirring plate can minimize this effect.

• Verify calibration every two hours by placing electrodes in a
fresh aliquot of the first standard solution used for calibration.
If value has changed, recalibrate the electrode.

• Always use fresh standards for calibration.

• After immersion in solution, check electrode for any air
bubbles on membrane surface and remove by shaking the
electrode as you would with a thermometer or lightly tapping
of the electrode.

• If electrode response is slow, the membrane may contain a
surface layer of contaminant. Restore performance by
soaking electrode in distilled water for about 5 minutes, then
rinse and soak in a standard solution for about 1 hour
before use.

Sample Storage

If possible, alkaline samples should be measured at once. The rate
of ammonia loss at room temperature from a stirred 100 mL basic
solution in a 100 mL beaker is about 50% in six hours. If samples
must be stored, make them slightly acidic (pH 6) by adding 0.5 mL
of 1 M HCl to each liter of sample, and place them in tightly capped
vessels. Make stored samples basic with pH-adjusting ISA
immediately before measurement.

9

Analytical Procedures

Analytical Techniques

A variety of analytical techniques are available to the analyst. The
following is a description of these techniques.

Direct Calibration is a simple procedure for measuring a large
number of samples. Only one meter reading is required for each
sample. Calibration is performed in a series of standards. The
concentration of the samples is determined by comparison to the
standards. ISA is added to all solutions to ensure that samples and
standards have similar ionic strength.

Incremental Techniques provide a useful method for measuring
samples, since calibration is not required. As in direct calibration,
any convenient concentration unit can be used. The different
incremental techniques are described below. They can be used to
measure the total concentration of a specific ion in the presence of
a large (50-100 times) excess of complexing agents.

• Known Addition is an alternate method useful for measuring
dilute samples, checking the results of direct calibration
(when no complexing agents are present), or measuring the
total concentration of an ion in the presence of an excess
complexing agent. The electrodes are immersed in the
sample solution and an aliquot of a standard solution
containing the measured species is added to the sample.
From the change in potential before and after the addition, the
original sample concentration is determined.

• Known Subtraction is useful as a quick version of a titration,
or for measuring species for which stable standards do not
exist. It is necessary to know the stoichiometric ratio
between standard and sample. For known subtraction, an
electrode sensing the sample species is used. Stable
standards of a species reacting completely with the sample in
a reaction of known stoichiometry are necessary.

• Analate Addition is often used to measure soluble solid
samples, viscous samples, small or very concentrated
samples, to diminish the effects of complex sample matrices,
or to diminish the effects of varying sample temperatures.
This method is not suitable for dilute or low concentration
samples. Total concentration is measured even in the
presence of complexing agents. The electrodes are immersed
in a standard solution containing the ion to be measured and
an aliquot of the sample is added to the standard. The original
sample concentration is determined from the change in
potential before and after the addition.

10

• Analate Subtraction is used in the measurement of ions for
which no ion-selective electrode exists. The electrodes are
immersed in a reagent solution which contains a species that
the electrode senses, and that reacts with the sample. It is
useful when sample size is small, or samples for which a
stable standard is difficult to prepare, and for viscous or very
concentrated samples. The method is not suited for very
dilute samples. It is also necessary to know the
stoichiometric ratio between standard and sample. Specific
instructions for known subtraction, analate addition and
analate subtraction, can be found in the Orion meter
instruction manuals. Titrations are quantitative analytical
techniques for measuring the concentration of a species by
incremental addition of a reagent (titrant) that reacts with the
sample species. Sensing electrodes can be used for
determination of the titration end point. Ion-selective
electrodes are useful as end point detectors, because they
are unaffected by sample color or turbidity. Titrations are
approximately 10 times more precise than direct calibration,
but are more time-consuming.

Direct Calibration

Set Up

1. Prepare the electrode as described in Electrode Assembly
and Electrode Preparation.

2. Connect electrode to meter.

3. Prepare two standards which bracket the expected sample
range and differ in concentration by a factor of ten. Standards
can be prepared in any concentration unit to suit the
particular analysis requirement. All standards should be at
the same temperature as the samples. (For details on
temperature effects on electrode performance, refer to
Temperature Effects.)

11